The welding forces and torque in refill friction stir spot welding (RFSSW) play a crucial role in optimizing welding process parameters, understanding material deformation and flow mechanisms, improving welding equipment, and analyzing tool wear. To measure the welding forces and torque during pure copper RFSSW, an octagonal ring dynamometer (ORD) was designed and manufactured based on ring deformation theory. A measurement system for welding forces and torque was established, and the ORD was calibrated and validated. Welding experiments were conducted under different process parameters to measure the welding forces and torque during pure copper RFSSW. The variation patterns of welding forces and torque throughout the welding process were analyzed, and the relationships between welding parameters and the peak welding forces and torque were established. The results show that the calibration errors for axial force, torque, and lateral force were 0.97%, 3.78%, and 1.56%, respectively, with cross-sensitivity errors below 5%. The primary welding force during the RFSSW was the axial force, with a peak value more than 10 times that of the lateral force. The clamping, plunging, and refilling stages were the key phases where both welding forces and torque increased significantly. The maximum welding forces occurred near the second dwell stage before the end of welding, while the maximum torque was observed during the plunging stage. Axial force was most influenced by plunge depth, torque was mainly affected by rotational speed, and lateral force was strongly impacted by the combined effects of plunge depth and rotational speed. The average prediction errors for the axial force, torque, and lateral force models were 0.61%, 2.25%, and 1.20%, respectively, with a maximum error of 4.67%. These findings are instrumental in optimizing process parameters to achieve higher weld quality. The study has practical implications for industrial applications, particularly in manufacturing electrical connectors and thin-sheet heat exchangers, where high-quality copper welds are essential. This work also provides a foundation for future research into enhancing the efficiency and reducing the energy consumption of the RFSSW process.

In this study, under varying PDSP (plunging depths of stirring pin) and process parameters, refill friction stir spot welding tests were performed on 6061-T6 aluminum alloy, relying on a stirring tool with a 12 mm sleeve diameter and an 8 mm stirring pin diameter. The results manifested the internal defects in the weld zone when PDSP was 0, notwithstanding the alterations in process parameters. However, these flaws disappeared when PDSP was 0.5 mm and 1 mm. In the weld zone, PDSP exerted a dramatic effect on the internal metal flow state, particularly the curvature of the “Hook” shape and the width of the bonding ligament. It changed the downward bending of the ‘Hook’ into an upward one, influencing the fracture behavior of the weld zone and elevating the ULSF (ultimate lap shear force) by up to 20% (PDSP = 0.5 mm, welding speed = 30 mm/min, rotation speed is 1200 rpm). Besides, the PDSP intensified the PAZ (pin affected zone) pressure, induced more metal flowing into the SAZ (sleeve affected zone), thus reinforced the SAZ-TMAZ(thermomechanically affected zone) bonding strength, and upgraded the region’s microhardness. In summary, the PDSP is commendable for bolstering the weld zone’s performance, but excessively large PDSP values incur drastic indentations in the PAZ, which diminish the ULSF.

This study investigates refill friction stir spot welding (RFSSW) of 2 mm thick ZK61M-T5 magnesium alloy. Sound joints were obtained at rotational speeds of 1000 rpm and 1500 rpm with welding speeds of 30–50 mm/min. At 1000 rpm, micro-pores appeared at the sleeve-affected zone bottom, exhibiting a full-fillet fracture and a maximum ultimate lap shear force (ULSF) of 8.3 kN. Increasing the speed to 1500 rpm eliminated the pores and changed the fracture mode to partial fillet, but reduced the ULSF to 7.7 kN. Higher welding speeds caused the Hook feature to shift from upward to downward. Compared with the base material, grains in the thermomechanically and sleeve-affected zones were refined, while those in the pin-affected zone coarsened with increasing rotational speed. Overall, higher rotational speed increased heat input and mitigated internal defects, but enhanced annealing and Zr segregation, leading to reduced mechanical performance.

Due to the unique physicochemical properties of copper (Cu), conventional fusion welding faces certain challenges when welding this material. In this study, Cu-Cu joints were fabricated using refill friction stir spot welding (RFSSW) at five different plunge depths. The macroscopic morphologies of the welded joints (WJs) under various welding conditions were examined, and the microstructural differences in representative zones of the WJ were analyzed. The microhardness and tensile-shear failure load (T-SFL) of the WJs were evaluated to assess the mechanical performance, and the failure mechanisms were explored. The results indicated that with increasing plunge depth, the microstructure tended to coarsen, accompanied by a gradual decrease in microhardness. The T-SFL initially increased and then decreased with increasing plunge depth, with the optimal depth found to be 1.4 times the plate thickness. For Cu-Cu joints, the softening effect in the heat-affected zone (HAZ) outside the stirring zone is identified as a key factor limiting the load-bearing capacity of the WJs.