Investigation of the age hardening and operative deformation mechanism of 7075 aluminum alloy under creep forming

Creep forming is a process where plastic deformation is applied at the materialʹs ageing temperature. In order to study the effect of microstructure changes on the hardening and creep deformation behavior of 7075 aluminum alloys, creep forming experiments have been performed at temperatures of 150 °C and 190 °C for 6–72 h. Results indicated that with increasing of forming time and temperature the size of globular shaped phase (known as GP zones) increased and transformed into η′ precipitates. Meanwhile, the η-MgZn2 phase with almost the same size formed along the grain boundaries with quite narrow precipitate free zones. Further increasing of forming time, led to a decrease in the number of η′ precipitates and an increase in precipitate spacing, where the large equilibrium η phase formed and the precipitate free zones became wider. Moreover and creep curves can be divided into two stages. During the first stage, higher creep rate occurred, which increased with temperature but decreased with forming time. While during the second stage, the creep rate reached its lowest value and kept constant. The variations of hardness and creep strain have been discussed in detail by taking account of the evolution of precipitate microstructure.