A comparative study on cryogenic formability and paint baking property of AA7075 alloy sheets tempered at fast retrogressed and pre-aged conditions

Forming at cryogenic temperatures provides a new approach to the formability improvement of 7000-series high-strength aluminum alloys. However, the cryogenic forming process based on solution-treated (W) or peak artificial aged (T6) alloys cannot meet the demands of autobody manufacturing for good formability, high post-formed strength and high efficiency simultaneously. The fast retrogression (FR) treatment of AA7075-T6 and pre-aging (PA) treatment of AA7075-W are elaborately designed to tailor a pre-hardened AA7075 alloy, which has good cryogenic formability and relatively high initial strength, and can be further strengthened by paint baking in high efficiency. The results show the optimum fast retrogression temperature for AA7075-FR is 400 °C, at which the sample can acquire a cryogenic elongation of 29.2% and obtain 90% of initial T6 sheet tensile strength by the combined effect of cryogenic pre-deformation and subsequent paint baking treatment. The optimum pre-aging temperature for AA7075-PA is 120 °C, at which the sample can acquire a cryogenic elongation of 31.0% and exceed 96% of tensile strength of AA7075-T6 after paint baking treatment. The comparative study demonstrates that the supersaturated solid solution matrix with an appropriate proportion of GP zones and η′ phases, but few Cr-containing dispersoids and precipitate free zones, is an ideal pre-hardened microstructure to get a good combination of cryogenic formability and final strength. The precipitation of GP zones and η′ phases is prior to the formation of precipitate free zones during pre-aging of AA7075-W, making the pre-aged hardening cryogenic forming process more suitable for automobile industry.