Optimization of Induction Heating for Container-Less Melt Extraction from a Metallic Sheet
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THE extraction of metallic fibers from a melt belongs to the class of near-net-shape casting processes in which the final geometry is almost reached in one production step. A rapidly rotating wheel with edges machined into its perimeter draws filaments from the surface of a melt pool. This extraction is managed by a ‘‘just touching’’ contact between the edges running azimuthally with respect to the wheel and the melt surface. Details of the fiber formation process are described in Reference 2. Two techniques have been established on an industrial scale, which differ fundamentally in terms of the surface area. The interest is not with the extraction from the relatively large surface out of a crucible; rather, it is with pulling the fibers from a strictly confined melt volume, e.g., from the molten tip of a thin rod. Because the melt puddle is at the lower end of the rod in most cases, this variant of making fibers is called pendant drop melt extraction (PDME).[3–5] PDME is the sole method for the extraction of ultrathin fibers, which is explained with the higher maximum extraction speed. Under the assumption of a continuously JONG-SOO PARK, formerly Research Fellow, Department of Magnetohydrodynamics, Forschungszentrum Dresden-Rossendorf, 01314 Dresden, Germany, is now Researcher, POSCO Technical Research Laboratories, Process Solutions Research Group, Pohang, Korea. JOSEF PAL, Senior Research Scientist, ANDREAS CRAMER, Group Leader, and GUNTER GERBETH, Department Head, are with the Department of Magnetohydrodynamics, Forschungszentrum Dresden-Rossendorf. Contact e-mail: [email protected]. Manuscript submitted March 30, 2010. Article published online July 15, 2010. 1074—VOLUME 41B, OCTOBER 2010
running process* and provided a constant material flux *Albeit this supposition often does not hold,[2] it should be allowed here for the contemplation of a tendency.
rate from the melt to the wheel, the fiber cross-section is related inversely to the linear velocity at the circumference of the wheel. The large surface of a crucible is prone to a variety of instabilities, which limit the maximum possible velocity. In PDME, on the contrary, the capillary force becomes strong within the small drop and, thus, keeps the surface in shape at significantly higher velocities. Also in the case of a high melting point or that a highly reactive material demands containerless processing, only PDME is available as an established technique. Besides these advantages, PDME is mitigated by its low productivity. The capability of manufacturing during extraction out of a crucible is limited only by the size of that crucible and mechanical construction. Often, several edges are machined in the wheel, whereas the small molten tip of a rod allows only one extracting edge. Yet, the single rod is problematic with respect to heating its tip in contact with the extraction wheel. No matter whether acetylene-oxygen torches, laser beams, or electron beams are employed to melt the tip of the rod, the demand for focusing heat sources renders PDME extremely expensiv
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