EFFECT OF WELD STRUCTURE ON FATIGUE LIFE OF FRICTION STIR SPOT WELDING IN MAGNESIUM AZ31 ALLOY

In this paper the fatigue behavior in friction stir spot welded coupons of magnesium AZ31 alloy manufactured under different welding conditions are investigated. Two sets of lap-shear coupons were welded based on varying the plunge depth and tool geometry. Metallographic analysis of the untested lap-welds revealed differences in microstructural and geometrical features. Results from the load controlled cyclic tests showed that one set of welds exhibited better fatigue performance compared to the other set. Optical fractography of the failed fatigue coupons revealed that fatigue cracks initiated at the weld interface in both sets of coupons. However, the fracture mode showed variability between the two sets of coupons. As such, the main conclusion of this study is that the effective top sheet thickness, which is largely determined by the shoulder plunge depth, plays a significant role in the fatigue behavior of the friction stir spot welds in magnesium alloys.