Beyond the spark: the impact of tool electrode materials, polarities, and pulse distribution on machined surface texture in the electrical discharge machining of Inconel 718 superalloy

Sinking electrical discharge machining (sinking EDM) invariably results in surfaces with a thin layer of resolidified material (recast layer) after successive discharges. This phenomenon persists irrespective of the discharge energy. The need to investigate this layer frequently arises to determine the surface integrity of the component, with particular emphasis on texture and roughness parameters during finishing machining processes. This investigation is particularly crucial in research involving the use of negative polarity in metallic electrodes. However, the impact of different tool electrode materials and their polarities on texture in the Inconel 718 superalloy has yet to be thoroughly explored, and this is the central focus of the present study. Sinking EDM experiments were conducted at four duty factor levels, with input variables including an open circuit voltage of 120 V and a discharge current of 2.4 A. Special graphite, copper, and copper-tungsten alloy (CuW) were used as tool electrode materials, with both positive and negative polarities in the experiments. Morphological analysis, surface texture, and three-dimensional roughness parameters were innovatively compared for surface characterizations. In terms of average roughness, the graphite electrode (cathode) exhibited the highest amplitude deviation, with Sa = 2360 μm. In contrast, the metallic electrodes demonstrated superior roughness, with copper at Sa = 1063 μm and CuW at Sa = 1047 μm, representing reductions of 55.0% and 55.6%, respectively. A significant correlation was observed among the variables under study, with a correlation coefficient ( ρ ) of 0.98 between the parameters Sdq-Sdr, indicating a very strong positive correlation, and a ρ of −0.91 between Smr2-Sdr, revealing a very strong negative correlation. The texture assessment prioritized volume analysis and revealed that the peak volume ( µm³/µm²) was evenly distributed among different electrode materials and polarities. Metallic electrodes exhibited higher average volumetric amplitudes under positive polarity. Conversely, an alteration in average volumes was observed for valley voids, although these changes fell within the standard deviation range. Notably, valleys appeared more pronounced when using the graphite (+) electrode, showing a 200% increase, as opposed to a 125% increase with the graphite (−) electrode.