Fast aging kinetics of the AA6016 Al-Mg-Si alloy and the application in forming process

Abstract

Preaging treatment of AA6000 alloys has been a focus of alloy development for automotive applications. A much better bake-hardening response can be achieved with preaged materials compared to those in the naturally aged state. Recently, fast aging kinetics have been found in preaged AA6016 alloys. A heat treatment at 230 °C to 260 °C for over 60 seconds can increase the strength of the material, in particular, the yield strength, significantly. Observations of high-resolution transmission electron microscopy (HRTEM) indicate that relatively large clusters exist in the preaged state; they can grow rapidly into needle-shaped semicoherent precipitates at 230 °C for over 60 seconds, leading to the rapid changes in the mechanical properties. During heat treatment at higher temperatures, the preaged material behaved in a similar manner as the naturally aged material. The fast aging kinetics have been applied in making thermal-treated blanks with microstructural gradients. The blanks were treated locally at 230 °C for approximately 90 seconds to increase the strength of the local region. A heat treatment at temperatures higher than 350 °C for approximately 10 seconds can effectively reduce the strength. A combination of these two treatments applied to different locations of a blank, introducing strong gradients in the mechanical properties, can improve the forming performance of the material significantly, relative to the homogeneous blanks in the original condition.