Effects of substrate surface conditions on heat transfer and shell morphology in the solidification of a copper alloy
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I. INTRODUCTION
DIRECT casting of strips 1- to 5-mm thick for sheetmetal applications could substantially reduce the required hot rolling of thicker products and yield large savings in capital and operating costs. This implies that the as-cast surface quality of strips must be at least equal to that of hotrolled products; otherwise, surface conditioning may jeopardize the economic viability of the process. In spite of this stringent requirement, strip casting is, nevertheless, attracting considerable interest. Among the various technologies investigated, the one using two equal-diameter rolls has received the most attention, owing to its heat-extraction capacity and ability to cast in the targeted thickness range.[1] In this process, the heat flow is virtually unidirectional, and the contact time of the alloy with the rolls is in the order of 1 second. Mathematical modeling has shown that in this time scale, the thermal resistance at the mold-metal interface controls heat transfer and, thus, the productivity.[2,3] Considering that a high heat transfer and high surface quality are simultaneously desired, the surface condition of the rolls could then be one of the most important features of the process, because it has repercussions on both of these factors. Results of studies dealing with heat-transfer and surface-quality issues of mold-metal interactions are summarized in Table I, and it is observed they can vary to a fair extent. The comparison is complicated by the fact that the heat flux and surface condition of solidified alloys are not only affected by the surface finish of the mold, but also by other factors. The chemical composition of the liquid metal, the superheat, the wetting characteristics of the alloy with the chill, the applied pressure, the mold geometry, and the fluid flow all have a significant effect. Moreover, the evaluation of the surface condition of solidified alloys is sometimes carried out using different criteria: for example, the unevenness ratio,[4] crack index,[5] arithmetic average roughness,[6,7,8] and maximum profile depths[9] have been-utilized. DOMINIQUE BOUCHARD, FRANC¸OIS G. HAMEL, JEAN-PAUL NADEAU, and DANIEL SIMARD, are with the Industrial Materials Institute, National Research Council of Canada, Boucherville, PQ, Canada J4B 6Y4, and SIMON BELLEMARE, FABRICE DRENEAU, and DAVIDALEXANDRE TREMBLAY, are Undergraduate University Students. Manuscript submitted March 27, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS B
Other important studies have been conducted in this field but were focused on the effect of the mold surface conditions on the surface quality of solidified alloys[10–13] or the heat flow, [14–20] but not both. As stated earlier, in a process such as strip casting, the effect of the mold surface condition on these two aspects should not be treated separately. In this study, copper substrates with various surface conditions were immersed into a bath of a high-strength modified (HSM) copper alloy (UNS designation:C19400) for a controlled time period. The morphology of the
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