Comparative study on mechanical properties and thermal responses in conventional and underwater friction stir welding of dissimilar aluminium alloys

Abstract

This study investigates the influence of process parameters on the mechanical and thermal behaviour of dissimilar AA6061-T6, and AA5083-H12 aluminium alloy joints fabricated by conventional friction stir welding (CFSW) and underwater friction stir welding (UFSW). The combined effects of tool rotational speed and welding speed were analyzed using the response surface methodology (RSM), and the developed quadratic models showed excellent agreement with the experimental results (R² at 95%). The response surfaces for tensile strength (TS) and percentage elongation (PE) exhibited clear parabolic trends, with both responses increasing up to an optimum combination of 1120 rpm and 63 mm/min, beyond which they decreased due to excessive heat input and softening. Under these optimal parameters, the UFSW joint achieved a maximum TS of 196 MPa and PE of 9.8%, which were approximately 11.4% and 6.5% higher, respectively, than those of the corresponding CFSW joint. The underwater environment reduced the peak temperature by about 15–20% and increased nugget hardness by 6 to 8%, indicating improved heat management and microstructural refinement. The integrated RSM analysis confirmed a strong thermal mechanical correlation between process parameters, temperature control, hardness distribution, and weld performance.