Stabilizing the direct melt extraction of intermetallic fibres by magnetic fields

In the direct melt extraction process a quickly rotating wheel draws fibres out of a liquid metal pool which may contain any pure metal or alloy. One common application of such fibres is the production of highly porous metallic substrates, which, in the case of Ni-Al is ideally suited to serve as a highly heat- and corrosion-resistant catalyst carrier. But this demands the fibres to have diameters in the range of 50 to 100 microns and a small width distribution of their cross section. The main limitations of this process are due to turbulence within the melt and a wavy motion on the surface which both inhibit to fulfil these requirements.
Many patents are concerned with the stabilisation of the melt pool or at least of the contact region between metal surface and the extraction wheel. They propose submersing mechanical parts directly into, or placing them at least in close vicinity to 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 reliable at all. The present work describes a non invasive control mechanism by means of magnetic fields of different strength and orientation. They can be applied either 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. In a second setup the influence of this globally applied field on the fibres have been investigated (low melting Sn-Pb). Though the otherwise wavy surface was damped down to a nearly mirror-like plane the goal of significant smaller fibre cross-section was not reached by the global stabilisation alone. The clearly observable positive tendency towards smaller fibres is presently investigated with two series of model experiments with 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.