Heat Transfer Coefficient Measurements (HTC) at the Metal-Mould Interface in Permanent Mould Casting

Formerly based on empirical knowledge, casting is now inseparable from a scientific approach. Numerical simulation is commonly used to predict and understand the behaviour of alloys and moulds. Even if models used in software are accurate, one of the main difficulties in obtaining good results is the lack of databases. Among the needed data, the knowledge of interfacial thermal properties is one of the most significant in permanent moulding. Indeed, the cooling and solidification of the alloy are controlled by the interfacial behaviour with the mould and impact directly on the microstructure and on the formation of defects (shrinkage…). In die casting thermal conductivity at the interface is controlled by the use of different kinds and thicknesses of coatings. The fact that coatings are used in thin layers and the existence of bonding resistance increase the lack of accuracy of the data. So it appears necessary to be able to determine quickly and easily the HTC for different materials, thicknesses and process parameters. In industrial conditions it is difficult to control and reproduce the coatings. It is also important to be able to measure the HTC directly in the workshop. Many results concerning these problems are available in the literature, but very few able to quickly lead to reliable results. The experimental device which is proposed in the present study allows the determination of the HTC. The design of the software was done to allow the full automation of measurements and calculations in the experiment. Analytical calculation of the HTC is not sufficient to give relevant results due to the phase transformations, so an indirect numerical solution based on Beck’s method has been introduced with the development of a finite difference thermal solver. The three dimensional solver was designed to take into account the heat loss (radiation and convection) and the nonlinearity of thermal equations. Calculations performed from the results of experimental casting temperatures show good match and stability.