Numerical simulation of thermal cycle and void closing during friction stir spot welding of AA-2524 at different rotational speeds

Highlights: • Based on ABAQUS/Explicit, the voids closure mechanism in RFSSW of 2524 aluminum alloy was revealed. • The simulation results of temperature and stress field in welding process are analyzed. • The relationship between the grain size at different rotational speeds and the Z parameter is examined by the simulation and experimental results. A three dimensional and Coupled Eulerian–Lagrangian (CEL) model for the refill friction stir spot welding (RFSSW) of 2524-T3 aluminum alloy was developed based on the ABAQUS/Explicit. The distribution of stress and temperature field at the joint area, at five different rotational speeds, were simulated and verified using experimental studies. The simulation results showed that with increasing rotational speed, heat production by friction and plastic deformation was enhanced, while the stress was reduced. Moreover, internal voids, the typical defect, were found on the bottom edge of the stir zone, and the propensity for the voids were the lowest at the rotational speed of 2800 rpm (rpm); this was verified using the immersion ultrasonic C-scan test (IUCT) test results. Based on the simulation results, isothermal compression test results and electron backscatter diffraction (EBSD) analysis results of the joint, the relationship between grain size and the Zener–Hollman parameter at the stir zone (SZ) was also established, thereby providing an in-depth understanding and detailed rationalization of the RFSSW processes.