Si Piston Alloys Research Articles

EPMA analysis of calcium-rich compounds in near eutectic Al–Si alloys

The effect of Ca impurity on solidification structure of near eutectic Al–Si piston alloy (ZL109), the origin of the presence of calcium (Ca) and the transformation of Ca compounds in the alloy were investigated. It has been found that many kinds of Ca compounds exist in commercial pure silicon, which are the main source of the Ca impurity in ZL109 alloy. It has also been shown that calcium can modify eutectic Si and enhance the formation of α-Al dendrites and fibrous eutectic Si in the alloy, even retaining the structural characteristics after addition of sodium. The abnormal phenomenon is due to the formation of Ca x Si y P z compounds which are more stable than AlP phases in the melt. However, the Ca level can be reduced from 216 ppm to 30 ppm and then the P modification efficiency can be recovered after a refinement treatment by C 2Cl 6 because of the reaction: Cl 2 + Ca → CaCl 2(s), and the transformation: Al + Mg + Ca x Si y P z (s) + Cl 2 → (Al 5, Mg 0.85 + Ca 0.15) P(s) + CaCl 2(s) + MgCl 2(s), the (Al 5, Mg 0.85, Ca 0.15) P compounds serving as heterogeneous nucleating sites of the primary Si phases.