Evaluating weld properties of conventional and swept friction stir spot welded 6061-T6 aluminium alloy

6061-T6 aluminium alloy is a tempered grade aluminium material that is extensively used, particularly in space and automotive applications. The conventional and swept friction stir spot welding (FSSW) joints are prepared by varying tool rotational speed at four different levels. The mechanical properties, microstructure and mode of failure in both types of FSSW are evaluated and reported. The high plastic deformation and fine grains influenced the increase in hardness of the weld joints based on the Hall-Petch equation. The highest lap shear strength of 5.31 kN is observed in swept FSSW sample prepared at 1400 rpm. Interestingly, 1600 rpm decreased the lap shear strength due to insufficient friction between base metal and tool in the higher tool rotational speed. The minimum microhardness is noticed in heat-affected zone (HAZ) of both cases. Conventional FSSW samples showed shear fracture, nugget pullout fracture and circumferential fracture under lap shear loading, whereas swept FSSW showed only the circumferential fracture.