A Study on the Cobalt-Chromium Alloy Casting Body by means of High Frequency Centrifugal Casting

Abstract

In recent years the cobalt-chromium alloys are widely used as cast denture bases and the author is concerned here with the composition analysis of present alloy and casting body, such mechanical properties as hardness, tensile strength, elongation together with its structure both microscopically and roentgenologically on two representative casting alloys; Mitsubishi dentallium (the former) of Japan and ticonium (the latter) of U. S. A. The flow of melting alloy was also observed with the use of a radioisotope 63Ni. As a result, much difference was found in the compositions of these two different alloys. The latter contained Mo, Al, Ti and Be which are related to such factors as uniformity, minuteness of crystal grains and melting temperature of alloys, while the former contained W instead of Mo. The former contained a greater quantity of C which plays an important part in mechanical properties as compared with the latter. On the other hand, less than half the quantity of Ni was found in the former as against the latter and Co content was slightly more in the former. As regards Cr, nearly the same quantity was observed in both alloys. Si content as a deoxidizer was more in the former than the latter, but this was reverse in the case of Mn. The composition of casting body contained a higher ratio of Ni, Cr, Co and W as compared with present alloy. As for hardness, the former showed a greater value than the latter but a smaller hardness value was measured in the area of sprue attachment in common with both alloys. In the tensile strength and elongation tests, a slightly smaller value was measured in the former as against the latter. The structure images of present alloys and casting bodies were either solid solution structure or eutectic consisting of one kind of solid solution and another kind of solid solution, where the crystal grains were larger in the former and they were arranged in an irregular fashion. Casting body equipped with the reservoir was found to have somewhat larger crystal grains than non-reservoir casting body. In non-reservoir casting body, casting porosity and coarse image were concentratedly formed in the area of sprue attachment and within a casting body among the sprue attachment, scarcely being found elsewhere. In casting body with the reservoir, on the other hand, casting porosity and coarse image were not recognized, . they were formed in the reservoir concentratedly. The flow of melting alloy was uniform in casting body with the reservoir, while the flow was ununiform in non-reservoir casting body. Also in the case of one-piece cast denture base, casting porosity took place in the area of sprue attachment and its neighborhood without reservoir, hardness value in these areas being correspondingly smaller. Generally speaking, the tip of clasp showed a greater hardness value than in the base itself. It was impossible to determine any appreciable difference in respect of difficulty and ease in operation between the two alloys, when high frequency centrifugal casting was effected.