Heat transfer and microstructure during the early stages of metal solidification
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INTRODUCTION
THE advent of near-net-shape casting for the commercial production of thin metal slabs and worldwide efforts directed at casting thin strip have elevated the importance of cast microstructure. Elimination of successive hot-rolling operations in the production of strip requires that the microstructure of near-net-shape cast material yields mechanical properties that are comparable, or superior to, conventionally processed material. Given this challenge, factors influencing cast microstructure are being scrutinized to elicit knowledge that will aid in the design of new near-net-shape casting machines. Of central importance is the magnitude of heat transfer during the early stages of solidification because of its profound influence on cast structure. Lessons from conventional continuous casting all point to the need for control of heat transfer during solidification to prevent crack formation, to promote uniform shell growth, and to control cast structure. Considerable strides have been made over a period of three decades on mold design and operation and mold powder development for the continuous casting of steel slabs, which have led to better control of heat transfer and improved quality of the cast product. Although this knowledge base is invaluable in the design of new casting processes, many near-net-shape casting operations under development for the production of strip are considerably different from conventional continuous casting, and in most cases, molten metal is C.A. MUOJEKWU, Graduate Student, I.V. SAMARASEKERA, Professor, and J.K. BRIMACOMBE, Director and Alcan Chair in Materials Process Engineering, are with the Centre for Metallurgical Process Engineering, The University of British Columbia, Vancouver, BC, Canada V6T 1Z4. Manuscript submitted May 19, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS B
allowed to solidify directly on a substrate without a lubricant; under these circumstances, control is likely to be more difficult. Common to all casting processes is the fact that overall heat flow is affected by a series of thermal resistances, shown in Figure 1, of which the interfacial resistance between the mold and the newly forming shell, which is primarily a gap, is generally the largest. In the absence of a lubricant such as a mold flux, the surface roughness of the substrate, coatings, and superheat of the metal are likely to have a strong influence on the interfacial resistance and, consequently, on heat extraction and cast microstructure. The present study focuses on transient heat transfer in the early stages of solidification of an alloy on a substrate and the consequent evolution of microstructure, quantified in terms of the secondary dendrite arm spacing (SDAS). The primary objective was to identify the effect of process variables, such as mold surface roughness, mold material, metal superheat, alloy composition, and lubricant on heat transfer and microstructure. This knowledge, embodied in a mathematical model of solidification, serves as a powerful tool in the development
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