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Top1. Introduction
Electro-discharge machining (EDM) process is based on spark erosion phenomenon under controlled condition. The process is used for machining of conducting materials that are difficult to machine by cutting with hard tools. Die-sinking EDM is widely used in industries as it has the capability to machine complex 3D shapes such as die and mould cavities in hardened steels (Kiyak and Cakir, 2007; Bose and Pain, 2016; Kharola, 2016). Apart from metals, different types of metal matrix composites have also been machined by EDM successfully (Bhuyan et al., 2014; Dixit and Awasthi, 2015). In certain applications, sinking of holes with sizes going down to micro level is also carried out (Jahan, et al., 2009; Porwal, et al., 2014; Kibria, et al., 2014). The die-sinking EDM process has been investigated for its performance with different materials for tool and work, different types of dielectric media, different powders mixed in dielectric media and ultrasonic assistance. In many research attempts, a set of operating parameters is selected on the basis of settings available in the machine. The setting parameters are voltage (Vs), current (Cs), pulse on time (Ton), pulse off time (Toff) and dielectric pressure (Abbas, et al., 2007). Puertas and Luis (2003) have studied surface finish in EDM and attempted to optimize the process from surface finish point of view. Numerical simulation of surface roughness formation has been attempted by Suryavanshi, et al., (2014). Klocke et al., 2013 have analyzed the process for material removal rate (MRR). Usually, one set of experiment is planned according to design of experiment (DoE) covering the operating range, and setting parameters are identified for MRR and surface finish (Lin et al., 2006; Barenji, et al., 2016). During interaction with practitioners in the industry, it has come to our knowledge that the practitioners use different strategies for rough and finish machining in EDM. It seems logical as maximum amount of material is removed in rough EDM, while improvement of surface finish becomes the objective in the finish EDM.
It is seen from the literature that investigations have been carried out on the effect of powder-mixed dielectric on EDM performance, taking conducting powders like as graphite, copper, aluminum, tungsten, titanium, chromium, etc. and non-conducting powders like as alumina, silicon carbide, titanium carbide, tungsten carbide, molybdenum disulphide, etc. (Kozak, et al., 2003; Rehbein, et al., 2004; Çoğun et al. 2006; Marashi et al., 2016). Under certain combination of powder-mix, the surface finish is also found to improve (Wong et al., 1998; Peças and Henriques, 2008). While conducting powders are found to assist in the sparking phenomenon and enhance material removal rate, surface modification is done using appropriate powders that get deposited on the workpiece surface. It is also seen that graphite powder is used widely for different steels to enhance process performance (Jeswani, 1981; Fong and Chen, 2005; Lin et al., 2006). In an earlier work by Jeswani (1981), concentration of graphite in dielectric is varied from 0.25 to 6.0 gm/l and the material removal rate is found to improve with the concentration. From a practical point of view, a fixed concentration of graphite is preferred in the industry.
This research work also focuses on ultrasonic assisted EDM. From the literature survey, it is seen that ultrasonic vibration is imparted to the tool electrode (Kremer et al., 1989; Goiogana, et al. 2016) or workpiece immersed in the dielectric medium (Shabgard, et al., 2011). Numerical studies have also been carried out by Shervani-Tabar and Mobadersany (2013) on the effect of ultrasonic vibration on dielectric medium in the electrode gap. In the experiments reported in the literature, the frequency is taken as 20 or 40 kHz, while the amplitude varies from 6 to 15 µm.