Insights into the dual effects of Ti on the grain refinement and mechanical properties of hypoeutectic Al–Si alloys

Hypoeutectic Al–Si alloys are becoming increasingly popular in automotive and aerospace engineering fields due to their excellent overall performance, and grain refinement is regarded as an important way to improve casting and mechanical properties. Titanium (Ti) is a basic element for grain refinement; thus, a certain amount of Ti is often included in Al–Si alloys. In the present work, the changes in the grain refinement, mechanical, and casting properties of Al–Si alloys with different Ti concentration levels under various grain refinement conditions were systematically investigated. The specific roles of Ti in the heterogeneous nucleation of α-Al grains were summarized, and the formation mechanism of Ti-rich zones in Al–Si alloys was revealed. Excess Ti concentration could not efficiently reduce the grain size of Al–Si alloys and eventually resulted in inferior mechanical and casting qualities; hence, the recommended Ti concentration level for the aluminum alloy grades of A356 and A357 is ≤ 0.1 wt%. Furthermore, an optimized technique for the grain refinement of hypoeutectic Al–Si alloys was presented. A small amount of an Al–TCB master alloy was introduced to achieve the best grain refinement and mechanical properties in a trace Ti environment. The addition of 0.5 wt% of the Al–TCB master alloy at the Ti concentration level of 0.06 wt% increased the ultimate tensile strength, elongation, and quality index of the Al–7Si–0.45Mg alloy to 328.8 ± 5.0 MPa, 14.4 % ± 0.6 %, and 970.7 ± 33.1 MPa, respectively.