Simulation analysis is a key technical means for studying the internal metal flow patterns in refill friction stir spot welding zones. This study used DeformV11.0 software to establish an accurate and reliable numerical simulation model for 6061-T6 aluminum alloy refill friction stir spot welding. The microstructure of different stages during actual welding was obtained using the stop method, and combined with the simulation results, shows that the temperature in the spot welding zone is highest during the dwell stage, with a high degree of match between the temperature distribution and actual measurements. This stage is also crucial for affecting the refill process. The results indicate that the metal flow rate in the center of the spot welding zone is slow and the pressure is low, while the flow rate on both sides is fast, and the temperature and pressure are high. In addition, the metal in the weld zone flows plastically in a shear friction and in situ spinning manner, and the weld zone achieves connection in a form similar to “complete friction plug riveting”. A “spiral suction–refill injection layer stacking” model was established to describe the forming mechanism of refill friction stir spot welding.

Refill friction stir spot welding (RFSSW) is an effective technique for achieving high-quality joints in metallic materials, with rotational speed being a critical parameter influencing joint quality. Current research on RFSSW has primarily focused on low-melting-point materials such as aluminum alloys, while limited attention has been given to pure copper, a material characterized by its high-melting-point and high-thermal-conductivity. This study aims to investigate the effects of rotational speed on the microstructure and mechanical properties of RFSSW joints in pure copper. To achieve this goal, welding experiments were conducted at five rotational speeds. The welding defects, microstructure, and hook morphology of the welded joints were analyzed, while the variations in axial force and torque during welding were studied. The influence of rotational speed on the microhardness and tensile-shear failure load of the welded joints was explored, and the fracture modes of the welded joints at different rotational speeds were discussed. The results indicated that the primary welding defects were incomplete refill and surface unevenness. Higher rotational speeds resulted in coarser microstructures in the stir zones. As the rotational speed increased, the hook height progressively rose, the peak axial force showed an increasing trend, and the peak torque continuously decreased. The high microhardness points in the welded joints were predominantly located at the top of the sleeve stir zone (S-Zone), while the low microhardness points were observed at the center of the pin stir zone (P-Zone) and in the heat-affected zone (HAZ). The tensile-shear failure load of the welded joints initially increased and then decreased on the whole with the rising rotational speed, peaking at 5229 N at a rotational speed of 1200 rpm. At lower rotational speeds, the fracture type of the welded joints was characterized as plug fracture. Within the rotational speed range of 1200 rpm to 1600 rpm, the fracture type transitioned to upper sheet fracture. The initial fractures under different rotational speeds exhibited ductile fracture. This study contributes to advancing the understanding of RFSSW characteristics in high-melting-point and high-thermal-conductivity materials.