Influence of liquid dielectric medium on microwave-metal discharge-based drilling of AISI 304 stainless steel

Microwave-metal discharge-based drilling of metallic materials involves use of thermal energy from the plasma column for material removal. However, poor control over plasma generated in atmospheric air causes wide heat-affected zone (HAZ) and poor surface integrity around the drilled hole. The present work is an attempt to minimize drilling-related defects by incorporating circulating liquid dielectric as the drilling media. Performance of microwave drilling was assessed after machining in liquid dielectric and atmospheric air media. Stainless steel (AISI 304) and pure tungsten were chosen as the work and tool materials, respectively. Process performance parameters, such as diametrical overcut, circularity, HAZ and material removal rate (MRR), were evaluated for the drilling intervals of 40, 50 and 60 s. It was observed that the dissipation of microwave power within the liquid dielectric medium reduces the intensity of the plasma column. In addition, pressure applied by the liquid dielectric medium over the plasma column aids in its confinement. As a result, a relatively less intense and more uniform plasma column is generated in the liquid dielectric medium, which reduces erosive action on the work material. This controlled removal of work material results in a significant reduction (up to 90%) in HAZ. The uniform nature of plasma column yielded better hole circularity in liquid dielectric medium.