Influence of thermal stress on material removal of Cf_SiC composite in EDM

This paper proposes an electrical discharge machining (EDM) for use with carbon-fiber-reinforced silicon carbide (Cf-SiC) composite, which are materials that are extremely difficult to machine. The preliminary machining experiment indicates that EDM is a promising method to fabricate the Cf-SiC composite. However, owing to the distinct material properties of the Cf-SiC composite, the machining characteristics are quite different from those of metal materials in the EDM process, which mainly reflects the difference in the material removal process. To address the above issue, this study investigated the influence of thermal stress on the removal of material during EDM of the Cf-SiC composite. In the scanning electron microscopy (SEM) result, the morphology of the machined surface demonstrates the fracture removal of the carbon fiber with a discharge on the parallel surface and small-area spalling removal on the cross-section of the carbon fiber with a discharge on the vertical surface. In addition, it also exhibits a brittle fracture of the SiC coating and SiC matrix. Furthermore, a novel observation method that can avoid the interference of the discharge plasma by using a distinct band pass filter and laser illumination of a specific wavelength, is employed to investigate the discharge process of the Cf-SiC composite. It further confirms the fracture removal process of the SiC coating and carbon fibers. Finally, a thermal-electric-mechanical coupling simulation is conducted to investigate the stress distribution on the discharge surface of the Cf-SiC composite. It is found that the thermal stress on the machined surface exceeds the tensile strength of the SiC coating, carbon fiber, and SiC matrix. Thus, based on the above results, it is concluded that the thermal stress plays a significant role in high efficiency material removal from the Cf-SiC composite during EDM.