Inverse Segregation in Unidirectionally Solidified Al–Cu Alloy Ingots
Transactions of the Japan Institute of Metals
Several experiments were carried out to study the formation mechanism of inverse segregation in Al-Cu alloy ingots. Al-4 mass%Cu alloy ingots solidified horizontally in a stainless steel mold showed only a columnar zone and inverse segregation. However, an Al-14 mass%Cu alloy exhibited either only equiaxed crystals or a columnar-equiaxed transition structure depending on the cooling temperature. No clear inverse segregation occurred in these ingots. It is considered that equiaxed crystals moved
... axed crystals moved with the flow of the melt, and these crystals were irregularly distributed when it solidified. All ingots solidified from the bottom or from the molten surface in a stainless steel mold or a graphite mold showed inverse segregation. In particular, an equiaxed zone in the Al-14 mass%Cu alloy solidified from the molten surface in the graphite mold had an extremely low Cu concentration. On a chill face of ingots decanted during the unidirectional solidification, the Cu concentration was always higher than the mean concentration of the alloy. This suggests that the solute Cu was already concentrated near the chill face at the initial stage of solidification. It is considered that the solute was trapped among the crystals of the stable solid shell formed on the mold wall.