| Attempts have been made to evaluate the relative stability of surface layer reactions of cast metal with mold material in commercially pure titanium and Ti-6Al-4V alloy castings. For melting of titanium alloys, plasma arc melting process was used and the extent of the surface reaction of castings was determined by optical metallography and microhardness profiles. The mold materials being examined included ZrO₂, Al₂O₃, CaO stabilized ZrO₂, CaZrO₃ and CaO. The machined graphite mold was used for comparison. Moreover, it was to investigate the effect of the binder on metal-mold reaction in case of Al₂O₃ mold. The results showed that titanium castings with ZrO₂, Al₂O₃ mold had clear reaction layer(α-case), whereas negligible reaction with titanium occurred in CaO stabilized ZrO₂, CaO, CaZrO₃ and graphite mold. The grading of thermal stability of mold material does not follow the free energy data of the formation of these oxide. CaZrO₃ is regarded as a promising mold material for titanium investment castings. In additions, metal-mold reaction of titanium castings was affected sufficiently by the binder as well as the mold material. |
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