Friction Stir Spot Welding (FSSW) has been practically used to the construction of aluminum car-body in the automotive application. In this study, the weld strength and factors governing the weld strength in friction stir spot welded aluminum alloy 6061-T6 was examined. The weld strength increased with the process time during FSSW up to 3 s, beyond which it decreased. The fracture path changed from the lapped interface into the shoulder edge as the process time increased, and the maximum strength was obtained at the process time when the transition of the fracture path occurred. The cross-sectional microstructure depended on the threads on the probe surface, i.e. the elliptical zone was formed in the stir zone by the threads, but effect of the thread on the weld strength was hardly found. The present study suggests that the weld strength was strongly related with size of the well-consolidated region which was larger than the elliptical zone observed in the vicinity of the exit hole.
