EFFECTS OF Mg AND Cu CONTENT ON QUENCH SENSITIVITY OF Al-Si-Mg ALLOY

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.