Effects of dielectric fluids on surface integrity for the recast layer in high speed EDM drilling of nickel alloy

Dielectric fluids affect not only the discharge status but also the structure and composition of recast layer in the electrical discharge machining (EDM) process. In order to reduce the thickness of recast layer and improve the surface quality of film cooling holes of nickel alloy, in this paper, the effects of four different types of dielectric fluids on surface integrity of the recast layer formed during high speed EDM drilling process were investigated; the formation mechanism of the recast layer on nickel alloy was discussed by comparing the surface characters of the bulk and those of the recast layer using various characterization methods. The dielectric fluids with larger electric conductivity (emulsion and water solution) were found to have higher sensitivity to the single pulse discharge energy. With the increase in electric conductivity of dielectric fluids, the recast layer became thicker accordingly. The thickness of the recast layer when using kerosene fluid was the smallest (4.8 μm), which was nearly one third of the that of using water-based fluid. The cooling rate of the dielectric fluid affected the microstructure of the recast layer, where the surface of recast layer was apt to form fine grain zone and the bottom formed columnar grain.