The effect of thermal contact resistance on the solidification process of a pure metal
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INTRODUCTION
THE solidification of materials is of particular interest in materials processing, metallurgy, purification of metals, solidification of castings and ingots, and various other solidification technologies. Recently, solidification has received more attention due to its major practical application to casting. The structures and properties of a casting are mainly dependent on the rate at which solidification occurs in the mold. Many researchersO.2.3] have demonstrated that the properties of a casting can be improved by increasing the rate of solidification. The desired increase in properties is often achieved by the use of permanent mold or chills to remove more rapidly the latent heat of fusion. If the permanent mold or chill is used as a mold, there exists a thermal contact resistance at the casting-mold interface due to the formation of an air gap. The air gap leads to a decrease of solidification rate, and as a result, the properties of the casting are reduced. The heat-transfer coefficient at the casting-mold interface is determined by the roughness of the mold surface, the kinds of coating material and their thickness, etc. If the heat-transfer coefficient is very small, the solidification time will be very long. In producing the fine cast metal, it is therefore important to understand the mechanism of heat transfer through the metal-mold interface. There are many works concerning the heat transfer at the interface between casting and mold. Sully t41 measured the heat-transfer coefficient with time at the casting-mold interface for the various molds. To examine the quantitative effect of an air gap upon the rate of solidification, Durham and Berry[5] determined the extent of this effect and also investigated the role which the degree of superheat has in solidification of a pure metal. Robertson and Fascetta[61 presented the approximate solution in which, knowing the solidified thickness vs time, the heat-transfer coefficient can be found by finding the first zero of a transcendental function. Ho and Pehlke tT,sl worked on the process of air gap formation and heat transfer through the gap for a cylindrical J. LEE, Professor, is with the Department of Mechanical Engineering, Yonsei University, Seoul 120-749, Korea. J.H. MOH, Assistant Professor, is with the Department of Mechanical Engineering, Wonkwang University, Jeon Buk 570-749, Korea. K.Y. HWANG, Senior Researcher, is with the Agency for Defense Development, Taejon 305-600, Korea. Manuscript submitted July 12, 1994. METALLURGICALAND MATERIALS TRANSACTIONS A
coordinate system. Nishida et aL t91 investigated the effect of the casting shape on the air-gap formation and the heattransfer mechanism through the gap. In modeling heat transfer across the casting-mold interface, the air gap is assumed to exist. Usually, the air gap width is either assumed to remain constant or is obtained from an experiment. Huang et al. t~~ described the variation and distribution of the air gap heat-transfer coefficient at the casting-mold interface in shaped casti
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