Relationship of Ca, B, and AlP in Al–12.6Si alloy

P modification has been widely used in Al–Si piston industry, but trace of Ca element has great influence on the P modification efficiency. In this work, it is found that primary Si can be heterogeneously nucleated by AlP in near eutectic Al–12.6Si alloy, but Ca element may destroy the P modification efficiency, whereas the addition of B can recover the P modification efficiency in near eutectic Al–12.6Si alloy with high Ca containing. The microstructure transformation was related to the reaction of Ca, B, and AlP. According to the thermodynamic calculation, Ca may react with AlP and form Ca₃P₂ compound in Al–Si alloy, whereas, when B was added into the melt, AlP could be reformed. The reaction of Ca, B, and AlP can be shown as follows: $ 2{\text{AlP}} + 3{\text{Ca}} \Rightarrow {\text{Ca}}_{3} {\text{P}}{}_{2}^{{}} + 2{\text{Al}} $ ; $ {\text{Ca}}_{3} {\text{P}}_{2} + 18{\text{B}} + 2{\text{Al}} \Rightarrow 3{\text{CaB}}_{6} + 2{\text{AlP}} $ . In addition, with B added into the Al–12.6Si alloy with Ca and P addition, the mechanical properties were improved compared with single Ca and/or P addition.