Microstructure evolution and Interface morphology optimization in synergistic double-sided probeless friction stir spot welding of dissimilar 7075/6061 aluminum alloys

This study introduces Synergistic Double-Sided Probeless Friction Stir Spot Welding (SDP-FSSW) as a novel solid-state technique for joining dissimilar aluminum alloys, specifically AA7075-T8 and AA6061-T4, which differ markedly in their mechanical properties. The primary objective was to systematically investigate how varying the rotational speeds of the upper and lower tool shoulders affects weld formation, microstructural evolution, and joint mechanical performance. By independently controlling the rotation speeds of both tool shoulders, the process enabled precise regulation of interfacial morphology, heat input, and plastic flow, resulting in flat, defect-free welds with controlled hook formation and significant material intermixing. Tensile-shear tests showed that all joints exhibited strengths above 5 kN. Under optimal conditions—600/200 rpm with a dwelling time of just 3 s—the joint achieved a maximum tensile-shear strength of 9585.9 N. These results demonstrate that SDP-FSSW provides superior control over interface morphology and joint properties, offering significant potential for fabricating high-strength, lightweight, multi-material structures in advanced manufacturing.