The relationship between microstructural characteristics and galvanic effect, SCC behavior of friction stir welded joint in as-welded and heat-treated conditions

摘要

The effects of microstructural evolution on mechanical properties, macro/micro electrochemical corrosion behavior and str ess corrosion cracking behavior of friction stir welding (FSW) AA6061 -T6 joint after post -weld solution and aging treatments were investigated. The inherent microstructural gradients in FSW joint lead to dramatic degradation of mechanical properties and the presence of macro-galvanic effect [1], with the latter exacerbating anodic dissolution in heat-affected zone (HAZ) induced by micro-galvanic corrosion and inhibiting pitting corrosion in SZ. Post-weld heat treatment (PWHT) causes the formation of matrix precipitates with similar densities in different sub -regions, resulting in optimized precipitate distribution [2], comprehensive hardness recovery, and diminished macro -galvanic effect. Grain boundary misorientation angle, grain size and pre -existing dislocations synergistically influence the evolution of grain boundary precipitates (GBPs) and precipitation -free zones (PFZs) during the PWHT. As a result, pitting corrosion is the dominant corrosion form in SZ due to the narrowest PFZ width and dispersed GBPs, while intergranular corrosion is caused by continuous GBPs in other sub -regions. This study verified the dominant role of macro -galvanic effect and micro -galvanic effect in the corrosion process of FSW joint and FSW -PWHT joint, respectively. The maximum SCC susceptibility at HAZ in As-FSWed joint is dominated by enhanced anodic dissolution due to macro-galvanic effect. The SCC sensitivity of FSW -PWHT joint is higher than that of FSW joint due to high electrochemical activity and corrosion rate caused by the severe stress concentration between sub -grains and recrystallized grains at the TMAZ/SZ interface.