Titanium investment casting is used in aerospace technology to cast large and complicated structures. The applicability in jewelry would give more freedom of design if the challenges of the alloy and the casting process (high affinity for oxygen and strong crucible and investment reactions) can be mastered. The aim of this study was to assess the suitability of selected commercial ceramic crucibles for the investment casting of Ti6Al4V and NiTi alloys for jewelry purposes. A commercial vacuum tilt casting machine and a centrifugal casting machine were employed to cast the alloys in cold copper molds or in refractory investment shells. The form filling of the shell and the crucible and investment reactions were assessed by microstructural investigation, chemical analysis and hardness profiles of the as-cast items as a function of shell temperature. The reaction of the alloys strongly depended on the alloy, the crucible material, the coating of the crucibles and the temperature of the ceramic shell. Optimum results were achieved with yttria and yttria-coated Al titanate crucibles for Ti6Al4V and NiTi alloys, respectively. Improvement of the materials and process costs are necessary to allow the cost-effective casting of jewelry made from titanium alloys.

Author: Dr. Ulrich E. Klotz