金銀パラジウム合金に及ぼす繰返し鋳造の影響

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Remelting of used casting Au-Ag-Pd alloy is very common in dental practice. In such a case, it has been recommended that new alloys should be added up to at least the same amount as the residual button of the previous casting to be used. Many investigators have studied the change in properties of this alloy system by recasting, but not all the reports have given similar tendencies. This may be due to different casting conditions in each case. When the alloy is practically cast, it is usually heated in a reducing atmosphere or with flux in air. Such conditions may be helpful for the durability of the alloy to remelting. But, in the present study, it was scheduled to find out the potential changes in properties of Au-Ag-Pd alloy by repeated melting and casting under a severe condition, with gas-air blowtorch adding neither flux nor any new alloy. Chemical analysis of the alloy revealed that approximately one third of Zn content was lost by evaporation during the first casting. No further decrease in Zn content was found by recasting from the second to even the tenth, indicating that the residual Zn remained in the alloy as oxide and could no longer be expected to act as a deoxidizer. Although no loss of Cu was recognized by chemical analysis, it was suggested through the observation of the surface appearance and EDMA that Cu-oxide markedly increased on the surfaces of the casting and button as the alloy was repeatedly cast. It is, therefore, supposed that the substantially effective Cu content in the alloy might be considerably decreased by recasting. Melting temperature range of the alloy tended to rise gradually as the alloy was repeatedly melted, because of the loss of Zn and Cu elements which were both useful for lowering the melting temperature of this alloy. However, the rise was only 3.8℃ for the liquidus temperature and 2.5℃ for the solidus even at the tenth remelting. On the other hand, the casting temperature might be more affected by the formation of oxides. The temperature optimum for casting judged from the appearance of the molten alloy was 20℃ higher than the liquidus temperature at the first melting. It began to increase from the third remelting and became about 40℃ higher at the fourth remelting, showing no further increase after that up to the tenth. The mechanical properties of the cast alloy were significantly changed by recasting. Hardness at room temperature increased little by little as the alloy was repeatedly cast. Marked increases in the hardness at 400 and 600℃ were recognized for the specimens after the seventh recasting. It may correspond to the change in transformation temperature of the alloy. Both tensile strength and elongation tended to decrease by recasting. Large deviations of the strength and elongation were often found in the specimens recast. On the fractured surface of those which showed markedly low elongation, a number of large porosities or inclusions were observed. The inclusions were found by EDMA to be cristobalite particles from the investment mold, or Cu-oxide and Zn-oxide with the fragment of asbestos sheet laid on the bottom of crucible. Porosities formed in the cast crown showed a tendency to decrease in number, size and volume fraction as casting was repeated up to three times. Then they increased rapidly. This tendency was especially marked at the occlusal subsurface layer. No significant differences were detected in the castabilities of the repeatedly cast alloys, so far as the meshes and wires were employed as the pattern.
九州歯科学会の論文
1984-12-25

金銀パラジウム合金に及ぼす繰返し鋳造の影響

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