Improved solution treatment for an as-rolled Al–Zn–Mg–Cu alloy. Part II. Microstructure and mechanical properties

In this work, improved solution treatments and their effects on the microstructure and mechanical properties of as-rolled Al–6.22Zn–2.11Mg–2.39Cu alloy have been investigated. By exploring the dissolution process of soluble constituent particles and monitoring changes in the grain structure, a solution treatment sequence with a good balance between a minimum volume fraction of remaining constituents and a partially recrystallised grain structure has been obtained. The results indicate that MgZn2 (η) particles can be completely dissolved after holding at 475 °C for only 5 min, whereas the dissolution of Al2CuMg (S) particles is relatively difficult and their complete dissolution requires a stepped solution treatment. By conducting a final solution treatment step at an increased temperature of 495 °C, all the S-phase particles can be dissolved, whereas the recrystallised fraction can still be controlled to be less than 50%. After retrogression and re-ageing (RRA), samples subjected to a final-stage solution treatment at 495 °C have a relatively higher volume fraction of η′ and η precipitates and can therefore have better mechanical properties.