Strength calculation and microstructure characterization of HAZ softening area in 6082-T6 aluminum alloy CMT welded joints

Abstract

Degradation of mechanical properties in the heat-affected zone (HAZ) is a common issue when welding 6082-T6 aluminum alloy. Even for cold metal transfer (CMT) welded joints with little heat input, it is always challenging to avoid this problem. In this study, we examined the effects of various heat inputs on the softening of CMT welded 6082-T6 aluminum alloy joints as well as the mechanism underlying the softening of HAZ. The hardness of the welded joint at the HAZ softening area directly decreases as the heat input increases. The softening of HAZ is the most severe under the impact of thermal cycling, with a softening zone width of up to 5.5 mm and a minimum hardness of about 61 HV, reaching 58.65% of the base material (BM). According to the strength model established in the study, the most significant factors affecting the strength of welded joints were the weakening of precipitation strengthening and solid solution strengthening due to welding thermal cycling. These factors accounted for 51.51% and 30.26% of the total strength, respectively. Compared with BM, the contribution of precipitation strengthening and solid solution strengthening in HAZ softening area decreased by 85 MPa and 23 MPa, respectively. Where the weakening of precipitation strengthening is mainly due to the β" and β' phase transitions induced by welding thermal cycling. The decrease in solid solution strengthening is then mainly influenced by the change in the content of Mg and Si elements in the matrix. The revelation of softening mechanism provides theoretical support for how to control the loss of mechanical properties of welded joints of Al-Mg-Si aluminum alloy.