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.

Methods of X-ray diffraction and transmission electron microscopy were used to study the microstructure of dispersion-strengthened Cu-Al2O3nanocomposites obtained by the method of simultaneous...

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.

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.