Mold filling and dimensional accuracy of titanium castings in a spinel-based investment

摘要

Objectives. Aim of the study was to analyze the mold filling capacity and the dimensional accuracy of a spinel-based investment for titanium castings.rnMethods. Expansion of the investment in dependence of the preheating temperature was measured in a dilatometer. The degree of transformation of MgO and Al_2O_3 to spinel (MgAl_2O_4) was evaluated by means of X-ray powder diffraction. Mold filling capacity was assessed by casting a grid and calculating the percentage of completed segments. Dimensional accuracy was analyzed by casting a hollow cylinder and measuring the difference between the inner diameter of the resin pattern and the resulting titanium casting. Results. Spinel formation starts at 819℃. Diffraction patterns prove the formation of spinel from MgO and Al_2O_3. The amount of spinel increases with increasing preheating temperature. The final expansion of the investment at the end of the preheating cycle at 450℃ shows a linear correlation to the maximum preheating temperature. The degree of mold filling is reciprocal to the preheating temperature. The dimensional accuracy shows a linear correlation to the amount of spinel. Best dimensional accuracy was obtained at about 900℃. After a preheating temperature of 884℃, as recommended by the manufacturer, the cast specimens showed a slightly lower inner diameter as compared to the resin patterns. Significance. The results suggest that with the spinel investment analyzed an excellent accuracy of titanium castings may be obtained.