PSI - Issue 10

E.L. Papazoglou et al. / Procedia Structural Integrity 10 (2018) 235–242

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E.L. Papazoglou et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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arises for industry to develop new non-conventional machining processes for efficient and effective machining of these materials. One basic and major difference between conventional and non-conventional machining is the type of energy which is used, with the conventional machining processes primarily been based on the interaction of mechanical energy with the workpiece. On the other hand, non-conventional machining makes use of the direct energy source concept, where there is no intimate contact between the energy source and the workpiece (Payal and Sethi (2003)). Electrical Discharge Machining (EDM) is one of the earliest, but still widely used non-conventional machining processes, based on thermoelectric energy between the workpiece and the electrode (Mohd Abbas et al. (2007)). The major advantage of EDM is the ability to machine any electrical conductive material, including the carbon fiber reinforced composites (CFRC) (Gourgouletis (2011)), irrespectively of whether it is difficult to be machined because of its hardness, strength or another mechanical property. Additionally, EDM is capable of machining parts and com ponents with complex geometries, maintaining high level of dimensional accuracy and surface finish, and is used extensively in die and mould, aerospace, automotive, micro-electronics, and biomedical industries (Jahan (2015)). There are two main types of EDM, the die sinking EDM and the wire EDM, with the basic principle of both being the same: material is removed when several consecutive and rapidly repetitively discharges occur between a tool electrode and a workpiece electrode, which are both immersed in a dielectric medium - fluid (Weingärtner et al. (2012)). EDM uses thermoelectric energy to remove material from the workpiece, without any mechanical contact between the electrodes during the whole process. A voltage difference is applied between the working electrode and the workpiece, which are separated by a small gap filled with dielectric fluid (oil or deionised water), resulting to the ionisation of the latter. When the electric field reaches a critical value, streamers are forming, which establish a plasma channel between the tool and the workpiece (Jahan (2015)). The plasma channel, with temperatures between 6,000 and 12,000 K, causes strong heating of both electrode and workpiece materials, creating a small molten metal pool at the surface. Some materials, due to the extreme heating, may even be directly vaporized. The plasma implodes under the pressure imposed by the surrounding dielectric and consequently, the molten metal pool is strongly sucked up into the dielectric, leaving a small crater at the workpiece surface. Every single spark removes a very small volume of material, typically in the range of 10 -6 - 10 -4 mm 3 , with the process being repeated a few thousand times per second (Choudhary and Jadoun (2014)). The gap phenomena such as the plasma column formation in the dielectric, interaction between electrons and ions, heat transfer and material rejection, present a stochastic nature and they have not been fully explained and described with a common accepted theory (Rajurkar et al. (2006)). Performance parameters are an indicator of the EDM results, and of the procedure efficiency and effectiveness. Material removal ratio (MRR) is a measure for the erosion rate of the workpiece, typically used to quantify the machining speed. It is defined as the volume of material removed over a unit period of time, and commonly expressed in mm 3 /min. Tool wear ratio (TWR) is defined as the ratio of volume of materials removed from the tool electrode to that of workpiece and is commonly expressed as percentage, with low values of TWR representing more stable and economical machining. TWR greatly depends on the operating parameters. The surface quality (SQ) of the machined workpiece is a broad and extremely important factor of the EDM process, including aspects such as surface roughness (SR), extend of heat affected zone (HAZ), recast layer thickness, namely the white layer (WL) and micro cracking density (Choudhary and Jadoun (2014); Jahan (2015)). Like most of the machining processes, EDM can be approached experimentally and/or theoretically. During last decades, effort is being made for a better understanding of the phenomena that occur, in order to fully exploit the potential of EDM. Perez et al. (2007) investigated, through local micron scale temperature measurements and numerical methods, the heat influence on the metallurgical and mechanical properties of steel, which have been sub mitted to EDM. Gosavi and Gaikwad (2016) tried to predict the optimum EDM parameters through a thermo mechanical analysis for EN31 tool steel as workpiece material and copper electrode. The comparison of the theoretical and experimental results concluded a high agreement between theoretical and experimental values. Tang and Yang (2016) established a thermo-hydraulic coupling numerical model of discharge crater formation and analysed it with the finite element method. Liu and Guo (2016) presented a numerical approach to predict and analyse the formation mechanism of the WL, the HAZ and the corresponding phase transformations due to EDM for ASP 2023 tool steel. Also, the effects of EDM conditions on the thickness of WL and HAZ were investigated. Extremely few experimental studies have been carried out during the last decades for machining AISI O1, one of the most common work steels, with EDM. A study was conducted by Lee et al. (1988), where the surface transformation and damage in AISI O1, A2, D2, and D6 tool steels after EDM were investigated. Through detailed surface examinations and data analyses