Understanding novel assisted electrode from a theoretical and experimental perspectives for EDM of aluminum nitride ceramics

Although ceramic materials are known for their high-temperature stability, high hardness, high strength-to-weight ratio, and high resistivity to wear, their inherent brittleness makes them difficult to machine. Electrical discharge machining (EDM) is a noncontact machining process that can machine any workpiece irrespective of its hardness, as long as the material is electrically conductive. Therefore, the challenge that comes with the EDM of ceramics is the electrical nonconductivity. This study has focused on implementing and modifying assisted electrode method to machine aluminum nitride (AlN) ceramics by the EDM process with a goal of machining through holes. A successful sandwich structured multilayer removable coating assistive electrode method has been developed, and the effectiveness of assistive electrode in the material removal process has been investigated from experimental and theoretical perspectives. A physics-based model has been proposed to understand the material removal mechanism and how the proposed assistive electrode can lead to successful machining in electrically nonconductive ceramics. Melting and thermal spalling have been identified as dominant forms of material removal during EDM of nonconductive ceramics. The amount of carbon deposition on the walls of the machined surface is a key parameter that dictates the feasibility and outcome of the method. Deposition of carbon introduces electrical conductivity to the machining zone that allows machining to go through. According to the model, plasma channel radius is reduced due to the low conductivity of the workpiece, which aids in concentrated energy transfer from electrode to workpiece. In addition, the maximum heat flux generated for a particular peak current also decreases with the increment of pulse-on time. The proposed physics-based model was found to be effective in predicting the performance of assistive electrode for EDM of ceramics.