Bipolar nano-second pulse power supply for electrochemical micromachining of tungsten carbide without tool wear

Abstract

Electrochemical machining (ECM) of tungsten carbide (WC) alloy using bipolar pulses with a neutral electrolyte (NaNO3 aq) was successfully proposed in recent years. However, tool electrode wear always occurs during machining in the period of negative pulses. To solve this problem, a method of ECM with sacrificial auxiliary electrode using bipolar current is presented. This paper is focusing on the development of nano-second pulse bipolar power supply for this method. Firstly, positive and negative nanosecond pulses are obtained with the method of DC voltage chopping by using a multi-channel high-speed switch composed of gallium nitride (GaN) transistors. Then, these pulses are accordingly distributed onto the electrodes between tool electrode, workpiece electrode and auxiliary electrode during ECM in different time. During the period of positive pulses, the workpiece anode and tool cathode form a circuit for ECM, which dissolve the workpiece and form a passive film (tungsten oxide) on tungsten carbide; during the period of negative pulses, the workpiece cathode forms a loop with the auxiliary anode, resulting in hydrogen evolution reaction on the surface of workpiece, so as to weaken the adhesion of the passive film and improve the pH value of the electrolyte close to the machining area on workpiece surface to dissolve the tungsten oxide. ECM experiments were carried out with the developed power supply, and the micro holes are successfully machined on the surface of tungsten carbide without tool wear, which verifies the feasibility of the presented method and designed bipolar pulse power supply.