Optimization of Solution Heat Treatment Processes for AA7075-H18 Alloy Sheets

The solution heat treatment (SHT) is an important step in hot forming and in-die quenching (HFQ) process. Herein, the SHT of cold-rolled AA7075-H18 alloy sheets is carried out in the temperature range of 462-518 °C for 0.9-29 min, and a response surface model based on strength and toughness is established. The results reveal that the maximal strength and toughness are achieved after SHT at 490.3 °C for 19.1 min and 475.1 °C for 21.1 min, respectively. One should note that the mechanical properties are closely related to the microstructural evolution during the SHT process. In particular, the dissolution amount of MgZn₂ particles increases with the increase of SHT temperature or time, which further increases the strength of AA7075 alloy after age-hardening treatment. However, the alloy is overheated when the solution temperature is close to 510 °C, which deteriorates its strength and toughness. On the other hand, the cold-rolled texture is disappeared after the SHT process and a random recrystallization orientation is formed. Moreover, an excellent consistency between the variation trend of recrystallized grains and the response surface model is obtained, confirming that the response surface model is reliable. The experimental results show that the mechanical properties under different SHT process conditions depend on the coupling effect of precipitation strengthening and grain size strengthening. Finally, the process windows for optimizing the SHT process to balance the strength and toughness of sheets are defined. The current research provides crucial theoretical support and systematic guidance for the manufacturing of hot-formed AA7075 alloy components.