Effect of preheat & post-weld heat treatment on the microstructure and mechanical properties of 6061-T6 aluminum alloy welded sheets

MIG welding is one of the most predominantly welding techniques in manufactories due to its high automation. However, aluminum alloy welded joints fabricated using MIG welding often exhibit a reduction in mechanical properties. In this paper, an attempt has been made to combine preheat and post-weld aging treatment in double-pulse MIG (DP-MIG) welding process to produce 6061-T6 aluminum alloy welded sheets with high mechanical properties. Response surface methodology (RSM) was used to study the main effect and interactive effect of heat treatment parameters including preheat temperature, aging temperature and time on mechanical properties. On this basis, the optimum heat treat parameters that maximize both strength and ductility of welded joints have been determined using multi-objective optimization algorithm. Additionally, the influence of heat treatment parameters on the microstructural evolution and mechanical properties of welded joints was further investigated, by systematic experimental and theoretical analysis. The results showed the preheat temperature affects the grain size and precipitation of the welded joint, while the aging heat treatment mainly affects the precipitation and transformation of the strengthen phase. Since the aging temperature is of greater significance for the improvement of mechanical properties of welded joints, compared with other heat treatment parameters and their interactions, indicating the strengthening of precipitates determines the mechanical properties of welded joints.