Influence of Mg and Cu on precipitation behaviors and mechanical properties of Al-Si alloys

Al-8Si-0.5 Mg, Al-8Si-2Cu, and Al-8Si-2Cu-0.5 Mg alloys have been designed in this study, aiming to reveal the influence of Mg and/or Cu additions on microstructures and mechanical properties of Al-Si alloys. Transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and the Kampman-Wagner Numerical (KWN) model have been used to investigate age-hardening behaviors as well as their relation to mechanical properties. Microstructure analysis suggest that beta'', theta ' and QP precipitates have formed in peak-aged Al-8Si-0.5 Mg, Al-8Si-2Cu and Al-8Si-2Cu-0.5 Mg alloys, respectively. Notably, the Al-8Si-2Cu-0.5 Mg alloy exhibits faster precipitation kinetics, larger age-hardening response and remarkable thermal stability compared to the other two alloys. Theoretical calculations indicate that the nucleation of QP precipitates is easier than beta'' and theta ' precipitates, but the slower diffusivity of Cu limits the growth rate of QP precipitates during the growth stage. Furthermore, the contribution of beta'', theta ' and QP precipitates to yield strength of peak-aged Al-Si alloys has been quantified, which shows a significant variation. beta'' precipitates in the Al-8Si-0.5 Mg alloy exhibit the highest strengthening effect (similar to 186.4 MPa), followed by QP precipitates in Al-8Si-2Cu-0.5 Mg (similar to 160.2 MPa) and theta ' precipitates in the Al-8Si-2Cu alloy (similar to 13.4 MPa). This work provides valuable insights into the influence of Mg and/or Cu additions on the aging response and mechanical properties of Al-Si alloys, offering a reference for further enhancing the performance of Al-Si alloys.