Abstract: The detailed precipitation behavior and grain structure in different sub-regions of friction stir welding (FSW) AA6061-T6 joint after post-weld solution and aging treatments were explored. And the effects of microstructural evolution on mechanical properties, macro/micro electrochemical corrosion behavior and stress corrosion cracking behavior were investigated. The inherent microstructural gradients in FSW joint lead to dramatic degradation of mechanical properties and the presence of macro-galvanic effect, with the latter exacerbating anodic dissolution in heat-affected zone (HAZ) induced by micro-galvanic corrosion and inhibiting pitting corrosion in stirred zone (SZ). Post-weld heat treatment (PWHT) causes the formation of matrix precipitates with similar densities in different sub-regions, resulting in optimized precipitate distribution, 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 thermomechanical affected zone (TMAZ)/SZ interface.

Key words: Aluminum alloys, Friction stir welding, Galvanic corrosion, Post-weld heat treatment, Microstructure evolution