Mechanical performance optimization and microstructure analysis of similar thin AA6061-T6 sheets produced by swept friction stir spot welding

In order to increase the bonding area of friction stir spot welding (FSSW) welds, AA6061-T6 aluminum alloy was successfully welded in swept friction stir spot welding (SFSSW) process. The orthogonal experiment design was used to investigate the effect of welding parameters (tool rotation speed (omega), welding traverse speed (F), and swept radius (R)) on the joint lap shear and cross-tension performances. SFSSW welds present typical defects of keyhole, flash, and hook. Multi-objective particle swarm optimization (MOPSO) was used to determine the Pareto front and compromise solutions for the maximum lap shear failure load (LSFL) and cross-tension failure load (CTFL) simultaneously. The optimum LSFL and CTFL were 6.916 and 4.11 kN obtained under the tool rotation speed of 2050.4 rpm, the welding traverse speed of 95.48 mm/min, and the swept radius of 2.96 mm. Compared to the initial solution, the LSFL and CTFL were increased by 83.76% and 34.77% respectively. Under optimal welding parameters, the lap shear and cross tension of SFSSW welds exhibit shear and nugget pull-out fracture modes, respectively.