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

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• This work combines controlling of kinetic factors and addition of a small amount of Zr element in Al-Si solvent refining process to obtain a super high B removal rate. • The interaction between Zr, B, and Si, and the morphology evolution of ZrB 2 and ZrSi 2 phases and the engulfing behavior of primary Si on impurity phases are investigated. • Based on theoretical calculation, the formation temperatures of ZrB 2 and ZrSi 2 in the Al-Si melt are evaluated and a competitive reaction mechanism has been discovered. 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 ZrB 2 and ZrSi 2 phases are revealed. Based on theoretical calculations, the formation temperature of ZrB 2 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 ZrB 2 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 ZrSi 2 phases. A model is established to describe the different trapping/ engulfing/ pushing situations of ZrB 2 and ZrSi 2 phases, which determine the final impurity content in the refined primary Si phases.