Grindability of Cast Ti-Hf Alloys

Objective: As part of our systematic studies characterizing the properties of titanium alloys, we investigated the grindability of a series of Ti-Hf alloys. Methods: Buttons of titanium and titanium alloys (10, 20, 30, and 40 mass% Hf) were made using an argon-arc melting furnace. Wax patterns (3.5 mm x 8.5 mm x 30.5 mm) were invested in a magnesia investment material (Selevest CB, Selec). Each ingot was cast into the mold using a dental casting machine (Ticast Super R, Selec) at a mold temperature of 200°C. After casting, the molds were kept at room temperature. Prior to testing, the hardened surface layers (250 mm) were removed. Three specimens were made for each metal. The specimens were ground using a carborundum wheel (#4, Shofu) on an electric dental handpiece (LM-I, GC) at five different speeds (500, 750, 1000, 1250, or 1500 m•min^-1) with 0.98 N. Grindability was evaluated as volume of metal removed during one minute of grinding (grinding rate) and volume ratio of metal removed compared to wheel material lost (grinding ratio). The test was performed twice for each specimen and grinding speed. The bulk hardness of the specimens was measured. Data were analyzed using ANOVA followed by Scheffé's test (a = 0.05). Results: There were no significant differences in the grinding rates and ratios between the titanium and the alloys for all speeds (p > 0.05). The hardness of the alloy increased as the concentration of hafnium increased. The values for all the alloys were significantly higher (p < 0.05) than that for the titanium. Conclusion: The addition of hafnium contributed little to improving the grindability under the present conditions. However, hafnium can be used as a hardener without worsening the grindability. This study was partially funded by NIH/NIDCR grant DE11787.