Relating Quench Sensitivity to Microstructure in 6000 Series Aluminium Alloys

Abstract

In high strength 6000 series alloys dispersoids that form during heating to the homogenization temperature are used to improve fracture toughness and suppress grain growth during the extrusion process. However, these dispersoids can act as heterogeneous nucleation sites for non-hardening Mg-Si precipitates if quenching after extrusion is delayed. This leads to a reduced level of Mg and Si in solid solution and hence lower achievable strength and hardness. This phenomenon is called quench sensitivity. In this study, the hardening response of several 6000 series aluminium alloys is related to microstructural features, especially dispersoid density. Therefore, the alloys were quenched at varying rates after extrusion and age hardened to peak strength. Quench sensitivity is related to dispersoid density as well as to the enthalpy related to the precipitation of Mg-Si phase measured by DSC. The results suggest that in alloys containing dispersoids, quench sensitivity is primarily determined by the number density of dispersoids. However, effects associated with elements in solid solution cannot be ruled out. TEM investigations suggest that not only the general reduction of Mg and Si is responsible for the reduced mechanical properties, but that an inhomogeneous distribution of hardening precipitates might be another determining factor.