The effect of multistage ageing on microstructure and mechanical properties of 7050 alloy

In the present work, the multistage ageing process involving pre-ageing, heating process and re-ageing treatment was proposed for an Al-Zn-Mg-Cu alloy aiming to promote comprehensive properties. The age-hardening and the conductivity variation were examined to evaluate the potential in the strength and the corrosion resistance, respectively. The evolution of precipitation microstructure during multistage ageing process was investigated systematically through TEM observation. The current study suggests that the variations of hardness and conductivity are affected by heating rates. Tensile testing shows the multistage ageing process (120 °C/24 h + 120-200 °C/40 °C/h+120 °C/24 h) is capable of enduing high strength equivalent to the T6 temper, together with the RRA corrosion resistance levels. TEM micrographs during multistage ageing process have revealed that the variation in size of intra-grain precipitates and also the size and distribution of grain boundary precipitates are affected by heating temperature and heating rate.