The development and experimental validation of a numerical model of an induction skull melting furnace
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The Development and Experimental Validation of a Numerical Model of an Induction Skull Melting Furnace V. BOJAREVICS, R.A. HARDING, K. PERICLEOUS, and M. WICKINS Induction skull melting (ISM) is a widely used process for melting certain alloys that are very reactive in the molten condition, such as those based on Ti, TiAl, and Zr, prior to casting components such as turbine blades, engine valves, turbocharger rotors, and medical prostheses. A major research project has been undertaken with the specific target of developing improved techniques for casting TiAl components. The aims include increasing the superheat in the molten metal to allow thin section components to be cast, improving the quality of the cast components and increasing the energy efficiency of the process. As part of this, the University of Greenwich (United Kingdom) has developed a dynamic, spectral-method-based computer model of the ISM process in close collaboration with the University of Birmingham (United Kingdom), where extensive melting trials have been undertaken. This article describes in detail the numerical model that encompasses the coupled influences of turbulent flow, heat transfer with phase change, and AC and DC magneto-hydrodynamics (MHD) in a time-varying liquid metal envelope. Associated experimental measurements on Al, Ni, and TiAl alloys have been used to obtain data to validate the model. Measured data include the true root-meansquare (RMS) current applied to the induction coil, the heat transfer from the molten metal to the crucible cooling water, and the shape of the semi-levitated molten metal. Examples are given of the use of the model in optimizing the design of ISM furnaces by investigating the effects of geometric and operational parameter changes.
I. INTRODUCTION
AS the result of an extensive worldwide research effort to develop gamma-titanium-aluminide (-TiAl) alloys for high-performance components, the small-scale production of certain components such as turbine blades, exhaust valves, and turbocharger rotors has started.[1,2] However, there are various technological and economic barriers to more widespread production and there is a particular need to develop robust foundry techniques suitable for the high-volume production of (near)-net-shape components. The induction skull melting (ISM) process is currently the most effective means of melting these alloys, which are then usually investment cast into ceramic shell molds. Melting is carried out in a water-cooled copper crucible with the power supplied by a high-intensity induction field (Figure 1). In early furnace designs, the first metal to melt immediately resolidified on the inner wall of the crucible to form a “skull,” which acted as a protective container for the remainder of the melt. However, the energy efficiency of such furnaces was poor and the superheat was typically 10 °C to 20 °C when melting Ti alloys. The low superheat made it difficult to fill thin section castings unless rapid pouring was used, but this led to various
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