SIMPLIFICATION IN POLISHING METAL MOLD OF CEMENTED CARBIDE MACHINED BY EDM USING OXIDATION

近年来，硬质合金因其优异的耐磨性和耐热性在各种型坯模具中得到了广泛的应用。然而，在金属模具制造过程中，硬质合金在电火花加工(EDM)过程中会产生大量裂纹和微凹坑的热影响区(HAZ)，目前的HAZ是通过抛光来去除的，这依赖于手工精加工。然而，该工艺延长了金属模具的加工时间，增加了金属模具的生产成本，因此提出了一种新的表面改性方法，即氧化表面改性法来代替机械去除表面改性法。该方法通过对硬质合金表面进行高温氧化处理，软化了硬质合金表面气孔较多的硬质合金层，便于用钢丝刷摩擦去除。结果表明：(1)硬质合金电火花加工产生的热影响区可以完全消除。氧化后的表面由钴和钨氧化物组成。(2)去除氧化物后残留的晶体结构转变为碳化钨六方晶格，结构稳定。因此，表面改性方法有望极大地促进金属模具制造过程中的抛光。

Recently, the use of cemented carbides has become widespread for various kinds of meal molds because of their superior wear and heat resistance. However, the heat-affected zones (HAZ), in which ihere are many cracks and microcraters, are generated by electrical discharge machining (EDM) of cemented carbides during the metal mold manufacturing process.Under the present situation, HAZ are removed by polishing, which is dependent on hand finishing. However, this process extends the machining time and increases the cost of production of metal molds.Therefore, a surface modification method using oxidation is proposed as a new surface modification method instead of the mechanical removal one. The proposed method uses a high-temperature oxidation of the surface of cemented carbides, softening the layrs with many pores so that they can be easily removed by rubbing with a wire brush.The following results were obtained :(1) The heat-affected zones generated by EDM of cemented carbides could be completely removed by this oxidizing treatment. The oxidzed surface is comprised of cobalt and tungsten oxides.(2) The crystal structures remaining after oxide removal are transformed into the hexagonal lattice of tungsten carbide, WC,in a stable structure. Consequently, the surface modification method is expected to greatly facilitate polishing during the metal mold manufacturing process.