Refill friction stir spot welding of high-strength aluminium alloys: Linking hook formation and calculated heat input to microstructure and mechanical properties

Refill Friction Stir Spot Welding is a promising technique for joining lightweight aluminium alloys such as AA7075-T6. However, hook defects—microstructural discontinuities at the weld interface—can significantly compromise joint integrity. Additionally, variations in heat input during welding influence microstructure evolution and mechanical properties. This study investigates the influence of process parameters and the hook defect on the quasi-static performance of AA7075-T6 RFSSW joints. Joints with various parameter combinations were analysed using optical microscopy, electron backscatter diffraction, tensile lap shear testing, and microhardness mapping, alongside thermocouple measurements of process temperatures. A semi-quantitative heat input model was developed to provide new insights into thermal characteristics and their effects on microstructural evolution. The obtained results show a decrease in ultimate lap shear force for high upward or downward-directed hooks. A welding time of 4 s and rotational speed of 2200 rpm resulted in optimal joint performance, achieving an ultimate lap shear force of 12.2 kN and sufficient heat input to prevent refill defects. Extended welding durations led to overheating and additional defects in the stir zone, while low welding time or rotational speed significantly influenced welding temperatures, heat input, and microstructural characteristics at the shoulder plunge path periphery. The findings emphasise the critical role of dynamic recrystallisation in the resulting microstructures and their impact on mechanical performance. Fatigue strength of the optimised joints exceeds RFSSW joints reported in the literature, underscoring the effectiveness of the selected process parameters in enhancing joint durability. Furthermore, the established semi-quantitative heat input model links energy partitioning to defect formation and microhardness of the joints. This provides new insights into the RFSSW process and enables knowledge transfer to other RFSSW applications.