Role of heat treatment in the improvement of mechanical properties of a high-quality Al-11Si-1.5Cu-0.3Mg casting alloy

The purpose of this study is to prepare a high-quality Al-11Si-1.5Cu-0.3Mg casting alloy with a good combination of strength and ductility. The microstructures of as-cast alloy were tailored by employing combined additives of Al-3wt.% B refiner, Al-10wt.%Sr modifier and trace addition of La element. By using OM and SEM, the characteristics of the morphologies of eutectic Si particles and the fracture surfaces of the alloys after solution treatment and aging treatment were measured. The mechanical properties of the alloys after single-step or two-step solution treatment were investigated by tensile testing, and the quality of casting samples were evaluated by quality index, Q. The results indicate that the alloy with substantially modified microstructures displays an improvement in mechanical properties of 270 MPa in ultimate tensile strength and 6% in elongation. After an optimized two-step solution treatment of 490 degrees C/4h + 505 degrees C/4h, the ultimate tensile strength and elongation can reach 346 MPa and 10%, respectively. Under the aging condition, the elongation maintains a relative high value of 5% together with the strength of about 400 MPa, which is the outstanding combination of strength and ductility.