The Effects of Surface Roughness and Metal Temperature on the Heat-Transfer Coefficient at the Metal Mold Interface
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ansfer coefficient at the metal mold interface plays a very important role in predicting the solidification rate and grain structure of a metal. In solidification modeling, the heat-transfer coefficient represents an important input parameter. The accuracy of this input parameter affects the accuracy of the solidification modeling predictions. Many factors affect the value of a heat-transfer coefficient at the metal mold interface, and there is a need to investigate, and if possible, to quantify these factors.[1,2] Sully[3] reported that during casting solidification, heat flow reaches a maximum within a few seconds and then falls, sometimes at continuously changing rates. The latter is reportedly associated with the separation of the casting surface from the mold surface. This physical separation results in the formation of an air gap, which incidentally reduces the heat-transfer rate.[4–8] The mechanism governing heat transfer between a solidifying liquid metal against a metal mold/chill has been the subject of intense investigation by Ho and Pehlke[9,10] and Pehlke.[11] They suggested that heat transfer is initiated by conduction: (1) through contact between a BASIL COATES, Formerly Graduate Student, Department of Material Science and Engineering, University of Toronto, is with the Hentel Litecare Corporation, Brooklyn, NY 11224. STAVROS A. ARGYROPOULOS, Professor, is with the Department of Material Science and Engineering, University of Toronto, Toronto, ON, Canada M5S 3E4. Contact e-mail: [email protected] Manuscript submitted July 1, 2005. Article published online April 4, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS B
thin solidified skin and the peaks of the rough surface and (2) through the gas contained in the voids between the contact areas. Further, as solidification progresses, relative thermal expansion and contraction of the chill and casting can result in a reduction in the contact areas resulting in an increase in void. Eventually this leads to a complete separation and the formation of an air gap. The formation of an air gap has a dramatic effect on the reduction of heat-transfer coefficient values. The value of the heat-transfer coefficient can be influenced further by several factors, including the presence and thickness of surface coatings,[12–15] casting surface orientation,[16,9,17] casting size,[15,16,18] mold material,[8,16,9] liquid alloy surface tension,[19] mold preheat,[20,21] alloy superheat,[8,22,23] and chill surface roughness.[2,8,12,23–26] The fundamentals of the casting process dictate that some amount of superheat must be applied in order to effectively fill any mold cavity. The effects of superheat on heat-transfer coefficient values have been investigated extensively by numerous scientists.[8,18,22] Muojekwu et al.[8] conducted experiments with Al-Si alloys cast against water-cooled chills made from copper, brass, steel, and cast iron. The results presented showed that interfacial heat flux and heat-transfer coefficients increased with increased superheat. The increase in heat flow was attri
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