Coarsening behavior of Mg2Si precipitates during post homogenization cooling process in Al-Mg-Si alloy

The temperature and time dependence of the radii of Mg2Si precipitates in 6082 Al alloy under different post-homogenization cooling rates over a wide range of 32–120,000 °C/h were investigated by scanning and transmission electron microscopy. The equivalent radius of the Mg2Si precipitate decreased from 488 to 65 nm, associated with an increase in the cooling rate. The precipitating phase is primarily equilibrium β‐Mg2Si, as inferred from differential scanning calorimetry and X-ray diffraction. With the increasing cooling rate, the hardness of the alloy after T5 heat treatment initially increased and then plateaued. The critical cooling rate was identified to be 760 °C/h. A new thermodynamic particle coarsening model was designed to predict the evolution of particle size during the cooling process, and it presented appropriate agreement with the experimental data. This model can be applied to optimize alloys after the homogenization cooling process to ensure low deformation resistance, improve extrudability, and achieve maximum age-hardening effect in subsequent heat treatments.