Synthesis, Characterization and Cold Workability of Cast Copper-Magnesium-Tin Alloys
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Cu-Pb alloys with low Sn content have been used for the last 100 years as the functional material in journal bearings. The mechanical resistance comes from the Cu-Sn matrix, while the Pb phase provides the higher tribological compatibility with iron. Considering that these alloys have served their purpose quite well during this time, research, and development of high strength triboalloys has received very little attention. However, the actual trends to reduce energy costs by raising machine efficiency and lowering friction and wear, as well as the concern about the toxicity of Pb, create demand for higher strength and environmentfriendly triboalloys.[1] Based on Rabinowicz’s[2] tribological compatibility studies and considering that either steel or cast iron are used for the crankshaft and other components that may come in contact with the journal bearing’s functional alloy, the possible elements that could replace Pb are In, AGUSTI´N EDUARDO BRAVO BE´NARD, DAVID MARTI´NEZ HERNA´NDEZ, ARMANDO ORTIZ PRADO, and RAFAEL SCHOUWENAARS FRANSSENS, Professors, are with the Departamento de Materiales y Manufactura, Facultad de Ingenierı´ a, Edificio O, Universidad Nacional Auto´noma de Me´xico, Avenida Universidad 3000, 04510 Mexico DF, Mexico. Contact e-mail: [email protected] JOSE´ GONZALO GONZA´LEZ REYES, Professor, is with the Instituto de Investigaciones en Materiales, Universidad Nacional Auto´noma de Me´xico, Circuito Exterior, Ciudad Universitaria, Coyoaca´n, 04510 Mexico DF, Mexico. Manuscript submitted August 28, 2012. Article published online September 17, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
Cd, Ag, Zr, Bi, Sn, and Mg. The first two are toxic, Ag and Zr are relatively expensive and Bi does not look to be a very promising solution.[3] Cu-Mg alloys are already used in similar applications,[4–8] which means that Mg may be another alternative to substitute Pb in leaded tin bronzes for improved tribological properties and environmental safety. The preferred method to manufacture Al-Sn alloys used for journal bearings is thin slab casting followed by cold rolling.[9,10] This process is relatively simple and cost-effective. Powder metallurgy is used to manufacture Cu-Pb journal bearings. However, the latter is more difficult to accomplish in Mg containing alloys due to this elements high oxygen affinity at high temperatures. This affinity also affects the casting process. For Mg castings, there has been an enormous amount of research to find an efficient melt protection alternative to SF6 which is highly contaminating.[11] The amount of Mg used in commercial Cu-Mg alloys is usually less than 1 pct in weight.[4–8] This results in a very small volume fraction of the second phase due to the solubility of Mg in Cu. In Cu-Sn alloys, the formability is limited for Sn contents above 10 pct.[12] Even though the Cu-Sn binary phase diagram has been well studied and the thermodynamic assessment recently done by Liu et al.[13] appears to be the most reliable,[14,15] slight differences still appear in the literature. The most rec
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