42nd International Deep Drawing Research Group Conference - IDDRG 2023

Currently, due to the requirement of lightweight in the transportation field, the demand for high-strength aluminum alloy is increasing, but it is difficult for the traditional forming processes to balance the three components of formability, strength and efficiency. In order to solve this problem, a novel stamping process for high-strength AA7075 aluminum alloy sheets is proposed, termed Pre-hardened Cryogenic Forming (PCF) process. PCF process is to fulfill simultaneously the demands of good cryogenic formability and high post-formed strength through a suitable ratio of microstructure between the solution-treated matrix and the pre-aged second phases, thereby cancelling the additional artificial aging treatment and achieving high-efficiency. The good cryogenic formability is assured by solution-treated matrix, while the initial strength and further artificial age-hardening during paint baking are regulated by means of pre-aged second phases. A critical issue for PCF process is to determine the pre-aging process window, i.e., a suitable combination of pre-aging temperature and time period. The results show that pre-aging at 120 °C for 1h can achieve excellent comprehensive mechanical properties of cryogenic plasticity and post-baking strength, where the mixed structure of the GP zones and the η' phases for PA-120-1h alloy is the suitable combination of strengthening precipitates for balancing cryogenic plasticity and post-baking strength. In addition, based on the newly-designed cooling system (cooling channel and local spraying for LN2), stamping tests are conducted on a W-shaped beam component to verify the engineering feasibility. The cryogenically stamped W-shaped components show good surface quality and uniform thickness distribution. The final post-baking strength exceeds 92% of the tensile strength of AA7075-T6 alloy without extra heat treatment, and the thickness reduction rate is within 10%. Regardless of the immersion time of the sheets in liquid nitrogen, the PCF process can be finished within 2 min, greatly improving the forming efficiency.