Molecular Dynamisc Simulation Of Discharge Characteristics And Ablation Mechanism In Micro-EDM

Electrical discharge machining(EDM) itself has very complicated interelectrode phenomenon and electrical discharge phenomena. And EDM is a transient phenomena which occurs in a extremely short time and in a small discharge gap, accompanied by the electrode material removal, thus making experiment observations extremely difficult. So the mechanism of electrical discharge machining(EDM) has not yet to be fully understood clearly, which seriously restricts its development.To analyze the material removal process and the mechanism of material removal in EDM from a microscopic and dynamic view. This study modified heat source model and the heat source model, and simulated the process of electrode material ablation by using Molecular Dynamics(MD) method. From the simulation results, it can be found that immediately after discharge is ignited, material ablation occurs, and it occurs mostly during the discharge duration. And the discharge crater become deep at first, and then become shallow again. It can be considered due to three reasons. The first is that the partial ablated material returns to the discharge crater again. The second is that during discharge duration the melting area flows outside the electrode and protrudes into the gap forming the bulge around the crater, but after discharge ends the melting area flows back slightly into the bottom of discharge crater again. The last is that the disorder of the atomic lattice arrangements of the re-solidified layer formed on the surface of discharge crater brings about the decrease in the density of the material.In this study, the hydrostatic pressure was also calculated. It can be found that extremely high pressure is generated inside the melting area after discharge ignited. This calculated result indicates that the increasing pressure distribution along the depth direction of melting area can be considered the material removal motivity, which is the main mechanism of material removal in EDM.In addition, this study also analyzed the surface metamorphic layer in EDM, including heat affected zone and re-solidified layer. The results showed that, at the beginning of the discharge under the effect of the linear expansion of gauss heat source, the surface metamorphic layer expaned along the depth direction dramatically. With the continuous expansion of plasma discharge channel, the expansion of surface metamorphic layer along the radial direction is obvious, and after the discharge surface metamorphic layer continues to expand. This is because after discharge the heat conduction does not disappear, changing the material microstructure. And the melting area reaches the maximum size at the end of the discharge duration and the most of melting area has not been removed but be leaved forming the re-solidified layer. After discharge the heat affected zone is still expanding, but the melting area is still greater than the heat affected zone.Moreover, the simulation results of the case of the minimum energy in EDM showed that if the discharge power is selected appropriately in micro EDM, the material will only melt but not be removal. This provides a new method for EDM. This study also analyzed the ablation of polycrystalline material in EDM. In addition to the simulation of single crystal material ablation, immediately after discharge is ignited, material ablation also occurs. But due to the presence of grain boundary, compared with single crystal material ablation, the ablation of polycrystalline material is mostly ablated in form of cluster. After the discharge, due to the effect of heat conduction of the heat source, the material microstructure changes, forming relatively significant heat affected zone.