Strain-Controlled Low-Cycle Fatigue Behavior of Friction Stir-Welded AZ31 Magnesium Alloy
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MAGNESIUM (Mg) alloys are very attractive candidates for structural applications in the aerospace and automobile industries because of their weight reduction and energy saving characteristics. Therefore, a reliable welding process is urgently required to realize these applications, especially for the wrought Mg alloys. However, conventional fusion welding methods have some problems for the welding of Mg alloys such as porosity, oxidization, and high residual stress. Friction stir welding (FSW), as a solid joining method[1] can effectively avoid the drawbacks of the fusion welding, producing sound joints of Mg alloys. As a widely used wrought Mg alloy with a good combination of strength and ductility, AZ31 has been widely subjected to FSW investigations, including studies of the texture distribution,[2–4] precipitation,[5] material flow,[6,7] residual stress,[8] and mechanical properties.[9–14] It was reported that high-quality FSW AZ31 joints with excellent mechanical properties could be obtained by optimizing the welding parameters.[9,15] From an engineering design perspective, it is important to understand the fatigue behavior of FSW joints. Recently, some studies have evaluated the fatigue J. YANG, formerly Postgraduate with Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, People’s Republic of China, is now Postdoctoral Fellow with the University of Science and Technology Beijing, 30 Xueyuan Road, Beijing 10083, People’s Republic of China. D.R. NI, Associate Professor, D. WANG, Assistant Professor, and B.L. XIAO and Z.Y. MA, Professors, are with the Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences. Contact e-mail: [email protected] Manuscript submitted February 14, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
resistance of FSW AZ31 joints, including the behavior of stress-controlled failure[16–18] and fatigue crack propagation.[19] Padmanaban et al.[18,19] found that FSW AZ31 joints had a higher fatigue strength and lower fatigue crack growth exponent than the joints formed by pulsed current gas tungsten arc welding. Chowdhury et al.[17] reported that the fatigue strength of FSW AZ31 joints was affected by the pin thread orientation. However, investigations into the low-cycle fatigue (LCF) behavior of FSW AZ31 joints, which is of practical importance in estimating the component lifetime during the service process,[20,21] are lacking. It is well documented that wrought Mg alloys have strong textures. For FSW Mg alloy joints, it was reported that the intense material flow during FSW generated a special texture distribution in the nugget zone (NZ), with the basal plane rotating around the pin column surface.[2,3] This texture distribution influenced the tensile properties[14,22] and fracture behavior[10,23] of the FSW Mg alloy joints. For example, the ultimate tensile strength (UTS) of FSW AZ31 joints increased as the rotation rate increased, attributable to
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