Calculation of physical-chemical interaction of titanium alloys with the materials of the mold

Thermochemical inertness of molds is one of the principal conditions for obtaining high-quality castings in titanium alloys. Refractory and binder materials used in foundry molding actively interact with titanium. Therefore, thermochemical resistance of molds can be improved by selecting molding and binder materials the most inert to titanium and by development of optimum technological process. To assess the possibility of interaction of titanium when poured into molds with molding materials of Al2O3, ZrO2, Y2O3, and a binder based on SiO2 the main possible reactions are discussed and the changes of isobaric-isothermal potential (Gibbs energy) are calculated. The course of reactions was considered at 1700 °C (melting point 1668 °C), with the condition that titanium is in liquid state. Based on the calculated values of ΔZ (ΔG) it is shown that the reaction of reduction of yttrium, zirconium and aluminum oxides by titanium is thermodynamically impossible. In Russian conditions it is not economic to use yttrium and zirconium oxides as molding materials to produce shell molds for casting titanium alloys. The appearance of α layer on titanium alloy castings when using a SiO2 based binder is caused, as shown by thermodynamic calculations, by reactions forming aluminum silicates that worsen chemical resistance and refractoriness of the mold. To obtain high-quality castings of titanium alloys it is economically feasible to use shell molds based on fused alumina with the mandatory replacement of SiO2 based binders with aluminia sols.