that the casting would experience in the less conductive, but more com- monly used, sand mold. Te mac- rostructure appeared very similar to the alloy cast in the conductive steel mold but, unlike the alloys cast in the conductive mold, the alloys were not completely cellular. Te solidification structure in
this alloy started as a course cel- lular structure and then turned into a dendritic structure very early in the solidification process. Te slower cooling rate of the more insulat- ing mold seemed to favor dendritic solidification. Still the narrow freez- ing range was narrow enough to sup- press the formation of microshrink- age porosity, which is ultimately the most important characteristic of this as-cast alloy.
Density & Mechanical Behavior
Te near-congruent copper- manganese alloys exhibit attractive mechanical properties for structural applications, including about twice the hardness (strength) of commercially pure copper (Table 1) while maintain- ing high ductility (22%) to comple- ment the increased castability. Te Cu-35Mn alloy with low
carbon (93 HV) compares favorably to other common cast copper alloys. Cast yellow brass, the closest analog commercial brass, containing 35% Zn and 1% each of lead and tin, exhibits 83 HV (Table 1). Since the relative contributions of lead and tin are small and at least partially offsetting, this difference directly reflects the more potent solid solution strengthening effect of manganese compared to zinc. In addition to yellow brass, the cast copper-manganese alloys compare favorably with bearing bronze C932 (83-7Sn-7Pb-3Zn), bismuth bronze C89833 (88-5Sn-2.2Bi-4Zn-1Ni) and leaded red brass C836 (85-5Sn-5Pb- 5Zn). Te higher solute content of the Cu-Mn solid solution gives higher hardness, even without the manganese carbides, compared to these alloys of notoriously low castability. A final alloy for comparison in Table 1 is the silicon brass C875 (82-4Si-14Zn), which has higher strength and hardness. Tis alloy is also noted for the absence of
Table 1. Vickers Hardness Measured for Copper-Manganese Alloys and Handbook Values for Other Cast Copper Alloys
Alloy Cu-35Mc
Cu-35Mn, carbide containing C-110, recast
C857 yellow brass C932 bearing bronze C836 red brass
C898 Bi-Sn bronze C875 silicon brass
HV* (kg/mm2 93 ± 4.1
111 ± 1.9 56 ± 1.6 83 73 65 65
131
*Commercial alloy values from Metals Handbook, converted from Brinell (500 kg) values using ASTM conversion table for catridge brass.
)
Pb, although it nevertheless has a wide freezing range. While the formation of the
carbides in the high carbon alloys was not expected, they increased the hardness of the alloy by 19%. Given the size and distribution of the carbides, this effect is most likely due to the higher hardness or composite effect of the carbide particles them- selves, similar to the effect of primary carbides in tool steels. No data have been found yet on the hardness of Mn7C3, but comparison with other transition metal carbides and obser- vations of relief in polishing suggest their greater hardness compared to the alloy matrix. Furthermore, the small amount of manganese that goes to carbide formation does not significantly reduce the high concen- tration of manganese in the matrix alloy, which is more than offset by the hardening (strengthening) effect of the carbides. Based on the hard- ness and tensile properties results and distinct absence of microporos- ity, the near-congruent cast copper- manganese alloys also are expected to perform well in fatigue compared to other cast copper alloys Near-congruent composition cop- per-manganese alloys were prepared by air melting and conventional cast- ing, and the resultant solidifiprimary conclusions of this study are: • A cellular solidification morphol- ogy, rare in conventional casting, was found to be attainable in
copper-manganese binary alloys having a range of compositions of approximately 3% Mn about the congruent point.
• No microporosity was observed due to the narrow freezing range and cellular solidification mor- phology.
• Manganese carbide particles with two different morphologies (angu- lar and globular) form in the alloy from melts prepared in contact with carbon. The formation of these particles was effectively con- trolled through changes in crucible chemistry, temperature, and melt- ing time, enabling clean copper- manganese alloys to be prepared by air melting, with the presence of little secondary phases.
• Through comparison to other alloys, manganese was found to be a potent solid solution strengthener while maintaining high ductility, leading to favorable mechanical properties, with a further increase in hardness (strength) being pos- sible through the formation of the carbide phase.
• The combination of high castabil- ity and good mechanical properties of this system leads to an alloy that may be well suited for an array of applications.
Tis article is based on the presenta- tion “Structure and Properties of Cast Near-Congruent Copper-Manganese Alloys,” which was delivered at the 119th Metalcasting Congress in April.
June 2015 MODERN CASTING | 35
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