The equilibrium solidification phase diagram of Al-11.91Si-3.5Cu-1.69Ni-0.75 Mg was simulated by using phase diagram calculation software Pandat

and heating DSC curve was used to determine the distribution of endothermic peak of the alloy.The first endothermic peak occurs at 502.7 ℃

corresponding to the Al2 Cu transition temperature based on analysis of the phase diagram.In order to prevent the over-burning from melting of Al2 Cu

the solid solution temperature should be lower 5

1

0℃than the beginning of the first endothermic peak.Based on the above results

a single step solution such as at495 ℃for 6h、10h、14h

was presented.The DSC results also reveal that the temperature of the first endothermic peak is increased to 522.1 ℃ after solution at 495 ℃for 2h

which is beneficial for aging treatment as a result of high solution temperature promoting high element solution.Based on the above results

the two step solution

such as at 495℃for 2hand at 515℃for 4h、8h、12h

was designed to compare with single step solid solution results.Artificial aging at 200 ℃ for all the sample after the completion of solution treatment was conducted to draw the aging curves

and tensile properties were measured under the peak-aged condition.The optimized heat treatment was presented as follows:two step solution at 495℃for 2hand at 515℃for 8hwith aging at 200℃for 4h

where the ultimate tensile strength reaches 367 MPa from 212 MPa and the elongation reaches 0.66% from 0.44% at as-cast condition.