Investigation on Sustainable Refractory Systems for Investment Casting Manufacturing of Titanium Alloys

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The light-weight titanium alloys are promising for high temperature application owing to their low density, high strength and excellent corrosion resistance. However, the difficulty of casting titanium alloys arise from their reactivity with nitrogen and oxygen even when in low concentrations in standard vacuum furnaces. Melting and casting needs to be done in furnaces with very high vacuum or inert gas to avoid dissolving oxygen and nitrogen in the molten titanium. In addition to gaseous elements and species, the major challenge for the investment casting foundry is the high reactivity between molten titanium and silica based ceramic moulds. It has been found that molten titanium alloys will react with silica, freeing oxygen which dissolves into solution and forms an embrittled surface layer upon cooling, commonly referred as alpha case. Increasingly inert refractory systems are being devised to accommodate these requirements and this study would bring understanding to a proposed cost effective route to minimizing alpha case formation. Traditionally the material selection is based on free energy of formation of the oxides from Ellingham diagrams. Although yttria appeared to be the best candidate for replacing silica as a face-coat material, limiting factors on the use of this oxide are that yttria sols are exceptionally unstable and very expensive. Pure oxides such as alumina and zirconia still have significant interaction with the alloy which need to be removed by chemical milling. The proposed investigation will lead to a sustainable non-silica based multi-mineralic ceramic system with reduction in metal/ceramic interaction for casting reactive alloy. In order to support this study, tools capable of handling multicomponent, multi-phase (i.e. multi mineral) systems will also be developed. These models allow the calculation of realistic activities of alloy components and multiple oxides. All the necessary information can be obtained from Gibbs energies, as used in the Calphad approach. While the Ellingham diagram covers formation of pure oxides, from pure elements and oxygen, the Calphad method will go beyond this and treat non-ideal systems. This information is invaluable when rationalising the reactivity of new slurry and shell systems and crucial in tailoring shell systems for specific alloys. Studying the colloidal properties of selected systems will give fundamental understanding on the stability of current and future slurry systems, which is crucial for the investment casting community.