Warm forming of AA7075-T6: optimizing the heating time to maintain T6 condition

Warm forming at 200 degrees C is an interesting way of improving the formability of AA7075T6. The T6 condition offers the highest ultimate and yield strengths of this aluminium alloy. It is therefore important to maintain these excellent mechanical properties at the end of the forming process. With the hot forming process at temperature above 450 degrees C, it is necessary to add a heat recovery treatment obtained during paint baking to keep this T6 state, which is costly in terms of time and energy. In warm forming process, it is possible to maintain the T6 condition by controlling the heating time to avoid precipitation changes. The objective of this study is to find these optimal heating time conditions to maintain the T6 state during warm forming multi-step process. Different heating times were reproduced using a Gleeble 3500 machine. Electrical conductivity and hardness were measured after these different conditions to make the link with the evolutions of precipitates of AA7075-T6. Tensile tests were also performed to characterize the mechanical behavior at the end of these heating cycles. A holding time of less than 10 seconds is determined to maintain the T6 state at 200 degrees C. Two multi-step warm forming devices (a cylindrical cup in two steps and a U-channel part) were finally tested to validate these optimal time forming conditions.