Modeling of the Discharge-Sample Interaction in the Electron Discharge Machining (EDM) Process

Summary form only given. We present a global model of the discharge-sample interaction in the EDM environment. Using this model, important quantities that are of crucial importance for the development of EDM like the microstructure and the residual stress in the resolidified layer as well as the mass transfer controlling the erosion rate can be calculated accurately. A thermal model has been first elaborated on the basis of local temperature measurements made with a home made microthermocouple device and with a fast, high resolution IR CCD camera. These instruments allowed to determine experimentally the cathode plasma expansion rplasma(t) of a single discharge. This expansion is well described by a power law rplasmaproptn with n=0.2. The fraction of the discharge power transferred to the sample has been found to be 0.10 to be compared to 0.149 obtained in multiple spark experiment. Using this model, the thermal history of the resolidified layer (called white layer) is simulated and a multilayer structure is obtained in agreement with experimental observations. This multilayer is predicted to be composed of martensite and residual austenite. On the basis of this model, a stress analysis has been performed. This analysis demonstrates that the residual stress is due to a coupled effect of the martensite formation in the HAZ and the thermal history. This is in agreement with a model of crack formation in the white layer developed recently. The effect of multiple sparks has been simulated. The formation of continuous white and heat affected zone layers is obtained. This result accounts well for observations of the global EDM process. Finally, the same thermal model has been used to calculate numerically the convection flow in the melt pool created by the discharge. For the first time, it is shown that convection is important as early as 50 mus after the beginning of the spark. The role of the convection flow on the matter removal process will be discussed. These results open the route for novel applications of EDM for high quality surface treatment. Examples of these applications will be presented