Florian Bulling* a, Ulrich E. Klotz a, Lisa Freitag b, Christos G. Anezirisb

a fem Research Institute for Precious Metals and Metals Chemistry, Katharinenstrasse 17, Schwaebisch Gmuend, Germany

b Institute of Ceramics, Refractories and Composite Materials, TU Bergakademie Freiberg, Agricolastraße 17, 09599 Freiberg, Germany

* Corresponding author:

E-Mail: bulling@fem-online.de

Tel.: 07171 1006-722

Fax: 07171 1006-900

Abstract

The investment casting of titanium and its alloys requires a high resistance of the crucibles and shells in terms of temperature and reactivity. Hence, mainly cold copper crucibles in vacuum arc remelting (VAR) or induction skull melting (ISM) plants are used in current investment casting processes. However, such processes suffer from a low energy efficiency, caused by the fact that a simultaneous cooling and heating takes place. The availability of ceramic crucibles that offer sufficient resistance to the titanium melt enables vacuum induction melting (VIM), which is more energy efficient and provides higher homogeneity of the melt temperature compared to cold crucible techniques. CaZrO3 is an artificial type of ceramic that is synthesized from mixtures of CaO and ZrO2. It shows a high corrosion resistance, even under very high temperatures up to 1800°C, which corresponds to the titanium casting temperatures. Crucibles and shell moulds of CaZrO3 were successfully produced and used in subsequent casting trials, which were conducted with a tilt casting machine and an electric arc casting machine. Metallographic examinations, hardness measurements and chemical analyses of the casting parts followed to evaluate the quality of the casting parts. Careful control of the melt temperature and melting duration is required to avoid crucible reactions that otherwise compromise the oxygen and zirconium content of the melt. The results in casting Ti-6Al-4V with CaZrO3 crucibles and shells moulds show a good quality of the casting parts. The impurity limits of oxygen and zirconium according to ASTM B367-09 for cast titanium parts could be met. Furthermore, the formation of the well-known alpha case could be avoided under appropriate casting conditions in terms of temperature, melting duration and melting machine capacity.