An Experimental Study of the Phenomenon of Surface Alloying by EDM Process Using Inconel Tool Electrode

Electrical Discharge Machining (EDM) has emerged as a very important machining process due to its numerous advantages. It is extensively used by the die and toolmaking industry for the accurate machining of complex internal profiles. Although EDM is essentially a material removal process, it has been used successfully for improving the surface properties of the work materials after machining. As the dissolution of the electrode takes place during the process, some of its constituents may alloy with the machined surface under appropriate machining conditions. Additive powders in the dielectric medium may form part of the plasma channel in the molten state and produce similar alloying effect. The breakdown of the hydrocarbon dielectric under intense heat of the spark contributes carbon to the plasma channel. Sudden heating and quenching in the spark region also alters the surface properties. This paper reports the results of an experimental study into electrical discharge machining of H13 hot die steel with Inconel (an alloy of chromium, nickel and iron) tool electrode under machining conditions favouring high electrode wear. The results show improvement in micro-hardness after machining by as much as 88%. Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) analysis of the machined surfaces show transfer of chromium and nickel from the tool electrode. Both these elements form intermetallic compounds as well as solid solution with iron and strengthen it. It was found that percentage of chromium increased from 5.39% to 6.52% and that of nickel increased from 0.19% to 4.87%. The favourable machining conditions for surface alloying were found to be low value of peak current, shorter pulse on-time, longer pulse off-time and negative polarity of the tool electrode.