Criterion for Constitutional Supercooling at Solid-Liquid Interface in Initial Transient Solidification with Varying Solute Content at Interface

Abstract

A criterion for appearance of the constitutional supercooling at the solid-liquid interface in the initial transient solidification is discussed theoretically and experimentally. First, a relation between the moving velocity of the interface and the solute content was analyzed to derive a moving velocity of the interface under a simple model of the linear change in the solute content at the interface. And, a criterion for appearance of the constitutional supercooling at the planar interface was analyzed to obtain the distance of the stable growth of the interface with the planar shape. Then, the solidification experiment was carried out with the Al-4 mass% Cu alloy: the aluminum alloy was inserted in the alumina tube of 0.4 to 2 mm in inner diameter and heated for 2:5$4 h under a temperature gradient to obtain the stationary interface, and then the alumina tube was cooled in the furnace for 0 to 45 s. After furnace cooling, the alumina tube was quenched in water to observe the interface. The interface with the planar shape appeared for 20$30 s after the start of furnace cooling, and then the columnar structure grew ahead of the interface. Then the solute content in the solid behind the interface was analyzed to show that the solute content in the specimen quenched after furnace cooling was different from that in the specimen quenched without furnace cooling. The experimental results were compared with the theoretical calculations to infer that the interface moved with the planar shape for a short time after the start of furnace cooling, and then the interface became unstable to form the columnar structure.

Influence of Casting Conditions on Initially Solidified Structures Formed during Permanent-Mold Casting of Zn–Al and Al–Cu Alloys

“…Normally, during the initial transient solidification, a solid phase with a solute content kC 0 appears and grows with increasing solute content at the planar interface, 28) where k is the partition coefficient at the solidliquid interface and assumed to be less than unity in the present discussion. When the molten alloy was poured on the chill block, however, the solute content decreased in the depth direction, as shown in Fig.…”