Pure titanium casting into zirconia-modified magnesia-based investment molds

Objective. Molten titanium is highly reactive with common mold materials at elevated temperatures. The aim of this investigation was to improve the accuracy of pure titanium casting by adding unreactive zirconia into magnesia-based investment material. Methods. An automatic thermal expansion laser-recording machine (TEM-1000/Pantos, Nippon Co.) was used to measure thermal expansion of investment materials. An automatic argon-casting machine (Castmatic-S, Iwatani Co.) was used to cast pure titanium samples. A stereomicroscope was used (Nikon SM-2T, Japan) to measure marginal discrepancy on a metal die. A Vickers microhardness indenter (MXT-50, Matsuzawa Seiki Co.) determined the Vickers hardness (VH) of the titanium samples. Interfacial reactivity of the titanium was evaluated with an X-ray diffractometer (Rigaku D/max VIII, Tokyo, Japan). A dental X-ray machine was used to examine internal porosity of the castings. Data was analysed with paired t-test (p<0.05), ANOVA/Tukeyʹs HSD test (p<0.001) and Spearmanʹs correlation test (p<0.05). Results. Addition of 5.0–6.0 mass% of zirconia to a magnesia-based investment material significantly increased its thermal expansion value (p<0.05). ANOVA indicated significant marginal discrepancy differences within the zirconia-added group (p<0.001). There was a significant correlation between thermal expansion at 750 °C and marginal discrepancy (p<0.05). The addition of zirconia decreased interfacial reactivity and the VH of titanium. Significance. Under appropriately adjusted conditions, the addition of zirconia to magnesia-based mold materials may be used to produce high quality pure titanium castings.