The quench sensitivity of cast Al-7 wt pct Si-0.4 wt pct Mg alloy

The effect of quenching condition on the mechanical properties of an A356 (Al-7 wt pct Si-0.4 wt pct Mg) casting alloy has been studied using a combination of mechanical testing, differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). As the quench rate decreases from 250 °C/s to 0.5 °C/s, the ultimate tensile strength (UTS) and yield strength decrease by approximately 27 and 33 pct, respectively. The ductility also decreases with decreasing quench rate. It appears that with the peak-aged condition, both the UTS and yield strength are a logarithmic function of the quench rate,i.e., UTS orσ _y =A logR +B. The termA is a measure of quench sensitivity. For both UTS and yield strength of the peak-aged A356 alloy,A is approximately 32 to 33 MPa/log (°C/s). The peak-aged A356 alloy is more quench sensitive than the aluminum alloy 6063. For 6063,A is approximately 10 MPa/log (°C/s). The higher quench sensitivity of A356 is probably due to the high level of excess Si. A lower quench rate results in a lower level of solute supersaturation in the α-Al matrix and a decreased amount of excess Si in the matrix after quenching. Both of these mechanisms play important roles in causing the decrease in the strength of the peak-aged A356 with decreasing the quench rate.