Thermo-mechanical numerical simulation and spot-weld formation mechanism of 15 mm-diameter refill friction stir spot welding of 6061-T6 aluminum alloy

This study investigates 15 mm-diameter refill friction stir spot welding (RFSSW) of 6061-T6 aluminum alloy using a coupled thermo-mechanical finite element model developed in Deform V11.0. The predicted weld profile and thermal cycle agreed well with experimental results, with a maximum peak-temperature deviation of 6 °C and a relative error below 2%. The weld zone experienced rapid heating, a fluctuating high-temperature stage and subsequent cooling, with the peak temperature occurring near the transition from dwell to early refilling. Particle tracking showed that plasticized material transport involved coupled axial feeding, radial displacement and circumferential stirring under the coordinated motion of the pin and sleeve. Based on the evolution of the original faying interface, joint residual line and stage-dependent microstructures, the large-diameter weld was inferred to form through shear-extrusion shaping, pressure-assisted solid-state bonding and friction plug-riveting accumulation. These findings clarify the thermo-mechanical behavior and formation mechanism of large-diameter RFSSW joints and support the design of high-load-bearing welded structures.