%0 journal article %@ 0013-7944 %A Examilioti, T.N., Karanikolas, D., Riekehr, S., Al-Hamdany, N., Papanikos, P., Klusemann, B., Kashaev, N., Alexopoulos, N.D. %D 2024 %J Engineering Fracture Mechanics %P 109811 %R doi:10.1016/j.engfracmech.2023.109811 %T Effect of filler materials on the tensile properties and fracture toughness of laser beam welded AA2198 joints under different ageing conditions %U https://doi.org/10.1016/j.engfracmech.2023.109811 %X The influence of different filler materials on the microstructure, tensile mechanical properties, and fracture toughness of laser beam welded AA2198 alloy as well as the effect of different artificial ageing heat treatments were investigated in this contribution. The welded joints were produced when exploited either, Al-Si (AA4047) or Al-Cu (AA2319) filler wires. It was shown that the Al-Si filler wire gave higher hardness values in the fusion zone when compared to the Al-Cu filler wire. The post heat treatment of the welded specimens increased by approximately +100 % the yield stress and by +20 % the ultimate tensile strength with increasing ageing time, in a similar way to the non-welded material. Elongation at fracture decreased in an inverse proportional manner to yield stress. Artificial ageing before welding gave improved elongation at fracture for the over-aged condition only. The quality index concept showed that the artificial ageing before the welding did not succeed in giving a higher quality of the welded joints, for both filler materials investigated. The opposite was shown on the post heat treatment, where the peak-aged condition increased substantially the ‘quality’ of the welded joints with both filler materials. The critical stress intensity factor was increased by +25 % for the under-aged condition for the post-welded condition and both investigated filler wires as a result of the balance between medium values in strength and ductility, respectively. %0 journal article %@ 2238-7854 %A Kuliiev, R., Keller, S., Kashaev, N. %D 2024 %J Journal of Materials Research and Technology : JMRT %P 1975-1989 %R doi:10.1016/j.jmrt.2023.11.168 %T Identification of Johnson-Cook Material Model Parameters for Laser Shock Peening Process Simulation for AA2024, Ti-6Al-4V and Inconel 718 %U https://doi.org/10.1016/j.jmrt.2023.11.168 %X This paper addresses the identification of Johnson-Cook material model parameters for the simulation of high strain rate processes such as laser shock peening. A combined numerical and experimental approach is described for the identification of the material parameters for AA2024-T3, Ti–6Al–4V, and Inconel 718 alloys. Validation of the parameters was performed based on depth-resolved residual stress profile evaluation after both experimental and numerical laser shock peening application. However, it has been observed that the strain rate-dependent coefficient identified at low strain rates is insufficient for accurately representing the behavior of the Ti–6Al–4V alloy. Consequently, there is a need to identify and determine the appropriate parameter specifically tailored to the strain rates that are relevant to the intended application of the material. The outcomes of this study provide significant insights into the accurate identification of Johnson-Cook material model parameters for laser shock peening simulations. The findings emphasize the critical importance of considering the strain rate-dependent coefficient C in the Johnson-Cook material model to enhance the accuracy and precision of representing material behavior in simulation efforts. %0 journal article %@ 2213-8463 %A Sala, S.T., Körner, R., Huber, N., Kashaev, N. %D 2023 %J Manufacturing Letters %P 60-64 %R doi:10.1016/j.mfglet.2023.09.006 %T On the use of machine learning and genetic algorithm to predict the region processed by laser peen forming %U https://doi.org/10.1016/j.mfglet.2023.09.006 %X Laser peen forming uses laser-pulse-induced strains to deform sheets by adjusting laser parameters and peening patterns. Finding an optimal pattern in a vast space of practically infinite solutions is challenging. This study presents a workflow using a simplified model to predict deformation. A machine learning-based cellular automata neural network (CANN) and genetic algorithm (GA) were used for pattern prediction. Experiments showed high process uncertainty, justifying simplified modeling. The CANN predicted patterns reliably but lacked generalization due to insufficient deformation data for various process parameters. The GA required optimization efforts to reduce computation time but was successful at generalizing pattern prediction. %0 journal article %@ 1572-8145 %A Sala, S.T., Bock, F.E., Pöltl, D., Klusemann, B., Huber, N., Kashaev, N. %D 2023 %J Journal of Intelligent Manufacturing %R doi:10.1007/s10845-023-02240-y %T Deformation by design: data-driven approach to predict and modify deformation in thin Ti-6Al-4V sheets using laser peen forming %U https://doi.org/10.1007/s10845-023-02240-y %X The precise bending of sheet metal structures is crucial in various industrial and scientific applications, whether to modify deformation in an existing component or to achieve specific shapes. Laser peen forming (LPF) is proven as an innovative forming process for sheet metal applications. LPF involves inducing mechanical shock waves into a specimen that deforms the affected region to a certain desired curvature. The degree of deformation induced after LPF depends on numerous experimental factors such as laser energy, the number of peening sequences, and the thickness of the specimen. Consequently, comprehending the complex dependencies and selecting the appropriate set of LPF process parameters for application as a forming or correction process is crucial. The main objective of the present work is the development of a data-driven approach to predict the deformation obtained from LPF for various process parameters. Artificial neural network (ANN) was trained, validated, and tested based on experimental data. The deformation obtained from LPF is successfully predicted by the trained ANN. A novel process planning approach is developed to demonstrate the usability of ANN predictions to obtain the desired deformation in a treated region. The successful application of this approach is demonstrated on three benchmark cases for thin Ti-6Al-4V sheets, such as deformation in one direction, bi-directional deformation, and modification of an existing deformation in pre-bent specimens via LPF. %0 journal article %@ 1059-9495 %A Kuliiev, R., Riekehr, S., Ventzke, V., Keller, S., Kashaev, N. %D 2023 %J Journal of Materials Engineering and Performance %P 10843-10856 %R doi:10.1007/s11665-023-08385-4 %T On the Effect of Testing Frequency on High and Very High Cycle Fatigue Behavior of AA2024-T3; Ti-6Al-4V; and Inconel 718 %U https://doi.org/10.1007/s11665-023-08385-4 %X An increase of testing frequency to reduce the testing time could affect the fatigue behavior of metallic materials. The current paper investigates the effect of testing frequency on the high and very high cycle fatigue behavior of AA2024, Ti-6Al-4V, and Inconel 718 alloys tested at 20, 90, and 1000 Hz. To quantify the effect of testing frequency a two-parameter Weibull distribution and Basquin’s equation were utilized as well as the threshold stress intensity factor range was determined. Fatigue crack initiation sites were evaluated on the fracture surface with respect to different testing frequencies. The results showed that changes in the testing frequency significantly influence the mechanical response of the Ti-6Al-4V alloy, while no substantial impact on the fatigue properties of AA2024 and Inconel 718 alloys was identified. The findings of the study contribute to a more complete knowledge of the frequency sensitivity of the alloys and provide valuable insights for the design and evaluation of materials used in high-frequency applications. %0 journal article %@ 2238-7854 %A Examilioti, T.N., Li, W., Kashaev, N., Ventzke, V., Klusemann, B., Alexopoulos, N.D. %D 2023 %J Journal of Materials Research and Technology : JMRT %P 895-908 %R doi:10.1016/j.jmrt.2023.02.206 %T On anisotropic tensile mechanical behavior of Al-Cu-Li AA2198 alloy under different ageing conditions %U https://doi.org/10.1016/j.jmrt.2023.02.206 %X The anisotropic mechanical behavior of Al–Cu–Li (AA2198) alloy under different artificial ageing conditions as well as for different thicknesses of the material was examined in the present investigation. Material characterization was performed for three different sheet directions, namely, longitudinal (0°), diagonal (45°) and transverse (90°) to the rolling direction. The results showed that the grain structure did not have significant changes by applying different artificial ageing times. In T3 heat treatment condition, only δ′ (Al3Li) phase was observed, while with increasing the ageing time, the major precipitates were T1 (Al2CuLi) phases. The latter was found to increase in size with artificial aging time. Almost similar yield strength values were observed for all investigated thicknesses, while the higher thickness specimens showed higher elongation at fracture values. Anisotropy was slightly higher at T3 condition, while the lowest anisotropy degree was noticed at the peak-aged condition. The specimens extracted in the diagonal direction showed high variation in mechanical properties, when compared with the respective results at the other two investigated directions, independently from the thickness of the specimens as well as from heat treatment condition. The smaller thickness specimens presented lower elongation at fracture values and for all investigated sheet directions, since they undergo plane stress conditions. Analysis of work hardening behavior showed that Bauschinger effect, which was not observed in T3 condition, became increasingly more prominent with artificial ageing time in all directions investigated. %0 journal article %@ 1526-6125 %A Wang, M., Kashaev, N. %D 2023 %J Journal of Manufacturing Processes %P 286-299 %R doi:10.1016/j.jmapro.2023.01.011 %T On the optimal process window for powder-based laser-directed energy deposition of AA7050 under different robot programs and scanning strategies %U https://doi.org/10.1016/j.jmapro.2023.01.011 %X Powder-based laser-directed energy deposition (L-DED) has drawn lots of attention during the last years because of the multiple application areas, such as coating, repairing, and building 3D structures. One challenge in L-DED is the limited variety of applicable materials, especially in the case of aluminum alloys. To broaden the material spectrum, the processability and the optimal process window of high-strength Al-alloy AA7050 are investigated in the present study. Besides the process parameters, special emphasis was paid to the scanning strategy and the robot program controlling. The present results show a strong effect of scanning strategy and robot program on buildability, geometrical accuracy, melt pool visibility, porosity level, and crack initiation. Hot cracks can be reduced or eliminated by choosing an appropriate combination of scanning strategy and robot program. Meanwhile, the geometrical accuracy of specimens with any height was well maintained. Based on the systematic experimental study, appropriate L-DED process parameters were identified for the deposition of structures up to 24 layers with a lower porosity level (about 3.7 ± 0.8 %) and lower amount of hot cracks, which are comparable with that in the 12-layer structure. %0 journal article %@ 0924-0136 %A Sala, S., Keller, S., Chupakhin, S., Pöltl, D., Klusemann, B., Kashaev, N. %D 2022 %J Journal of Materials Processing Technology %P 117578 %R doi:10.1016/j.jmatprotec.2022.117578 %T Effect of laser peen forming process parameters on bending and surface quality of Ti-6Al-4V sheets %U https://doi.org/10.1016/j.jmatprotec.2022.117578 %X Laser peen forming (LPF) is a metal forming process that utilizes laser-induced mechanical shock waves to form desired shapes or modify bent structures. The present work focuses on the applicability of LPF to Ti-6Al-4V sheets, to identify an optimal LPF process parameter window and achieve desired bending without compromising the surface quality within the peened region. The effect of LPF process parameters, i.e. laser power density, overlap, type of sacrificial overlay, and the number of peening sequences was investigated for specimens with different thicknesses. The laser power density and number of peening sequences were the most influential parameters that affect the bending of the specimens. Using sacrificial overlay has a significant effect on the bending and surface quality of the specimens. Surface quality after LPF was assessed by measuring the roughness in the peened region. In experiments without a sacrificial overlay, a black titanium oxide residue on the peened region was observed and additionally, small micro-cracks were found in the near surface region. Further characterization of the peened region revealed that the average crack length increased with increase in laser power density. Two possible LPF process parameter combinations were identified to obtain bending in the peened region, where LPF with sacrificial overlay resulted in no surface damage. Furthermore, residual stresses were determined at various LPF process parameters by incremental hole-drilling method in the peened region. %0 journal article %@ 0925-8388 %A Wang, H., Keller, S., Chang, Y., Kashaev, N., Yan, K., Gurevich, E.L., Ostendorf, A. %D 2022 %J Journal of Alloys and Compounds %P 163011 %R doi:10.1016/j.jallcom.2021.163011 %T Effect of laser shock peening without protective coating on the surface mechanical properties of NiTi alloy %U https://doi.org/10.1016/j.jallcom.2021.163011 %X We study the effect of laser shock peening (LSP) without protective coating on the surface mechanical property of NiTi alloy. The Vickers microhardness and wear resistance are measured to determine the mechanical property of NiTi samples treated with different LSP parameters (3 J with 10 ns and 5 J with 20 ns). From the electron backscatter diffraction (EBSD) analysis, it can be found that the laser shock peening does not induce obvious grain refinement in the surface region of NiTi alloy. Both compressive and tensile residual stress in the top layer are determined using the hole drilling method. The results show that the LSP treatment without a protective coating increases the roughness and enhances the surface mechanical properties of NiTi alloy. %0 journal article %@ 0966-9795 %A Panov, D., Naumov, S., Stepanov, N., Sokolovsky, V., Volokitina, E., Kashaev, N., Ventzke, V., Dinse, R., Riekehr, S., Povolyaeva, E., Nochovnaya, N., Alekseev, E., Zherebtsov, S., Salishchev, G. %D 2022 %J Intermetallics %P 107466 %R doi:10.1016/j.intermet.2022.107466 %T Effect of pre-heating and post-weld heat treatment on structure and mechanical properties of laser beam-welded Ti2AlNb-based joints %U https://doi.org/10.1016/j.intermet.2022.107466 %X The effect of pre-heating and post-weld heat treatment on microstructure and mechanical properties of laser-welded joints in a Ti–23Al–23Nb-1.4V-0.8Zr-0.4Mo-0.4Si (at.%) alloy was studied. Laser beam-welding was carried out at room temperature as well as after pre-heating up to 800°С. The post-weld heat treatment comprised either air quenching from 920°С followed by aging at 800°С or only aging at 800°С. The microstructure of the fusion zone consisted of columnar β-grains after welding at room temperature and 400 °C or both the columnar and large equiaxed crystals at 600 and 800 °C. An increase in the pre-heating temperature caused the columnar β-crystals growth as well as an increase in the fusion zone and heat-affected zone widths. Meanwhile, a decrease in the Al and Ti content, as well as an increase in both the porosity and gaseous elements content (O and N) after welding at 600–800 °C were found. The microhardness of each joint obtained after welding with pre-heating temperatures up to 600 °C was lower than that of the base material. All the welded joints showed the yield strength and ultimate tensile strength levels between 1070 and 1110 MPa, which correspond to approximately 80% of the base metal level. Reasonable total elongation of the joint was achieved after welding at 400 °C (4.3%). The post-weld heat treatment involving air quenching from 920 °C with subsequent aging at 800 °C for 6 h demonstrated the best results. The heat treatment resulted in the precipitation of the O- and α2-phases and an increase in total elongation to 6.5%. %0 journal article %@ 0925-8388 %A Mironov, S., Ozerov, M., Kalinenko, A., Stepanov, N., Plekhov, O., Sikhamov, R., Ventzke, V., Kashaev, N., Salishchev, G., Semiatin, L., Zherebtsov, S. %D 2022 %J Journal of Alloys and Compounds %P 163383 %R doi:10.1016/j.jallcom.2021.163383 %T On the relationship between microstructure and residual stress in laser-shock-peened Ti-6Al-4V %U https://doi.org/10.1016/j.jallcom.2021.163383 %X The relationship between residual stress and microstructure evolution during laser shock peening (LSP) of Ti-6Al-4V was investigated. To this end, the program material was processed using a 5-J Q-switched Nd:YAG laser with a wavelength of 1064 nm and a pulse duration of 20 ns. The residual stresses developed during LSP were determined by means of the incremental-hole-drilling method, and the corresponding microstructures were established using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). From these observations, it was deduced that deformation and the resulting microstructure evolution during LSP were controlled by an inhibition of dislocation cross-slip, which, in turn, was attributed to the extremely short duration of the process. Hence, it was surmised that the unique residual-stress state generated during LSP is associated with two intrinsic characteristics of this technique, i.e., the very high imposed energy and the extremely short time scale. The large and non-uniform mechanical energy input gives rise to the residual stresses while the limited time span prevents stress relief via dislocation cross slip and climb. %0 journal article %@ 2075-4701 %A Pozdnyakov, V., Keller, S., Kashaev, N., Klusemann, B., Oberrath, J. %D 2022 %J Metals %N 1 %P 107 %R doi:10.3390/met12010107 %T Coupled Modeling Approach for Laser Shock Peening of AA2198-T3: From Plasma and Shock Wave Simulation to Residual Stress Prediction %U https://doi.org/10.3390/met12010107 1 %X Laser shock peening (LSP) is a surface modification technique to improve the mechanical properties of metals and alloys, where physical phenomena are difficult to investigate, due to short time scales and extreme physical values. In this regard, simulations can significantly contribute to understand the underlying physics. In this paper, a coupled simulation approach for LSP is presented. A global model of laser–matter–plasma interaction is applied to determine the plasma pressure, which is used as surface loading in finite element (FE) simulations in order to predict residual stress (RS) profiles in the target material. The coupled model is applied to the LSP of AA2198-T3 with water confinement, 3×3mm2 square focus and 20 ns laser pulse duration. This investigation considers the variation in laser pulse energy (3 J and 5 J) and different protective coatings (none, aluminum and steel foil). A sensitivity analysis is conducted to evaluate the impact of parameter inaccuracies of the global model on the resulting RS. Adjustment of the global model to different laser pulse energies and coating materials allows us to compute the temporal pressure distributions to predict RS with FE simulations, which are in good agreement with the measurements. %0 journal article %@ 1755-5817 %A Odermatt, A., Dorn, F., Ventzke, V., Kashaev, N. %D 2022 %J CIRP Journal of Manufacturing Science and Technology %P 443-453 %R doi:10.1016/j.cirpj.2022.02.019 %T Coaxial laser directed energy deposition with wire of thin-walled duplex stainless steel parts: Process discontinuities and their impact on the mechanical properties %U https://doi.org/10.1016/j.cirpj.2022.02.019 %X The topic of this paper is the process stability and its influence on the tensile material properties of duplex stainless steel manufactured via coaxial laser directed energy deposition with wire. The influence of slight changes in the prescribed offset per layer and laser power on the process stability, occurrence of discontinuities, and mechanical properties of duplex stainless steel 2209 parts was investigated. If the prescribed offset or laser power was too high, overheating of the melt-pool was observed. Otherwise, plunging of the wire through the melt-pool occurred. Stable processing was achieved by the adaptation of prescribed offset and laser power according to limited exponential growth laws. The overheating of the melt-pool leads to unsalvageable instabilities, whereas plunging of the wire through the melt-pool leads to self-stabilization by lowering the effective deposition rate and layer height. The discontinuities at sites, where wire plunged through the melt-pool are limited to the surface of the parts. The microstructure and tensile mechanical properties are unaffected by the presence of the surface discontinuities when they are removed by a machining step. The yield strength, ultimate tensile strength, and fracture strain exhibit a strong directionality, which can be explained by the anisotropy and inhomogeneity of the microstructure consisting of austenite-rich layers and ferrite-rich layer bands. %0 journal article %@ 0142-1123 %A Wang, Q., Huber, N., Liu, X., Kashaev, N. %D 2022 %J International Journal of Fatigue %P 106940 %R doi:10.1016/j.ijfatigue.2022.106940 %T On the analysis of plasticity induced crack closure in welded specimens: A mechanism controlled by the stress intensity factor resulting from residual stresses %U https://doi.org/10.1016/j.ijfatigue.2022.106940 %X The aim of this paper is to clarify how and to what extent varied welding residual stresses (WRSes) affect the plane stress plasticity induced crack closure (PICC) simulation. A well-characterized, representative WRS field with tensile and compressive reversals was imported and the predicted PICC results were evaluated against the relevant WRS-free ones. It turned out that instead of WRS typically reported in the literature, there is actually a Kres (stress intensity factor resulting from residual stresses) controlled crack closure mechanism when investigating the PICC behavior in the presence of a WRS field, i.e., WRS influences PICC in the form of Kres. Four major categories where the crack tip was located in different Kres zones were defined using MT (middle tension) and SENT (single edge notch tension) specimens to investigate the mechanism in detail. In summary, the Kres value determined the general steady-state PICC level in the WRS field, i.e., a diminished steady-state opening stress level was predicted in the WRS field with a positive Kres regardless of whether the crack is located in a tensile or a compressive WRS zone, and an elevated value was estimated when the Kres was negative. Besides, the initial transient period was scrutinized that occurred in some WRS scenarios and the concept of primary and secondary plastic wake evolution was proposed to explain its formation. It was found that the transient behavior could be eradicated by tailoring the secondary plastic wake employing appropriate constitutive models and mesh refinement levels. The results attained in the present work provide some guidelines for the WRS-PICC modeling and simulation in the fracture mechanics community. %0 journal article %@ 2452-3216 %A Prokhorov, A., Vshivkov, A., Plekhov, O., Kashaev, N. %D 2022 %J Procedia Structural Integrity %P 540-546 %R doi:10.1016/j.prostr.2022.01.120 %T Fatigue peculiarity of metals treated by laser shock impact %U https://doi.org/10.1016/j.prostr.2022.01.120 %X The work is devoted to experimental investigation of laser shock peening (LSP) effect on the thermo-mechanical properties of metals. ARMCO-iron and Titanium Grade 2 are chosen for the study. Samples were subjected to laser shock peening. After that, investigation of residual stress on heat generation during fatigue loading was performed. To study the damage-induced heating the fatigue tests were coupled with infrared thermography measurements. The results of the study have shown that LSP procedure qualitatively changes temperature evolution in both materials under cyclic loading. The heating (energy dissipation) of specimens after LSP was several times higher than in initial state. %0 journal article %@ 2238-7854 %A Examilioti, T., Papanikos, P., Kashaev, N., Klusemann, B., Alexopoulos, N. %D 2022 %J Journal of Materials Research and Technology : JMRT %P 2431-2446 %R doi:10.1016/j.jmrt.2022.05.197 %T Experimental and numerical investigation of laser beam-welded Al–Cu–Li joints using micro-mechanical characteristics %U https://doi.org/10.1016/j.jmrt.2022.05.197 %X The local tensile mechanical properties of laser beam-welded joints of AA2198 alloy with Al-Si filler wire were experimentally investigated. For this purpose, micro-flat tensile specimens were extracted from the fusion zone and the heat-affected zone. The chemical composition of the filler wire affects the local mechanical properties in the fusion zone, showing an approximately 26 % decrease in yield strength from the radiation exposure side to the weld root side. The effect of post-weld heat treatment on the tensile mechanical behavior was additionally investigated for different heat treatment artificial ageing conditions. The maximum yield strength increase was noticed for 48 h of artificial ageing for the weld root side of the fusion zone. Several approximations were proposed to correlate the hardness measurements with the local tensile mechanical properties of the welded joint that allow for a fast assessment of the tensile mechanical behaviour of the welded joint. To evaluate the effect of (i) artificial ageing and (ii) geometrical imperfections of the weld on the mechanical behavior of the welded joint, finite element analyses were performed, using the local mechanical properties as input to the model. It is shown that the local mechanical properties of the fusion zone play a pivotal role on the strain localization and fracture of the welded joint. %0 journal article %@ 0142-1123 %A Azevedo, L., Kashaev, N., Horstmann, C., Ventzke, V., Furtado, C., Moreira, P., Tavares, P. %D 2022 %J International Journal of Fatigue %P 107226 %R doi:10.1016/j.ijfatigue.2022.107226 %T Fatigue behaviour of laser shock peened AISI D2 tool steel %U https://doi.org/10.1016/j.ijfatigue.2022.107226 %X Laser shock peening (LSP) is an advanced surface enhancement technique capable of imparting beneficial compressive residual stresses, thereby improving metal parts’ fatigue resistance and crack propagation resistance. This study presents the fatigue behaviour of AISI D2 tool steel samples subjected to LSP with a power density of 15 GW/cm2 and a double laser scanning sequence. The effect of LSP on quasi-static tensile properties was also addressed. The fatigue crack propagation was suppressed in LSP-treated samples when tested at = 10 MPa. The fatigue strength of treated samples was 32.9% higher compared to untreated samples. The LSP-treated samples have shown a larger fatigue fracture surface area than the as-received (AR) sample. In addition, the tendency to form secondary cracks in the fatigue fracture paths decreased due to LSP treatment. LSP treatment can effectively improve the fatigue resistance and crack propagation behaviour of AISI D2 steel. These improvements are due to significant compressive residual stresses in a superficial layer of the material. The present study provides a reference for improving the mechanical properties of AISI D2 steel by laser shock peening. %0 journal article %@ 2212-8271 %A Wang, M., Kashaev, N. %D 2022 %J Procedia CIRP %P 218-223 %R doi:10.1016/j.procir.2022.08.053 %T Investigation of process window for AA7075 considering effects of different wire feed directions in lateral Laser Metal Deposition %U https://doi.org/10.1016/j.procir.2022.08.053 %X This work addresses the influences of the feed direction of AA7075 high-strength aluminum alloy in wire-based lateral laser metal deposition (LMD). The additive manufacturing process is investigated for the deposition of thin-walled structures. The interaction between wire and laser beam as well as the evolution of the melt pool are in-situ monitored by a high-speed camera. The consequences of different feed directions are analyzed in terms of processability, surface morphology, geometry, and porosity. Besides, the appropriate process parameters for AA7075 in lateral wire LMD are also studied. A maximal relative density level of about 99.7 % can be reached. No macro-cracks on the specimen surface or inside the specimen are observed. Optical microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy are employed to characterize the microstructure and measure the chemical composition of specimen produced with the optimal process parameters. The plateau-like distribution of the hardness evolution and the uniform layer thickness in the middle of the thin-walled structure indicate that a stable LMD-process can be achieved. %0 journal article %@ 2238-7854 %A Wang, M., Ventzke, V., Kashaev, N. %D 2022 %J Journal of Materials Research and Technology %P 388-403 %R doi:10.1016/j.jmrt.2022.09.051 %T Wire-based laser directed energy deposition of AA7075: effect of process parameters on microstructure and mechanical properties %U https://doi.org/10.1016/j.jmrt.2022.09.051 %X The process development for wire-based laser directed energy deposition of AA7075 is studied. Thin-wall structures are produced to investigate the process-microstructure-mechanical performance relationship. By optimizing the process parameters and building strategies, the minimal porosity level of 0.8% and 0.3% can be achieved in the continuous and discontinuous building strategies, respectively. The porosity level exhibits a primary dependence on the specific energy and a secondary dependence on the ratio between wire feed rate and laser scan speed. Thin-wall structures show an average hardness of 115 HV0.1. In two optimized building strategies, the ultimate tensile strength of 400 MPa is achieved without the cost of ductility (fracture strain of 9.2%). Large columnar grains with preferential orientation and the distribution of secondary phases relative to the loading direction during tensile tests contribute to superior mechanical properties. %0 journal article %@ 2214-8604 %A Sandmann, P., Keller, S., Kashaev, N., Ghouse, S., Hooper, P., Klusemann, B., Davies, C. %D 2022 %J Additive Manufacturing %P 103204 %R doi:10.1016/j.addma.2022.103204 %T Influence of laser shock peening on the residual stresses in additively manufactured 316L by Laser Powder Bed Fusion: A combined experimental–numerical study %U https://doi.org/10.1016/j.addma.2022.103204 %X Detrimental subsurface tensile residual stresses occur in laser powder bed fusion (LPBF) due to significant temperature gradients during the process. Besides heat treatments, laser shock peening (LSP) is a promising technology for tailoring residual stress profiles of additively manufactured components. A multi step process simulation is applied aiming at predicting the residual stress state after applying LSP to a cuboid shaped specimen manufactured by LPBF in two different building directions as well as comparing it with a post-build heat treatment. The validity of the numerical simulation is evaluated based on comparisons of residual stresses determined by incremental hole drilling technique within different stages of the multi step process: in the as-build condition, after subsequent heat treatment as well as after applying LSP to the as-build and heat treated specimens, showing overall a good experimental-numerical agreement throughout each of the process stages. Applying a heat treatment to the as-build LPBF sample at 700 °C for 6 h showed not to be effective in eliminating the surface tensile stress entirely, reducing the tensile residual stresses by 40%. However, the application of LSP on LPBF components showed promising results: LSP was able even to convert the detrimental near surface tensile residual stresses in the LPBF component into compressive residual stresses next to the surface, which is known to be beneficial for the fatigue performance. %0 journal article %@ 0021-8944 %A Gachegova, E.A., Sikhamov, R., Ventzke, V., Kashaev, N., Plekhov, O.A. %D 2022 %J Journal of Applied Mechanics and Technical Physics %N 2 %P 335-342 %R doi:10.1134/S0021894422020171 %T Influence of laser shock peening on low- and high-cycle fatigue of an OT4-0 titanium alloy %U https://doi.org/10.1134/S0021894422020171 2 %X Effect of laser shock peening on the fatigue life of an OT4-0 titanium alloy is studied. Laser peening is carried out using a Q-switched Nd:YAG laser operating at a pulse repetition rate of 10 Hz. It is suggested by the analysis of the effect of various laser treatment parameters on the magnitude and distribution of residual stresses over the sample thickness that an optimal type of laser shock treatment makes it possible to create a compressive residual stress region whose depth reaches up to 1 mm and whose maximum value is 600 MPa. The results of the study of the fracture surface structure show that the fatigue fracture mechanism changes, while the service life of samples significantly increases during both low- and high-cycle fatigue after laser shock peening. %0 journal article %@ 0939-1533 %A Seiler, M., Keller, S., Kashaev, N., Klusemann, B., Kästner, M. %D 2021 %J Archive of Applied Mechanics %N 8 %P 3709-3723 %R doi:10.1007/s00419-021-01897-2 %T Phase-field modelling for fatigue crack growth under laser shock peening-induced residual stresses %U https://doi.org/10.1007/s00419-021-01897-2 8 %X For the fatigue life of thin-walled components, not only fatigue crack initiation, but also crack growth is decisive. The phase-field method for fracture is a powerful tool to simulate arbitrary crack phenomena. Recently, it has been applied to fatigue fracture. Those models pose an alternative to classical fracture-mechanical approaches for fatigue life estimation. In the first part of this paper, the parameters of a phase-field fatigue model are calibrated and its predictions are compared to results of fatigue crack growth experiments of aluminium sheet material. In the second part, compressive residual stresses are introduced into the components with the help of laser shock peening. It is shown that those residual stresses influence the crack growth rate by retarding and accelerating the crack. In order to study these fatigue mechanisms numerically, a simple strategy to incorporate residual stresses in the phase-field fatigue model is presented and tested with experiments. The study shows that the approach can reproduce the effects of the residual stresses on the crack growth rate. %0 journal article %@ 0142-1123 %A Sun, R., Keller, S., Zhu, Y., Guo, W., Kashaev, N., Klusemann, B. %D 2021 %J International Journal of Fatigue %P 106081 %R doi:10.1016/j.ijfatigue.2020.106081 %T Experimental-numerical study of laser-shock-peening-induced retardation of fatigue crack propagation in Ti-17 titanium alloy %U https://doi.org/10.1016/j.ijfatigue.2020.106081 %X Residual stresses induced by laser shock peening in Ti-17 titanium specimens were experimentally and numerically investigated to identify the mechanisms and generation conditions of the retardation of fatigue crack propagation (FCP). The retardation was experimentally observed with fatigue life prolonged by 150%. A multi-step simulation strategy for fatigue life prediction is applied, which successfully predicts the experimentally observed FCP behavior. The fractographic observations and numerical simulation indicate that crack closure, as opposed to other microstructural influences, is the dominant effect on retardation. The studies of multi-FCP aspects show that significant retardation occurs in specimens at high values of residual stresses, small peening gap distances, and lower externally applied loads. %0 journal article %@ 0013-7944 %A Keller, S., Klusemann, B. %D 2021 %J Engineering Fracture Mechanics %P 107415 %R doi:10.1016/j.engfracmech.2020.107415 %T Application of stress intensity factor superposition in residual stress fields considering crack closure %U https://doi.org/10.1016/j.engfracmech.2020.107415 %X The correlation between stress intensity factor (SIF) range and fatigue crack growth is a powerful tool for fail–safe design approaches applied to lightweight structures. The key role is the precise calculation of the SIFs of fatigue load cycles. Advanced material processing can shape residual stresses and make the SIF calculation a challenging task. While the consideration of tensile residual stresses is successfully tackled by the SIF superposition, the treatment of compressive residual stresses needs still clarification. This work demonstrates the application of the SIF superposition principle in regions containing high compressive residual stresses leading to crack closure effects. Crack closure depends on the combined load of residual and applied stresses and is interpreted as a change of crack geometry in this work. Thus the relation between the source, i.e. the applied or residual stress, and its consequence, i.e. the corresponding SIFs, depends on the interaction of the sources, i.e. the combined load. Due to this interaction, residual stress–induced changes of the fatigue behaviour cannot be linked to the residual or applied SIF only. This work proposes two alternative definitions of applied and residual SIF, allowing a clear correlation between either the residual or the applied SIF to fatigue behaviour changes. %0 journal article %@ 8756-758X %A Kashaev, N., Groth, A., Ventzke, V., Horstmann, M., Riekehr, S., Staron, P., Huber, N. %D 2021 %J Fatigue and Fracture of Engineering Materials and Structures %N 4 %P 887-900 %R doi:10.1111/ffe.13400 %T Effect of laser heating on mechanical properties, residual stresses and retardation of fatigue crack growth in AA2024 %U https://doi.org/10.1111/ffe.13400 4 %X Local laser heating treatment using a defocussed laser beam was applied to the surface of 2‐mm‐thick AA2024‐T3 sheets. Two different treatment designs—namely, lines and circles—as well as the positioning and number of treatments were investigated regarding their potential to retard the propagation of through‐thickness fatigue cracks. The highest fatigue crack growth life extension of 285% was achieved by the application of four laser heating lines or two circles on each specimen side. The induced compressive residual stress field through the LH process is primarily responsible for an improvement in fatigue crack growth life. An emphasis was placed on investigating the effect of the treatment on the possible reduction of tensile and fatigue strength (S‐N life). If only one line was applied transverse to the loading direction or only one circle was used, the reduction of fatigue strength was comparable to the reduction of fatigue strength resulting from the stress concentration introduced due to the presence of a rivet hole. %0 journal article %@ 0079-6425 %A Zerbst, U., Bruno, G., Buffiere, Y., Wegener, T., Wu, T., Zhang, X., Kashaev, N., Meneghetti, G., Hrabe, N., Madia, M., Werner, T., Hilgenberg, K., Koukolíková, M., Procházka, R., Džugan, J., Möller, B., Beretta, S., Evans, A., Wagener, R., Schnabel, K. %D 2021 %J Progress in Materials Science %P 100786 %R doi:10.1016/j.pmatsci.2021.100786 %T Damage tolerant design of additively manufactured metallic components subjected to cyclic loading: State of the art and challenges %U https://doi.org/10.1016/j.pmatsci.2021.100786 %X We see that many of the classic concepts need to be expanded in order to fit with the particular microstructure (grain size and shape, crystal texture) and defect distribution (spatial arrangement, size, shape, amount) present in AM (in particular laser powder bed fusion). For instance, 3D characterization of defects becomes essential, since the defect shapes in AM are diverse and impact the fatigue life in a different way than in the case of conventionally produced components. Such new concepts have immediate consequence on the way one should tackle the determination of the fatigue life of AM parts; for instance, since a classification of defects and a quantification of the tolerable shapes and sizes is still missing, a new strategy must be defined, whereby theoretical calculations (e.g. FEM) allow determining the maximum tolerable defect size, and non-destructive testing (NDT) techniques are required to detect whether such defects are indeed present in the component. Such examples show how component design, damage and failure criteria, and characterization (and/or NDT) become for AM parts fully interlinked. We conclude that the homogenization of these fields represents the current challenge for the engineer and the materials scientist. %0 journal article %@ 1526-6125 %A Bock, F., Herrnring, J., Froend, M., Enz, J., Kashaev, N., Klusemann, B. %D 2021 %J Journal of Manufacturing Processes %P 982-995 %R doi:10.1016/j.jmapro.2021.02.016 %T Experimental and numerical thermo-mechanical analysis of wire-based laser metal deposition of Al-Mg alloys %U https://doi.org/10.1016/j.jmapro.2021.02.016 %X A finite element model is employed to perform a sequentially coupled thermo-mechanical analysis for enabling rapid process simulations of temperature fields, residual stresses and distortions for the production of additively manufactured parts via laser metal deposition. Experimental identification of characteristic process features such as temperature distribution, melt pool dimensions and bead geometries were used for the initial built-up and calibration of the model. The addition of material during process simulation is realised through reactivating inactive elements during the transient heat transfer analysis and through reactivating a combination of inactive and quiet elements during the mechanical analysis. The travelling heat source is geometrically bounded to precisely control the volume of its energy distribution. The results of the transient heat transfer analysis are sequentially coupled to a mechanical analysis for obtaining information on the resulting residual stresses and deformation. Based on the good agreement between numerical and experimental results of the thermal analysis, conclusions on the corresponding residual stress distributions and deformation are made. It is shown that the model represents an efficient tool for process prediction regarding thermal history, residual stresses and final-part deformations. Finally, the model is utilised to identify parameters and conditions of the process that lead to reduced residual stresses and deformations of the investigated additive part. %0 journal article %@ 1438-1656 %A Spoerk-Erdely, P., Staron, P., Liu, J., Kashaev, N., Stark, A., Hauschildt, K., Maawad, E., Mayer, S., Clemens, H. %D 2021 %J Advanced Engineering Materials %N 11 %P 2000947 %R doi:10.1002/adem.202000947 %T Exploring Structural Changes, Manufacturing, Joining, and Repair of Intermetallic γ-TiAl-Based Alloys: Recent Progress Enabled by In Situ Synchrotron X-Ray Techniques %U https://doi.org/10.1002/adem.202000947 11 %X Intermetallic γ‐TiAl‐based alloys are a promising class of materials for lightweight high‐temperature applications. Following intensive research and development activities, they have recently entered service in the automotive and aircraft engine industries. In the course of the past decades, the development of these complex multiphase alloys has benefited considerably from the application of (in situ) X‐ray scattering and diffraction techniques. Herein, a practical introduction and overview of recent progress in this field of research are provided. In particular, four case studies taken from various stages in the alloy development (i.e., fundamental research—manufacturing, joining, and repair—and application) illustrate current prospects at modern synchrotron radiation sources, including detailed information on available setups for in situ high‐energy X‐ray diffraction and small‐angle X‐ray scattering experiments and a discussion of potential limitations in the use of these techniques. %0 journal article %@ 0924-0136 %A Schwab, K., Keller, S., Kashaev, N., Klusemann, B. %D 2021 %J Journal of Materials Processing Technology %P 117154 %R doi:10.1016/j.jmatprotec.2021.117154 %T Tailoring of residual stresses by specific use of defined prestress during laser shock peening %U https://doi.org/10.1016/j.jmatprotec.2021.117154 %X The aim of the present study is to tailor laser shock peening-induced residual stresses by applying defined prestress. For this purpose, elastic prestress is introduced during laser shock peening application and subsequently released. The influence of prestress on the resulting residual stresses is investigated experimentally by a four-point bending device that allows to prestress the specimen during laser shock peening. Furthermore, a semi-analytical model of laser shock peening, extended by a contribution accounting for the prestress, is used to determine the prestress—residual stress relationship. A linear relation between prestress and compressive residual stress is found when the resulting compressive residual stresses are in the range of 20% to 100% of the yield strength. Generally, tensile prestress leads to a higher magnitude and penetration depth of resulting compressive residual stress after laser shock peening. As a proof of concept, prestress was used to alter a non-equibiaxial residual stress profile into an equibiaxial one, demonstrating the applicability of prestress as effective tool for residual stress design. %0 journal article %@ 2075-4701 %A Ozerov, M., Povolyaeva, E., Stepanov, N., Ventzke, V., Dinse, R., Kashaev, N., Zherebtsov, S. %D 2021 %J Metals %N 3 %P 506 %R doi:10.3390/met11030506 %T Laser Beam Welding of a Ti-15Mo/TiB Metal–Matrix Composite %U https://doi.org/10.3390/met11030506 3 %X A Ti-15Mo/TiB metal–matrix composite was produced by spark plasma sintering at 1400 °C. The fractions of the elements in the initial powder mixture were 80.75 wt.% Ti, 14.25 wt.% Mo, and 5 wt.% TiB2. The initial structure of the synthesized composite was composed of bcc β titanium matrix and needle-like TiB reinforcements with an average thickness of 500 ± 300 nm. Microstructure and mechanical properties of the composite were studied after laser beam welding (LBW) was carried out at room temperature or various pre-heating temperatures: 200, 400, or 600 °C. The quality of laser beam welded joints was not found to be dependent noticeably on the pre-heating temperature; all welds consisted of pores the size of which reached 200–300 µm. In contrast to acicular individual particles in the base material, TiB whiskers in the weld zone were found to have a form of bunches. The maximum microhardness in the weld zone (~700 HV) was obtained after welding at room temperature or at 200 °C; this value was ~200 HV higher than that in the base material. %0 journal article %@ 1996-1944 %A Bock, F., Keller, S., Huber, N., Klusemann, B. %D 2021 %J Materials %N 8 %P 1883 %R doi:10.3390/ma14081883 %T Hybrid Modelling by Machine Learning Corrections of Analytical Model Predictions towards High-Fidelity Simulation Solutions %U https://doi.org/10.3390/ma14081883 8 %X Within the fields of materials mechanics, the consideration of physical laws in machine learning predictions besides the use of data can enable low prediction errors and robustness as opposed to predictions only based on data. On the one hand, exclusive utilization of fundamental physical relationships might show significant deviations in their predictions compared to reality, due to simplifications and assumptions. On the other hand, using only data and neglecting well-established physical laws can create the need for unreasonably large data sets that are required to exhibit low bias and are usually expensive to collect. However, fundamental but simplified physics in combination with a corrective model that compensates for possible deviations, e.g., to experimental data, can lead to physics-based predictions with low prediction errors, also despite scarce data. In this article, it is demonstrated that a hybrid model approach consisting of a physics-based model that is corrected via an artificial neural network represents an efficient prediction tool as opposed to a purely data-driven model. In particular, a semi-analytical model serves as an efficient low-fidelity model with noticeable prediction errors outside its calibration domain. An artificial neural network is used to correct the semi-analytical solution towards a desired reference solution provided by high-fidelity finite element simulations, while the efficiency of the semi-analytical model is maintained and the applicability range enhanced. We utilize residual stresses that are induced by laser shock peening as a use-case example. In addition, it is shown that non-unique relationships between model inputs and outputs lead to high prediction errors and the identification of salient input features via dimensionality analysis is highly beneficial to achieve low prediction errors. In a generalization task, predictions are also outside the process parameter space of the training region while remaining in the trained range of corrections. The corrective model predictions show substantially smaller errors than purely data-driven model predictions, which illustrates one of the benefits of the hybrid modelling approach. Ultimately, when the amount of samples in the data set is reduced, the generalization of the physics-related corrective model outperforms the purely data-driven model, which also demonstrates efficient applicability of the proposed hybrid modelling approach to problems where data is scarce. %0 journal article %@ 2075-4701 %A Abbaszadeh, M., Ventzke, V., Neto, L., Riekehr, S., Martina, F., Kashaev, N., Hönnige, J., Williams, S., Klusemann, B. %D 2021 %J Metals %N 6 %P 877 %R doi:10.3390/met11060877 %T Compression Behaviour of Wire + Arc Additive Manufactured Structures %U https://doi.org/10.3390/met11060877 6 %X Increasing demand for producing large-scale metal components via additive manufacturing requires relatively high building rate processes, such as wire + arc additive manufacturing (WAAM). For the industrial implementation of this technology, a throughout understanding of material behaviour is needed. In the present work, structures of Ti-6Al-4V, AA2319 and S355JR steel fabricated by means of WAAM were investigated and compared with respect to their mechanical and microstructural properties, in particular under compression loading. The microstructure of WAAM specimens is assessed by scanning electron microscopy, electron back-scatter diffraction, and optical microscopy. In Ti-6Al-4V, the results show that the presence of the basal and prismatic crystal planes in normal direction lead to an anisotropic behaviour under compression. Although AA2319 shows initially an isotropic plastic behaviour, the directional porosity distribution leads to an anisotropic behaviour at final stages of the compression tests before failure. In S355JR steel, isotropic mechanical behaviour is observed due to the presence of a relatively homogeneous microstructure. Microhardness is related to grain morphology variations, where higher hardness near the inter-layer grain boundaries for Ti-6Al-4V and AA2319 as well as within the refined regions in S355JR steel is observed. In summary, this study analyzes and compares the behaviour of three different materials fabricated by WAAM under compression loading, an important loading condition in mechanical post-processing techniques of WAAM structures, such as rolling. In this regard, the data can also be utilized for future modelling activities in this direction. %0 journal article %@ 1617-7061 %A Seiler, M., Keller, S., Kashaev, N., Klusemann, B., Kästner, M. %D 2021 %J PAMM: Proceedings in Applied Mathematics and Mechanics %N 1 %P e202100210 %R doi:10.1002/pamm.202100210 %T Simulation of fatigue crack growth in residual-stress-afflicted specimen with a phase-field model %U https://doi.org/10.1002/pamm.202100210 1 %X Laser shock peening (LSP) is a promising technique to systematically introduce local compressive residual stresses in metal sheets, inhibiting fatigue cracks in these areas. We model fatigue crack growth in these specimen with the help of a phase-field model for fatigue fracture [1]. First, we parametrise the model using untreated aluminium specimens. In a second step, we use the determined parameters to simulate residual-stress-afflicted specimens, qualitatively reproducing the crack inhibition due to LSP. %0 journal article %@ 1044-5803 %A Examilioti, T., Kashaev, N., Ventzke, V., Klusemann, B., Alexopoulos, N. %D 2021 %J Materials Characterization %P 111257 %R doi:10.1016/j.matchar.2021.111257 %T Effect of filler wire and post weld heat treatment on the mechanical properties of laser beam-welded AA2198 %U https://doi.org/10.1016/j.matchar.2021.111257 %X The mechanical behavior of autogenously and non-autogenously laser beam-welded joints of Al-Cu-Li alloy AA2198 and the effect of post weld heat treatment are examined in this contribution. The deformation texture of the base material does not present any significant change in the macrotexture with applying different artificial ageing times. Autogenously and non-autogenously laser beam-welded joints present a decrease in yield stress and ultimate strength in the as-welded condition, approximately 45% and 36–38%, respectively, when compared with AA2198-Τ3 base material. The addition of the AA4047 filler wire increases the Si and Cu content in the grain interior and in the grain boundaries of the fusion zone of the welded joint. Micro-hardness measurements for autogenously laser-welded joints showed a decrease in hardness by 27% for the fusion zone and 42% for the heat-affected zone, when compared with the non-autogenously laser beam-welded joints. Α quality index was exploited to evaluate the tensile mechanical performance of the welded joints. It is observed that the non-autogenously welded joints always show a higher ‘quality’ than the respective autogenously welded joints and the highest quality index in terms of mechanical performance is achieved for the as-welded and the peak-aged conditions, respectively. Regardless of the post weld heat treatment condition, both autogenously and non-autogenously laser-welded specimens fractured in between the equiaxed and fusion zone during tensile loading. %0 journal article %@ 2075-4701 %A Prokhorov, A., Vshivkov, A., Plekhov, O., Kashaev, N., Fomin, F., Ozerov, M., Zherebtsov, S. %D 2021 %J Metals %N 8 %P 1198 %R doi:10.3390/met11081198 %T The Effect of LSP on the Structure Evolution and Self-Heating of ARMCO Iron under Cyclic Loading %U https://doi.org/10.3390/met11081198 8 %X This work is devoted to the experimental investigation of the effect of laser shock peening (LSP) on the thermo-mechanical properties of metals. ARMCO iron was chosen as the model material for the study. Samples were subjected to LSP, and were tested following the procedure of the self-heating (Risitano) technique. To investigate the damage that was induced by heating, the fatigue tests were coupled with infrared thermography measurements. The results of the study showed that the LSP procedure qualitatively changes the temperature evolution in ARMCO iron during cyclic loading. The heating (energy dissipation) of the LSP treated specimen was several times higher than that of the specimen in the initial state. To explain the structural mechanisms of energy dissipation, the microstructure of the specimens was examined using transmission (TEM) and scanning (SEM) electron microscopy, as well as electron backscattering diffraction (EBSD). The results of the structural investigation confirm the qualitative change of defect evolution caused by LSP treatment. %0 journal article %@ 8756-758X %A Kashaev, N., Keller, S., Staron, P., Maawad, E., Huber, N. %D 2021 %J Fatigue and Fracture of Engineering Materials and Structures %N 12 %P 3463-3481 %R doi:10.1111/ffe.13579 %T On the prediction of fatigue crack growth based on weight functions in residual stress fields induced by laser shock peening and laser heating %U https://doi.org/10.1111/ffe.13579 12 %X This study deals with fatigue crack growth prediction for thin sheet AA2024 specimens with residual stresses introduced through laser shock peening and laser heating treatments. Different weight functions are used to calculate the stress intensity factor due to the presence of residual stresses. A superposition principle is used by calculating the total stress intensity factor considering the applied loads and residual stresses. The fatigue crack growth is predicted using the Paris' law based on the effective stress intensity factor range, in that the effect of residual stresses is considered by changing the total stress intensity factor ratio. It is concluded, that weight functions are a powerful tool to predict fatigue crack growth in AA2024 containing residual stress fields, as long as the gradient in the loading direction, as induced by laser shock peening and laser heating, is moderate. %0 journal article %@ 1526-6125 %A Odermatt, A., Ventzke, V., Dorn, F., Dinsé, R., Merhof, P., Kashaev, N. %D 2021 %J Journal of Manufacturing Processes %P 148-158 %R doi:10.1016/j.jmapro.2021.10.020 %T Effect of laser beam welding on microstructure, tensile strength and fatigue behaviour of duplex stainless steel 2205 %U https://doi.org/10.1016/j.jmapro.2021.10.020 %X This study focuses on the microstructure, tensile and fatigue properties of laser beam welded butt joints in 4 mm thick sheets of duplex stainless steel 2205. The microstructural characteristics of the joints were investigated via optical microscopy and electron backscattered diffraction. A 300 μm wide heat affect zone with increased ferrite content and a nearly fully ferritic 800 μm wide fusion zone were found. No porosity could be found with X-ray radiography. Microhardness measurements revealed increased strength in the fusion and heat affect zones of the weldments. Uniaxial tensile and stress controlled fatigue tests were performed in order to characterize the mechanical properties of specimens containing laser beam welded joints and the base material. The specimens containing weldments were stronger, but less ductile than the base material, due to the weld metal restricting deformation. The as-welded joints exhibited worse fatigue performance than the base material due to the notch at the excess weld metal. The fatigue properties of the specimens containing joints could be elevated to the base material level by a laser surface remelting treatment. %0 journal article %@ 1044-5803 %A Liu, J., Wu, T., Wang, M., Wang, L., Zhou, Q., Wang, K., Staron, P., Schell, N., Huber, N., Kashaev, N. %D 2021 %J Materials Characterization %P 111371 %R doi:10.1016/j.matchar.2021.111371 %T In situ observation of competitive growth of α grains during β → α transformation in laser beam manufactured TiAl alloys %U https://doi.org/10.1016/j.matchar.2021.111371 %X Low ductility has long been the bottleneck for high temperature application of TiAl alloys. It is reported that grain refinement through boride could improve its mechanical property. However, this refinement is suppressed at high cooling rate. This article delineates for the first time an in situ observation by synchrotron X-ray diffraction. It illustrates the mechanism of competitive nucleation and grain growth of Burgers and non-Burgers α grains during β → α transformation in a Ti–45Al–5Nb–0.2C–0.2B alloy (TNB-V5). Comparing with those in base material, the volume fraction and size of borides are significantly reduced in the welding zone. The non-Burgers α grains nucleate earlier than Burgers α. However, Burgers α grains grow much faster than non-Burgers α during β → α transformation in the welding zone, due to a high thermodynamic driving force of Burgers α grains. No orientation relationships between α and borides or between β and borides are observed in the fast cooling. %0 journal article %@ 2075-4701 %A Sikhamov, R., Fomin, F., Klusemann, B., Kashaev, N. %D 2020 %J Metals %N 4 %P 495 %R doi:10.3390/met10040495 %T The Influence of Laser Shock Peening on Fatigue Properties of AA2024-T3 Alloy with a Fastener Hole %U https://doi.org/10.3390/met10040495 4 %X The objective of the present study was to estimate the influence of laser shock peening on the fatigue properties of AA2024-T3 specimens with a fastener hole and to investigate the possibility to heal the initial cracks in such specimens. Fatigue cracks of different lengths were introduced in the specimens with a fastener hole before applying laser shock peening. Deep compressive residual stresses, characterized by the hole drilling method, were generated into the specimens by applying laser shock peening on both sides. Subsequently, the specimens were subjected to fatigue tests. The results show that laser shock peening has a positive effect regarding the fatigue life improvement in the specimens with a fastener hole. In addition, laser shock peening leads to a healing effect on fatigue cracks. The efficiency of this effect depends on the initial crack length. The effect of laser shock peening on the fatigue life periods was determined by using resonant frequency graphs. %0 journal article %@ 8756-758X %A Kashaev, N., Ushmaev, D., Ventzke, V., Klusemann, B., Fomin, F. %D 2020 %J Fatigue and Fracture of Engineering Materials and Structures %N 7 %P 1500-1513 %R doi:10.1111/ffe.13226 %T On the application of laser shock peening for retardation of surface fatigue cracks in laser beam‐welded AA6056 %U https://doi.org/10.1111/ffe.13226 7 %X The present study aims to investigate the extent to which the fatigue behaviour of laser beam‐welded AA6056‐T6 butt joints with an already existing crack can be improved through the application of laser shock peening. Ultrasonic testing was utilized for in situ (nondestructive) measurement of fatigue crack growth during the fatigue test. This procedure allowed the preparation of welded specimens with surface fatigue cracks with a depth of approximately 1.2 mm. The precracked specimens showed a 20% reduction in the fatigue limit compared with specimens without cracks in the as‐welded condition. Through the application of laser shock peening on the surfaces of the precracked specimens, it was possible to recover the fatigue life to the level of the specimens tested in the as‐welded condition. The results of this study show that laser shock peening is a very promising technique to recover the fatigue life of welded joints with surface cracks, which can be detected by nondestructive testing. %0 journal article %@ 1475-1305 %A Sandmann, P., Nielsen, M., Keller, S., Maawad, E., Staron, P., Klusemann, B. %D 2020 %J Strain : the journal of the British Society for Strain Measurement %N 4 %P e12338 %R doi:10.1111/str.12338 %T Combined experimental–numerical study on residual stresses induced by a single impact as elementary process of mechanical peening %U https://doi.org/10.1111/str.12338 4 %X Peening processes can be used as a fatigue enhancement treatment for metallic structures by locally introducing compressive residual stresses. A combined experimental–numerical study on a single‐impact process with a drop tower on the aluminium alloy AA5754, representing the elementary process of mechanical peening, has been performed to investigate different impact parameters on the residual stress profile. Residual stresses have been measured using high‐energy X‐Ray diffraction. A three‐dimensional finite element model is used to predict the residual stresses numerically. The elastic strain components from the numerical results are used to calculate residual stresses by assuming either a plane stress or a plane strain state for different specimen thickness to assess the validity of respective assumption. The validity of the numerical simulation is evaluated based on comparisons of the elastic strain profiles and the percentage loss in kinetic energy of the steel ball due to the impact for four different energies, showing overall a good agreement in the experimental–numerical comparisons. %0 journal article %@ 0268-3768 %A Examilioti, T., Kashaev, N., Enz, J., Klusemann, B., Alexopoulos, N. %D 2020 %J The International Journal of Advanced Manufacturing Technology %P 2079-2092 %R doi:10.1007/s00170-020-05893-8 %T On the influence of laser beam welding parameters for autogenous AA2198 welded joints %U https://doi.org/10.1007/s00170-020-05893-8 %X The effects of different autogenous laser beam welding process parameters on the fusion zone (FZ) geometry, microstructure, and tensile mechanical properties were investigated for 5-mm-thick AA2198 alloy sheets. Porosity formation and hot cracking are observed for low laser powers and welding velocities, while the porosity level is essentially reduced with increasing laser power. The characteristic cross-sectional geometry of the welded joints changes with increasing laser power, taking shapes from narrow V shape to rectangular I shape, and the results are discussed based on the “closed” and “open” keyhole formation during laser beam welding. A methodology is exploited in terms of quantifying the geometrical dimensions of the cross-section of the FZ in order to promote the welded joints with a narrow width as well as with a rectangular shape. The optimal process parameters, leading to FZ close to the desirable rectangular I shape and with a low number of defects, are identified. Microstructural analyses reveal a pronounced transition zone in between the FZ and the heat-affected zone, which is subdivided into two narrow zones, the partially melted zone (PMZ) and the equiaxed zone. The narrow width of the FZ and PMZ, as well as the rectangular shape of the FZ, enables the autogenous welded joint to reach good tensile deformation properties. %0 journal article %@ 0030-3992 %A Ageev, E., Andreeva, Y., Ionin, A., Kashaev, N., Kudryashov, S., Nikonorov, N., Nuryev, R., Petrov, A., Rudenko, A., Samokhvalov, A., Saraeva, I., Veiko, V. %D 2020 %J Optics and Laser Technology %P 106131 %R doi:10.1016/j.optlastec.2020.106131 %T Single-shot femtosecond laser processing of Al-alloy surface: An interplay between Mbar shock waves, enhanced microhardness, residual stresses, and chemical modification %U https://doi.org/10.1016/j.optlastec.2020.106131 %X Ambient-air single-shot ablative femtosecond laser surface processing of an advanced Al-alloy AA5083 was characterized by non-contact broadband ultrasonics, scanning electron microscopy with energy-dispersive x-ray spectroscopy, x-ray diffraction, and Vickers microhardness tests. The characterization indicates that the generated Mbar-level shock waves induce not only a strong structural modification of the processed micron-thick surface layer with sub-GPa-level residual compressive and tensile stresses, raising its microhardness by almost 45%, but also significant depth-dependent chemical modification within the layer. %0 journal article %@ 0921-5093 %A Odermatt, A., Richert, C., Huber, N. %D 2020 %J Materials Science and Engineering A %P 139700 %R doi:10.1016/j.msea.2020.139700 %T Prediction of elastic-plastic deformation of nanoporous metals by FEM beam modeling: A bottom-up approach from ligaments to real microstructures %U https://doi.org/10.1016/j.msea.2020.139700 %X For the prediction of elastic-plastic deformation behavior of nanoporous materials, a computationally efficient method is needed that integrates the complex 3D network structure and the large variation of ligament shapes in a representative volume element. Finite element simulations based on beam elements are most efficient, but for a quantitative prediction, a correction is required that accounts for the effects of the mass around the junctions. To this end, a nodal correction is presented that covers a wide range of parabolic-spherical ligament shapes. Smooth functions are provided that define the extension of the nodal corrected elements along the ligament axis, their radius and their yield stress as functions of the individual ligament shape. It is shown that the increase in radius of the nodal beam elements can be replaced by scaled material parameters. Simulations of randomized FEM beam networks revealed that, in relation to the randomization of the ligament axis, the distribution of the ligament shape has a stronger impact on the macroscopic stress–strain response and should thus receive particular attention during the characterization of nanoporous microstructures. This also emphasizes the importance of the thickness analysis via image processing. Combining a nodal corrected FEM beam model with geometry data derived from image multiplication of the skeleton and Euclidean distance transform of a nanoporous gold FIB-SEM tomography dataset significantly improves the prediction of the stress–strain curve. The remaining deviation is expected to stem from the known underestimation of the real ligament diameter by the Euclidean distance transform as well as local variations in the circularity of the ligaments. %0 journal article %@ 0921-5093 %A Froend, M., Ventzke, V., Dorn, F., Kashaev, N., Klusemann, B., Enz, J. %D 2020 %J Materials Science and Engineering A %P 138635 %R doi:10.1016/j.msea.2019.138635 %T Microstructure by design: An approach of grain refinement and isotropy improvement in multi-layer wire-based laser metal deposition %U https://doi.org/10.1016/j.msea.2019.138635 %X The additive production of metallic components with high-throughput is usually associated with high process temperatures and slow cooling rates. This typically results in strongly oriented columnar grain growth along the building direction of the structure having exceedingly large grain sizes. As a result, such structures show typically low strength and anisotropic mechanical behaviour in as-deposited condition. Consequently, post-processing is commonly performed to homogenize and eventually increase the mechanical properties of the deposited structures. In this regard, precise control of the applied process energy allows a modification of the local temperature distribution and cooling conditions during the additive manufacturing process, which strongly influence the resulting solidification microstructure. The aim of the present study is the development of an approach that allows to influence the solidification conditions in wire-based laser metal deposition of an Al-Mg alloy through specific adjustments of the laser irradiation. It was found that significantly different solidification microstructures in as-deposited condition can be achieved by adjusting the laser beam irradiance within a range resulting in conduction mode welding conditions while keeping the heat input constant. The application of high laser beam irradiances, close to the transition to keyhole mode welding, results in structures with a homogeneous large-grained solidification microstructure exhibiting a degree of anisotropy of around 12% between building direction and the direction of deposition. In contrast, the use of low laser beam irradiance close to the lower limit of stable melting, results in structures with a significantly refined microstructure. Consequently, an increase of yield strength of up to around 20% and microhardness of up to 13%, as compared to structures processed with high laser beam irradiance, could be obtained. Moreover, the anisotropy of the as-deposited structure was reduced to a degree lower than 2%. %0 journal article %@ 1059-9495 %A Abbaszadeh, M., Hönnige, J.R., Martina, F., Neto, L., Kashaev, N., Colegrove, P., Williams, S., Klusemann, B. %D 2019 %J Journal of Materials Engineering and Performance %N 8 %P 4931-4942 %R doi:10.1007/s11665-019-04249-y %T Numerical Investigation of the Effect of Rolling on the Localized Stress and Strain Induction for Wire + Arc Additive Manufactured Structures %U https://doi.org/10.1007/s11665-019-04249-y 8 %X Cold rolling can be used in-process or post-process to improve microstructure, mechanical properties and residual stress in directed-energy-deposition techniques, such as the high deposition rate wire + arc additive manufacturing (WAAM) process. Finite element simulations of the rolling process are employed to investigate the effect of rolling parameters, in particular rolling load and roller profile radius on the residual stress field as well as plastic strain distribution for the profiled roller. The results show the response to rolling of commonly used structural metals in WAAM, i.e., AA2319, S335JR steel and Ti-6Al-4V, taking into account the presence of residual stresses. The rolling load leads to changes in the location and the maximum value of the compressive residual stresses, as well as the depth of the compressive residual stresses. However, the roller profile radius only changes the maximum value of these compressive residual stresses. Changing the rolling load influences the equivalent plastic strain close to the top surface of the wall as well as in deeper areas, whereas the influence of the roller profile radius is negligible. The plastic strain distribution is virtually unaffected by the initial residual stresses prior to rolling. Finally, design curves were generated from the simulations for different materials, suggesting ideal rolling load and roller profile combinations for microstructural improvement requiring certain plastic strains at a specific depth of the additive structure. %0 journal article %@ 0142-1123 %A Keller, S., Horstmann, M., Kashaev, N., Klusemann, B. %D 2019 %J International Journal of Fatigue %P 265-276 %R doi:0.1016/j.ijfatigue.2018.12.014 %T Experimentally validated multi-step simulation strategy to predict the fatigue crack propagation rate in residual stress fields after laser shock peening %U https://doi.org/0.1016/j.ijfatigue.2018.12.014 %X Laser shock peening (LSP) is a promising technology to retard the fatigue crack propagation (FCP) in metallic lightweight structures. A multi-step simulation strategy to predict FCP in LSP-induced residual stress fields is proposed and applied. The simulation strategy involves an LSP process simulation, a transfer approach to include the plastic strains in a C(T) specimen model to calculate the residual stresses and a FCP simulation to determine the stress intensity factors. The FCP rate is finally determined via FCP equations. The validity of the simulation strategy including the crack driving quantities prediction is experimentally demonstrated by a novel ‘simulation’ approach. %0 journal article %@ 0268-3768 %A Chupakhin, S., Klusemann, B., Huber, N., Kashaev, N. %D 2019 %J The International Journal of Advanced Manufacturing Technology %N 5 - 8 %P 1567-1581 %R doi:10.1007/s00170-018-3034-2 %T Application of design of experiments for laser shock peening process optimization %U https://doi.org/10.1007/s00170-018-3034-2 5 - 8 %X Laser shock peening—a very promising life enhancement technique—has demonstrated great success regarding the improvement of fatigue behavior via deep compressive residual stresses. However, the prediction and adaption of residual stress fields on basis of the laser peening parameters are still not comprehensively established. The aim of the current work is to investigate the effects of the laser pulse energy, the number of treatment overlaps as well as the laser spot size on the resulting residual stress distribution, characterized by following quantities: the residual stress close to the surface, the maximum compressive residual stress, and the integral compressive stress area over the specimen depth. For a systematic investigation of all main and interaction-based process parameter effects, and a subsequent parameter optimization, the general full factorial design is employed. The results show that laser shock peening with different process parameter combinations, inducing residual stresses with comparable integral stress area, can lead to a minimum fatigue life extension of approx. 100,000 cycles, representing a minimum fatigue life of 250% of the base material. The experimental scatter in the number of cycles to failure follows the Weibull distribution which qualitatively correlates with the standard deviation of the integral stress area. %0 journal article %@ 2075-4701 %A Kallien, Z., Keller, S., Ventzke, V., Kashaev, N., Klusemann, B. %D 2019 %J Metals %N 6 %P 655 %R doi:10.3390/met9060655 %T Effect of Laser Peening Process Parameters and Sequences on Residual Stress Profiles %U https://doi.org/10.3390/met9060655 6 %X Laser Peening (LP) is a surface modification technology that can induce high residual stresses in a metallic material. The relation between LP process parameters, in particular laser sequences, as well as pulse parameters and the resulting residual stress state was investigated in this study. The residual stress measurements, performed with the hole drilling technique, showed a non-equibiaxial stress profile in laser peened AA2024-T3 samples with a clad layer for certain parameter combinations. Shot overlap and applied energy density were found to be crucial parameters for the characteristic of the observed non-equibiaxial residual stress profile. Furthermore, the investigation showed the importance of the advancing direction, as the advancing direction influences the direction of the higher compressive residual stress component. The direction of higher residual stresses was parallel or orthogonal to the rolling direction of the material. The effect was correlated to the microstructural observation obtained via electron backscattered diffraction. Additionally, for peening with two sequences of different advancing directions, the study showed that the order of applied advancing directions was important for the non-equibiaxiality of the resulting residual stress profile. %0 journal article %@ 0966-9795 %A Burkhardt, I., Ventzke, V., Riekehr, S., Kashaev, N., Enz, J. %D 2019 %J Intermetallics %P 74-83 %R doi:10.1016/j.intermet.2018.09.012 %T Laser welding and microstructural characterization of dissimilar γ-TiAl-Ti6242 joints %U https://doi.org/10.1016/j.intermet.2018.09.012 %X Crack-free dissimilar TNM-Ti6242 joints were successfully obtained by laser welding with preheating temperatures of 400 °C, 600 °C and 800 °C, in spite of the differing thermophysical properties of the used alloys. The microstructure of the joints was investigated by optical microscopy, SEM, EDX, EBSD and the local mechanical properties were determined by microhardness measurements. The microstructure coarsens with increasing preheating temperature. Homogenous mixture of both alloys in the fusion zone was achieved, even so local microsegregations of Ti, Al and Nb occurred. The fusion zone mainly consists of α2 (Ti3Al). A transition zone, present as a β/βO seam, was revealed for preheating temperatures of 400 °C and 600 °C. For preheating to 800 °C the β/βO seam was penetrated with α2 lamellas. Since the alloying elements of the other alloy were detected in the heat-affected zone, diffusion processes occurred. The average microhardness in the fusion zone of the dissimilar TNM-Ti6242 joints is independent of the preheating temperature and is higher than the base material microhardness. Comparing the microhardness in the fusion zone of similar Ti6242 and TNM joints with dissimilar TNM-Ti6242 joints, the microhardness values of the dissimilar TNM-Ti6242 joints range between the values of the similar Ti6242 and TNM joints. However, the values in the fusion zone are closer to the microhardness values of the similar TNM joint, which is caused by the mainly α2 containing fusion zone of the dissimilar TNM-Ti6242 joints. %0 journal article %@ 2214-8604 %A Froend, M., Ventzke, V., Kashaev, N., Klusemann, B., Enz, J. %D 2019 %J Additive Manufacturing %P 100800 %R doi:10.1016/j.addma.2019.100800 %T Thermal analysis of wire-based direct energy deposition of Al-Mg using different laser irradiances %U https://doi.org/10.1016/j.addma.2019.100800 %X The wire-based direct energy deposition of metallic lightweight materials such as titanium or aluminium alloys has recently received increasing attention in industry and academia. However, high-throughput deposition is mostly associated with process-limiting phenomena such as the development of high temperatures resulting in poor surface quality as well as coarse and unidirectional solidification microstructures. In this regard, laser systems, which are already widely used in industrial processes, allow for a great variety in the controllability of energy inputs, thereby enabling the control of process temperatures and resulting microstructures. The subject of the current study is the detailed elucidation and evaluation of important features such as the development of temperature gradients, resulting cooling rates and thermal cycles for different laser beam irradiances. Significant heat accumulation and process instabilities as well as inhomogeneous thermal profiles along the length and height of the parts were observed at a high laser beam irradiance. In contrast, lower laser beam irradiance resulted in a more stable process with increased cooling rates, which favourably influenced the refinement of the solidification microstructure. %0 journal article %@ 1617-7061 %A Pozdnyakov, V., Keller, S., Kashaev, N., Klusemann, B., Oberrath, J. %D 2019 %J PAMM: Proceedings in Applied Mathematics and Mechanics %N 1 %P e201900497 %R doi:10.1002/pamm.201900497 %T Two-step simulation approach for laser shock peening %U https://doi.org/10.1002/pamm.201900497 1 %X Laser shock peening (LSP) is a surface modification technique to introduce compressive residual stresses (RS) with a high magnitude in the near surface region of the material. Due to non‐linear interactions (e.g. laser absorption by plasma, shock wave propagation, etc.) and a high number of parameters, it is difficult to study and optimize the process based on experiments alone. Therefore, a two‐step simulation approach is proposed in this paper, where two models are combined, because one model of the complete process is difficult to derive, due to the different characteristics of the plasma formation and the shock wave propagation in the material. On one hand, a global model including plasma and shock wave descriptions is applied for the LSP of an aluminium sample with water confinement. The numerical solution of this model, applied for a 3×3 mm2 focus size, 5 J and 20 ns (full width at half maximum (FWHM)) laser pulse, allows to determine the temporal plasma pressure evolution on the material surface. On the other hand, a finite element simulation is used to calculate the RS distribution within the target material, where the plasma pressure is applied as a surface loading for the aluminium alloy AA2198‐T3. The simulated residual stresses are fitted to measurements via parameter variation of the global model. The identified values and the two‐step simulation approach can be used in future work to predict stress states of materials after LSP for various process parameters variations. %0 journal article %@ 0921-5093 %A Kashaev, N., Ventzke, V., Petrov, N., Horstmann, M., Zherebtsov, S., Shaysultanov, D., Sanin, V., Stepanov, N. %D 2019 %J Materials Science and Engineering A %P 138358 %R doi:10.1016/j.msea.2019.138358 %T Fatigue behaviour of a laser beam welded CoCrFeNiMn-type high entropy alloy %U https://doi.org/10.1016/j.msea.2019.138358 %X Laser beam welding was used to produce butt joints from the CoCrFeNiMn-type high entropy alloy. The alloy in the initial condition had an fcc single-phase coarse-grained structure. Laser welding resulted in the M7C3-type carbides precipitation in the fcc matrix. The carbide particles precipitation resulted in a considerable increase in microhardness from 150 HV 0.5 for the as-sintered condition to 205 HV 0.5 in the fusion zone. Laser beam welding had a negligible effect on both static mechanical properties and fatigue behaviour of the alloy. The endurance limit of either type of specimens (i.e. with and without welding seam) was 200 MPa. Fracture of all specimens with the laser beam welded seams occurred in the base material area during both tensile and fatigue testing. Weak effect of welding on static/fatigue behaviour of the alloy can be attributed to the higher hardness of the fusion zone, resulting in strain localization in the base material area. An increase in load resulted in activation of secondary slip systems and formation of deformation twins in fatigue specimens. %0 journal article %@ 0013-7944 %A Keller, S., Horstmann, M., Kashaev, N., Klusemann, K. %D 2019 %J Engineering Fracture Mechanics %P 106630 %R doi:10.1016/j.engfracmech.2019.106630 %T Crack closure mechanisms in residual stress fields generated by laser shock peening: A combined experimental-numerical approach %U https://doi.org/10.1016/j.engfracmech.2019.106630 %X Laser shock peening (LSP) is successfully applied to retard fatigue cracks in metallic lightweight structures by introducing specific, in particular compressive, residual stress fields. In this work, experiments and a multi-step simulation strategy are used to explain the fatigue crack retarding and accelerating mechanisms within these LSP-induced residual stress fields. Crack face contact is identified as main mechanism to retard the fatigue crack as the stress distribution changes and the stress intensity factor range decreases. Crack face contact is experimentally detected by load vs. crack opening displacement (COD) curves and scanning electron microscopy (SEM) of the crack faces, as well as during numerical simulations. The convincing agreement between experiment and simulation, especially regarding the specific crack face contact areas, allowed the proper evaluation of the stress intensity factors depending on the crack length. It is found that crack closure is indeed one of the main reasons for the efficient application of LSP for fatigue crack retardation. Furthermore, the occurrence of crack closure does not indicate a zero value stress intensity factor in complex residual stress fields, as the areas of crack face contact depend strongly on the LSP-induced compressive residual stresses. %0 journal article %@ 2296-8016 %A Richert, C., Odermatt, A., Huber, N. %D 2019 %J Frontiers in Materials %P 327 %R doi:10.3389/fmats.2019.00327 %T Computation of Thickness and Mechanical Properties of Interconnected Structures: Accuracy, Deviations, and Approaches for Correction %U https://doi.org/10.3389/fmats.2019.00327 %X tasks in terms of stiffness and strength. %0 journal article %@ 2075-4701 %A Panina, E., Yurchenko, N., Zherebtsov, S., Stepanov, N., Salishchev, G., Ventzke, V., Dinse, R., Kashaev, N. %D 2019 %J Metals %N 12 %P 1351 %R doi:10.3390/met9121351 %T Laser Beam Welding of a Low Density Refractory High Entropy Alloy %U https://doi.org/10.3390/met9121351 12 %X The effect of laser beam welding on the structure and properties of a Ti1.89NbCrV0.56 refractory high entropy alloy was studied. In particular, the effect of different pre-heating temperatures was examined. Due to the low ductility of the material, laser beam welding at room temperature resulted in the formations of hot cracks. Sound butt joints without cracks were produced using pre-heating to T ≥ 600 °C. In the initial as-cast condition, the alloy consisted of coarse bcc grains with a small amount of lens-shaped C15 Laves phase particles. A columnar microstructure was formed in the welds; the thickness of the grains increased with the temperature of pre-heating before welding. The Laves phase particles were formed in the seams after welding at 600 °C or 800 °C, however, these particles were not observed after welding at room temperature or at 400 °C. Soaking at elevated temperatures did not change the microstructure of the base material considerably, however, “additional” small Laves particles formed at 600 °C or 800 °C. Tensile test of welded specimens performed at 750 °C resulted in the fracture of the base material because of the higher hardness of the welds. The latter can be associated with the bcc grains refinement in the seams. %0 journal article %@ 0094-243X %A Álvarez, P., Escribano, R., Zubiri, F., Fomin, F., Kashaev, N., Bauer, S. %D 2019 %J AIP Conference Proceedings %P 070003 %R doi:10.1063/1.5112608 %T Development of laser straightening (LS) strategies to remove distortion in welded aeronautical structures %U https://doi.org/10.1063/1.5112608 %X Manufacturing of stiffened aeronautical structures requires the joining of stringers to thin skin. Structures targeted in this work are composed by Ti-6Al-4V (Grade 5) stringers and commercially pure Ti (cp-Ti, Grade 2) skin. The stringers are laser beam welded (LBW) to the skin in T-joint configuration. Despite the highly concentrated and relatively low heat input of the laser source, the reduced thickness of the skin (0.8 mm) leads to significant angular distortion and so- called “Zeppelin effect”. Moreover, buckling distortion is observed in medium size panels (500 x 500 mm) including several stringers. Within this study, laser straightening (LS) process has been developed to mitigate as-welded distortion. The process is applied on the reverse of the skin and is based on the temperature gradient mechanism (TGM). The same welding source (disk laser) and optic head were employed for both LBW and LS. Consequently, the complete manufacturing of the structure can be done in just one working station. Different LS strategies were applied with the aim of removing welding induced distortion. They included both constant and variable laser power runs, single and multiple runs and different scanning sequences. After optimization of LS process, medium size panels with maximum distortion of less than 2.6 mm were obtained. Initial distortion was higher than 11 mm, meaning that approximately 80% of welding induced distortion was effectively removed. %0 journal article %@ 0268-3768 %A Enz, J., Carrarin, C., Riekehr, S., Ventzke, V., Kashaev, N. %D 2018 %J The International Journal of Advanced Manufacturing Technology %N 1-4 %P 299-310 %R doi:10.1007/s00170-017-1197-x %T Hot cracking behaviour of an autogenously laser welded Al-Cu-Li alloy %U https://doi.org/10.1007/s00170-017-1197-x 1-4 %X AA2198 is a relatively new light-weight and high-performance Al-Cu-Li alloy considered for aviation and space applications. However, Al-Cu-Li alloys generally exhibit severe weldability problems for all fusion-welding techniques, such as laser-beam welding. In particular, porosity formation and hot cracking are observed for the laser-beam welding of these alloys. A common remedy for hot cracking is the use of an appropriate filler wire with a high Si content. In the present study, three different approaches for improving the hot cracking susceptibility of AA2198 laser beam welded without any filler material are presented. For this purpose, pre-heating of the weld samples to elevated temperatures, pre-loading of the weld samples perpendicular to the welding direction, or an optimization of the laser-beam welding parameters were conducted. The autogenously welded samples were assessed with regard to the resulting total crack length and their mechanical properties. It was demonstrated that all of the presented approaches led to a reduction of hot cracking. However, the largest effect was observed for the use of low levels of laser power and welding velocity. The mechanical properties of the optimised autogenously welded samples are only marginally inferior as for the samples laser welded with the Al-Si filler wire AA4047. %0 journal article %@ 0924-0136 %A Keller, S., Chupakhin, S., Staron, P., Maawad, E., Kashaev, N., Klusemann, B. %D 2018 %J Journal of Materials Processing Technology %P 294-307 %R doi:10.1016/j.jmatprotec.2017.11.023 %T Experimental and numerical investigation of residual stresses in laser shock peened AA2198 %U https://doi.org/10.1016/j.jmatprotec.2017.11.023 %X Laser shock peening (LSP) is a surface treatment which improves the fatigue performance of metallic structures by introducing compressive residual stresses. The aim of this paper is the investigation of LSP of the aluminium alloy AA2198. This investigation includes the variation of the laser power density (2.78 GW/cm2 to 25 GW/cm2) and the squared laser focus (1 mm × 1 mm and 3 mm × 3 mm). Additionally, two different temper stages (T3 and T8) and thicknesses (3.2 mm and 4.8 mm) of AA2198 are considered. The study of the LSP process is split into two parts; at first, LSP experiments are performed to clarify the influence of the temper stage, the focus size, the laser power density and the thickness of the specimen on the residual stress field. Secondly, a process model based on the finite element method is employed which requires in particular the adjustment of a suitable laser induced pressure pulse. Due to the different yield strength and strain hardening behaviour of the different temper conditions, AA2198-T8 shows a lower penetration depth of compressive residual stresses compared to AA2198-T3. A smaller focus size leads to higher compressive residual stresses near the surface but a lower penetration depth. To investigate possible shock wave reflections, different base layers in the LSP process are investigated considering a free, a clamped and a glued back-side of the specimen. No differences in terms of resulting residual stresses were observed. The experimental study provides some preliminary assumptions which are used to simplify the simulation set-up. Residual stresses are measured by the incremental hole drilling method using electronic speckle pattern interferometry (ESPI) as well as synchrotron X-ray diffraction. The calculated residual stresses in the simulation are averaged layer-wise over a sample area for comparison with the measured residual stresses. The model is used to simulate the LSP process for the considered temper stages and focus sizes to predict the resulting residual stresses. Simulated and measured residual stress profiles show for the different cases very good agreement. %0 journal article %@ 8756-758X %A Lu, J., Huber, N., Kashaev, N. %D 2018 %J Fatigue and Fracture of Engineering Materials and Structures %N 5 %P 1183-1195 %R doi:10.1111/ffe.12762 %T Improving the fatigue performance of airframe structures via the hybridized application of geometrical modifications and laser heating %U https://doi.org/10.1111/ffe.12762 5 %X This study aimed to investigate an optimization method that can maximize fatigue crack retardation based on the concepts of crenellation and residual stress engineering. By applying both concepts, fatigue crack retardation was achieved by the systematic modulation of the panel thickness and by the superposition of a beneficial residual stress field induced by laser heating. To identify an optimized implementation of both concepts, an advanced finite element method-genetic algorithm coupled approach was proposed, where each possible configuration in terms of the crenellation geometry and the positioning of the laser heating lines was encoded in a binary string. The inclusion of the residual stress field induced by the laser heating in the finite element method model was achieved by the inherent strain approach. It was found that the optimized configurations showed from 38% up to 77% fatigue life extensions, which were much larger than the linear superimposition of the fatigue life improvements by each individual technique. %0 journal article %@ 0959-6526 %A Gialos, A.A., Zeimpekis, V., Alexopoulos, N.D., Kashaev, N., Riekehr, S., Karanika, A. %D 2018 %J Journal of Cleaner Production %P 785-799 %R doi:10.1016/j.jclepro.2017.12.151 %T Investigating the impact of sustainability in the production of aeronautical subscale components %U https://doi.org/10.1016/j.jclepro.2017.12.151 %X The aim of this paper is to investigate the impact of sustainability aspects in the production of aeronautical subscale components by comparing the traditional riveted versus the innovative Laser Beam Welding (LBW) process in industrial conditions. We adopt a quantitative assessment methodology for both processes, by taking into account a series of manufacturing scenarios with six multi-dimensional aeronautical subscale components in different annual production rates (mass production). The results reveal that the exploitation of the LBW technology can provide weight savings up to 28%, by exploiting lower density AlCuLi alloys, while the time savings during the manufacturing process can be up to 67%. Furthermore, the total manufacturing cost of the LBW process can be reduced up to 40% for the case of long structures, when compared to the corresponding riveted structures. In terms of environmental friendliness, the LBW process results to increased CO2e emissions by 124% during the manufacturing process when compared to the riveting process. However, this difference is lower than 60% when longer structures with smaller count of stringers are used. Finally, despite the high carbon footprint emission during the manufacturing phase, when the life cycle of aircrafts is assessed, the LBW joining process can contribute to lighter components, resulting to less weighted aircrafts whose engines consume less fuel, contributing in that way to energy and GHG emissions reductions. %0 journal article %@ 2212-8271 %A Froend, M., Ventzke, V., Riekehr, S., Kashaev, N., Klusemann, B., Enz, J. %D 2018 %J Procedia CIRP %P 131-135 %R doi:10.1016/j.procir.2018.08.062 %T Microstructure and hardness evolution of laser metal deposited AA5087 wall-structures %U https://doi.org/10.1016/j.procir.2018.08.062 %X Wire-based laser metal deposition enables to manufacture structures with very high deposition rates in comparison to powder-based laser additive manufacturing. However, this advantage is generally accompanied with a high energy input. Thus, an accumulation of heat within the structure can result. In addition, the heat conduction conditions can also change with increasing structure height, leading to inhomogeneous microstructural formation along the part. The present study deals with the evolution of the microstructure and hardness in laser metal deposited AA5087 wall structures. In this regard, two samples processed at adapted parameters for different deposition rates are investigated. %0 journal article %@ 1526-6125 %A Froend, M., Riekehr, S., Kashaev, N., Klusemann, B., Enz, J. %D 2018 %J Journal of Manufacturing Processes %P 721-732 %R doi:10.1016/j.jmapro.2018.06.033 %T Process development for wire-based laser metal deposition of 5087 aluminium alloy by using fibre laser %U https://doi.org/10.1016/j.jmapro.2018.06.033 %X In recent decades, laser metal deposition, as a part of additive manufacturing, developed into a promising methodology in industrial fields. In recent years, there has been an increased interest in the processability of lightweight high-strength structural materials, such as aluminium alloys. However, in terms of wire-based laser metal deposition, there is still a lack of knowledge with regard to the processability of aluminium alloys. In this research, the process development for wire-based laser metal deposition of a 5087 aluminium alloy (AlMg4.5 MnZr) has been conducted. It is observed that pre-heating is beneficial in terms of porosity and distortion reduction. Within optimized parameter ranges, it is possible to control the geometric shape, dilution, and aspect ratios of the deposited layers in a systematic way. Accordingly, defect-free layers with tailored geometrical features can be processed and adapted to specific process requirements. %0 journal article %@ 0966-9795 %A Kashaev, N., Ventzke, V., Stepanov, N., Shaysultanov, D., Sanin, V., Zherebtsov, S. %D 2018 %J Intermetallics %P 63-71 %R doi:10.1016/j.intermet.2018.02.014 %T Laser beam welding of a CoCrFeNiMn-type high entropy alloy produced by self-propagating high-temperature synthesis %U https://doi.org/10.1016/j.intermet.2018.02.014 %X Fiber laser beam welding of a CoCrFeNiMn-type high entropy alloy (HEA) produced by self-propagating high-temperature synthesis (SHS) was reported in this work. The SHS-fabricated alloy was characterized by both ∼2 times reduced Mn content in comparison with that of the other principal components and the presence of impurities including Al, C, S, and Si. The as-fabricated alloy was composed of columnar fcc grains with coarse precipitates of MnS and fine Cr-rich M23C6 carbides. Successful defect-free butt joint of the alloy was obtained using a laser power of 2 kW and a welding speed of 5 m/min. Welding resulted in changes in texture and structure of the fcc matrix. In addition, precipitation of nanoscale B2 phase particles in the weld zone was observed. A pronounced increase in microhardness from (153 ± 3) HV 0.5 (base material) to (208 ± 6) HV 0.5 (fusion zone) was observed. The B2 phase precipitation after welding was found to be in a reasonable agreement with the ThermoCalc predictions. Quantitative analysis demonstrated that the increase in hardness can be associated with the B2 phase precipitation. Possibilities of the development of HEAs with intrinsic hardening ability after laser processing are discussed. %0 journal article %@ 2212-8271 %A Enz, J., Lolos, C., Riekehr, S., Ventzke, V., Kashaev, N. %D 2018 %J Procedia CIRP %P 127-130 %R doi:10.1016/j.procir.2018.08.060 %T Influence of different Al-Cu substrates on the properties of laser metal deposited Al coatings %U https://doi.org/10.1016/j.procir.2018.08.060 %X The application of coatings to structures is generally done in order to locally tailor properties. For this purpose, powder-based laser metal deposition (LMD) can be utilized. The resulting properties of the coatings are not only affected by the LMD process parameters, but also by the substrate material. In this regard, the chemical composition as well as its initial microstructure of the substrate has a significant influence. In the present study two Al-Cu alloy substrates, 2139 and 2198, are used for the LMD of pure Al powder. The resulting coatings possess clearly different properties, although the same process parameters are used. %0 journal article %@ 2212-8271 %A Burkhardt, I., Visone, R., Riekehr, S., Rackel, M.W., Kashaev, N., Enz, J. %D 2018 %J Procedia CIRP %P 176-179 %R doi:10.1016/j.procir.2018.08.088 %T Parameter development and characterization of laser metal deposited high-temperature Ti alloy powders %U https://doi.org/10.1016/j.procir.2018.08.088 %X Powder-based laser metal deposition (LMD) of Ti alloys enables the manufacturing of geometrical complex structures with tailored properties for high-temperature applications. The most important parameters for the LMD-process are the laser power, process velocity, laser focus position, gas flow and powder feed rate. Compared to the predominantly used Ti-6Al-4V, Ti-6Al-2Sn-4Zr-2Mo shows a higher oxidation resistance and a lower creep rate. For determining suitable process conditions, a parameter study using Ti-6Al-2Sn-4Zr-2Mo powder is performed. Subsequent, the parameter transfer to a γ-TiAl alloy is examined. The assessment of the dissimilar-LMD-structures is conducted by visual inspection, radiography inspection, microstructural analysis and hardness measurements. %0 journal article %@ 0255-5476 %A Froend, M., Bock, F., Ventzke, V., Riekehr, S., Kashaev, N., Klusemann, B., Enz, J. %D 2018 %J Materials Science Forum, THERMEC 2018 %P 988-994 %R doi:10.4028/www.scientific.net/MSF.941.988 %T Experimental Investigation of Temperature Distribution during Wire-Based Laser Metal Deposition of the Al-Mg Alloy 5087 %U https://doi.org/10.4028/www.scientific.net/MSF.941.988 %X Wire-based laser metal deposition enables to manufacture large-scale components with deposition rates significant higher compared to powder-based laser additive manufacturing techniques, which are currently working with deposition rates of only a few hundred gram per hour. However, the wire-based approach requires a significant amount of laser power in the range of several kilowatts instead of only a few hundred watts for powder-based processes. This excessive heat input during laser metal deposition can lead to process instabilities such as a non-uniform material deposition and to a limited processability, respectively. Although, numerous possibilities to monitor temperature evolution during processing exist, there is still a lack of knowledge regarding the relationship between temperature and geometric shape of the deposited structure. Due to changing cooling conditions with increasing distance to the substrate material, producing a wall-like structure results in varying heights of the individual tracks. This presents challenges for the deposition of high wall-like structures and limits the use of constant process parameters. In the present study, the temperature evolution during laser metal deposition of AA5087 using constant process parameters is investigated and a scheme for process parameter adaptions in order to reduce residual stress induced componential distortions is suggested. %0 journal article %@ 2589-1529 %A Herrnring, J., Staron, P., Kashaev, N., Klusemann, B. %D 2018 %J Materialia %P 243-255 %R doi:10.1016/j.mtla.2018.08.010 %T Multiscale process simulation of residual stress fields of laser beam welded precipitation hardened AA6082 %U https://doi.org/10.1016/j.mtla.2018.08.010 %X In this study, a multiscale modelling approach for the determination of residual stresses for the laser beam welded, precipitation hardened aluminium alloy AA6082-T6 is presented and applied. The material behaviour is described by an elasto-visco-plastic material model, specially suited for fusion welding processes. The microstructure evolution during the welding process has a direct influence on the macroscopic mechanical properties. The modelling approach accounts for the change in the microstructure via a Kampmann-Wagner Numerical model which takes into account the kinetics of the precipitates. The macroscopic mechanical properties are determined via classic dislocation theory, which accounts for the interaction between dislocations and precipitates. The temperature field of the welding process is described by a highly efficient semi-analytical approach. The solution of the temperature field in connection with a three dimensional moving heat source is achieved by using the method of Green’s functions. By employing the method of Green’s functions, it is possible to reduce the numerical effort significantly. The results of this modelling approach are compared to temperature, hardness as well as residual stress measurements, obtained from synchrotron X-ray diffraction, for welded sheets to clarify the accuracy of the applied model. %0 journal article %@ 1526-6125 %A Kashaev, N., Ventzke, V., Cam, G. %D 2018 %J Journal of Manufacturing Processes %P 571-600 %R doi:10.1016/j.jmapro.2018.10.005 %T Prospects of laser beam welding and friction stir welding processes for aluminum airframe structural applications %U https://doi.org/10.1016/j.jmapro.2018.10.005 %X FSW, as a solid-state joining process, has the advantage that the joining is conducted at temperatures below the melting point of the materials to be joined. Therefore, improved mechanical performance of joints is expected compared to that of fusion joining processes such as LBW. Furthermore, better mechanical properties can be obtained when heat input during joining is reduced by employing stationary shoulder FSW and/or external cooling. On the other hand, LBW offers several advantages such as low distortion, high strength of the joint, and high welding speeds due to its low localized-energy input. Thus, LBW - as a high-speed and easily controllable process - allows the welding of optimized complex geometrical forms in terms of mechanical stiffness, strength, production velocity, and visual quality. Both joining processes have advantages and disadvantages, depending on joint geometries and materials. They both have the potential to reduce the total weight of the structure. The FSW process (particularly lower heat input stationary shoulder FSW process) is more advantageous in producing long-distance straight-line butt joints or overlapped joints of aircraft structures, whereas the high-speed and easily controllable LBW process allows the joining of complex geometrical forms due to its high flexibility, particularly in the new generation high strength Al-alloys (such as AA2198), the strengthening phases of which are more heat resistant. %0 journal article %@ 0255-5476 %A Herrnring, J., Kashaev, N., Klusemann, B. %D 2018 %J Materials Science Forum, THERMEC 2018 %P 1411-1417 %R doi:10.4028/www.scientific.net/MSF.941.1411 %T Precipitation Kinetics of AA6082: An Experimental and Numerical Investigation %U https://doi.org/10.4028/www.scientific.net/MSF.941.1411 %X The development of simulation tools for bridging different scales are essential for understanding complex joining processes. For precipitation hardening, the Kampmann-Wagner numerical model (KWN) is an important method to account for non-isothermal second phase precipitation. This model allows to describe nucleation, growth and coarsening of precipitation hardened aluminum alloys based on a size distribution for every phase which produces precipitations. In particular, this work investigates the performance of a KWN model by [1-3] for Al-Mg-Si-alloys. The model is compared against experimental data from isothermal heat treatments taken partially from [2]. Additionally, the model is used for investigation of the precipitation kinetics for a laser beam welding process, illustrating the time-dependent development of the different parameters related to the precipitation kinetics and the resulting yield strength. %0 journal article %@ 0255-5476 %A Burkhardt, I., Ventzke, V., Riekehr, S., Kashaev, N., Enz, J. %D 2018 %J Materials Science Forum, THERMEC 2018 %P 1404-1410 %R doi:10.4028/www.scientific.net/MSF.941.1404 %T Development of an optimised shielding strategy for laser beam welding of Ti6Al2Sn4Zr2Mo %U https://doi.org/10.4028/www.scientific.net/MSF.941.1404 %X Ti6Al2Sn4Zr2Mo exhibits improved oxidation and creep properties compared to Ti6Al4V. Laser beam welding (LBW) is an approved process to receive narrow weld seams at high welding speeds with low heat input. Almost distortion free complex shaped structures can be joined with optimal parameters. For the optimisation of the LBW process the most relevant parameters are the welding speed, the laser input power and the gas shielding strategy. Using a fibre laser, the laser radiation is attenuated by a welding plume the so-called metal-vapour cloud (MVC). The MVC has a large influence on the laser input power. Therefore, an approach for reducing the MVC by optimising the shielding strategy using an additional gas flow in opposite welding direction is examined. Utilizing high-speed camera records, the effectiveness of the approach is assessed. Welded samples are evaluated by visual and radiographic inspection, metallographic assessment as well as microhardness measurements with regard to weld seam geometry, defects, microstructure and local mechanical properties. The obtained results are correlated to the used laser welding parameters. %0 journal article %@ 2452-3216 %A Fomin, F., Klusemann, B., Kashaev, N. %D 2018 %J Procedia Structural Integrity %P 273-278 %R doi:10.1016/j.prostr.2018.12.046 %T Surface modification methods for fatigue properties improvement of laser welded Ti64 butt joints %U https://doi.org/10.1016/j.prostr.2018.12.046 %X Surface and internal defects formed upon laser beam welding (LBW) have been recognized as a serious problem because they cause stress concentration leading to premature failure of a welded component. This paper seeks to remedy these weld imperfections by applying various post-weld treatments and analyzing their effect on the high cycle fatigue (HCF) performance of welded joints. High efficiency of laser-based post-processing techniques after welding such as laser surface remelting (LSR) and laser shock peening (LSP) was demonstrated and compared with conventional approaches. The study reveals that welding porosity determines the internal crack initiation of the surface-treated weldments. Influence of process parameters on porosity level and the HCF properties is presented in detail. Based on an extensive experimental study, practical guidelines needed to mitigate the notch effect from defects and to maximize the fatigue performance of the laser-welded Ti-6Al-4V butt joints are given. %0 journal article %@ 0142-1123 %A Fomin, F., Horstmann, M., Huber, N., Kashaev, N. %D 2018 %J International Journal of Fatigue %P 22-35 %R doi:10.1016/j.ijfatigue.2018.06.012 %T Probabilistic fatigue-life assessment model for laser-welded Ti-6Al-4V butt joints in the high-cycle fatigue regime %U https://doi.org/10.1016/j.ijfatigue.2018.06.012 %X The present paper focuses on the effect of inherent welding-induced defects on the high-cycle fatigue behaviour of laser-welded Ti-6Al-4V butt joints. The transition of the crack origin from the surface to the subsurface occurs upon removal of the surface stress concentrators. Under these circumstances, fatigue cracks nucleate at subsurface porosity and show a typical fish-eye pattern of fracture surface. A fatigue-life assessment model has been developed for internally flawed materials based on a fracture-mechanics approach, which takes effects of short-cracks into account. A novel approach for the simplified construction of the cyclic resistance curve of internal cracks is proposed herein. Using statistical methods, the experimentally determined porosity distribution has been incorporated into the model to predict the fatigue scatter range. The presented methodology can potentially be used to achieve a required reliability of the welded joints with respect to fatigue as a design criterion. %0 journal article %@ 0268-3768 %A Fomin, F., Froend, M., Ventzke, V., Alvarez, P., Bauer, S., Kashaev, N. %D 2018 %J The International Journal of Advanced Manufacturing Technology %N 5-8 %P 2019-2031 %R doi:10.1007/s00170-018-1968-z %T Metallurgical aspects of joining commercially pure titanium to Ti-6Al-4V alloy in a T-joint configuration by laser beam welding %U https://doi.org/10.1007/s00170-018-1968-z 5-8 %X The present paper focuses on the metallurgical and microstructural characterization of the laser beam-welded T-joints between commercially pure titanium (CP-Ti) and Ti-6Al-4V alloy. The weld regions were comprehensively studied and the mechanisms leading to the final morphology within each weld region were described. The link between microstructural features and local mechanical properties was demonstrated. Owing to different constitution, the responses of the two titanium alloys to thermal cycles imposed by laser welding are completely different. A strong interface with no dilution zone between the two alloys was observed. The cooling rate during the welding process is high enough for diffusionless martensitic transformation in the Ti-6Al-4V part of the fusion zone. In contrast, no evidence of martensite was found in the CP-Ti because of low solute content and, consequently, much higher critical cooling rate. Plausible reason for some controversy found in the literature on the resulting transformation products after laser processing of CP-Ti was given. The present findings might have important industrial implications because careful microstructural characterization revealed the real position of the skin fusion line, which is of great importance for fulfillment of the weld quality criteria. %0 journal article %@ 1044-5803 %A Froend, M., Ventzke, V., Riekehr, S., Kashaev, N., Klusemann, B., Enz, J. %D 2018 %J Materials Characterization %P 59-67 %R doi:10.1016/j.matchar.2018.05.022 %T Microstructure and microhardness of wire-based laser metal deposited AA5087 using an Ytterbium fibre laser %U https://doi.org/10.1016/j.matchar.2018.05.022 %X Wire-based additive manufacturing has been increasingly investigated in recent years. Although it is possible nowadays to manufacture structures that are free from inner defects such as porosity and cracks using wire and arc additive manufacturing, there is still a lack of knowledge regarding wire-based laser metal deposition of aluminium and its alloys. In order to be able to produce locally tailored part properties, it is necessary to understand the process parameter to material property relationships. Using laser energy sources, it becomes possible to analyse in detail the heat input and to observe occurring microstructural evolutions. This work includes a microstructural and mechanical characterization of an AA5087 wall structure. Detailed analyses of the chemical composition, texture, and microhardness of the structure have been performed. The microstructure contains different grain orientations as well as grain shapes and sizes along the structure, resulting in locally different material properties. The results have been analysed and discussed in reference to fundamental theories such as the Hall–Petch and Orowan mechanisms. %0 journal article %@ 0921-5093 %A Kashaev, N., Pugachev, D., Ventzke, V., Fomin, F., Burkhardt, I., Enz, J., Riekehr, S. %D 2017 %J Materials Science and Engineering A %P 110-120 %R doi:10.1016/j.msea.2017.03.115 %T Microstructure and mechanical performance of autogenously fibre laser beam welded Ti-6242 butt joints %U https://doi.org/10.1016/j.msea.2017.03.115 %X This work deals with the effects of laser beam power, focus position and advance speed on the geometry, microstructure and mechanical properties such as the tensile strength and microhardness of autogenously fibre laser beam welded Ti-6Al-2Sn-4Zr-2Mo (denoted as Ti-6242) butt joints used for high temperature applications. The Ti-6242 sheet employed here is characterized by a globular (α+β) microstructure. Laser beam welded butt joints consisted of a martensitic fusion zone, inhomogeneous heat affected zones and equiaxed base materials. The microhardness increased from 330 HV 0.3 in base material to 430 HV 0.3 in fusion zone due to the martensitic transformation. Butt joints showed the base material level of strength in tensile test. The local increase in microhardness provided a shielding effect that protected the Ti-6242 butt joint against mechanical damage during the static tensile load test. The predicted critical total underfill depth that does not reduce the tensile strength of the weld was determined to be 25% of the specimen thickness. %0 journal article %@ 0255-5476 %A Kashaev, N., Pugachev, D., Riekehr, S., Ventzke, V. %D 2017 %J Materials Science Forum, THERMEC 2016 %P 903-908 %R doi:10.4028/www.scientific.net/MSF.879.903 %T Fiber Laser Beam Welding of Ti-6242 - Effect of Processing Parameters on Microstructural and Mechanical Properties %U https://doi.org/10.4028/www.scientific.net/MSF.879.903 %X The present work investigates the effects of laser beam power, focus position and advance speed on the geometry, microstructure and mechanical properties of fiber laser beam welded Ti-6Al-2Sn-4Zr-2Mo (denoted as Ti-6242) butt joints used for high temperature applications. Detailed microstructural and mechanical studies were performed on welds produced using optimized parameters (a laser beam power of 5 kW, a focus position of 0.0 mm and an advance speed of 6.2 m/min). The Ti-6242 base material is characterized by a globular (α+β) microstructure. The heat input during laser beam welding led to the formation of a martensitic α’-phase fusion zone. The heat affected zone consisted of globular grains and acicular crystallites. These local transformations were connected with a change in the micro-texture, average grain size and β-phase content. Furthermore, the microhardness increased from 330 HV 0.3 to 450 HV 0.3 due to the martensitic transformation. The mechanical behavior of the laser beam welded Ti-6242 butt joint loaded in tension was determined by the properties of the Ti-6242 base material. The local increase in hardness provided a shielding effect that protected the Ti-6242 butt joint against mechanical damage. %0 journal article %@ 0142-1123 %A Kashaev, N., Ventzke, V., Horstmann, M., Chupakhin, S., Riekehr, S., Falck, R., Maawad, E., Staron, P., Schell, N., Huber, N. %D 2017 %J International Journal of Fatigue %P 223-233 %R doi:10.1016/j.ijfatigue.2017.01.042 %T Effects of laser shock peening on the microstructure and fatigue crack propagation behaviour of thin AA2024 specimens %U https://doi.org/10.1016/j.ijfatigue.2017.01.042 %X Laser shock peening (LSP) was performed on aluminium alloy AA2024 with a thickness of 2.0 mm. Microstructural studies using the electron back scatter diffraction (EBSD) technique were performed to quantify the micro-texture changes in the material through LSP. A residual stress analysis was performed using synchrotron radiation and a hole drilling technique. Fatigue crack propagation (FCP) tests were performed to investigate the retardation effect of LSP residual stresses. Load versus crack opening displacement curves were analysed to obtain the corrected values of load opening levels considering the effects of the residual stresses. Specimens with the LSP treatment reveal a significant retardation of the FCP. The presence of compressive residual stresses caused the crack closure effect, which increased the level of crack opening load and therefore reduced the effective load range. An original methodology to consider LSP-induced residual stresses on the FCP behaviour was proposed. %0 journal article %@ 1526-6125 %A Enz, J., Kumar, M., Riekehr, S., Ventzke, V., Huber, N., Kashaev, N. %D 2017 %J Journal of Manufacturing Processes %P 272-280 %R doi:10.1016/j.jmapro.2017.07.030 %T Mechanical properties of laser beam welded similar and dissimilar aluminum alloys %U https://doi.org/10.1016/j.jmapro.2017.07.030 %X Two approaches were used for laser beam welding of similar and dissimilar joints of AA7075 and AA5182 that aim to overcome the weldability problems of high-strength Al-Zn-Mg-Cu alloys. The first approach implies the use of vanadium foil as additional filler material, while the second implies the use of a fiber laser with a large beam diameter and a top-hat beam profile. Although both approaches result in an improved weld quality, in terms of weld appearance, porosity and cracking, the resulting mechanical properties differ considerably. The addition of vanadium leads to a local increase of microhardness in the fusion zone. However, the tensile strength of these joints is lower as for fiber laser welded joints. In direct comparison fiber laser welded joints exhibit also higher formability as the joints welded with vanadium foil. The highest formability is obtained for dissimilar joints with the medium-strength Al-Mg alloy. Due to the unavoidable softening in the weld zone of heat treatable aluminum alloys, the formability of the joints is inferior in comparison to the base materials. In addition, the positive effect of post-weld heat treatment, surface milling and warm forming on the resulting mechanical properties of similar and dissimilar joints is discussed. %0 journal article %@ 0030-3992 %A Froend, M., Fomin, F., Riekehr, S., Alvarez, P., Zubiri, F., Bauer, S., Klusemann, B., Kashaev, N. %D 2017 %J Optics and Laser Technology %P 123-131 %R doi:10.1016/j.optlastec.2017.05.017 %T Fiber laser welding of dissimilar titanium (Ti-6Al-4V/cp-Ti) T-joints and their laser forming process for aircraft application %U https://doi.org/10.1016/j.optlastec.2017.05.017 %X The weldability of dissimilar T-joints between commercially pure titanium (cp-Ti) Grade 2 skin and Ti-6Al-4V Grade 5 stringer using a continuous wave 8 kW ytterbium fiber laser as well as the possibility of subsequent laser straightening process of these joints were investigated. Based on the industrial standards ISO 4578:2011 and AWS D17.1:200, process development to compensate inherent angular distortion after welding by laser heating with the same equipment as for welding was carried out. The obtained results were effectively transferred to a 6-stringer-demonstrator with a length up to 500 mm. To investigate the shape and morphology of the welding seam as well as to verify its freedom from defects using the defined process parameters, metallographic transverse cross-sections and X-ray analyses were realized. In addition, the behavior of the welding seam geometry and the bending behavior of the specimens for varied process parameters were elucidated. For the welding process special attention to the shielding conditions and to the local and angular laser beam positioning was payed. To straighten the welded joints, laser straightening parameters inducing no microstructural changes were identified. %0 journal article %@ 2452-3216 %A Fomin, F., Kashaev, N. %D 2017 %J Procedia Structural Integrity %P 415-422 %R doi:10.1016/j.prostr.2017.11.107 %T Influence of Porosity on the High Cycle Fatigue Behaviour of Laser Beam Welded Ti-6Al-4V Butt Joints %U https://doi.org/10.1016/j.prostr.2017.11.107 %X Surface defects and internal discontinuities are inevitable results of the laser beam welding (LBW) process regardless of the material used. Comprehensive understanding of the fatigue degradation caused by these defects is of major concern for the introduction of LBW into manufacturing or repair processes. The present paper focuses on the effect of inherent welding-induced material flaws on the high cycle fatigue behaviour of the laser welded Ti-6Al-4V butt joints. If the surface quality of the welded joint is sufficiently high, the transition of the crack origin from the surface to the subsurface occurs. The mechanisms of internal fatigue crack formation and growth at the early stages were studied. A typical fish-eye pattern of fracture surface was observed in close proximity to the crack origin. The model based on fracture mechanics for durability prediction in the presence of randomly distributed porosity was developed. The link between theory, modelling and experiment was successfully demonstrated. %0 journal article %@ 0309-3247 %A Chupakhin, S., Kashaev, N., Klusemann, B., Huber, N. %D 2017 %J The Journal of Strain Analysis for Engineering Design %N 3 %P 137-151 %R doi:10.1177/0309324717696400 %T Artificial neural network for correction of effects of plasticity in equibiaxial residual stress profiles measured by hole drilling %U https://doi.org/10.1177/0309324717696400 3 %X The hole drilling method is a widely known technique for the determination of non-uniform residual stresses in metallic structures by measuring strain relaxations at the material surface caused through the stress redistribution during drilling of the hole. The integral method is a popular procedure for solving the inverse problem of determining the residual stresses from the measured surface strain. It assumes that the residual stress can be approximated by step-wise constant values, and the material behaves elastically so that the superposition principle can be applied. Required calibration data are obtained from finite element simulations, assuming linear elastic material behavior. That limits the method to the measurement of residual stresses well below the yield strength. There is a lack of research regarding effects caused by residual stresses approaching the yield strength and high through-thickness stress gradients as well as the correction of the resulting errors. However, such high residual stresses are often introduced in various materials by processes such as laser shock peening, for example, to obtain life extension of safety relevant components. The aim of this work is to investigate the limitations of the hole drilling method related to the effects of plasticity and to develop an applicable and efficient method for stress correction, capable of covering a wide range of stress levels. For this reason, an axisymmetric model was used for simulating the hole drilling process in ABAQUS involving plasticity. Afterward, the integral method was applied to the relaxation strain data for determining the equibiaxial stress field. An artificial neural network has been used for solving the inverse problem of stress profile correction. Finally, AA2024-T3 specimens were laser peened and the measured stress fields were corrected by means of the trained network. To quantify the stress overestimation in the hole drilling measurement, an error evaluation has been conducted. %0 journal article %@ 0255-5476 %A Staron, P., Liu, J., Riekehr, S., Schell, N., Huber, N., Kashaev, N., Mueller, M., Schreyer, A. %D 2017 %J Materials Science Forum, Mechanical Stress Evaluation by Neutrons and Synchrotron Radiation VIII %P 114-119 %R doi:10.4028/www.scientific.net/MSF.905.114 %T In Situ Experiment for Laser Beam Welding of Ti Alloys Using High-Energy X-Rays %U https://doi.org/10.4028/www.scientific.net/MSF.905.114 %X The laser beam welding (LBW) process has many advantages for industrial production; however, it still has to be optimized for two-phase Ti alloys. Phase transformations and residual stresses play a crucial role for welding these alloys. Specific questions about the development of phase content during fast heating with a laser and rapid cooling can only be addressed with time-resolved in-situ experiments, avoiding artefacts from quenching. Also the residual stress development during cooling depends on the occurring phase transformations. Thus, an LBW chamber employing a fibre laser was developed for use with high-energy X-rays from a synchrotron source. Bead-on-plate welding experiments with 2.5 mm thick samples were carried out at the HZG high-energy materials science beamline (HEMS) at DESY, Hamburg. The first experiments focused on the solid-solid phase transformations in a Ti-6Al-4V alloy. Moreover, residual stresses developing during cooling were studied. %0 journal article %@ 0143-8166 %A Kashaev, N., Ventzke, V., Fomichev, V., Fomin, F., Riekehr, S. %D 2016 %J Optics and Lasers in Engineering %P 172-180 %R doi:10.1016/j.optlaseng.2016.06.004 %T Effect of Nd:YAG laser beam welding on weld morphology and mechanical properties of Ti–6Al–4V butt joints and T-joints %U https://doi.org/10.1016/j.optlaseng.2016.06.004 %X A Nd:YAG single-sided laser beam welding process study for Ti–6Al–4V butt joints and T-joints was performed to investigate joining techniques with regard to the process-weld morphology relationship. An alloy compatible filler wire was used to avoid underfills and undercuts. The quality of the butt joints and T-joints was characterized in terms of weld morphology, microstructure and mechanical properties. Joints with regular shapes, without visible cracks, pores, and geometrical defects were achieved. Tensile tests revealed high joint integrity in terms of strength and ductility for both the butt joint and T-joint geometries. Both the butt joints and T-joints showed base material levels of strength. The mechanical performance of T-joints was also investigated using pull-out tests. The performance of the T-joints in such tests was sensitive to the shape and morphology of the welds. Fracture always occurred in the weld without any plastic deformation in the base material outside the weld. %0 journal article %@ 0278-6125 %A Alexopoulos, N.D., Gialos, A.A., Zeimpekis, V., Velonaki, Z., Kashaev, N., Riekehr, S., Karanika, A. %D 2016 %J Journal of Manufacturing Systems %P 38-52 %R doi:10.1016/j.jmsy.2016.02.002 %T Laser beam welded structures for a regional aircraft: weight, cost and carbon footprint savings %U https://doi.org/10.1016/j.jmsy.2016.02.002 %X Laser beam welded structures offer great opportunities for the lightweight design of fuselage structures in order to reduce structural weight for increased fuel efficiency. Our main objective is to validate and demonstrate that laser beam welding (LBW) technology provides the best opportunities in terms of weight reduction, production time and energy consumption for manufacturing aircraft components. To this end, a comparison in terms of energy, process time, cost and carbon footprint is assessed against the ‘conventional’ manufacturing process of riveting, to prove that LBW is actually an environmental friendly process. Manufacturing of a four-stringer stiffened flat subscale component was the case of the present work that was called in the Clean Sky Eco-Design Airframe (EDA) project as the B1 demonstrator (742 mm × 384 mm). The LBW process has been broken down into several sub-processes and activities according to the Activity Based Costing (ABC) methodology and the weight reduction, production time and energy consumption results were compared against the respective of the riveting process. It was proved that for the specific subscale LBW component, it consumes half the energy and can be processed in less than half the time needed (in serial processing of the component) with riveting. Manufacturing of the component with the LBW process (door to door approach) is more environmentally friendly, since it produces 53% less CO2e emissions than the respective riveted process. This is a clear advantage to this manufacturing process in order to assure a sustainable life cycle of the final product. %0 journal article %@ 1073-5623 %A Enz, J., Riekehr, S., Ventzke, V., Huber, N., Kashaev, N. %D 2016 %J Metallurgical and Materials Transactions A %N 6 %P 2830-2841 %R doi:10.1007/s11661-016-3446-2 %T Laser Weldability of High-Strength Al-Zn Alloys and Its Improvement by the Use of an Appropriate Filler Material %U https://doi.org/10.1007/s11661-016-3446-2 6 %X Heat-treatable Al-Zn alloys are promising candidates for use as structural lightweight materials in automotive and aircraft applications. This is mainly due to their high strength-to-density ratio in comparison to conventionally employed Al alloys. Laser beam welding is an efficient method for producing joints with high weld quality and has been established in the industry for many years. However, it is well known that aluminum alloys with a high Zn content or, more precisely, with a high (Zn + Mg + Cu) content are difficult to fusion weld due to the formation of porosity and hot cracks. The present study concerns the laser weldability of these hard-to-weld Al-Zn alloys. In order to improve weldability, it was first necessary to understand the reasons for weldability problems and to identify crucial influencing factors. Based on this knowledge, it was finally possible to develop an appropriate approach. For this purpose, vanadium was selected as additional filler material. Vanadium exhibits favorable thermophysical properties and, thereby, can improve the weldability of Al-Zn alloys. The effectiveness of the approach was verified by its application to several Al-Zn alloys with differing amounts of (Zn + Mg + Cu). %0 journal article %@ 0261-3069 %A Maawad, E., Gan, W., Hofmann, M., Ventzke, V., Riekehr, S., Brokmeier, H.-G., Kashaev, N., Mueller, M. %D 2016 %J Materials and Design %P 137-145 %R doi:10.1016/j.matdes.2016.03.148 %T Influence of crystallographic texture on the microstructure, tensile properties and residual stress state of laser-welded titanium joints %U https://doi.org/10.1016/j.matdes.2016.03.148 %X Preferred grain orientations (crystallographic texture) of base materials could influence on mechanical properties, microstructure and residual stresses of welded joints. This should be considered for design purposes, in particular for materials having non-cubic crystal structures. A multitude of experiments have been carried out in this field of study without considering the crystallographic texture based anisotropy of base materials. In the present work, commercially pure titanium (CP-Ti) rolled sheets were laser welded along various directions with respect to the sample orientations, namely rolling direction (RD), transverse direction (TD) and 45° to RD. Three-dimensional strain profiles and the local texture around the weld were measured by neutron diffraction. Furthermore, grain orientation mapping within the base material, the heat-affected zone and the fusion zone was investigated using the electron back-scatter diffraction (EBSD) technique. The results revealed that tensile properties of the samples are different, while no significant statistical differences in residual stresses were observed. %0 journal article %@ 1073-5623 %A Liu, J., Staron, P., Riekehr, S., Stark, A., Schell, N., Huber, N., Schreyer, A., Mueller, M., Kashaev, N. %D 2016 %J Metallurgical and Materials Transactions A %N 12 %P 5750-5760 %R doi:10.1007/s11661-016-3745-7 %T Phase Transformation and Residual Stress in a Laser Beam Spot-Welded TiAl-Based Alloy %U https://doi.org/10.1007/s11661-016-3745-7 12 %X The microstructure, chemical composition, residual stress, and lattice parameter evolution of the welding zone (WZ) and heat-affected zone (HAZ) of a laser-beam-welded TiAl-based alloy were investigated. It was found that both α2 and γ phases remain highly restrained in the WZ edge, and the stresses are relieved in the HAZ. A grain refinement mechanism is proposed, which works by heating the material to the β or α + β phase field for a short time. The lamellar colonies are refined by the Nb-enriched segregations. %0 journal article %@ 0924-0136 %A Enz, J., Riekehr, S., Ventzke, V., Huber, N., Kashaev, N. %D 2016 %J Journal of Materials Processing Technology %P 155-162 %R doi:10.1016/j.jmatprotec.2016.06.002 %T Fibre laser welding of high-alloyed Al–Zn–Mg–Cu alloys %U https://doi.org/10.1016/j.jmatprotec.2016.06.002 %X The theoretical fundamentals of laser weldability of metals are surveyed and relevant thermophysical parameters are identified – such as vapour pressure, keyhole pressure, beam irradiance, surface tension and viscosity. The derived approach for improving the laser weldability implies the use of a Yb fibre laser with an initial large beam diameter, a top-hat beam profile and a high laser power, which enables the formation of a large and stable keyhole during deep penetration welding. For validating the effectiveness of the developed approach, it is applied to various high-alloyed and hard-to-weld Al–Zn–Mg–Cu alloys. Defect-free welds are obtained even for AA7034 – the alloy with the highest (Zn + Mg + Cu) content commercially available. As reference, the same alloys are welded by using a conventional Nd:YAG laser with a small beam diameter, a Gaussian beam profile and medium laser power. The laser weldability deteriorates with increasing (Zn + Mg + Cu) content in terms of porosity and excess of penetration. The obtained results highlight the importance of the laser system used on the laser weldability of Al–Zn–Mg–Cu alloys. %0 journal article %@ 0309-3247 %A Chupakhin, S., Kashaev, N., Huber, N. %D 2016 %J Journal of Strain Analysis for Engineering Design %N 8 %P 572-581 %R doi:10.1177/0309324716663940 %T Effect of elasto-plastic material behaviour on determination of residual stress profiles using the hole drilling method %U https://doi.org/10.1177/0309324716663940 8 %X The hole drilling method is a well-known technique for the determination of non-uniform residual stress profiles by measuring relaxation distortions caused by the presence of the hole. The integral method, an inverse calculation technique on which the hole drilling method is based, assumes linear elastic material behaviour and is therefore limited to the measurement of residual stresses below 60% of the yield strength. The aim of this study is to investigate the effects of elastic–plastic material behaviour on the determined non-uniform residual stress profile when the residual stresses exceed the given 60% limit. To this end, compressive residual stress profiles, as they are typically induced by laser shock peening, are investigated using finite element simulations followed by an analysis with the integral method. The obtained results from the analysis are compared to the applied residual stress profiles. An evaluation of the deviation between these two profiles provides detailed insight into the expected error as a function of hole drilling depth and the ratio of residual stress magnitude to yield strength. As an additional benefit of the presented approach, it also provides an indication of the range of depth at which the non-uniform residual stress profile should be corrected to reduce measurement error. %0 journal article %@ 1073-5623 %A Tian, Y., Robson, J.D., Riekehr, S., Kashaev, N., Wang, L., Lowe, T., Karanika, A. %D 2016 %J Metallurgical and Materials Transactions A %N 7 %P 3533-3544 %R doi:10.1007/s11661-016-3509-4 %T Process Optimization of Dual-Laser Beam Welding of Advanced Al-Li Alloys Through Hot Cracking Susceptibility Modeling %U https://doi.org/10.1007/s11661-016-3509-4 7 %X Laser welding of advanced Al-Li alloys has been developed to meet the increasing demand for light-weight and high-strength aerospace structures. However, welding of high-strength Al-Li alloys can be problematic due to the tendency for hot cracking. Finding suitable welding parameters and filler material for this combination currently requires extensive and costly trial and error experimentation. The present work describes a novel coupled model to predict hot crack susceptibility (HCS) in Al-Li welds. Such a model can be used to shortcut the weld development process. The coupled model combines finite element process simulation with a two-level HCS model. The finite element process model predicts thermal field data for the subsequent HCS hot cracking prediction. The model can be used to predict the influences of filler wire composition and welding parameters on HCS. The modeling results have been validated by comparing predictions with results from fully instrumented laser welds performed under a range of process parameters and analyzed using high-resolution X-ray tomography to identify weld defects. It is shown that the model is capable of accurately predicting the thermal field around the weld and the trend of HCS as a function of process parameters. %0 journal article %@ 1073-5623 %A Liu, J., Staron, P., Riekehr, S., Stark, A., Schell, N., Huber, N., Schreyer, A., Mueller, M., Kashaev, N. %D 2016 %J Metallurgical and Materials Transactions A %N 12 %P 5761-5770 %R doi:10.1007/s11661-016-3726-x %T Phase Transformations During Solidification of a Laser-Beam-Welded TiAl Alloy—An In Situ Synchrotron Study %U https://doi.org/10.1007/s11661-016-3726-x 12 %X An in situ highly time-resolved, high-energy X-ray diffraction investigation was carried out to observe the phase transformations of a TiAl alloy during laser beam welding. The diffraction patterns are recorded every 0.1 seconds by a fast area two-dimensional detector and plotted according to time, yielding the solidification pathway, the solid phase volume fraction, and the lattice parameter variation of different phases during the solidification and cooling process. Moreover, it is the first study that can demonstrate that the α phase without any Burgers orientation relationship, the so-called non-Burgers α, precipitates appear earlier than the Burgers α. The non-Burgers α grains are found to nucleate on the primary borides. %0 journal article %@ 1438-1656 %A Kashaev, N., Ventzke, V., Horstmann, M., Riekehr, S., Yashin, G., Stutz, L., Beck, W. %D 2015 %J Advanced Engineering Materials %N 3 %P 374-382 %R doi:10.1002/adem.201400202 %T Microstructure and Mechanical Properties of Laser Beam Welded Joints between Fine-Grained and Standard Ti-6Al-4V Sheets Subjected to Superplastic Forming %U https://doi.org/10.1002/adem.201400202 3 %X A fine-grained Ti–6Al–4V sheet that has been developed for superplastic forming (SPF) was joined to a standard Ti–6Al–4V sheet using a Nd:YAG laser and alloy compatible filler wire. The microstructural and mechanical properties of dissimilar laser beam welded butt joints were investigated to determine their behavior under static and cyclic loads and for SPF. The filler wire affected the heat input and reduced the increase in the hardness within the fusion zone compared to that in the heat-affected zone. The laser beam welding process activated local microstructure transformations that were associated with local changes in the microtexture, the β content, and the grain size. The mechanical behavior of a dissimilar laser beam welded butt joint under a static tensile load was controlled by the properties of the standard Ti–6Al–4V sheet. Laser beam welded specimens showed inferior fatigue behavior. Removing the geometrical notches did not significantly improve the fatigue behavior because local microstructural and microtextural changes still created metallurgical notches. SPF was observed in the fine-grained Ti–6Al–4V sheet without crack formation in the heat-affected zones or the fusion zone. The welding seam of the dissimilar fine-grained-standard butt joint was resistant to SPF. %0 journal article %@ 0261-3069 %A Enz, J., Khomenko, V., Riekehr, S., Ventzke, V., Huber, N., Kashaev, N. %D 2015 %J Materials and Design %P 110-116 %R doi:10.1016/j.matdes.2015.03.049 %T Single-sided laser beam welding of a dissimilar AA2024–AA7050 T-joint %U https://doi.org/10.1016/j.matdes.2015.03.049 %X In the aircraft industry double-sided laser beam welding of skin–stringer joints is an approved method for producing defect-free welds. But due to limited accessibility – as for the welding of skin–clip joints – the applicability of this method is limited. Therefore single-sided laser beam welding of T-joints becomes necessary. This also implies a reduction of the manufacturing effort. However, the main obstacle for the use of single-sided welding of T-joints is the occurrence of weld defects. An additional complexity represents the combination of dissimilar and hard-to-weld aluminium alloys – like Al–Cu and Al–Zn alloys. These alloys offer a high strength-to-density ratio, but are also associated with distinct weldability problems especially for fusion welding techniques like laser beam welding. The present study demonstrates how to overcome the weldability problems during single-sided laser beam welding of a dissimilar T-joint made of AA2024 and AA7050. For this purpose a high-power fibre laser with a large beam diameter is used. Important welding parameters are identified and adjusted for achieving defect-free welds. The obtained joints are compared to double-sided welded joints made of typical aircraft aluminium alloys. In this regard single-sided welded joints showed the expected differing weld seam appearance, but comparable mechanical properties. %0 journal article %@ 0003-6900 %A Sticchi, M., Schnubel, D., Kashaev, N., Huber, N. %D 2015 %J Applied Mechanics Reviews %N 1 %P 010801 %R doi:10.1115/1.4028160 %T Review of Residual Stress Modification Techniques for Extending the Fatigue Life of Metallic Aircraft Components %U https://doi.org/10.1115/1.4028160 1 %X A major challenge for the aircraft industry in the future will be the development of effective strategies for maintaining and extending the service life of a growing fleet of aging aircraft. In this context, residual-stress-based approaches for extending the fatigue life of aircraft components are believed to have great potential for providing cost-effective solutions. This paper reviews residual-stress-based life extension techniques and published work on the use of these techniques in aerospace applications. The techniques reviewed include cold expansion, shot peening, laser shock peening, deep rolling and heating. Comparisons of the various techniques with regard to current applications and limitations are given. %0 journal article %@ 1613-6810 %A Schaefer, H., Hess, C., Tobergte, H., Volf, A., Ichilmann, S., Eickmeier, H., Voss, B., Kashaev, N., Nordmann, J., Akram, W., Hartmann-Azanza, B., Steinhart, M. %D 2015 %J Small %N 8 %P 931-935 %R doi:10.1002/smll.201303930 %T Ultrafine Sanding Paper: A Simple Tool for Creating Small Particles %U https://doi.org/10.1002/smll.201303930 8 %X A top-down approach, i.e., creating small particles by mechanical force starting from bulk materials, probably presents the most logical approach to particle size reduction and, therefore, top-down techniques are among the first to achieve small particles. A new solvent-free, amazingly simple approach is reported, suitable to achieve nanoparticles and sub-micro particles. %0 journal article %@ 1358-8265 %A Steglich, D., Tian, X., Bohlen, J., Riekehr, S., Kashaev, N., Kainer, K.U., Huber, N. %D 2015 %J International Journal of Crashworthiness %N 2 %P 177-190 %R doi:10.1080/13588265.2014.996319 %T Experimental and numerical crushing analyses of thin-walled magnesium profiles %U https://doi.org/10.1080/13588265.2014.996319 2 %X In order to assess the crashworthiness of simple magnesium structures the axial deformation behaviour of different hollow rectangular profiles produced from wrought magnesium alloys Mg–3wt.%Al–1wt.%Zn–0.3wt.%Mn and Mg–1wt.%Zn–0.4wt.%rare earth mischmetal were investigated under quasi-static compressive loading conditions. Laser beam welding was applied to build the crush configurations from plane rolled sheets; indirect extrusion was used to manufacture seamless profiles. Numerical simulations were conducted to predict and assess the crush behaviour. The simulation results revealed that the material work hardening rates evidenced in uniaxial compression tests together with the cross section influenced the buckling modes as well as the energy dissipation. The performance of the magnesium profiles in terms of dissipated specific energy is better than that of aluminium profiles for small compressive displacements. However, for large displacements, shear-compressive failure limited the crush displacement and hence the energy dissipation. The weld itself did not influence the failure and the energy dissipation of the respective structure. For the alloy and process development of wrought magnesium, prospective improvements towards higher dissipated energy can be realised by increasing not only the strength but also the hardening rate of the material. %0 journal article %@ 0257-8972 %A Coratella, S., Sticchi, M., Toparli, M.B., Fitzpatrick, M.E., Kashaev, N. %D 2015 %J Surface and Coatings Technology %P 39-49 %R doi:10.1016/j.surfcoat.2015.03.026 %T Application of the eigenstrain approach to predict the residual stress distribution in laser shock peened AA7050-T7451 samples %U https://doi.org/10.1016/j.surfcoat.2015.03.026 %X Laser Shock Peening allows the introduction of deep compressive residual stresses into metallic components. It is applicable to most metal alloys used for aerospace applications. The method is relatively expensive in application, and therefore development studies often rely heavily on Finite Element Modeling to simulate the entire process, with a high computational cost. A different approach has been used recently, the so-called eigenstrain approach. The present study looks at the feasibility of applying the eigenstrain method for prediction of the residual stress in a sample that contains curved surface features. The eigenstrain is determined from a simple geometry sample, and applied to the more complex geometry to predict the residual stress after laser shock peening. In particular the prediction of residual stress at a curved edge, and for different values of material thickness, have been studied. The research has demonstrated that the eigenstrain approach gives promising results in predicting residual stresses when only the thickness is altered, but when the geometry of the peened surface is altered the eigenstrain method seems to slightly overestimate the residual stresses. This highlights a limitation of the eigenstrain method where the change in geometry means that the inelastic strain field as a consequence of a treatment – in this case laser shock peening – no longer has similitude. %0 journal article %@ 1877-7058 %A Lu, J., Kashaev, N., Huber, N. %D 2015 %J Procedia Engineering %P 248-254 %R doi:10.1016/j.proeng.2015.08.065 %T Crenellation Patterns for Fatigue Crack Retardation in Fuselage Panels Optimized via Genetic Algorithm %U https://doi.org/10.1016/j.proeng.2015.08.065 %X Crenellation is a promising technique to effectively improve the fatigue life of fuselage panels. It systematically varies the thickness of the fuselage skin at a constant structural weight. In the design of the crenellation patterns, the schemes of redistributing the skin material between different thickened and thinned regions can be innumerable. In order to select the optimum design from the huge searching space, a genetic algorithm was used in this study, which was coupled with FEM simulations used to predict the fatigue life of different crenellation designs. To accelerate the optimization process, a progressively refined searching approach and an old-individual-filtering technique were used. The suggested approach leads to both a reduced computational cost and improved solution quality. %0 journal article %@ 0261-3069 %A Kashaev, N., Ventzke, V., Riekehr, S., Dorn, F., Horstmann, M. %D 2015 %J Materials and Design %P 73-81 %R doi:10.1016/j.matdes.2015.04.051 %T Assessment of alternative joining techniques for Ti–6Al–4V/CFRP hybrid joints regarding tensile and fatigue strength %U https://doi.org/10.1016/j.matdes.2015.04.051 %X CFRP and titanium joints are used in the aerospace industry. These materials are usually joined by titanium rivets which are inserted into holes drilled through both materials. Conventional riveted hybrid joints of CFRP and titanium parts fail under quasi static loading due to the uneven load distribution at the titanium rivets. Under cyclic loading, the fatigue failure occurs mainly in the titanium part because of the higher notch sensitivity. The aim of this work is the comparison of different joining concepts in terms of stiffness, strength and fatigue limit. First, laser riveting, here titanium pins are Nd:YAG laser beam welded to the Ti–6Al–4V parts. Second, conventional riveted hybrid joint is combined with adhesive bonding. Third, surface structuring of the Ti–6Al–4V parts is used to enhance friction in the riveted joint. Tensile and fatigue tests as well as fractographical examinations are performed to establish the process–property–performance relationship of the hybrid joints. Laser riveting leads to higher stiffness but equal strength, when compared to conventional riveted joints. Fatigue life is improved by the implementation of adhesive bonding and surface structuring. %0 journal article %@ 2051-3305 %A Sticchi, M., Staron, P., Sano, Y., Meixer, M., Klaus, M., Rebelo-Kornmeier, J., Huber, N., Kashaev, N. %D 2015 %J The Journal of Engineering %N 13 %P 97-105 %R doi:10.1049/joe.2015.0106 %T A parametric study of laser spot size and coverage on the laser shock peening induced residual stress in thin aluminium samples %U https://doi.org/10.1049/joe.2015.0106 13 %X Laser Shock Peening is a fatigue enhancement treatment using laser energy to induce compressive Residual Stresses (RS) in the outer layers of metallic components. This work describes the variations of introduced RS-field with peen size and coverage for thin metal samples treated with under-water-LSP. The specimens under investigation were of aluminium alloy AA2024-T351, AA2139-T3, AA7050-T76 and AA7075-T6, with thickness 1.9 mm. The RS were measured by using Hole Drilling with Electronic Speckle Pattern Interferometry and X-ray Diffraction. Of particular interest are the effects of the above mentioned parameters on the zero-depth value, which gives indication of the amount of RS through the thickness, and on the value of the surface compressive stresses, which indicates the magnitude of induced stresses. A 2D-axisymmetrical Finite Element model was created for a preliminary estimation of the stress field trend. From experimental results, correlated with numerical and analytical analysis, the following conclusions can be drawn: increasing the spot size the zero-depth value increases with no significant change of the maximum compressive stress; the increase of coverage leads to significant increase of the compressive stress; thin samples of Al-alloy with low Hugoniot Elastic Limit (HEL) reveal deeper compression field than alloy with higher HEL value. %0 journal article %@ 0255-5476 %A Enz, J., Riekehr, S., Ventzke, V., Kashaev, N. %D 2015 %J Materials Science Forum, Light Metals Technology 2015 %P 389-394 %R doi:10.4028/www.scientific.net/MSF.828-829.389 %T Laser Weldbability of different Al-Zn alloys and its improvement %U https://doi.org/10.4028/www.scientific.net/MSF.828-829.389 %X Weld defects - such as porosity and hot cracking - occur especially during the laser beam welding of high-alloyed Al-Zn alloys. This significantly limits the application range of these promising high-strength alloys. In the present study the laser weldability of Al-Zn alloys was investigated regarding welding parameters and chemical composition of the alloys. In addition, the novel approach of the Helmholtz-Zentrum Geesthacht for overcoming the weldability problems was applied to the different Al-Zn alloys in order to assess its capability. It was shown that the laser weldability of Al-Zn alloys deteriorates with an increasing amount of Zn, Mg and Cu. The variation of laser welding parameters did not lead to any improvement of weldability. Only the use of a V foil as additional filler material resulted in promising welding results even for high-alloyed Al-Zn alloys. %0 journal article %@ 0966-9795 %A Liu, J., Staron, P., Riekehr, S., Stark, A., Schell, N., Huber, N., Schreyer, A., Mueller, M., Kashaev, N. %D 2015 %J Intermetallics %P 27-35 %R doi:10.1016/j.intermet.2015.03.003 %T In situ study of phase transformations during laser-beam welding of a TiAl alloy for grain refinement and mechanical property optimization %U https://doi.org/10.1016/j.intermet.2015.03.003 %X In situ time-resolved X-ray diffraction by synchrotron radiation was used to monitor the phase transformations and grain-refining processes during laser-beam welding of a γ-TiAl-based alloy. The heating rate plays an important role of grain refinement. A high heating rate suppresses solid–solid phase transformations. The superheated γ grains serve as heterogeneous nuclei for β grains on subsequent solidification and refine the lamellar colonies. At low heating rate, diffusion-based transformations are observed on heating and coarse lamellae are formed after welding. The refined lamellar colonies improve the mechanical properties. %0 journal article %@ 0870-8312 %A Lu, J., Huber, N., Kashaev, N. %D 2015 %J Ciencia & Tecnologia dos Materiais %N 2 %P 100-107 %R doi:10.1016/j.ctmat.2015.06.003 %T Influence of the geometry on the fatigue performance of crenellated fuselage panels %U https://doi.org/10.1016/j.ctmat.2015.06.003 2 %X Crenellation is a novel local engineering technique aimed at improving the fatigue performance of the airframe structures without increasing the weight. In this concept, a systematic thickness variation is applied to the fuselage skin to retard the fatigue crack growth. In order to achieve the best retardation effect, it is necessary optimize the crenellation geometry. As a result, a parameter study characterizing three independent geometric aspects of the crenellations was performed: the crenellation ratio c, the periodic length λ and a position parameter. The study was based on a FEA model validated by experiments. It is expected to give a sufficiently accurate prediction on fatigue life of different crenellation patterns. The obtained knowledge concerning the impact of those geometrical factors could provide guidance for future crenellation designs for industrial applications. %0 journal article %@ 1864-5631 %A Schaefer, H., Kuepper, K., Wollschlaeger, J., Kashaev, N., Hardege, J., Walder, L., Beladi-Mousavi, S.M., Hartmann-Azanza, B., Steinhart, M., Sadaf, S., Dorn, F. %D 2015 %J ChemSusChem %N 18 %P 3099-3110 %R doi:10.1002/cssc.201500666 %T Oxidized Mild Steel S235: An Efficient Anode for Electrocatalytically Initiated Water Splitting %U https://doi.org/10.1002/cssc.201500666 18 %X The surface of steel S235 was oxidized by Cl2 gas and checked for its electrocatalytic efficiency regarding oxygen formation in aqueous solution. If exposed to humid Cl2 gas for 110 min, steel S235 became an electrocatalyst that exhibits an overpotential for the oxygen evolution reaction (OER) of 462 mV at 1 mA cm−2 at pH 7. The OER activity of the same sample at pH 13 was moderate (347 mV overpotential at 2.0 mA cm−2 current density) in comparison with OER electrocatalysts developed recently. Potential versus time plots measured at a constant current demonstrate the sufficient stability of all samples under catalysis conditions at pH 7 and 13 for tens of hours. High-resolution X-ray photoelectron spectra could be reasonably resolved with the proviso that Fe2O3, FeO(OH), MnO(OH), and Mn2O3 are the predominant Fe and Mn species on the surface of the oxidized steel S235. %0 journal article %@ 0255-5476 %A Riekehr, S., Ravasi, R., Enz, J., Ventzke, V., Kashaev, N. %D 2015 %J Materials Science Forum, Light Metals Technology 2015 %P 298-304 %R doi:10.4028/www.scientific.net/MSF.828-829.298 %T Mechanical Properties of Fibre Laser Welded AZ31B Sheets and their Dependence on the Spot-Size %U https://doi.org/10.4028/www.scientific.net/MSF.828-829.298 %X In the present work the mechanical behaviour of laser beam welded AZ31B alloy was studied, by changing systematically the spot size of the used fibre laser system between 200 µm and 1000 µm at different power levels between 2 kW and 8 kW. Maximum welding velocities with respect to imperfections were determined. The characterization of the obtained welds - in terms of Vickers hardness, UTS, Af and weld width, resp. weld area - was correlated with the micro-texture in dependence of the different Focus Spot Diameters and Laser Beam Power levels as well as the resulting cooling rates. Highest UTS of 94% of the base material was achieved with 200 µm Focus Spot Diameter and Laser Beam Power of 4 kW at welding velocity of 100 mm/s. By increasing the Focus Spot Diameter to 600 µm, the tensile strength was reduced to 86 % of the actual strength of the base material. %0 journal article %@ 0933-5137 %A Stutz, L., Beck, W., Arends, S., Horstmann, M., Ventzke, V., Kashaev, N. %D 2014 %J Materialwissenschaft und Werkstofftechnik %N 9 %P 841-846 %R doi:10.1002/mawe.201400292 %T Material saving and cost reduction with hot forming of U-shaped titanium part - Materialeinsparung und Kostenreduzierung durch Heissumformung von U-foermigen Titanbauteilen %U https://doi.org/10.1002/mawe.201400292 9 %X Titanium sheets of the work horse alloy Ti–6Al–4V were formed in a single stroke to a U-shaped component at process temperatures ranging from 750 to 890 °C. Specimens were extracted to validate the neglectable influence of the hot forming process on mechanical properties and fatigue behaviour. In conclusion, hot deep drawing of titanium sheets offers a cost efficient alternative to a gas pressure superplastic forming process, while maintaining its main benefits such as significantly improved formability, low residual stresses and tight tolerances. %0 journal article %@ 2211-8128 %A Qi, F., Staron, P., Ventzke, V. %D 2014 %J Procedia Materials Science %P 1834-1840 %R doi:10.1016/j.mspro.2014.06.296 %T Effect of Local Laser Surfacing on the Fatigue Crack Propagation Rate of Al-alloy 6056 and its Laser beam Weld %U https://doi.org/10.1016/j.mspro.2014.06.296 %X A new surfacing method based on the laser melting was investigated in this study in order to improve the fatigue crack propagation performance of high strength al-alloy and its laser beam weld. The main objective was to reduce the Fatigue Crack Propagation (FCP) rate and increase the fatigue life of Al-alloy welded structure. The fatigue crack propagation rate of Al-alloy 6056 was assessed using CT-100 specimen including base material, base material with Local Laser Surfacing (LLS), laser beam weld and laser beam weld with LLS respectively. The results show a significant reduction in fatigue crack propagation rate with LLS treatment because the fatigue crack retardation phenomenon is obvious when the fatigue crack close to or meet the laser surfacing lane. The fatigue life of BM with LLS specimen is higher than that of BM specimen. For the LBW, there shows same results. That is FCP rate of the specimen of laser beam weld with LLS treatment shows significant reduction compared with the specimen of laser beam weld without LLS treatment. The facture surface and microstructure of LLS and LBM were observed also in order to explain the basic mechanism of LLS improving the fatigue life. %0 journal article %@ 1612-3433 %A Ventzke, V., Riekehr, S., Horstmann, M., Haack, P., Kashaev, N. %D 2014 %J Welding and Cutting %N 4 %P 245-249 %T One-sided Nd:YAG laser beam welding for the manufacture of T-joints made of aluminium alloys for aircraft construction %U 4 %X In aircraft construction, T-joints between aluminium alloys are manufactured either by riveting or by two-sided laser beam welding. This article describes one-sided Nd:YAG laser beam welding as a method of manufacturing skin/clip joints between the AA6156-T4/AA6013-T6 and AA2139-T3/ AA6013-T6 aluminium material combinations in T-joint designs. One fundamental problem associated with this procedure is the high porosity level in T-joints executed by means of one-sided laser beam welding. It is shown that the formation of pores is determined not only by the types of the aluminium alloys, the variations in the welding directions and the preparation of the joining faces but also by an excessive angle of incidence between the laser beam and the skin field. The laser beam must be transmitted in the bonding region between the clip and the skin field in order to reduce the porosity level by decreasing the angle of incidence. %0 journal article %@ 2211-8128 %A Enz, J., Iwan, H., Riekehr, S., Ventzke, V., Kashaev, N. %D 2014 %J Procedia Materials Science %P 1828-1833 %R doi:10.1016/j.mspro.2014.06.295 %T Fracture Behavior of a Laser Beam Welded High-strength Al-Zn Alloy %U https://doi.org/10.1016/j.mspro.2014.06.295 %X Laser beam welding of butt joints made of the newly developed high-strength Al-Zn alloy PA734 is conducted. A new approach of the Helmholtz-Zentrum Geesthacht is used to solve the problems of weldability and softening. The results of the fatigue, fatigue crack propagation and fracture toughness tests are discussed relating to the microstructural characteristics and the mechanical properties of the laser welded joints and compared to the base material. The obtained data can be used for the assessment of the damage tolerance behaviour of the laser welded integral aircraft structures made of Al-Zn alloys. %0 journal article %@ 0032-678X %A Ventzke, V., Riekehr, S., Horstmann, M., Kashaev, N., Brokmeier, H.-G., Huber, N. %D 2014 %J Practical Metallography - Praktische Metallographie %N 6 %P 401-424 %T Prozessentwicklung zum Rotationsreibschweißen der Gamma-TiAl-Feingusslegierung Ti-47Al-3.5(Mn+Cr+Nb)-0.8(B+Si) mit Ti6Al4V: Teil: 2 - The Development of the Rotational Friction Welding Process for the Welding of Gamma-TiAl-Casting Alloy Ti-47Al-3.5(Mn+Cr+Nb)-0.8(B+Si) to Ti6Al4V: Part: 2 %U 6 %X This article reports on the development of the rotational friction welding process for the welding of γ-TiAl- casting alloy Ti-47Al-3.5(Mn+Cr+Nb)-0.8(B+Si) to Ti6Al4V base metal and on the characterisation of the mechanical and microstructural properties of dissimilar metal friction welding. Pre- and post welding heat treatment is necessary in order to reduce the occurrence of bonding defects. The mechanical behaviour of friction welded joints under tensile loading at room temperature is primarily determined by the properties of the inter-metallic γTiAl- casting alloy base metal and not by the bonding zone itself. At the relevant service temperature of 700 °C, failure by fracture does not take place on the Ti6Al4V side of the dissimilar metal friction welded joint due to the super-plastic deformation of this alloy. %0 journal article %@ 0032-678X %A Ventzke, V., Riekehr, S., Horstmann, M., Kashaev, N., Brokmeier, H.-G., Huber, N. %D 2014 %J Practical Metallography - Praktische Metallographie %N 5 %P 321-352 %R doi:10.3139/147.110266 %T Prozessentwicklung zum Rotationsreibschweißen der Gamma-TiAl-Feingusslegierung Ti-47Al-3.5(Mn+Cr+Nb)-0.8(B+Si) mit Ti6Al4V: Teil: 1 - The Development of the Rotational Friction Welding Process for the Welding of Gamma-TiAl-Casting Alloy Ti-47Al-3.5(Mn+Cr+Nb)-0.8(B+Si) to Ti6Al4V: Part: 1 %U https://doi.org/10.3139/147.110266 5 %X This article reports on the development of the rotational friction welding process for the welding of γ-TiAl- casting alloy Ti-47Al-3.5(Mn+Cr+Nb)-0.8(B+Si) to Ti6Al4V base metal and on the characterisation of the mechanical and microstructural properties of dissimilar metal friction welding. Pre- and post welding heat treatment is necessary in order to reduce the occurrence of bonding defects. The mechanical behaviour of friction welded joints under tensile loading at room temperature is primarily determined by the properties of the inter-metallic γTiAl- casting alloy base metal and not by the bonding zone itself. At the relevant service temperature of 700 °C, failure by fracture does not take place on the Ti6Al4V side of the dissimilar metal friction welded joint due to the super-plastic deformation of this alloy. %0 journal article %@ 0255-5476 %A Kashaev, N., Riekehr, S., Horstmann, M., Ventzke, V. %D 2014 %J Materials Science Forum, Thermec 2013 %P 2310-2315 %R doi:10.4028/www.scientific.net/MSF.783-786.2310 %T Fatigue, Fatigue Crack Propagation and Mechanical Fracture Behaviour of Laser Beam-Welded AZ31 Magnesium Sheets %U https://doi.org/10.4028/www.scientific.net/MSF.783-786.2310 %X lightweight construction. %0 journal article %@ 2212-8271 %A Enz, J., Riekehr, S., Ventzke, V., Sotirov, N., Kashaev, N. %D 2014 %J Procedia CIRP %P 203-208 %R doi:10.1016/j.procir.2014.06.132 %T Laser Welding of High-Strength Aluminium Alloys for the Sheet Metal Forming Process %U https://doi.org/10.1016/j.procir.2014.06.132 %X formability of the base materials, the results obtained for the welded joints are compared with base material properties. %0 journal article %@ 0036-7184 %A Ventzke, V., Riekehr, S., Horstmann, M., Haack, P., Kashaev, N. %D 2014 %J Schweissen und Schneiden %N 1-2 %P 22-26 %T Einseitiges Nd:YAG-Laserstrahlschweissen zum Herstellen von T-Stossverbindungen aus Aluminiumlegierungen fuer den Flugzeugbau %U 1-2 %X In aircraft construction, T-joints between aluminium alloys are manufactured either by riveting or by two-sided laser beam welding. The article describes one-sided Nd:YAG laser beam welding as a method of manufacturing skin/clip joints between the AA6156-T4/AA6013-T6 and AA2139-T3/AA6013-T6 aluminium material combinations in T-joint designs. One fundamental problem associated with this procedure is the high porosity level in T-joints executed by means of one-sided laser beam welding. It is shown that the formation of pores is determined not only by the types of the aluminium alloys, the variations in the welding directions and the preparation of the joining faces but also by an excessive angle of incidence between the laser beam and the skin field. The laser beam must be transmitted in the bonding region between the clip and the skin field in order to reduce the porosity level by decreasing the angle of incidence. %0 journal article %@ 1022-6680 %A Kashaev, N., Chupakhin, S., Enz, J., Ventzke, V., Groth, A., Horstmann, M., Riekehr, S. %D 2014 %J Advanced Materials Research, 11th International Fatigue Congress %P 1457-1462 %R doi:10.4028/www.scientific.net/AMR.891-892.1457 %T Fatigue and Fatigue Crack Propagation of Laser Beam-Welded AA2198 Joints and Integral Structures %U https://doi.org/10.4028/www.scientific.net/AMR.891-892.1457 %X To meet the future demands of the aerospace industry with respect to safety, productivity, weight, and cost, new materials and joining concepts have being developed. Recent developments in the metallurgical field now make it possible to use laser-weldable Al-alloys of the 2xxx series such as AA2198 with a high structural efficiency index due to their high strength and low density. AA2198 holds the promise of providing a breakthrough response to the challenges of lightweight design in aircraft applications. Laser beam welding as an efficient joining technology for fuselage structures is already established in the aircraft industry for lower fuselage panels because the welded panels provide a higher buckling strength and lower weight compared with the classical riveted designs. The key factor for the application of laser-welded AA2198 structures is the availability of reliable data for the assessment of their damage tolerance behavior. In the research presented, the mechanical properties with regard to fatigue and fatigue crack propagation of laser beamwelded AA2198 joints and four-stringer panels were investigated. It was found that the fatigue endurance limit of laser beamwelded AA2198T3 is approximately 25 % below the endurance limit of the base material. With regard to the fatigue crack propagation behavior, the laser beam welded four-stringer panels with T-joints show a fatigue life increased by a factor of 1.7 compared with the base material. This work shows that high-quality laser beam welds of AA2198 can be produced on a large scale using the laser beam welding facilities of the Helmholtz-Zentrum Geesthacht. %0 journal article %@ 1073-5623 %A Liu, J., Ventzke, V., Staron, P., Schell, N., Kashaev, N., Huber, N. %D 2014 %J Metallurgical and Materials Transactions A %N 1 %P 16-28 %R doi:10.1007/s11661-013-1886-5 %T Effect of Post-weld Heat Treatment on Microstructure and Mechanical Properties of Laser Beam Welded TiAl-based Alloy %U https://doi.org/10.1007/s11661-013-1886-5 1 %X Post-weld heat treatment is carried out on the laser beam welded γ-TiAl-based alloy Ti-48Al-1Cr-1.5Nb-1Mn-0.2Si-0.5B (at. pct). The macro/microstructure and mechanical properties of both as-welded and heat-treated specimens are investigated by radiography, SEM, and tensile tests. Moreover, high energy synchrotron X-ray diffraction is performed to measure the residual stresses and evaluate the microstructure evolution. It is found that the residual stresses are distributed in a three-peak shape in the region of the weld zone and heat-affected zone of the as-welded specimen due to the microstructural transformation and heat softening. The residual stresses are largely relieved after the heat treatment. The heat-treated specimens have a near fully lamellar microstructure and show balanced mechanical properties of strength and ductility. The diffraction shows that the phase transformation from α2 to γ takes place under tensile load at 1023 K (750 °C), and the grain size and lamellar spacing are refined in the weld zone. Finally, the fracture mechanisms are found to be controlled by the local stress concentration-induced strain misfit between α2 and γ phases in the near γ grains and delamination and debonding in the lamellae. Boride ribbons of 5 μm in the near fully lamellar microstructure are found not to be detrimental to the tensile properties. %0 journal article %@ 0966-9795 %A Liu, J., Dahmen, M., Ventzke, V., Kashaev, N., Poprawe, R. %D 2013 %J Intermetallics %P 65-70 %R doi:10.1016/j.intermet.2013.04.007 %T The effect of heat treatment on crack control and grain refinement in laser beam welded Beta-solidifying TiAl-based alloy %U https://doi.org/10.1016/j.intermet.2013.04.007 %X Investigations were conducted on the γ-TiAl-based alloy Ti–42Al–2.5Cr–1Nb–0.7Si–0.5B (at. %) to assess the influence of in situ and conventional post-weld heat treatment on the microstructure and microtexture transformations induced by laser beam welding. It was found that in situ post-weld heat treatment at 800 °C was important to inhibit weld seam cracking. EBSD and HEXRD results indicated that the welding zone mainly consisted of coarse α2 “textured colonies”. These textured α2 are found to be in Burgers orientation relationship with their parent β grains. After 1 h of conventional post-weld heat treatment at 1200 °C followed by furnace cooling, the textured colonies are refined. The α2 grains nucleate heterogeneously on borides so that the sharp texture of the weld zone was broken down. %0 journal article %@ 0255-5476 %A Steglich, D., Bohlen, J., Tian, X., Riekehr, S., Kashaev, N., Bargmann, S., Letzig, D., Kainer, K.U., Huber, N. %D 2013 %J Materials Science Forum, Light Metals Technology Conference, LMT 2013 %P 590-594 %R doi:10.4028/www.scientific.net/MSF.765.590 %T Crashworthiness of Magnesium Sheet Structures %U https://doi.org/10.4028/www.scientific.net/MSF.765.590 %X at progressive hardening along with high ductility for improving the bending and shear behaviour. %0 journal article %@ 2238-7854 %A Kashaev, N., Horstmann, M., Ventzke, V., Riekehr, S., Huber, N. %D 2013 %J Journal of Materials Research and Technology : JMRT %N 1 %P 43-47 %R doi:10.1016/j.jmrt.2013.03.003 %T Comparative study of mechanical properties using standard and micro-specimens of base materials Inconel 625, Inconel 718 and Ti-6Al-4V %U https://doi.org/10.1016/j.jmrt.2013.03.003 1 %X To improve innovative joining and deposition technologies for the construction of dissimilar joints, precise knowledge of the local mechanical properties of materials must be obtained. In the present article a comparative study of the tensile properties and fatigue behaviour in case of flat standard and micro-specimens of base materials Inconel 625, Inconel 718 and Ti-6Al-4V was accomplished. The aim of the study was to develop an efficient method for the investigation of the local mechanical properties by the use of micro-specimens subjected to electro discharge machining treatment and to obtain reliable tensile and fatigue test results. By the miniaturization of specimens a significant effect of roughness on mechanical properties was obtained. By considering a correction for the effective load-bearing cross-section the data obtained from micro-specimens are within 3% error with respect to standard specimen results. The results showed that the proposed technique can be successfully used to determine the tensile and fatigue properties of a small material volu