The movement of the concave casting surface during mushy-type solidification and its effect on the heat-transfer coeffic
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I.
INTRODUCTION
The resistance to heat transfer at the casting/mold interface decisively influences the solidification rate and the solidification time in metal molds. There are several factors which determine the thermal resistance and the heat flux at the interface, such as the imperfectness of the contact at the interface,[1] the coating material for the mold and their thicknesses, and the formation of an air gap[2] at the casting/mold interface due to deformation of the casting and mold during solidification. Thus, the overall heat transfer coefficient (h(t)) at the interface can be calculated if the thermal conductivity of the air gap and the coating material and its thickness are known. However, these parameters are difficult to determine, leading to other alternate methods to find h(t). The solidification rate of a casting in a metal mold depends on the air gap at the casting/mold interface, which varies with time and geometry.[3,4] Many researchers[2–5] have contributed to the understanding of the formation of the air gap and the heat flow at the convex casting interface to an outer mold. However, it was not possible to find any reports on gap formation between a concave casting interface and an inner mold or core. An air gap between a convex interface and an outer mold is usually formed as the mold expands and the casting contracts, which, on the other ZIN-HYOUNG LEE, Professor, is with the Department of Materials Science and Engineering, RASOM Korea Advanced Institute of Science and Technology, Taejon, 305–701, Korea. TAE-GYU KIM, formerly Postdoctoral Student, Department of Materials Science and Engineering, RASOM Korea Advanced Institute of Science and Technology, is Senior Researcher with SAMSUNG Research Center, Kiheung, 449-900, Korea. YUN-SUK CHOI, formerly Master’s Student, Department of Materials Science and Engineering, RASOM Korea Advanced Institute of Science and Technology, is Researcher with DAWOO Motor Co., Inchon, 403714, Korea. Manuscript submitted June 20, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS B
side, should cause any gap between the concave interface and the core to vanish and the contact pressure and the heattransfer coefficient to increase. However, this case was only valid for solid shell–forming solidification. For mushy-type solidification, the heat-transfer coefficient at the inner mold decreased unexpectedly during solidification, as shown in Figure 1.[11] The objective of the present study is to investigate the effect of solidification types on the air gap formation process at the interface between the concave casting surface and the core, during solidification of a hollow cylinder casting. II.
EXPERIMENTAL PROCEDURE
A set of molds for a hollow cylinder casting was made from a hot-working die steel (SKD61). Figure 2 shows the top view and the cross section of the casting and the molds, with the location of the inserted thermocouples. The displacement of the casting wall or the mold surface was measured using a gap sensor, as shown in Figure 3. A brass holder with a
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