Numerical Simulation of Melt-Pool Hydrodynamics in μ-EDM Process

Abstract

The machining efficiency and productivity of the micro-electrical discharge machining (μ-EDM) process can be enhanced by the fundamental insights on the plasma-material interaction, which can be appraised via experimental and numerical analysis of discharge crater formation. The high-power density with low pulse-duration results in the transformation of the solid workpiece into liquid and even vaporisation phase to generate a crater. The crater dimension largely depends on the extent of vaporization induced recoil pressure as well as the pressure exerted by the expanding plasma channel. Efforts have been made by the researchers to analyse the plasma characteristics viz. its temperature, pressure and the role of plasma pressure on the material removal. However, there is a scarcity of the realistic data pertaining to precise plasma pressure which consequences in less accurate estimation of the numerically simulated crater. Moreover, the extent of evaporation is delayed due to activation of plasma pressure. Therefore, it is imperative to conduct a sensitivity analysis of variation of plasma pressure to improve the predictive nature of three-phase multi-physics numerical modelling of the μ-EDM.In the present paper, an endeavour has been made to consider the multi-phase transformation corresponding to the high-power density applied to the workpiece. Further, the effect of parameters such as Marangoni convection, Young Laplace surface tension pressure, vapour induced recoil pressure and a time-varying plasma pressure on the melt-pool hydrodynamics are considered. A 2D phase-field numerical model for the machining of Ti-6Al-4V has been formulated considering the aforementioned parameters. The numerical model is implemented in the commercial finite element method (FEM) software COMSOL Multiphysics. The simulation results reveal that the material removal in the initial phase of pulse-duration is dominated by vaporisation along with the melt expulsion due to recoil pressure which governs the crater size. The vaporisation of the Ti-6Al-4V is delayed to 4250 K due to active plasma pressure in the melt-pool. In the remaining pulse period, the surface tension induced backflow of molten metal along with Marangoni convection result in non-uniform perturbation at the bottom of the crater. Finally, the simulated crater is validated with the experimentally determined one-off crater using a dedicated single spark EDM circuit.