Heat transfer and thermo-mechanical stresses in a gravity casting die: Influence of process parameters

The paper is concerned with thermal measurements in a gravity casting die of simple geometry. A inverse method, based on the Laplace Transform of the heat conduction equation, is used to determine the heat flux exchanged between the die and the aluminium product. This estimation is compared to the flux that can be determined from temperature difference between two thermocouples. A ratio between 1.5 and 2 is reported. In a specific experiment, both estimated heat flux are compared with the heat exchanged with an ice cylinder. This validates the measurements and the inverse method as far as order of magnitude is concerned. Thermal measurements are used to evaluate the thermal stresses in the mould, using a simple analytical formulation. The influence of two process parameters, the initial temperature of the die and the coating on the moulding surface, is studied. Basically the more conductive the coating and the lower the initial temperature of the die, the higher the heat transfer and the higher the thermal stresses. Less obvious is the influence of ageing for the graphite based coating. A phenomenon is observed from the heat flux vs. time curves, that can be related to recalescence. It is discussed in terms of reactivity of the coating, heterogeneous nucleation on the coating and supercooling of the melt.