Artificial ageing of Al–Si–Cu–Mg casting alloys

► Tensile testing of three Al–Si–Cu–Mg alloys, 3 SDAS, 2 ageing temperatures. ► YS after ageing is weakly dependent on the coarseness of the microstructure. ► The elongation of the Al–8Si–3Cu–0.5Mg with SDAS 10 μm recover on overageing. ► The Q index Q = YS + Kɛ can compare different Al–Si–Mg alloys but not Al–Si–Cu–Mg alloys. ► Al–Si–Mg alloys are preferred over Al–Si–Cu–Mg alloys for YS below about 300 MPa. The T6 heat treatment is commonly used for gravity cast Al–Si–Cu–Mg alloys. The influence of the alloying elements Cu and Mg and the artificial ageing temperature on the age hardening response were investigated. Artificial ageing was conducted at 170 °C and 210 °C for various times for three alloys, Al–7Si–0.3Mg, Al–8Si–3Cu and Al–8Si–3Cu–0.5Mg, cast with three different solidification rates (secondary dendrite arm spacing of about 10, 25 and 50 μm). The coarseness of the microstructure has a small influence on the yield strength, as long as the solution treatment is adjusted to obtain complete dissolution and homogenisation. The peak yield strength of the Al–Si–Mg alloy is not as sensitive to the ageing temperature as the Al–Si–Cu and Al–Si–Cu–Mg alloys are. The ageing response of the Al–Si–Cu alloy is low and very slow. When 0.5 wt% Mg is added the ageing response increases drastically and a peak yield strength of 380 MPa is obtained after 20 h of ageing at 170 °C for the finest microstructure, but the elongation to fracture is decreased to 3%. The elongation to fracture decreases with ageing time in the underaged condition as the yield strength increases for all three alloys. A recovery in elongation to fracture of the Al–Si–Cu–Mg alloy on overageing is obtained for the finest microstructure, while the elongation remains low for the coarser microstructures. The quality index, Q = YS + Kɛ, can be used to compare the quality of different Al–Si–Mg alloys. This is not true for Al–Si–Cu–Mg alloys, as K depends on the alloy composition. Overageing of the Al–Si–Mg alloy results in a decrease in quality compared to the underaged condition.