Refinement effect of two rare earth borides in an Al-7Si-4Cu alloy: A comparative study

In this paper, refinement performance of Al-La-B and Al-Ce-B refiners in Al-7Si-4Cu alloy and their effects on alloy defects and tensile properties were investigated. Due to the orientation relationships between rare earth borides and Al, CeB6 and LaB6 can act as the nucleation substrate of α-Al and refine Al-Si-Cu casting alloy effectively. Al-Ce-B refiner showed better refining efficacy than Al-La-B refiner because of the lower lattice mismatch and less angel difference of the certain orientation relationship between CeB6 and Al, demonstrated by TEM analysis. Tensile properties of Al-7Si-4Cu alloy were prominently improved from 204.4 ± 5.9 MPa to 250.6 ± 6.7 MPa in as-cast state and 335.7 ± 6.5 MPa to 386.0 ± 7.0 MPa in heat treatment state owing to the better refined microstructures and much fewer shrinkage defects. However, when the addition level of the refiners was higher than 1.0 wt%, the amount of gas pores in the alloy significantly increased. Hence, the best refiner addition level in this alloy is considered to be 1.0 wt%.