Effects of Mg and Cu Content on Quench Sensitivity of Al–Si–Mg Alloy

Abstract

Effects of Mg and Cu content on quench sensitivity of Al–Si–Mg-based cast alloys were investigated. Jominy end-quench test was performed to evaluate the quench sensitivity of Al–Si–Mg and Al–Si–Mg–Cu-based alloys. It was observed that the quench sensitivity of Al–Si–Mg alloy rises with the increase in Mg content. In general, quench factor values rise with the increase in distance away from the chill end of Jominy end-quench test bar. This is because the precipitation rate of strengthening particles increases with the increase in the concentration of quenched-in vacancy. The concentration of quenched-in vacancy declines with the decrease in cooling rate away from the chill end of test bar. The maximum quench factor value of Al–Si–0.57Mg alloy is 144.8, whereas those of Al–Si–0.35Mg and Al–Si–0.45Mg alloys are 38.5 and 34.8, respectively. This is in contrast to the quench factor values obtained in Al–Si–Mg alloy containing 0.8 %Cu. Results show that an addition of 0.8 % Cu to Al–Si–Mg alloy significantly reduces the quench sensitivity of the alloy. Quench factor values of Al–Si–Mg–0.8Cu alloy are in the range of 1–16. No significant reduction in the quench factor is observed on addition of 0.23 and 0.50 wt% of Cu to Al–Si–Mg alloy. Simulation results show that Guinier–Preston (GP) zones form in the Al matrix when the amount of Cu in the Al–Si–Mg alloy exceeds 0.57 wt% during the natural aging subsequent to quenching. These GP zones are well-known heterogeneous sites for nucleation of precipitates and are responsible for reducing quench sensitivity of the Al–Si–Mg–0.8Cu alloy.