Effect of pre-aging and paint baking processes on precipitation behavior in heat-affected zone of CMT welded Al–Mg–Si–Cu alloy joint

The heat-affected zone (HAZ) of the novel Al–Mg–Si–Cu alloy will undergo serious softening during welding. In this work, pre-aging (PA) and paint-baking (PB) processes are proposed to address this issue. The effects of different PA processes and the synergistic effect of PA and PB on the precipitates and properties of Al–Mg–Si–Cu alloy joint are studied. The results show that the degree and mechanism of HAZ softening vary with the distance from the weld center. The most severe softening occurs in HAZ Ⅱ, located 5–7 mm away from the weld center, where coherent β", QP1/2 and L phases transform into β and Q phases with a weak strengthening effect. The hardness of base metals changes in an S-shaped curve with increasing PA temperature. Compared to the T6 state, the tensile properties of HAZ can only be improved within a PA temperature range of 120–200 °C for 1–4 h. PA inhibits the coarsening of precipitates during welding and promotes the formation of more atomic clusters2 and GP zones in the HAZ, significantly enhancing its tensile properties. Combined with the PA state, the atomic clusters2 and GP zones transform into β" phases and Cu-containing phases after PB, which further improves the tensile properties of HAZ and significantly increases the hardness of base metals. The joint has the highest bake hardening response when PA is at 140 °C for 1 h. Finally, short-term PA and PB processes are optimized, resulting in a welded joint with a tensile strength of 304.7 MPa and a welding coefficient of 82.2%, demonstrating promising industrial application prospects.