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DUE to significant environmental challenges and ever-increasing energy prices faced by the transportation industry, lightweighting of motor vehicles is being considered as a key strategy for improving the fuel economy and lowering anthropogenic environmentdamaging emissions.[1–9] Of the prevalent materials for the next-generation transportation vehicles in the automotive and aerospace industries, magnesium alloys represent a lucrative option for the weight reduction owing to their low density, high strength-to-weight ratio, environmental friendliness, castability and recyclability.[1,10–22] As the structural application of magnesium alloys inevitably involves welding and joining in the manufacturing process, understanding the weldability of magnesium alloys plays an essential role in determining the viability of component lightweighting through replacement with magnesium alloys. As an enabling and green solid-state joining technology, friction stir welding (FSW)[21] has a high potential for the assembly of magnesium alloys because it can significantly mitigate the challenges normally associated S.H. CHOWDHURY, Graduate Student, D.L. CHEN, Professor and Ryerson Research Chair, and S.D. BHOLE, Professor, are with the Department of Mechanical and Industrial Engineering, Ryerson University, 350 Victoria Street, Toronto, ON M5B 2K3 Canada. Contact e-mail: [email protected] X. CAO, Research Officer, and P. WANJARA, Group Leader, are with the Aerospace Manufacturing Technology Centre, National Research Council Canada, 5145 Decelles Avenue, Montreal, QC H3T 2B2, Canada. Manuscript submitted April 15, 2012. Article published online August 24, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

with fusion welding processes, including susceptibility to hot cracking, compositional segregation of alloying elements and precipitation of divorced intermetallic particles.[18,22–29] As a thermomechanical process with optimal welding parameters, FSW has the advantage of refining the grain size in the weldment via severe plastic deformation and recrystallization,[30–37] which could contribute towards enhancing the formability and increasing the strength of the assembly. Naturally safety enhancement must be an integral part in any conceptual design, development, and deployment of lightweight, fuel-efficient and environmentally-sustainable next generation motor vehicles. In this regard, mechanical properties including strength, ductility and strain-hardening behavior of magnesium alloys used in the structural applications must be evaluated in relation to the microstructural changes to guarantee the integrity and reliability of the joint and assembly. While there are numerous investigations on the properties of magnesium alloys, only some limited studies on the properties of welded magnesium alloy joints are reported. Quan et al.[38] studied the effects of heat input on microstructure and tensile properties of a laser welded AZ31 Mg alloy. Liu and Dong[39] reported the effect of microstructural changes on the tensile properties of a non-autoge