Pre-aging to improve bake hardening in a twin-roll cast Al-Mg-Si alloy

Heat-treatable Al-Mg-Si aluminum alloys are increasingly used for automotive body panel applications. They are shipped and formed in T4 temper while still formable and are subsequently given a bake cycle to increase strength by age hardening while the paint is cured. The industrial paint-bake treatment, however, is too short to fully exploit the age hardening potential of these alloys. Hence, the microstructure in the conventional T4 temper must be modified by creating clusters that grow and readily transform into coherent precipitates upon paint baking. The present work was undertaken to improve the paint-bake response of a twin-roll cast 6016 sheet by employing artificial aging between the solution treatment and the paint-bake cycle. Pre-aging in a wide temperature range (60-200 °C) was effective in improving the bake hardening response of the twin-roll cast 6016 sheet which, when processed without any pre-aging, failed to meet in-service strength requirements with a rather poor bake hardening response of 35 MPa.