Formation of Metal Carbide Powder by Spark Machining of Reactive Metals
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I.
INTRODUCTION
SPARK machining, also called electrical discharge machining (EDM), is widely used in laboratories and in industry for the cutting of delicate materials and for the shaping of complex parts, but few people in western nations have given any serious consideration to the nature of the material removed by the sparking action. Interest in the process has increased recently because it has been shown that amorphous metal powders can be produced by spark machining.~'z With proper choice of the alloy and the dielectric fluid, the amorphous metal powder has a composition unchanged from that of the bulk alloy, but more typically reactions with the dielectric alter the composition of spark machined powders from that of the initial metal. As early as 1964 it was realized in Russia that fine particulates produced by high frequency spark erosion of metals contained carbide and oxide phases.3 Spark erosion techniques have been used in Russia to produce, among other things, metastable fcc structure PdC and NaC1 structure WC powders. "'s In Russian studies it was demonstrated that altering the dielectric fluid can alter the powder composition. For example, Lunina and Novozhilov 6 found that fine Ni powder prepared by a discharge technique contained 22.5 pct carbon when the erosion was done in toluene, but only 5.6 pct carbon when it was eroded in ethyl alcohol. Spark-generated carbides, oxides, and nitrides produce a surface hardening effect which has been employed commercially. In a review paper on spark-hardening of metals, 7 Welch attributes the first observation of this phenomenon to Rawdon in 1924.8 There have been numerous studies of the effect of processing variables upon the structure and hardness of sparked surfaces, but these authors are not aware of any previous investigation in which a correlation is made J.D. AYERS, Metallurgist, and KATHY MOORE, Physical Science Aide, are with Naval Research Laboratory, Code 6320, Washington, DC 20375. Manuscript submitted October 15, 1982.
METALLURGICAL TRANSACTIONS A
between the structure of spark-machined surfaces and the structure of powder particles generated by the machining. In this study it is shown that the structure and composition of carbide powders produced by the spark machining of elemental Ti, Zr, and W is strongly dependent upon the amount of carbon incorporated into the workpiece surface during machining. II.
EXPERIMENTAL
These experiments employed commercial purity Ti, iodide process Zr, and W in the form of 1.27 cm diameter rods of unknown purity. All samples were machined with an AGIE Type AB 30H electrical discharge instrument, a device which generates a square wave output of variable frequency. The work reported here was done at frequencies of 400 to 25,000 Hertz using a 50 pct duty cycle. The samples were machined in kerosene with a fiat, slotted graphite tool which was rotated to keep the gap clear of spark erosion products. Experiments were done with the tool polarized both positively and negatively with respect to the workpiece. After mach
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