Simultaneous Improvement of Micro-Hardness and Surface Finish in Die Steels by Powder-Mixed EDM Process

Electrical Discharge Machining (EDM) is widely used by the die and tool making industry for the accurate machining of complex internal profiles in various types of dies. Due to the absence of physical contact between the tool and the workpiece, the hardness of the workpiece is not a consideration in this process and die steels can be machined after hardening. Although EDM is a machining process, it has also been successfully used for improving the surface properties of dies and press tools. The intrinsic nature or the process causes some dissolution of the electrode and these particles may alloy with the machined surface under appropriate machining conditions. Breakdown of the hydrocarbon dielectric under intense heat of the spark contributes carbon to the plasma channel. Another method to produce similar alloying effect is the addition of powders of the desirable elements in the dielectric medium. If such powders are conductive in nature, they affect the energy distribution and sparking efficiency and consequently, the surface finish and micro-hardness. This paper presents the results of an experimental study into electrical discharge machining of H13 hot die steel with graphite powder mixed in the dielectric medium. Copper electrode and kerosene dielectric were used for the experiments and three operating parameters, namely peak current, pulse on-time and pulse off-time were varied. Results show increase in micro-hardness by 42% and improvement in surface finish by 68%. Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) analysis show smooth surface devoid of any craters and significant presence of carbon in the machined surface which is expected to provide self-lubricating properties to the die surface. Chemical composition of the machined surface checked with an optical emission spectrometer shows increase in percentage of carbon from 0.44% to 3.23%. All three operating parameters emerge as significant and the favorable machining conditions for surface alloying are found to be low value of peak current, shorter pulse on-time, longer pulse off-time and negative polarity of the tool electrode.