Revisiting the evolution of strength and microstructure of aluminum alloy 7055 during continuous retrogression and re-ageing treatment

The evolution of strength and microstructure in aluminum alloy 7055 during a continuous retrogression and re-aging (CRRA) treatment with two retrogression cooling rates was systematically investigated. The results reveal that GP II zones serve as the primary strengthening phase after the pre-ageing stage, characterized by Zn/Mg and (Zn + Cu)/Mg ratios of 1.24 and 1.36, respectively. During the retrogression treatment, these particles progressively transform into η′ phases, preventing strength reduction for the duration up to 90 min. Meanwhile, solute Cu atoms partition into aggregates (clusters, GP II zones, and η′ phases) by substituting for Zn and Al atoms rather than Mg atoms, resulting in compositional differences between aggregates in CRRA-treated samples and the pre-aged sample. Additionally, air cooling (3300°C/h) following the retrogression treatment facilitates strength improvement through secondary precipitation, while furnace cooling (45°C/h) further improves the electrical conductivity and increases Cu content in both matrix precipitates and grain boundary precipitates without deteriorating the yield strength.