The effect of increased pressure on interfacial heat transfer in the aluminium gravity die casting process

Abstract

Contraction and distortion of a casting during cooling within a mould can force their respective surfaces together, with the associated increased interfacial pressure resulting in increased interfacial heat transfer. This problem has been examined for the case of gravity and low pressure die casting of an Al alloy, where an insulating coating is applied to the die cavity to assist filling of the mould. The degree of interfacial pressure was estimated to be, for a typical small die casting, at most about 21 MPa. Repeated applications of a compressive load showed that a freshly applied die coating became thinner and smoother, until a stable situation was reached after about ten applications. The interfacial heat transfer coefficient was estimated to be increased by about 20%, with an increase in the applied pressure by a factor of two, from 7 MPa to 14 MPa, and increased by about 40%, with an increase in the applied pressure by a factor of three, from 7 MPa to 21 MPa. The heat transfer mechanisms between the casting and the die surfaces were evaluated to produce a simple model of interfacial heat transfer which included conduction through the points of actual contact, in parallel with conduction through the interfacial gas between the points of actual contact, both mechanisms being in series with the heat transfer by conduction through the die coating. Evaluation of the model produced agreement with experimentally determined values of the interfacial heat transfer coefficient to within about 15%.