The effect of silica-containing binders on the titanium/face coat reaction
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TRODUCTION
THREE factors have made the production of quality titanium castings a difficult task: (1) titanium has a high melting point; (2) it has low fluidity at pouring temperatures; and (3) it is highly reactive with nearly all gasses, liquids, or solids at temperatures above 500 7C.[1] Although each factor presents processing difficulties, it is the third which most hinders the capability of titanium casting technology. Liquid titanium has been termed ‘‘the universal solvent,’’[2] because violent reactions with gasses, liquids, and solids result in contamination of the titanium. Furthermore, titanium has a high affinity for interstitials such as nitrogen, oxygen, and carbon, and only small concentrations of these interstitials are enough to deleteriously affect its ductility.[3,4] During solidification, a reaction occurs between a titanium casting and its shell mold. The result of this interaction is an oxygen-enriched surface layer known as the alpha case. If a casting is to be used in a critical application, then this layer must be removed, usually by chemical milling. Because this removal process is waste producing, expensive, and limits the complexity and detail that can be achieved in a casting, much effort has gone into trying to develop a mold material that does not react with titanium. To date, however, no such shell mold has been developed for producing titanium investment castings that are free of the alpha case. A review of past articles, patents, and reports is given elsewhere.[5] The review also gleans information that resulted from efforts to develop a refractory crucible for containing liquid titanium. This information can be of benefit provided one is not too hasty to eliminate a material from consideration as a mold material because it was shown not C. FRUEH, Graduate Student, and D.R. POIRIER, Professor, are with the Department of Materials Science and Engineering, University of Arizona, Tucson, AZ 85721 and M.C. MAGUIRE, Senior Member of Technical Staff, is with the Liquid Metal Processing Department, Sandia National Laboratories, Albuquerque, NM 87185. Manuscript submitted May 29, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
to function well as a crucible. For example, Chapin and Friske[6,7,8] investigated the use of various oxides, carbides, borides, a sulfide, carbon, and graphite as possible container materials for titanium. They found that none of the selected materials was inert to titanium. Titanium melted in a thoria crucible was found to contain an average of 5.44 pct thoria; if thoria had been tested as a mold material, however, it is possible that very little or no contamination would have been detected. Thermodynamics can also help the investigator identify materials which should be tried. An analysis typically involves a comparison of the free energy of formation of the candidate refractory to that of the corresponding titanium phase. This is a rather simplistic approach because, as indicated by Saha et al.,[9] it neglects solution effects. In addition, most commercial binder
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