Rapid bake-hardening response of Al–Mg–Si alloy during two-stage pre-aging heat treatment

Abstract

A two-stage pre-aging (T4P2) treatment was conducted on Al–Mg–Si alloy, in order to obtain a rapid bake-hardening response (BHR), and a single-stage pre-aging (T4P) treatment was carried out for comparison. Effects of the second-stage pre-aging temperature on the mechanical properties and precipitation characteristics of the Al–Mg–Si alloy were systematically investigated using a tensile test, micro-hardness measurement, differential scanning calorimetry (DSC) tests and transmission electron microscopy (TEM). The BH value ranged from 90 MPa to 114 MPa after different T4P2 treatments, and the peak BH value was obtained at a second-stage temperature of 180 °C. There was an obvious reduction in the precipitation activation energy of β″ phase from 109.3 KJ mol−1 in T4P treatment to 62.22 KJ mol−1 in T4P2 treatment. The kinetic equations of β″ precipitates during T4P2 were established based on the Avrami-Johnson-Mehl method, to predict the precipitation behavior. It is proved that T4P2 could be a more viable approach than T4P in improving the BHR of Al–Mg–Si alloy, due to the formation of a large amount of diffused GP zones during the first stage, and due to the density of stable β″-nuclei at second-stage temperature higher than 180 °C.