Effects of artificial aging on microstructure, mechanical properties and stress corrosion cracking of a novel high strength 7A99 Al alloy

A recently developed 7A99 Al alloy was subjected to hot extrusion, solid solution and artificial aging. The microstructure evolution and precipitation behavior were examined, and their effects on mechanical properties and stress corrosion cracking were analyzed. The results showed that the coarse Mg(Zn, Cu)2 phases were broken and refined by hot extrusion. Most of the phases dissolved into Al matrix during solution, and only insoluble Al7Cu2Fe phase was remained. GPI zones and η′ appeared in the under-aged and peak-aged samples. With the extension of aging time, stable η phase with large size formed and became the dominant precipitate in the over-aged sample. It was concluded that the precipitation of 7A99 Al during aging at 150 °C followed the sequence of solid solution → GPI zones → metastable η′ → stable η. The peak-aged sample owned the high yield and ultimate tensile strength of 588 MPa and 622 MPa, and a low elongation of 10.2%. The under-aged and over-aged samples exhibited relatively lower strength and higher elongations. 2% pre-stretching prior to aging could further improve the tensile properties. Both the under-aged and peak-aged samples were susceptible to the stress corrosion cracking, and the intergranular cracking was observed. In contrary, the over-aged sample showed much better corrosion resistance with a typical ductile fracture, due to the coarsening and separation of grain boundary precipitations.