2012 Volume 84 Issue 2 Pages 81-88
Al-7%Si alloy was melted using 99.99%Al and 99.9999%Si, and specimens were prepared by adding phosphorus using a Cu-8%P alloy. Specimens were then cast into three casting molds with different cooling rates, and the relationship between the phosphorus content and cooling rate was clarified by measuring the eutectic silicon interlamellar spacing of each specimen. The mechanism by which phosphorus in the molten metal is partitioned to the α phase and silicon phase during solidification was considered by separating and quantitatively determining the phosphorus contents in the silicon phase and the α phase, and a parameter representing the eutectic structure was proposed in place of the modification/unmodification condition. The eutectic silicon interlamellar spacing is affected by the content of phosphorus in the silicon phase. The eutectic silicon interlamellar spacing is narrower (1―3μm) when the phosphorus content in the silicon phase is zero, and broader (4―12μm) when the phosphorus content is 3―5ppm. Partitioning of phosphorus in a molten metal to phosphorus (Psi) in the silicon phase and phosphorus (Pα) in the α phase is considered to occur in the following order : 1) solid solution or incorporation of phosphorus in the primary crystal α(Pα), 2) formation of AlP(Psi) for nucleation of the eutectic silicon, and 3) remaining eutectic α(Pα). The ratio eutectic silicon interlamellar spacing λ/dendrite arm spacing d2 was proposed as a parameter expressing the eutectic structure. The formula (λ/d2) ＜0.05 for the Al-7%Si-0%P alloy is considered an appropriate criterion for defining the improved structure.
