Casting-chill interface heat transfer during solidification of an aluminum alloy
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I. INTRODUCTION
MATHEMATICAL modeling as a productive tool is being employed nowadays in a huge number of industries around the world.[1,2,3] The principal uses of this technique are in[3] developing new products or processes, improving the quality of parts or products as the knowledge of the process is gained, reduction in production costs as the variability of the process is decreased, etc. Foundries can achieve the greatest benefits by implementing a direct approach to modeling,[4–9] in order to gain knowledge about the interaction between the different phenomena that take place during the solidification of complex pieces. The microstructure which develops in a cast depends on its solidification rate,[10,11,12] and, since the solidification rate depends on the rate at which the heat flows at the castingmold interfaces,[13–18] it is possible to establish a direct link between the microstructure and the type and characteristics of the mold employed to produce the pieces. The heat-transfer conditions during solidification are complex, since the boundary conditions between the piece being cast and its mold change with time. It can be assumed that an intimate contact between the metal and mold exists while the former is liquid, but, as solidification proceeds, the metal shrinks, forming an air gap at the interface, which reduces the heat-transfer rate to the surrounding media.[13–18] The goal of this work is to study the heat-transfer conditions that develop during solidification of an aluminum alloy poured into an experimental rig that forces unidimensional heat flow and how they are affected by solidification kinetics. EULOGIO VELASCO, Process Engineer, SALVADOR VALTIERRA, Research and Development Manager, and JUAN FRANCISCO MOJICA, Technology Vice-President, are with Corporativo Nemak, S.A. de C.V., ´ ´ ´ 66000 Garcıa, N.L., Mexico. JESUS TALAMANTES, Graduate Student, ´ ´ ´ formerly with Facultad de Ingenierıa Mecanica y Electrica, Universidad ´ ´ Autonoma de Nuevo Leon, is with the Department of Mechanical Engineering, The University of Sheffield, Sheffield S1 3JD, United Kingdom. SIGIFREDO CANO, Process Engineer, formerly with Corporation Nemak, is with Mahle, Inc., Morristown, TN 37815. RAFAEL COLAS, Professor, ´ ´ ´ is with the Facultad de Ingernierıa Mecanica y Electrica, Universidad ´ ´ Autonoma de Nuevo Leon, 66451 Cd. Universitaria, N.L., Mexico. Manuscript submitted November 12, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS B
II. EXPERIMENTAL AND MODELING PROCEDURES Three different solidification tests were made with the aid of a computer-controlled instrumented rig, which consisted of a 51-mm-thick steel plate and a mast used to hold a series of type-K (chromel-alumel) thermocouples at different heights. These thermocouples were connected to solid-state devices, which convert the electromotive force generated by the couple into a linear scale ranging from 0 to 5 V. The output from these devices was then fed into an analog-digital board installed in a compatible computer. Figure 1 shows, schematically,
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