Effect of natural ageing on the performance of pre-ageing to improve bake-hardening response of a twin-roll cast Al-Mg-Si alloy

Since a room-temperature delay between the solution heat treatment and the paint-bake cycle is unavoidable in industrial practice, it is essential to counteract the adverse effect of natural ageing on the bake-hardening response of Al-Mg-Si alloys. This is often accomplished by an additional ageing cycle, referred to as pre-ageing, before the paint-bake treatment. While pre-ageing treatments shortly after the solution treatment are very attractive, long delays prior to pre-ageing produce undesirable effects on the bake-hardening response. The best combination of a sufficiently high T8X and a reasonably low T4P yield strength for a twin-roll cast (TRC) Al-Mg-Si alloy is obtained when the sheet is pre-aged for 5 minutes at 180 &DE;; C with only a 10-minute delay after the solution treatment. Several hours of natural ageing can be tolerated before the T8X yield strength drops below 200 MPa with a similar heat treatment at 200 &DE;; C. Pre-ageing at even higher temperatures must be employed only after substantial natural ageing, of the order of several days, to avoid a too high T4P yield strength that would impair stampability.