Microstructural evolution and mechanical properties of bulk and powder metallurgy AA 7050 alloy after retrogression and re-aging heat treatment

Retrogression and re-aging (RRA) heat treatment effectively increases corrosion resistance and alloy strength simultaneously. This process involves re-dissolving and growing specific precipitates during retrogression after solid solution and artificial aging (T6) treatment, followed by the formation of nano-sized precipitates through re-aging. Although previous studies have explored the microstructure and mechanical properties of bulk alloys after RRA treatment, research on powder-sintered alloys remains limited. In this study, cast and sintered AA7050 alloys were prepared and subjected to RRA to examine their microstructural evolution and mechanical properties. Initial microstructures showed the formation of η-Mg(Zn,Cu,Al)₂, S-Al₂CuMg and θ-Al₂Cu phases, with each alloy had distinct solidification behaviours. After RRA, the grain boundary in the cast alloy changed from the η phase to the S phase, while the sintered alloy exhibited the η phase and precipitate-free zones (PFZs). The η′ phase precipitated in the Al matrix with average sizes of 21.7 nm and 14.0 nm in the cast and sintered alloy, respectively. These differences in microstructure affected hardness and electrical conductivity, demonstrating the advantages of sintered alloys in achieving refined precipitation and enhanced mechanical properties.