Effect of heat treatment on microstructure, mechanical properties and corrosion resistance of 7075 aluminum alloys fabricated by improved friction stir additive manufacturing

The use of an improved stirrng pin broadened the effective bonding area in the friction stir additive manufacturing (FSAM) of 7075 aluminum alloy. A systematic investigation was conducted to examine the effects of different heat treatment (T6, T73, and RRA) processes on the microstructure, tensile strength and corrosion resistance of as-fabricated (AS) specimen. The findings revealed that different heat treatments had minimal impact on the grain size, texture strength, and dislocation density. Variations in strength and corrosion resistance were primarily attributed to the size and distribution of precipitates. Following T6 heat treatment, the intragranular precipitates (IGPs) were smaller, with a higher proportion of )', while the grain boundary precipitates (GBPs) were coarser ), continuously distributed along grain boundaries. Consequently, T6 specimen showed the highest tensile strength, reaching 568 MPa, but had lower corrosion resistance than AS specimen due to precipitate-induced disruption of the passive film. The T73 heat treatment resulted in significant coarsening of IGPs, with a substantial transformation of )' to ), thereby reducing the precipitation strengthening effect. The coarsening of GBPs led to their discontinuous distribution along grain boundaries, which significantly increased the corrosion resistance. After RRA heat treatment, IGPs consisted of numerous fine )', while the coarser GBPs sporadically dispersed along grain boundaries. The strength of RRA specimen was only about 6 % lower than that of T6 specimen, but its corrosion resistance was significantly improved.