Bake hardening behavior and precipitation kinetic of a novel Al-Mg-Si-Cu aluminum alloy for lightweight automotive body

The natural aging (NA) phenomena and its negative effect on bake hardening (BH) response, and a wide temperature/time range of pre-aging (PA) treatment on a novel Al-Mg-Si-Cu automotive aluminum alloy were investigated systematically by micro-hardness measurement, tensile test, transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) analysis. The precipitation kinetic of main strengthening β″ phase was calculated according DSC curves based on JMA (Johnson-Mehl-Avrami) equation. The results indicate that artificial aging (AA) immediately after solution treatment is helpful to achieve good BH response, while aging temperature and time determine the value and arrival time of peak hardness. BH value is decreased with increase of NA time. BH value is ranging from 40 MPa to 110 MPa after different PA treatment, and the optimal one is obtained while PA at 160 °C for 10 min. The precipitation activation energy of β″ phase of T4P state (160 °C/10 min) is reduced about 42 kJ/mol compared to T4 state, which is attributed to the increase of GP region size during PA treatment.