Influence of Interrupted Quenching and Pre-aging on the Bake Hardening of Al-Mg-Si Alloy

Abstract 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 3 h to 45 min. 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.