A model of the interfacial heat-transfer coefficient for the aluminum gravity die-casting process

Interfacial heat-transfer coefficients were measured during the solidification of Al-Si alloys against coated die steel chills with varying chill temperature, coating thickness and coating type. Two principal resistances to heat transfer across the casting-chill interface were identified, namely, (1) the resistance to heat transfer of the coating itself and (2) the resistance to heat transfer of a layer of gas, (assumed to be air), trapped between the coating and casting surfaces by virtue of their roughness. These thermal resistances were evaluated by measurement of the coating thermal conductivity and determination of the thickness of the applied coatings and the thickness of the layer of air between the coating and casting surfaces. This produced a simple equation to predict the interfacial heat-transfer coefficient during the solidification of Al alloy die castings, which produced values that were found to agree well with the experimentally determined results. This equation was used to interpret the experimentally measured heat-transfer coefficients and to explain their variation with the different experimental conditions employed. A simple modification of the equation can also take into account the formation of an air gap, where the casting locally retreats away from the die surface, leading to a local reduction in the heat-transfer coefficient.