Stabilisation of the melt extraction process by magnetic fields


Stabilisation of the melt extraction process by magnetic fields

Cramer, A.; Gerbeth, G.; Bojarevics, A.; Gelfgat, Y.

When a quickly rotating wheel is brought in touching contact with the surface of a liquid metal pool which may contain any pure metal or alloy it draws material out of the melt. The molten metal solidifies at the, usually water-cooled, wheel, shrinks, and is flinged away by centrifugal forces. The shape of this rapidly quenched metal is determined by the geometry of the chill wheel. In the direct melt extraction of metallic and intermetallic fibres the extraction wheel is equipped with one or more edges aligned around the circumference of the wheel, the length of the fibres is determined by the distance of the grooves made into these edges.
The industrial production of melt-extracted fibres has two certain limitations which are inevitably coupled with the circumferential speed of the wheel. Besides the production rate most of the applications of such fibres demands them to have diameters in the range of 50mm to 100mm. Increasing the rotation rate decreases their cross-section, but far beyond the desired diameter respectively wheel speed the extraction process becomes unstable due to turbulence within the melt and a wavy motion on the surface.
Many patents are concerned with the stabilisation of the melt pool. They propose submersing mechanical parts directly into the melt to avoid the non-stationary conditions produced by the turbulence within the liquid metal pool. Even made of heat resistant ceramics they suffer from corrosion or cracks and do not work at all.
The present work describes contact-less control mechanism by means of steady magnetic fields of different strength and orientation. They can either be applied globally to damp the flow within the whole melt volume or locally to the meniscus region where the fibre is formed, to reach higher Lorentz forces in this very sensitive region.
Model experiments (In-Ga-Sn, liquid at room temperature; no extraction) with global stabilisation were carried out to study the calm down of the turbulent surface of an inductively stirred melt. The influence of the globally applied field onto the fibres has been investigated in a second model experiment using the low melting Sn-Pb and under real hazardous industrial conditions like induction heating and vacuum. Though the otherwise wavy surface was damped down to a nearly mirror-like plane the goal of significant smaller fibre cross-section could not be reached by the global stabilisation alone.
The clearly observable positive tendency towards smaller fibres is presently investigated with two series of model experiments with additional local stabilisation. The smaller volume which needs to be magnetised allows for a 5 times stronger field which can be achieved either by rare earth permanent magnets or a concentration of the globally applied field with magnetic field guides. First results using the second alternative of field concentration show a significant reduction of the fibre cross-section. Using a global field of 0.14T only we have been able to reach 0.7T at the edge of the wheel the outer parts of which including the edges were made of magnetic iron. The widespread range of extraction parameters, most of which have even been not mentioned here, is far from being investigated. As one typical result Sn-Pb fibres have been extracted with and without the locally concentrated magnetic field at a temperature of T=245oC, keeping even the circumferential speed of 7m/sec constant. The amount of fibres with diameter below 80mm increased from 9% to 51% by applying the magnetic field.

  • Lecture (Conference)
    International Workshop "Electromagnetic Control of Free Surface Flows in Materials Processing" (EFMP 2000) June 4-7, 2000, Ilmenau, Germany

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Publ.-Id: 3427