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Top1. Introduction
Micro-electrical discharge machining (micro-EDM) is one of the various established advanced micromachining processes. In principle, the technology of material removal for EDM and micro-EDM is same. However, there are several differences between these processes. These differences include ranges of applied current, gap voltage, pulse duration, pulse energy, flushing pressure, tool electrode size, plasma channel dimensions, flushing out of debris, etc. In micro-EDM, discrete and repetitive electrical discharges is made between micro-tool electrode and workpiece within a very small gap (called as inter electrode gap, IEG) by applying gap voltage and current between these electrodes (Sato et al., 1985). Due to conversion of electrical energy into thermal energy in the small gap, breakdown of dielectric fluid occurs and plasma channels are created in shortest gap region. Due to generation of very high temperature at the surface of both tool electrode and workpiece, material melts and vaporizes and craters are formed on both surfaces (Ossenbruggen, 1969). Debris particles formed are flushed away by flushing of dielectric fluid as well as pressure created by collapsing of plasma channels (Pandey and Singh, 2010). Due to non-contact and thermo-electric based material removal process, micro-EDM process is applied for the workpiece (conductive) material regardless of its hardness (Pham et al., 2004).
Amongst various process variables in micro-EDM, the type of dielectric fluid is an important factor. The nature of dielectric fluid certifies the micro-EDM process whether the discharge and material removal phenomena are effective or not (Chung et al., 2009). A good dielectric fluid must establish proper discharge gap between electrodes to avoid premature discharge. In addition, the debris particles should be efficiently ejected out from discharge gap to eliminate short-circuiting between electrodes. Moreover, after series of electrical discharges during pulse-on-time, the dielectric fluid should have the ability to recover the initial resistivity in the IEG. In many times, the use of hydrocarbon-based dielectrics produce organometallic compounds by reacting with metallic elements of both tool electrode and workpiece. Thus, the stability of discharge is not achieved in the short IEG. In addition, the use of hydrocarbon-based dielectrics produce harmful gases such as CO and CH4 which makes the machine environment toxic (Kunieda et al., 2005). At the same time, it is not safe to occur fire hazards. Other type of dielectrics such as mineral oil or an organic fluid also produces harmful fumes such as benzene, aromatic hydrocarbons, mineral aerosols, polycyclic aromatic hydrocarbons, etc. Thus, it produces environment pollution (Tonshoff et al., 1996). Thus, it is urgently needed to search an environment friendly dielectric fluid which promotes better machine environment as well as produces stable discharges between electrodes. Use of deionised water as dielectric fluid provides oxy-based compound by self-degrading at the discharge temperature and encourage higher machining rate (Kibria et al., 2010). However, the other machining characteristics such as tool wear, accuracy of micro-features, etc., may be inferior (Kibria and Bhattacharyya, 2011). Thus, proper understanding of various characteristics of dielectric fluid and selection of correct range of process parametric combination may augment the machining characteristics as well as qualitative factors of micro-EDM process.