Studies on electro discharge drilling of an Al2O3 –TiC composite

Ceramic matrix composites (CMCs) can be attractive for structural applications, but their machining by conventional methods is expensive and often critical. Complex geometries on advanced ceramics require contactless processes, such as electro discharge machining (EDM) or -drilling (EDD). These proved to be viable for CMCs with electrical resistivity below a critical value in the range of 1-3 Ωm. The condition is complied with by many CMCs: an example is alumina with titanium carbide. Material removal of ceramics by electrical discharges is a complex process involving different mechanisms, depending on the process setup. The paper describes an experimental study on EDD of 0.4 mm diameter holes with aspect ratio of 20 in Al2O3-TiC, using copper electrodes. Peak current (Ip), pulse-on time (ton) and pulse-off time (toff) are varied as independent variables. Four performance indicators are measured: material removal rate (MRR), electrode wear rate (EWrate), overcut (OC) and surface roughness (Ra, Sa). Empirical models are proposed to describe t the effect of process parameters on the output indicators. The analysis is supported by the observation of the surface and subsurface morphology, with the aim of investigating the material removal mechanisms and attaining a full comprehension of macroscopic results. It is found that removal mostly occurs by melting and evaporation and that surface morphology is determined by two phenomena ruled by pulse power. A process description is proposed, built around power as the ruling factor.