Residual Stress and Fatigue Strength of Hybrid Laser-MIG-Welded A7N01P-T4
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INTRODUCTION
5XXX, 6xxx, and 7xxx series aluminum alloys are widely used in the manufacturing of high-speed train bodies. Many welded components (e.g., underframes) of high-speed train bodies are composed of A7N01 aluminum alloy.[1] The metal inert gas (MIG) welding method is the major joining process used. Many other welding methods are considered to replace MIG welding in order to improve the welding efficiency and reduce the residual stress and deformation of the joint. The laser-arc hybrid welding method has been proved to be a welding method that can not only reduce the welding heat input, the welding residual stress, and deformation,[2] but it also can improve the properties of the joint.[3,4] Welding residual stress of the joint and its structure is closely related to heat source,[5] heat input per unit, materials,[6] fixtures, and sequence of welding.[7] James et al.[8] reported the residual stress distribution of the 5083-H321 Al alloy MIG-welded joint in the as-welded state and the effect of a sequence of applied fatigue loads on residual stress and strain values. Many researchers examined welding residual stresses of steel plates joined by laser welding[5] and laser-arc hybrid welding.[9,10] But
QIUYING WANG, Ph.D. Candidate, HUI CHEN, Professor, and ZONGTAO ZHU, Lecturer, are with the School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu 610031, P.R. China. Contact e-mail: [email protected] PEIXIAN QIU, Technologist, is with CRRC Qingdao Sifang Co., Ltd., Qingdao 266000, P.R. China. Manuscript submitted March 2, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
investigations on residual stresses of Al alloy hybrid laser-MIG-welded joints are far from complete. The fatigue properties of the welded joints and welded components are important issues for the high-speed train industry. Many researchers have studied the fatigue properties of Al alloy plates joined by MIG welding.[11,12] Yan et al.[3] demonstrated that the fatigue strength at Nf = 107 (the fatigue strength for a given fatigue life at 107 cycles) of hybrid laser-MIG-welded 5083 Al alloy plates could reach 84.6 pct of the base metal’s fatigue strength at Nf = 107. The fatigue property of the welded joint is intimately related to the microstructure, welding defect, and residual stress of the joint. Microstructure evolution caused by welding thermal cycles and softening behavior of the Al alloy hybrid laser-arc-welded joint is widely studied.[13–15] Defects induced by the welding process are always detrimental to the joint and usually the initiation sites of fatigue cracks. Moreover, residual stress induced by the welding process can affect the properties of the joint and its structure.[16] Pouget et al.[17] studied the fatigue crack growth rate of friction stir-welded 2050. They proved that compressive residual stresses in the vicinity of the weld led to a decrease in the fatigue crack growth rate. In the welded nugget, the fatigue crack growth rate increased due to both tensile residual stresses and microstructure cha
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