Hybrid Methods of Electric Discharging Machining for Products with Thermal Protective Coatings

Hybrid methods of electrical discharge machining for products with high-temperature protective coatings—aluminum oxide (Al2O3) and yttria-stabilized zirconia (YSZ)—are studied experimentally. The individual methods considered are the use of a secondary conducting surface with an aluminum mask and galvanic copper plating; laser-assisted electrical discharge machining; and the use of a suspension of thermally expanded graphite as a working fluid. Copper plating stabilizes the process but entails additional finishing to remove the copper layer from the surface of the protective coating. Laser assistance permits local opening of the coating but leads to the formation of thermal influence zones. When using a suspension of thermally expanded graphite as the working fluid, machining by the formation of conducting bridges is possible, but the conditions must be optimized so as to minimize the melting zone.