Warm formability and post-forming microstructure/property of high-strength AA 7075-T6 Al alloy

Abstract

Formability of peak-aged AA 7075-T6 sheet across a temperature range from room temperature to 250°C as well as post-forming microstructure/property were investigated in this paper. It showed that the optimized formability was obtained at 200°C, due to the higher ductility and work hardening capacity at this temperature. The dominant phases in 7075-T6, 200°C warm formed, and 250°C warm formed samples were fine η′ and GP zones, well-developed η′, and coarser η phases, respectively. Post-forming and post-paint-baking mechanical property of 7075 samples was closely associated with those corresponding microstructures. Specifically, 200°C warm forming caused limited coarsening of matrix precipitates (MPts) and generated a certain number of dislocations, due to the combined effect of the decreased precipitation strengthening and increased dislocation strengthening, the inherent high strength of 7075-T6 was perfectly preserved after 200°C forming. However, 250°C forming led to severely decreased hardness due to the sharply coarsened MPts. Furthermore, the paint-baking treatment exerted little influence to MPts, grain boundary precipitates (GBPs) and precipitate free zone (PFZ), thus the hardness loss caused by paint-baking was unobvious. Overall, 200°C was the appropriate temperature under which the peak-aged 7000 alloys exhibited enhanced formability and maintained high post-forming strength, after paint baking, the drawn component possessed microstructure/property similar to those of retrogression and re-aged (RRA) samples.