An efficient pre-hardened cryogenic forming process for AA7075 aluminum alloy sheets

Abstract

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 based on the W-temper alloy and 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, because there is a contradiction between the cryogenic formability and post-formed strength under the constraint of high efficiency. The results of tensile tests show that AA7075 aluminum alloy after pre-aging at 120 °C for 1 h (PA-120-1h) can achieve excellent comprehensive mechanical properties of cryogenic plasticity and post-baking strength. Combined with the analysis of micro-experimental results, the optimal pre-aging process window of AA7075 aluminum alloy is to ensure that the volume fraction of fine precipitates is between 2.5 % and 4 %, and the recommended ratio of the fine precipitates (<2 nm/≥2 nm) is about 1.5. 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. Based on the newly-designed cooling system, stamping tests are conducted on a W-shaped beam component to verify the engineering feasibility. Based on PA-120-1h alloy sheets, the component with improved surface quality can be successfully formed by PCF process. 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.