Surface Characterization of Electric Discharge Machined Surface of High Speed Steel

Abstract

A comprehensive study of the Surface Characterization of Electric Discharge Machined surface of High Speed Steel work piece was investigated in this Project. EDM tests on High Speed Steel work piece were conducted on a Minor model Modern Machine Tools make Electric Discharge Machine with a transistorized pulse generator having a maximum output of 85 V and 20 A. The test conditions used were Copper electrode with negative polarity, kerosene as dielectric, side flushing, Pulse Current settings of 2, 5, 10, 15 and 20 A, Pulse on-times of 50, 200, 400, 800 and 1600 s with duty factor of 0.5, and 85V open voltage. The Electric Discharge Machined surface morphology was examined with a Metallurgical Microscope and Scanning Electron Microscope (SEM-Leo-440 model) with EDX analysis. It was observed that at low pulse current and low pulse on-time, the craters were shallow and the density of global appendages and pockmarks were low, whereas at higher pulse current and pulse on time, the craters were deeper and global appendages were most evident. These global appendages were molten metals, which were expelled randomly during the discharge and later solidified on the work piece surfaces. It is also observed that the EDM conditions have no effect on the microstructure of the bulk material work piece, and the damage caused by the EDM surface is limited to certain depth only. These changes will depend on the variation of pulse current and pulse on-time. The development of new, advanced engineering materials and the need for precise and flexible prototypes and low-volume component production have made Electric Discharge Machining an important manufacturing process to meet such a demand. Electric Discharge Machining (EDM) is a thermo electric process that erodes work piece material by a series of discrete electrical sparks between the work piece and an electrode flushed by or immersed in a dielectric fluid. Unlike traditional cutting and grinding processes, which rely on the force generated by a harder tool or abrasive material to remove the softer work material, the EDM process utilizes electrical sparks or thermal energy to erode the unwanted material and generate the desired shape. The hardness and strength of the difficult to machine work material are no longer the dominating factors that affected the tool wear and hinder the machining process. This makes the EDM process particularly suitable for machining hard, difficult to machine material. The EDM process has the ability to machine precise, complex and irregular shapes with a CNC control system. In addition, the cutting force in EDM process is small, which makes it ideal for fabricating parts with miniature features. Several theories were proposed by early investigators to account for the erosion mechanism of the EDM process. It has been accepted generally that the metal removal phenomenon is predominantly thermal in nature [1]. The best explanation of EDM phenomena has been established by extensive experimental studies [2, 3]. Three stages can be distinguished: 1. Ionization and arc formation, at a localized area between the electrodes, following the application of a voltage exceeding the breakdown voltage. 2. The occurrence of the main discharge as an electron avalanche striking the anode, with low electrical resistance in the discharge channel. The cathode is struck by ions and is heated less rapidly than the anode.