Effect of pinless tool geometry and process parameters on the mechanical performance and microstructural evolution of friction stir spot welded AA7075-T6

Abstract Pinless friction stir spot welding (PFSSW) is a promising solid-state joining technique for high-strength aluminum alloys, in which joint performance is primarily governed by shoulder-induced material flow and process parameters. In this study, the combined effects of shoulder groove geometry, rotational speed, and dwell time on the mechanical and microstructural characteristics of 3-mm-thick AA7075-T6 aluminum alloy joints were systematically investigated. Two pinless tools with identical dimensions but different shoulder groove geometries (gamma and spiral) were employed to isolate the influence of groove-induced material flow. Welding was conducted at rotational speeds of 2,500–3,500 rpm and dwell times of 6–14 s. Joint performance was evaluated through lap-shear tensile testing, energy absorption capacity derived from load–displacement curves, effective weld width (Weff) measurements, macrostructural analysis, microhardness profiling, and fractographic observations. The results demonstrated that increasing rotational speed and dwell time generally enhanced Weff for both tools due to improved material plasticization and consolidation at the lap interface. However, the spiral tool consistently produced larger Weff values (419–852 µm) than the gamma tool (340–711 µm) under comparable conditions. A strong positive correlation was observed between effective weld width and energy absorption capacity, as confirmed by regression analysis. Microhardness profiles revealed a hardened region near the weld center (≈201–215 HV) and a softened heat-affected zone (≈150–162 HV), where dwell time primarily governed the extent of HAZ softening, while rotational speed influenced the peak hardness. Overall, the spiral groove geometry promoted more distributed material flow and superior energy absorption performance compared to the gamma geometry.