Tailored magnetic fields in the melt extraction of metallic filaments
Porous bodies that are resistant to corrosion at high temperatures and thermal shock may be produced from metallic fibers. In order to accomplish reasonable homogeneity and high porosity, the cross-sectional area of the fibers and the width of distribution thereof need to be small. This article studies two techniques for making fibers. Melt extraction out of a crucible yields filaments with a typical diameter ranging from 50 to 200 mu m, which is too thick. Also patented for a long time is the extraction from a pendant drop. Even though relatively fine fibers can be manufactured with this method, it never exceeded crucible extraction with respect to industrial importance owing to the low productivity of the process. The present article addresses the drawbacks of both variants of melt extraction of metallic filaments. Because metallic melts are electrically conducting, the use of magnetic fields allows for contactless process optimization. It is well believed that increasing the extraction speed diminishes the fiber diameter. Being not always true, at least in the case of crucible melt extraction, as indicated by the present findings, however, undesired fluid flow, i.e., turbulence, imposes an upper limit on the rotation rate of the extraction wheel. Application of a static magnetic field leads to both higher wheel speed and thinner filaments. The low productivity of extraction from the molten tip of a rod suffers from the fact that only one melt drawing edge can be used. As the bare rod is problematic with respect to heating its tip in contact with the extraction wheel, it is challenging to melt the entire edge of a sheet. A special design of the induction-heating magnetic field is also proposed to solve also this task.