Effect of two-stage overaging on microstructure and corrosion properties of an Al–Zn–Mg–Cu alloy

A two-stage overaging process was proposed to improve the electrical conductivity and stress corrosion cracking resistance (SCCR) of an Al–Zn–Mg–Cu alloy. The overaging process include a low-temperature aging at 120 °C for 5 h and a subsequent aging at 177 °C for 8, 14 and 24 h. The SCCR of the samples aged at 120 °C/5 h + 177 °C/24 h was higher than that of the T6 temper, at which the ISSRT is 1.2% and the electrical conductivity is 39.0%IACS. The increase of electrical conductivity is caused by the depletion of solution atoms during overaging. Meanwhile, the improvement of SCCR resistance is caused by the coarsening of grain boundary precipitates with higher Cu content. The tensile strength decreased from 663.5 MPa to 519.7 MPa, which is caused by the coarsen of strengthening precipitates in grains during overaging process.