Bi-metallic dies for rapid die casting

Abstract

Almost all casting processes involve placing the melt in a mould of lower thermal conductivity. As a result, high rates of energy extraction are effectively prevented. The reasons for this are all too apparent with materials of high thermal conductivity not having the necessary mechanical and thermal properties to withstand the rigours of the casting process. Moreover, slower solidification rates allow for minimal control and relatively simple design processes. The challenge is to design dies that allow for phenomenally higher rates of heat transfer. Dies are usually manufactured from hardened steel and incorporate cooling channels in order to increase the rate of energy extraction. Dies could plausibly be made from copper, which has a thermal conductivity some 16 times greater than conventional steels. However, copper cannot withstand the rigours of the casting process and in particular the abrasive action of the liquid melt albeit zinc or aluminium. This paper is concerned with the establishment of thermal models for copper-alloyed dies suitably protected with a thermally sprayed steel layer. Both steady state and transient thermal models are developed that are capable of predicting the time averaged and transient cyclic thermal behaviour of the new die designs. The models are based on the boundary element method. A perturbation approach is adopted for the prediction of transient die temperatures. Numerical experiments are performed and predicted temperatures are compared with thermocouple readings obtained from a copper–steel die on a commercial die casting machine.