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 β'', θ′ 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 β'' and θ′ precipitates, but the slower diffusivity of Cu limits the growth rate of QP precipitates during the growth stage. Furthermore, the contribution of β'', θ′ and QP precipitates to yield strength of peak-aged Al–Si alloys has been quantified, which shows a significant variation. β'' precipitates in the Al–8Si-0.5 Mg alloy exhibit the highest strengthening effect (∼186.4 MPa), followed by QP precipitates in Al–8Si–2Cu-0.5 Mg (∼160.2 MPa) and θ′ precipitates in the Al–8Si–2Cu alloy (∼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.