The surface texture properties after electrical discharge machining with negative polarity using graphite electrodes of different grain sizes

Abstract

Abstract Electrical discharge machining (EDM) is a precise, unconventional manufacturing technique widely used for machining difficult-to-cut materials. Hastelloy C-22, a nickel-chromium alloy, presents significant challenges for conventional machining due to its high hardness and melting point. Graphite electrodes are commonly employed in EDM because of their favourable technological and economic properties. In this study, two commercially available POCO graphite electrodes with different grain sizes were used: 1  $$\upmu$$ m (AF-5) and 10  $$\upmu$$ m (S-180). The investigation focused on the effect of graphite grain size and EDM electrical parameters on the surface roughness of Hastelloy C-22 under negative polarity, which is often selected to improve material removal efficiency and surface finish. Surface topography was quantitatively characterised using Sa , Sz , St , Sds , Sdr , and Sfd parameters. The results indicate that surface roughness is primarily determined by electrical parameters (current intensity, discharge duration, and time interval), with the influence of graphite grain size being relatively minor. Grain size affects tool wear and the stability of electrical discharges, which can induce surface irregularities. Statistical models were developed to predict surface roughness based on EDM parameters, providing guidance for optimising the machining of Hastelloy C-22 with different graphite electrodes.