Dynamic Model for Metal Cleanness Evaluation by Melting in a Cold Crucible
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are asking for increasingly clean alloys to avoid mechanical failure of components due to microstructural defects caused by oxide inclusions. Work has shown that by concentrating inclusions on the surface of a sample button by melting, either by cold crucible levitation melting or by other clean melting techniques, any small population of large, detrimental inclusions can be observed and analyzed far more effectively than using more traditional metallographic examination methods.[1] Cold crucible melting may also be used for element separation and concentration, for instance, in treating the nuclear fusion products generated after reprocessing of spent fuel.[2] Growing demands on metal cleanness and impurity control by size require a rapid analysis technique to determine the content of impurities in samples. Melting of small samples in the presence of an electromagnetic field can help to concentrate inclusions in specific positions on the surface. Barnard et al.[1] demonstrated experimentally that melting in a cold crucible with a high-frequency AC V. BOJAREVICS, Reader, and K. PERICLEOUS, Professor, are with the Centre for Numerical Modelling and Process Analysis, University of Greenwich, Park Row, London SE10 9LS, United Kingdom. Contact e-mail: [email protected] R. BROOKS, Group Leader of Advanced Engineered Materials, is with the National Physical Laboratory, Teddington, Middlesex TW11 0LW, United Kingdom. This article is based on a presentation given at the International Symposium on Liquid Metal Processing and Casting (LMPC 2007), which occurred in September 2007 in Nancy, France. Article published online March 4, 2009. 328—VOLUME 40B, JUNE 2009
field indeed brings particles to selected locations on the surface of consequently solidified metallic sample. Taniguchi and Kikuchi[3] obtained material with a nonconducting additive collected near the surface of a cylindrical shaft and proposed a theory for the particle concentration variation in the presence of the imposed electromagnetic field and steady melt flow. Toh et al.[4,5] repeated the cold crucible experiments and attempted to predict numerically the particle paths in a computed steady flow. They used the electromagnetic force expression derived by Leenov and Kolin[6] for the forces acting on particles of varying electrical conductivity in constant crossed electric and magnetic fields. Experimental observations of the particles of various shapes and electrical conductivities in the liquid metal carrying electrical current are described in Reference 7. An expression similar to Leenov and Kolin’s[6] was derived, but for the conditions of gradient magnetic field, which results in the increase of the effective force on the particles.[7] The process of sample melting in a cold crucible is dynamic, involving the melting stage, then quasi-stationary particle separation, and finally the solidification in the crucible. The final distribution of the initially uniformly distributed particles of various sizes and properties needs to be assessed. The proposed modeling technique
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