Numerical analysis of plasma channel characteristics and dynamic effects on molten pool in electrical discharge machining

The plasma channel provides the fundamental energy source and acting forces for material removal in electrical discharge machining (EDM). At present, the research on the characteristics of plasma channel is still in the exploratory stage. In this study, to clearly understand the characteristics of plasma channel and its dynamic effects on molten material, the transient mathematical model of plasma channel is first attempted and computed based on magneto-hydrodynamics (MHD). The breakdown process of plasma channel, as well as its inherent characteristics (heat flux, electromagnetic force and expansion pressure) are studied, based on which the dynamic effects of thermal input and force actions on anode molten pool are analyzed. Results show that the plasma channel sequentially experiences diabolo-shaped, lantern-shaped and finally stabilized horn-shaped stages, and the diameter expansion is nearly completed within 2μs which is independent of discharge current. The plasma channel presents time-variant characteristics in terms of heat flux distribution and energy distribution ratio into electrodes and gap dielectric. The electromagnetic force and expansion pressure not only determine the geometry evolution of plasma channel but also provide acting forces, which influence the distribution of hydrostatic pressure in molten pool and shear force on the interface. For preliminary verification purpose, the appearance of plasma channel is observed by high-speed photography, and the dimensions between simulation and experimental craters are compared. The feasibility of established plasma channel model is confirmed to a certain extent by showing an approximated channel appearance, as well as the maximal deviations of 10.8 % and 8.9 % in diameter and depth, respectively.