Investment casting of titanium alloys with calcium zirconate moulds and crucibles

Titanium alloys are very difficult to process via investment casting due to their high melting temperature and reactivity. The objective of the present work was to evaluate new shell mould materials for the suitability for titanium casting in comparison to conventional materials. A new corrosion-resistant material for crucibles and shell moulds based on calcium zirconate (CaZrO3) is proposed with the aim of improving the casting quality of titanium alloys. Tilt casting trials into CaZrO3 shell moulds using three different crucible materials were conducted. The results were compared to a commercial shell system containing silica. The reactivity with the titanium melt was extensively studied as a function of melt temperature, part dimensions and shell mould temperature by hardness profiles and oxygen measurements. The reaction of the CaZrO3 shell was very low, indicated by small oxygen contents and thereby low surface hardness. The formation of an α-case was avoided by choosing appropriate casting parameters.