Mechanism of boron removal of primary Si phases and morphology evolution of impurity phases during slow cooling solidification refining of Al-30wt.%Si alloy with Zr additions

Abstract

This work combines the control of kinetic factors and the addition of a small amount of Zr element in Al-Si solvent refining process to obtain a super high B removal rate. Impurity removal of primary Si phases in Al-30wt.%Si alloy is evaluated by solvent refining with slow cooling rates of 0.4, 2, 10, and 50 mK ⋅s−1, and Zr addition contents of 0, 330, 1000, and 3000 ppmw, respectively. With a 3000 ppmw Zr addition, the removal fraction of B in the primary Si is all over 93% and the best value reaches 99.36%. The P content is more sensitive to the cooling rate but does not change significantly with the increase of Zr. By morphology observation and ICP-OES analysis of the primary Si phases, the interaction between Zr, B, and Si, and the morphology evolution of ZrB2 and ZrSi2 phases are revealed. Based on theoretical calculations, the formation temperature of ZrB2 in the Al-Si melt is evaluated and a competitive reaction mechanism is discovered. Due to the excessive Zr addition, Zr first reacts with B and forms ZrB2 before the primary Si phase growth, which causes a decrease of the B content in the melt and contributes to the high B removal fraction. The remaining Zr in the melt combines with Si to form ZrSi2 phases. A model is established to describe the different trapping/ engulfing/ pushing situations of ZrB2 and ZrSi2 phases, which determine the final impurity content in the refined primary Si phases.