avoiding microporosity associ- ated with dendritic solidifica- tion is effectively wider than that for the first appearance of dendritic features. Tis means the narrow freezing range, when cast in an iron mold, was narrow enough to limit the porosity formation. A graphi- cal estimate from the phase diagram. Assuming circular liquidus and solidus curves near the congruent point, indicate a freezing range of 0.0004K for compositional deviations of 0.1% manganese from the congruent composition. Planar solidification was
Fig. 4. Magnification shows both globular and angular mangenese carbide morphologies.
the compositional requirements to obtain this beneficial solidification structure alloys were cast at different compositions around the copper- manganese congruent point. Alloys were cast from 39% Mn to 27% Mn. Tese alloys like all the alloys in this study were cast into a steel mold. Completely cellular structures were obtained in the study for alloys ranging over about 3% Mn near the congruent composition. For greater deviations, solidification was initially cellular, transitioning to mildly dendritic toward the center of the ingots, with a shallower depth of transition for increasing deviation from the congruent composition. Although the cellular-to-dendritic transition depends on the particular cooling conditions, as well as the solutal undercooling, the composi- tional tolerance appears to be a prac- tical range for commercial produc- tion. Furthermore, no microporosity was observed in the mildly dendritic structures. Te composition tolerance for
34 | MODERN CASTING June 2015
originally a goal of this study, but none was observed in any of the castings. Even though an alloy cast near a congruent minimum as shallow as the one observed in the copper-man- ganese system should tolerate larger deviations in composi- tion before the onset of non- planar growth, it appears that the required freezing range at these solidification conditions
was still too small to be achieved in this study. It is also possible that other impurities may dominate the solutal undercooling effects. In this regard, carbon has both direct and indirect effects on composi- tion variations in the alloy. Carbon dissolved in solution directly increases the freezing range. It also reacts with manganese, form- ing manganese carbide and reduc- ing the manganese concentration of the liquid during solidification.
The Effects of Carbon In some of the first castings the
presence of carbides was found. Unlike the castings discussed in the first section these alloys were cast in a clay-graphite crucible instead of a SiC crucible. Tese alloys were shown to have a smaller degree of cellular solidifi- cation. Te presence of carbon has an effect on the degree of cellular solidification. Previous work on Cu-C and Cu-Mn-C systems showed the
very low solubility of carbon in pure copper and increased solubil- ity when manganese is present. At 1473K (1200C), the carbon solubil- ity increases from 0.0008% in pure copper to 0.25% in Cu-35%Mn. Tis increased solubility will increase the solutal undercooling and promote dendritic solidification. An indirect of carbon as an
impurity is the formation of the Mn7C3 carbide that formed in these alloys and decreases the amount of manganese in the melt when alloy solidifies. Tese carbides formed as two separate morphologies in these castings. Te first is a large angular morphology and the second is a globular morphology. Te different shape hints that the carbides formed at two different times in the solidifi- cation process. Te presence of these carbides was shown to be controlled in our casting process by changing the crucible material and lowering the superheat.
The Effect of Mold Material Because most castings from com-
mercial applications are not poured into conductive permanent mold, it was deemed important to understand the effect of different cooling rates
Fig. 5. The higher manganese alloy (Cu-38.6Mn) shows mostly dendritic growth.
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