Development of an induction melting process for materials with low electrical conductivity or high melting point
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INTRODUCTION
I N D U C T I O N melting has been widely applied to refine or cast electrically conductive materials. However, in conventional induction melting, the chemical and mechanical contaminations from refractory or crucible are inevitable. Effective utilization of magnetic stress acting on the surface of molten metal enables one to confine the metal without any contact with the crucible or the induction coil. The cold crucible is a typical example of utilizing the magnetic stress and is composed of a segmented and water-cooled copper crucible which smooths the magnetic field distribution and transmits the magnetic energy of induction coil to the melt located in the crucible, t~] Since the electric current induced in the charge heats the charge itself due to joule heating and repels it from the surrounding conductive crucible, the charge is allowed to melt and solidify in the crucible without any contact with it. Furthermore, the magnetic induction mixing provides more intensive agitation of the melt in this method compared with electron beam or plasma melting methods, so that the alloying elements are more easily homogenized in the charge. Owning to these advantages, one can expect various applications of the cold crucible to material processes, such as the melting of chemically reactive metals and alloys, the melting of metals with a high melting point, and the treatment of radioactive materials. So far, the materials melted in the cold crucible, however, have been restricted to electrically conductive ones from the principle of induction heating. ~21In fact, it is difficult to melt some useful materials, such as semiconductors and oxides, e.g., Si and LiNbO3, which have poor electrical conductivities in solid state using the cold crucible. However, the electrical conductivities of these materials generally indicate an KAZUHIKO IWAI, Graduate Student, KENSUKE SASSA, Assistant Professor, and SHIGEO ASAI, Professor, are with the Department of Materials Processing Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01 Japan. RYOUJI TAMAOKI, formerly Undergraduate Student, Department of Materials Processing Engineering, Nagoya University, is a Member, Terminal Printer Group, Hirooka, Shiojiri-shi, Nagano 399-07, Japan. Manuscript submitted April 28, 1992. METALLURGICALTRANSACTIONSB
abrupt increase at the melting point, so that the joule heat generated by absorption of electric and magnetic energy in the molten part melts the residual solid part. [3] On the other hand, thermal plasma is well known as a tool to supply clean and high density energy with better controlability and has greater advantages compared with the electron and laser beams from the viewpoints of energy efficiency and power availability, t4J Especially, a nontransferred plasma cannot only melt metals with a high melting point but also dielectric materials. Therefore, developing a hybrid melting process which combines the cold crucible with the nontransferred plasma is considered very attractive when melting materials with a p
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