Study on the influence that hard water had in high-speed wire electrical discharge machining

Abstract

High-speed wire electrical discharge machining (HS-WEDM) using hard water working liquid results in the machined surface losing metallic luster and reducing the cutting speed comparing with distilled water working fluid. Scanning electron microscopy (SEM) was used to observe the microscopic surface of the workpiece. The chemical composition of the machined surface was analyzed via energy-dispersive X-ray spectroscopy (EDS). Results indicated that the primary cause for lost metallic luster of the machine’s surface was the Cl⁻ present in hard water, which was easily adsorbed on the surface of the workpiece under high-temperature and pressure inter-electrode conditions, resulting in pitting corrosion. By analyzing the conductivity and the continuous discharge waveforms of distilled water and hard water working liquid, it was determined that the reduced cutting speed was because the hard water working liquid contained ions such as Ca²⁺ and Mg²⁺, which were adsorbed by the colloid in the working fluid. This resulted in the loss of dielectric property for the working fluid. Furthermore, the conductivity of hard water working fluid is higher, making it easier to be broken down under the same voltage, which results in a larger discharge gap and reduced cutting speed. A method of distilling the hard water to remove the ions was presented to circumvent this; after which, the cutting surface recovered metallic luster and the general machining speed increased.