Friction stir spot welding (FSSW) process has been successfully applied for joining aluminum alloy 6061 to TRIP 780/800 steel. Effects of tool plunge speed and dwell time on the weld strength were studied through design of experiments and analysis of variance. It is shown that dwell time is a more dominant parameter in affecting the weld strength than plunge speed. Cross sections of weld specimens show the formation of hook with a swirling structure. Higher magnified SEM view with EDS analysis reveals the swirling structure to be composed of alternating thin layers of steel and Al-Fe intermetallic compounds (IMCs). During tensile shear test, cross nugget failure is the only failure mode. Cracks are initiated in the swirling structure at the tensile side of the weld nugget and cleavage feature can be observed on the fractured surface.

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