Microstructures and failure mechanisms of friction stir spot welds of aluminum 6061-T6 sheets

Microstructures and failure mechanisms of friction stir spot welds in aluminum 6061-T6 lap-shear specimens are investigated based on experimental observations. Optical micrographs of the cross-sections of friction stir spot welds in lap-shear specimens before and after failure are examined. These friction stir spot welds show the failure mode of nugget pullout under lap-shear loading conditions. The experimental observations suggest that under lap-shear loading conditions, the failure is initiated near the possible original notch tip in the stir zone (SZ) and the failure propagates along the circumference of the nugget to final fracture. Microindentation hardness data of base metal (BM), heat affected zone (HAZ), thermal–mechanical affected zone (TMAZ) and SZ are obtained. The interface between the HAZ and the TMAZ is the softest region, where the cracks of friction stir spot welds in the lap-shear specimens under the loadings initiate and lead to fracture of the specimens.