Graphite has a variety of uses and is extremely versatile. Electric discharge machining is one of the main uses. EDM allows the creation of intricate geometries and very precise tolerances. Tool can also be utilized to machine many materials, such as hard and soft alloys. The EDM process is especially useful for machining difficult-to-machine alloys. EDM creates a smooth surface that allows the use of larger tools.
EDM's physics is based upon the frictional melting phenomenon. A volume of melt is formed by an electric discharge that burns the material into the EDM. It is then cooled to create an isotropic, peaks-rich surface. Surface textures are formed when individual electrical discharge traces in the form of circular craters overlap. This spherical pattern in the surface of the machined part is directly related with the number and size electric discharges.
The porosity and grain size of graphite electrodes can have an impact on EDM's physics. In other words, by increasing the size of the grains, the chances that a short-circuit will occur increase, which reduces machining efficiency. Short-circuiting is further exacerbated by pulse duration and high current intensity.
This article, as part of an extensive research project, analyzes the effects of the graphite tool electrode's grain microstructure and the machining parameters used on Hastelloy C-22 for electrical discharge machining. In the experiments, two POCO graphite electrodes were tested with grain microstructures of 10 um and 1 um. The machining parameters investigated were the current intensity Ic, pulse duration ton, and time interval toff.
Results from the characterization and experimentation show that AF-5 is the graphite with the lowest Ra, MRR and TWR values of Hastelloy C-22. In contrast, S-180 graphite has a higher relative wear of the tool electrode compared to AF-5. The reason for the lower MRR and Ra values in AF-5 graphite comes from its grain size. It is also less porous and has a greater effective density.
The EDM performance for AF-5 graphite is improved by increasing the current intensity Ic, and the interval time ton. Longer pulse durations increase the EDM effects and heat penetration. Moreover, the duration of the pulse influences how large the craters are and the effect they have on the roughness of the surface. The electrode surface is affected by carbon deposition and other elements from the workpiece. It also raises the temperature at the sparking hole and helps stabilise the conditions of the plasma by deionizing it.
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