Boosting the grain refinement of commercial Al alloys by compound addition of Sc

For decades, Al-Ti-B has been widely used for refining commercial Al alloys. However, the addition of Al-Ti-B master alloys to recycled Al alloys suffers from Si -poisoning and cannot achieve desired refinement and thus final strength -ductility. In this study, a novel grain refiner made of Al-Ti-B-Sc has been developed and its application in refining recycled Al-Si-Cu alloys has been quantified using X-ray computed tomography (XCT). It has been found that the new Al-5Ti-1B-0.8Sc master alloy exceed the performance of Al-5Ti-1B in three aspects: (1) formation of Al3(Sc,Ti) heterogeneous nucleation particles other than Al3Ti; (2) dispersing TiB2 by forming TiB2/ Al3(Sc,Ti) clusters and increasing Al melt viscosity; (3) reducing the specific surface area of Al3Ti to improve its high -temperature stability. Surprisingly, the compound addition of Sc has been found to further reduce the grain and Si particle size due to accelerated kinetics of alpha-Al nucleation and diminished Si poisoning. Therefore, this study proposes not only a new master alloy but also a new grain refinement method, which can effectively increase the tensile strength and elongation of recycled Al alloys up to 433.2 +/- 10.5 MPa and 3.0 +/- 0.1 %, respectively.