Effect of Tool Rotation Speed on Defect Formation in Underwater Friction Stir Welding: A Numerical Study on AA5083-H12 and AA6061-T6

Underwater Friction Stir Welding (UFSW) is an advanced solid-state joining process that utilizes the cooling effect of water to minimize thermal distortion and improve joint quality in aluminium alloys. However, the effect of Tool Rotational Speed (TRS) on weld formation and mechanical behaviour in UFSW of dissimilar alloys remains inadequately understood. In this study, a combined numerical and experimental investigation is conducted to evaluate the influence of TRS on the weld characteristics of AA5083-H12 and AA6061-T6 aluminium alloys. A three-dimensional Computational Fluid Dynamics (CFD) model incorporating the Volume of Fluid (VOF) method is developed to simulate material flow and thermal behaviour under varying TRS conditions. The model is validated against experimentally measured temperature profiles with less than 2% deviation. Results show asymmetric temperature and strain rate distributions between the Advancing Side (AS) and Retreating Side (RS), with slightly higher temperature on the AS and higher strain rate on the RS. Material flow velocity at the tool workpiece interface increases with TRS up to 1120 rpm but becomes unstable at higher speeds (1400 and1600 rpm), leading to fluctuating slip rates and void formation near the RS. The optimum joint strength of 194 MPa is achieved at 1120 rpm, corresponding to refined equiaxed grains (8.9 µm) in the stir zone, while excessive rotation at 1600 rpm reduces strength to 150 MPa due to internal defects. Fracture analysis reveals a transition from ductile fracture at lower TRS to brittle failure at higher TRS. The study demonstrates that excessive tool rotation destabilizes material flow and promotes void formation, thereby reducing weld integrity.