Formation of Metal Carbide Powder by Spark Machining of Reactive Metals

Abstract

When metals or alloys are subjected to electrical discharges in a dielectric fluid, the molten metal frequently reacts with the fluid or with ionic fragments generated from the dielectric by the discharge. When hydrocarbon fluids are used as dielectrics, the reaction products are metal carbides. These carbides are present both as surface layers on the metal samples and as fine powder particles produced when material is ejected from the surface by the sparks. Results are reported here on a study of the formation of fine carbide powder by the spark machining of Ti, Zr, and W. Spark machining of Ti and Zr produced the NaCl structure carbide expected of these metals, though under some machining conditions the powder produced by the machining also contained fine dispersion of the parent metal or graphite. Machining of W produced hexagonal structure W₂C and WC in both the NaCl and the hexagonal structural forms. X-ray diffraction studies showed that the carbon concentration of these powders depended upon the frequency and polarity of the applied current pulses and upon other processing variables. The X-ray studies and scanning electron microscopy of the powders and of the surfaces from which they were generated showed that carburization reactions occurred both in the molten pools produced on the sample surfaces by the electric discharges and within fine particles after they were ejected from the surfaces.