Influence of interrupted quenching and pre-aging on the bake hardening of Al–Mg–Si Alloy

Thermal treatments such as pre-aging (PA) of Al–Mg–Si alloy sheet are important and necessary in the industrial production of automotive body panels. Interrupted quenching (IQ) treatments were investigated as a new strategy to improve the performance of Al–Mg–Si alloy during natural aging and paint-bake hardening. Direct comparisons were made to conventional pre-aging treatments for these alloys. The results show that IQ and PA treatments at 100°C both suppress natural aging and promote precipitation of β″ phase during paint baking. However, the 100°C IQ process has the potential to reduce the time from 3h to 45min. The accelerated formation of Cluster(2) induced by the slow quenching rate is the main reason for high efficiency. IQ treatments at 170°C produced less and coarser Cluster(2) which did not effectively improve the bake hardening response (BHR) after paint-bake cycle. The main disadvantage of IQ treatments is the precipitation of large quenching-induced precipitates and formation of precipitate-free zones (PFZ) at the grain boundaries which could decrease the fracture toughness of Al–Mg–Si alloys.