Tailored Magnetic Fields in the Melt Extraction of Metallic Filaments

Autor: Vladimir Galindo, Günter Stephani, Olaf S. Andersen, A. Cramer, Andris Bojarevics, Jānis Priede, Gunter Gerbeth, Yuri Gelfgat, Cris Kostmann
Přispěvatelé: Publica
Rok vydání: 2008
Předmět:
Zdroj: Metallurgical and Materials Transactions B 40(2009), 337-344
LMPC 2007 International Symposium on Liquid Metal Processing and Casting, 02.-05.09.2007, Nancy, FranceTailored magnetic fields in the melt extraction of metallic filaments, 305-311
LMPC 2007 International Symposium on Liquid Metal Processing and Casting, 02.-05.09.2007, Nancy, France
ISSN: 1543-1916
1073-5615
DOI: 10.1007/s11663-008-9158-2
Popis: Melt extraction is a near-net-shape casting process in which a swiftly rotating disc draws filaments out of a melt. The melt solidifies at the V-shaped circumferential edge upon first contact, and the layer grows while the disc moves further through the liquid pool. A disc may be equipped with several edges to increase the performance. Further, the perimeter may be notched to produce fibres having a certain length. During rapid cooling, the filament shrinks and is finally flung away by centrifugal force. Two different methods were developed in the seventies and established on industrial scale. Nonetheless, both still show their specific inadequacy. The extraction out of a crucible is characterised by a relatively large surface allowing multi-edge and thus efficient operation. Usually, the melt contained in a refractory is lifted slowly toward the disc. Induction heating and shear stress supplied by the disc lead to turbulent melt motion, which limits the process with respect to extraction velocity and, in turn, to relatively thick fibres. It is possible to extract ultra-fine filaments from a pending drop. That needs melting of a rod at its tip, which is usually accomplished by an oxygen-acetylene torch. Though the confined volume and capillary forces due to a small radius of curvature permit high extraction velocity, the productivity suffers from the fact that only one edge can be used. Damping of velocity fluctuations may be realised by a static magnetic field. Concerning the crucible extraction, extended series measurements showed that globally applying such a magnetic brake has two effects: (i) it is possible to achieve higher extraction speed, but (ii) the fibre diameter is barely affected. High speed video recordings revealed that extraction takes place only during duty cycles. Instead of producing thinner fibres, as has to be expected from higher circumferential speed in case of continuous extraction due to conservation of mass, the damping also reduces the mean velocity, and in turn the mass transport onto the disc; the globally applied magnetic brake influences the length of the duty cycle. Here, the tailored magnetic solution is a concentration of magnetic flux density to the small meniscus region of the extraction zone by means of ferromagnetic parts. This local stabilisation significantly reduces the fibre diameter. Poor efficiency of the pending drop technique can be overcome with extraction from a molten edge. The problem to be solved is melting the sheet directly at its edge. Owing to geometric restrictions, electromagnetic heating with a long inductor having two opposing branches would either release most of the heat at a certain distance from the edge, or in the disc. Passing the current in the same direction through the branches in combination with optimisation with respect to the skin effect allows moving the area of heat impingement almost entirely outside the inductor toward the edge of the jutting through sheet. In two experiments, a platinum sheet was molten directly at its edge and very thin tin fibres were produced.
Databáze: OpenAIRE
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