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A review on the use of environmentally-friendly dielectric fluids in electrical discharge machining
Journal of Materials Processing Technology 149 (2004) 341–346
A review on the use of environmentally-friendly dielectric fluids in electrical discharge machining Fábio N. Leão∗ , Ian R. Pashby School of Mechanical, Materials, Manufacturing Engineering and Management, University of Nottingham, University Park, Nottingham NG7 2RD, UK Accepted 31 October 2003
Abstract Electrical discharge machining became a commercial process after the discovery of the importance of the dielectric fluid, which affects factors such as productivity and quality. Health, safety and environment are also important factors, particularly when oil-based fluids are used. This paper presents a literature survey on the use of dielectric fluids that provide an alternative to hydrocarbon oil. It has been reported that water-based dielectrics may replace oil-based fluids in die sink applications. Gaseous dielectrics such as oxygen may also be the alternative. Nonetheless, these need further research in order to be commercially viable. © 2004 Elsevier B.V. All rights reserved. Keywords: Electrical discharge machining; Dielectric fluids; Die sink; Environment
1. Introduction Electrical discharge machining (EDM) is a highly developed technology which accounts for about 7% of all machine tool sales in the world [1]. EDM can be applied in a very wide range of operations including the manufacture of moulds and dies, surface texturing of steel rolls, surface alloying, production of aero engine components, production of components for electronic industries and manufacture of metallic prosthesis. The bases of EDM were probably established already in 1694, when Sir Robert Boyle described the phenomena of electric discharges in a gap [2]. However, it was only in early 1940 that electrical discharge machining started to become a well-know manufacturing process when Boris and Natalie I. Lazarenko discovered the decisive role of the dielectric fluid [3]. Since then, EDM has experienced a dramatic evolution. There are different variations of EDM processes; these can be classified according to the type of dielectric fluid used. Die sink EDM generally operates with hydrocarbon oil, while wire, micro-EDM and fast hole drilling usually work with deionised water. The dielectric fluid fulfils an extremely important function regarding productivity, costs and quality of the machined parts. Health, safety and environment are also important as∗ Corresponding author. Tel.: +44-115-9513738. E-mail address: [email protected] (F.N. Leão).
0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2003.10.043
pects, particularly when hydrocarbon oil is used. This work will cover a review on the performance of water-based dielectrics and gaseous dielectrics for die sink EDM, aiming to have a view on the potential of dielectrics alternatives to hydrocarbon oil.
2. Water-based dielectrics for die sink EDM Research over the last 20 years has involved the use of plain water, water mixed with organic compounds and commercial water-based dielectrics. 2.1. Performance of plain water The performance of plain water in terms of material removal rate and electrode wear is generally lower when compared to that obtained with hydrocarbon oils in die sink EDM. However, the use of deionised water or even tap water may result in higher levels of material removal rate in some special situations such as when a brass electrode at negative polarity is used [4]; pulse durations smaller than 500 s are employed [5] and machining of Ti–6A1–4V with a copper electrode [6]. Erden and Temel [4] showed that machining a steel workpiece with a negative brass electrode in deionised water and with pulse time ranging from 400 to 1500 s resulted in improved performance (higher material removal rate and lower
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electrode wear) when compared to performing the same operation in a hydrocarbon oil. For a pulse time of 800 s, material removal rate was approximately 60% higher and electrode wear 25% lower. Jilani and Pandey [5] reported that for pulse durations less than 500 s, the performance of tap water was better than distilled water and hydrocarbon oil, for surface roughness ranging from 40 to 60 m. Pulses of longer duration were associated with extensive gas accumulation in the gap, which affected the breakdown characteristics of the dielectric, decreasing the material removal rate. Distilled water was reported [6] as superior to hydrocarbon oil with a Ti–6Al–4V workpiece and copper electrode. When hydrocarbon oil was used, the decomposed carbon adhered to the surface of the electrode and titanium carbide (TiC) was formed on the workpiece surface. It was suggested that since the carbide has a higher melting point, the impulsive force of discharge is unstable, thus reducing the material removal rate. When distilled water was used, no carbon adhered to the surface of the electrode and titanium oxide (TiO2 ) was formed on the workpiece surface. As the oxide has a lower melting point than the carbide, the impulsive force of discharge was much more stable and the material removal rate was higher. Although the use of plain water results in a better performance in some situations, hydrocarbon oils are superior in a wider range of machining conditions. This has been attributed to the lower viscosity of water, which produces less restriction of the discharge channel, thus reducing the energy density and, as consequence, decreasing the material removal rate [7]. Moreover, the large amount of energy required to heat and vaporise water compared with oil results in lower gas pressure in the gap. Consequently the molten metal is not removed properly in every discharge, because of the insufficient pressure produced by the burst of water [8]. 2.2. Performance of water mixed with organic compounds In an attempt to improve the performance of deionised water, some authors [7,9,10] have studied the feasibility of adding organic compounds with large molecular structures such as ethylene glycol, glycerine, polyethylene glycol 200, polyethylene glycol 400, polyethylene glycol 600, dextrose and sucrose. Besides raising the viscosity of the working fluid (leading to more effective restriction of the discharge channel) [7], such compounds would be decomposed by the sparks producing gases with higher pressure than those produced by the decomposition of pure water. This would improve the removal of the molten metal out from the craters, increasing the material removal rate [9]. Masuzawa et al. [9] argued that solutions containing organic compounds with larger molecular weights were more efficient for material removal rate. A solution with high concentration of polyethylene glycol 600 had a performance compatible with Mitsubishi EDF oil.
König and Jörres [7] found that the best performance in terms of productivity/costs was achieved with a solution of glycerine with 87% of concentration. Rough machining of tool steel 56 NiCrMoV 7 with graphite electrodes resulted in higher material removal (40%) and lower relative wear (90%) when compared with hydrocarbon oil. In a more recent work, König et al. [10] reported that water-based dielectrics with a glycerine concentration ranging from 50 to 60% are suitable both for roughing and finishing machining. Moreover, an increase in the removal rate of up to 100% can be achieved in intensive machining of large areas such as forging dies. 2.3. Performance of commercial water-based dielectrics Commercial water-based fluids, whose performance have already been investigated include Elbolub (manufactured by Elotherm, Germany), Vitol QL (manufactured by Sodick, Japan) and Ionorex 500 plus. The latter has been recently developed by Oel-Held (Germany) and a consortium formed with other companies and universities, aiming to improve EDM technology (in economic and ecological terms) for production of moulds and dies. Dünnebacke [11] investigated the material removal rate and electrode wear achieved with Elbolub during the machining of parts such as crankshaft dies, forging dies for reversing ratchet handle and steering knuckle upper dies. In roughing operations, material removal rate achieved with Elbolub has been reported to be 2–3 times higher than that achieved with hydrocarbon oil. On the other hand, the high material removal rate was only achieved with graphite electrodes, which had higher wear than that seen when machining with oil. Dewes et al. [12] compared the performance of Ionorex 500 with hydrocarbon oil BP180 and deionised water mixed with a corrosion inhibitor when die sink machining Inconel 718. The lowest material removal rate was achieved with deionised water. Removal rates obtained with Ionorex were similar to those obtained with BP180; however, the highest values of relative electrode wear were achieved with BP180. In die sink EDM, it has been usual to keep the tool electrode and the workpiece immersed in the dielectric fluid. Sometimes EDM users apply the dielectric directly into the working gap by means of a jet, without submerging the electrode and the workpiece in the dielectric tank. Karasawa and Kunieda [13] have compared the machining performance of water-based dielectric Sodick VITOL-QL using a side jet, and submerging the workpiece and the electrode into the dielectric tank. The authors reported that the material removal rate achieved with the side jet was 20% higher than that of the submerged method, due to better flushing conditions. The improvement of performance using side jets may also be achieved in oil-based EDM, but fire risk is considerably higher when compared to the submerged method.
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2.4. Surface effects Workpiece surfaces machined with water-based dielectrics and hydrocarbon oil are different both in appearance and in terms of their surface roughness values. Hydrocarbon oil usually results in a dirty appearance with carbon particles inside and around the craters. Deionised water usually results in oxides on the machined surfaces (particularly when the workpiece material is steel) and lower values of surface roughness [4,7,8]. EDM spark temperatures may range from 9000 to 30 000 K during the on time and they will depend on the type of dielectric used, i.e., 20 850 K for water and 17 175 K for hydrocarbon oil [14]. Therefore, metallurgical alterations and, as a consequence, changes in the mechanical properties of the workpieces are expected to occur. Two distinct layers are identified in the surface of parts produced with EDM: the white or recast layer and the heat-affected zone [15]. Since water-based dielectrics and hydrocarbon oils have different chemical compositions and thermal conductivities, it is expected that the formation of the two layers and the associated mechanical properties will depend on the type of the dielectric used. It has been reported [16–18] that a process of carburisation occurs in the white layer of steel workpieces machined with hydrocarbon dielectric. In contrast, decarburisation of the white layer is observed when deionised water is used. Carburisation occurs due to the thermal decomposition of the carbon found in the hydrocarbon oil. Atoms of carbon transfer to the surface of the white layer, increasing the carbon concentration [18]. The increase of carbon content in the white layer may increase the microhardness [16]. Decarburisation is the decrease of carbon content of the white layer due to the binding of carbon (from steel) with hydrogen and oxygen found within water [18]. Kruth et al. [16] observed that the white layers of C 35 steel parts machined in hydrocarbon oil contained about four times more carbon than the base material. When machined in deionised water, the white layer underwent a decrease of nearly 50% in carbon content of the base material. One important aspect that characterises the white layer is the presence of micro-cracks, which decrease the fatigue strength of the machined parts. Kruth et al. [16] found a smaller number of cracks in the white layer of parts of steel machined with water, compared to those machined with oil. In contrast, Chen et al. [6] observed a larger number of cracks on the whiter layer of parts of Ti–6Al–4V machined with deionised water. Kranz et al. [17] studied the microstructure of the surface layer and the corresponding toughness of different types of tool steels machined with commercial water-based dielectric, Elbolub, and a hydrocarbon oil. The authors have demonstrated that the thickness of the heat-affected zone was more pronounced with Elbolub than with oil. This was attributed to the differences of viscosity and thermal conductivity of the dielectrics. The changes in microstructure,
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combined with cracks resulted in severe toughness losses for high-strength steels (X155CrVMo121 and S6-5-2), after machining with water-based dielectrics. On the other hand, the authors found that, for lower-strength steels (56 NiCrMoV 7) there were no significant changes in toughness.
3. EDM with gaseous dielectrics The use of a dielectric liquid as the working medium for EDM has never been questioned since Lazarenko discovered its decisive role in the performance of the process. One of the main functions of the dielectric is the restriction of the spark in order to have a higher density of energy and as consequence a higher performance. This has been reported to occur only when a dielectric liquid is used. If the sparks happened in the air, for example, the erosion effect would be very small because the electrical discharge would spread in the gap, loosing its energy. The bubble of vapour resulted from the spark into the dielectric liquid expands and the dynamic plasma pressure rises because the surrounding liquid dielectric restricts the plasma growth. When the temperature decreases during the off time, the bubble implodes removing the molten metal out of the crater. In 1985, however, a paper from NASA reported that a “dry EDM process” is possible if argon or helium is used as dielectric (cited by Kunieda and Furuoya [19]). Other working media that have been proposed to replace dielectric liquids include air and oxygen [20]. Kunieda and Yoshida [20] observed that the performance of EDM using gas (air and O2 ) can be better than that with a dielectric liquid under some especial situations, i.e., the use of a tubular electrode with very thin wall (<0.3 mm), negative polarity of the electrode, rotation/planetary motion of the electrode and high-speed gas flow. Material removal rate achieved with oxygen was higher than that achieved with air (55%) and EDM oil (21%). The greatest advantage of EDM in gas is the very low level of electrode wear (almost zero), which was reported to be independent on the pulse duration. The material removal rate of EDM with gas can be improved using ultrasonic vibrations of the workpiece, as it helps the flushing of the molten metal from the craters [21]. EDM with gaseous dielectrics could be applied for the production of small cavities (typical value of material removal rate is 8 mm3 /min). This technique, however, needs further developments before becoming commercially available.
4. Environmental aspects The manufacturing industry has been considered as one of the main sources of environmental pollution [22] and machining processes play an important role since it is the most widely used manufacturing process [23]. The minimisation
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Fig. 1. Environmental impact of die sink EDM.
of environmental impact has been an important topic for manufacturers all around the world, especially after the introduction of ISO 14000 environmental management system standards. In addition to maximising quality and cost, it is imperative for manufacturing industries to be concerned with minimising environmental impact of their processes and products. The approach that has been used in order to have a clean manufacturing that is in accordance with the requirements of the ISO 14000 is to identify and eliminate the source of pollution. If the source is inherent to the process itself, then alternative processes have to be considered [24]. One of the main sources of pollution in die sink electrical discharge machining is the dielectric fluid, particularly hydrocarbon oil. At the moment, there is no total clean manufacturing process that could replace EDM. The use of EDM in gas (air, oxygen) could be an alternative as it does not produce any waste and does not cause any adverse effect to health. However, this technique is not developed enough to be employed efficiently. The use of water-based dielectrics can be a solution to minimise the environmental problems in die sink EDM. The environmental impact resulting from the use of die sink EDM is shown in Fig. 1. During operation, the emissions resulting from the dielectric breakdown can be easily inhaled by the operator and may cause an adverse effect to health, especially when hydrocarbon oils are used. More-
over, hydrocarbon oils may cause skin problems such as dermatitis [25]. At the end of the operation, the sludge (materials removed from both the workpiece and the tool), dielectric waste, filter cartridges and deionising resins need to be disposed of appropriately; otherwise, there is a possibility of both the land and water being polluted. Wastes of dielectric oil are very toxic and cannot be recycled [26]; their disposal must follow environmental regulations. In contrast, deionised water wastes may simply be released via municipal sewer pipes, if the debris is effectively removed [27]. Although wastes of deionising resins and water-based dielectrics are less toxic when compared to hydrocarbon oil, they also need to be disposed of following environmental regulations. An approach that could be used to reduce wastes of water-based dielectric is the application of the fluid in the gap by means of a jet, instead of immersing the workpiece and the electrode in the dielectric tank. This would greatly reduce the volume of fluid used. This approach is not recommended for hydrocarbon oils due to the risk of fire. One of the main problems of die sink EDM is the high amount of energy consumed, especially when compared with conventional processes such as milling, for the same levels of material removal rate. This indirectly affects the environment, as more waste is produced in order to produce more electricity by means of thermal and/or nuclear power plants. The energy consumed in the spark gap, which is the effective energy for the erosion of the material, is usually less than 20% of the total input of electrical energy [28]. On the other hand, the energy consumed by the dielectric system may represent 50% of the total input of electrical energy, especially when low values of peak current are used [28]. The use of water as an alternative to hydrocarbon may affect those numbers as they have different values of specific weight, viscosity, dielectric strength and different ionisation mechanisms. The EDM process generates gases and fumes due to the thermal decomposition of the dielectric. Table 1 shows some of the substances which can be found in the emissions when the dielectric used is oil-based, deionised water and commercial water-based fluids. It can be seen that deionised water produces the lowest number of substances, which are much less hazardous to the operator and to the environment. Some of those substances, e.g., benzene, benzopyrene, are classified into norms and are submitted to severe prescriptions in matter of toxicology and limit values of concentration. Benzopyrene and benzene are considered as carcinogenic.
Table 1 Substances generated by die sink EDM with different types of dielectric fluid Oil-based [29–31]
Deionised water [27,30]
Commercial water-based [11,29]
Polycyclic aromatic hydrocarbons (e.g., benzopyrene); parafinnic vapours of hydrocarbons; oil mists (aerosol); metallic particles; nitroaromatic compounds; aldehydes; acetylene; ethylene; hydrogen; carbon dioxide; carbon monoxide; carbon black; xylene; butyl alcohols; butyl acetates
Water vapours; carbon monoxide; nitrogen oxide; ozone; metallic particles; chloride
Fluorite; chlorite; nitrite; bromide; nitrate; phosphate; sulphate; carbon dioxide; ozone; carbon monoxide; xylene; formaldehyde; toluene; benzol
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However, some studies [29,30] have shown that the concentration of both chemicals may be inferior to the MAK values (maximum allowable concentration).
Water-based dielectrics can replace hydrocarbon oils as they have higher performance and are more environmentally suitable.
5. Costs
Acknowledgements
Few publications are available on operating costs of die sink EDM using fluids alternative to hydrocarbon oil. Dünnebacke [11] found that the total operating cost when machining with commercial water-based Elbolub was ∼36% higher than with BP180 hydrocarbon. One of the most important costs was the water deionisation process. Costs that were found to be higher when water is used include equipment depreciation (21%), electrical energy (15%), dielectric losses (78%) and filtering aids (57%). However, the operating cost per part was ∼42% lower, since the material removal rate achieved with Elbolub was 2.33 times higher than that achieved with oil. Costs for waste disposal have been regarded as the same for both water-based dielectric and hydrocarbon oils [10]. Another important aspect that may greatly affect costs is the risk of fire of hydrocarbon oil.
The authors would like to thank CAPES Foundation (Brazil) for supporting this work.
6. Conclusions There is extensive evidence that hydrocarbon oils are more efficient than deionised/distilled water in die sink applications. However, water may result in higher levels of material removal rate in some situations. Some authors have studied the feasibility of adding organic compounds to deionised water and obtained a performance higher than that of hydrocarbon oils for roughing and finishing operations. Increase in material removal rates of up to 100% in roughing operations has been reported. Some commercial water-based fluids have performance similar or higher than that of hydrocarbon oils. A specific product may provide material removal rates 2–3 times higher than that achieved with hydrocarbon oil and lower cost per part (approximately 40%). Workpiece surface roughness/integrity is dependent on the type of dielectric fluid. Surface roughness produced with deionised water is generally lower than that of hydrocarbon oils. Steel parts machined with hydrocarbon have an increase in the carbon content. In contrast, parts machined with water have a decrease in the concentration of carbon. As a result, the microhardness of the white layer is generally higher when hydrocarbon oil is used as the dielectric. A thicker workpiece heat-affected zone with higher concentration of micro-cracks is produced with water-based dielectrics as opposed to hydrocarbon oils. Gaseous dielectrics such as air and oxygen can provide higher material removal rates than that with hydrocarbon oil, under some very special machining conditions. This application needs further research in order to be commercially viable.
References [1] H. Moser, Growth industries rely on EDM, Manuf. Eng. 127 (2001) 62–68. [2] D.F. Dauw, B.M. Schumacher, Milestones of worldwide EDM research activities, in: Proceedings of the Ninth International Symposium on Electromachining (ISEM 9), Nagoya, Japan, 1989, pp. 250– 254. [3] C. Frei, New methods of conditioning the dielectric liquid in EDM, in: Proceedings of the 10th International Symposium on Electromachining (ISEM X), Magdeburg, Germany, 1992, pp. 84– 87 [4] A. Erden, D. Temel, Investigation on the use of water as a dielectric liquid in electric discharge machining, in: Proceedings of the 22nd Machine Tool Design and Research Conference, Manchester, 1981, pp. 437–440. [5] S.T. Jilani, P.C. Pandey, Experimental investigations into the performance of water as dielectric in EDM, Int. J. Mach. Tool Des. Res. 24 (1984) 31–43. [6] S.L. Chen, B.H. Yan, F.Y. Huang, Influence of kerosene and distilled water as dielectrics on the electric discharge machining characteristics of Ti–6Al–4V, J. Mater. Process. Technol. 87 (1999) 107–111. [7] W. König, L. Jörres, Aqueous solutions of organic compounds as dielectrics for EDM sinking, Ann. CIRP 36 (1987) 105–109. [8] T. Masuzawa, Machining characteristics of EDM using water as dielectric fluid, in: Proceedings of the 22nd Machine Tool Design and Research Conference, Manchester, 1981, pp. 441–447. [9] T. Masuzawa, K. Tanaka, Y. Nakamura, Water-based dielectric solution for EDM, Ann. CIRP 32 (1983) 119–122. [10] W. König, F. Klocke, M. Sparrer, EDM—sinking using water-based dielectrics and electropolishing—a new manufacturing sequence in tool-making, in: Proceedings of the 11th International Symposium on Electromachining (ISEM XI), Lausanne, Switzerland, 1995, pp. 225– 234. [11] G. Dünnebacke, High performance electrical discharge machining using a water-based dielectric, in: Proceedings of the 10th International Symposium for Electromachining (ISEM X), Magdeburg, Germany, 1992, pp. 170–182. [12] R. Dewes, D. Aspinwall, J. Burrows, M. Paul, F. El-Menshawy, High speed machining-multi-function/hybrid systems, in: Proceedings of the Fourth International Conference on Industrial Tooling, Southampton, UK, 2001, pp. 91–100. [13] T. Karasawa, M. Kunieda, EDM capability with poured dielectric fluids without a tub, Bull. Jpn. Soc. Precision Eng. 24 (1990) 217– 218. [14] B.W. Pillans, M.H. Evensen, H.F. Taylor, P.T. Eubank, Fiber optic diagnostic techniques applied to electrical discharge machining sparks, J. Appl. Phys. 91 (2002) 1780–1786. [15] J.A. Mc Geough, Advanced Methods of Machining, Chapman & Hall, London, 1988 (ISBN 0412319705). [16] J.P. Kruth, L. Stevens, L. Froyen, B. Lauwers, K.U. Leuven, Study of the white layer of a surface machined by die-sinking electrodischarge machining, Ann. CIRP 44 (1995) 169–172.
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[17] R. Kranz, F. Wendl, K.-D. Wupper, Influence of EDM conditions on the toughness of tool steels, Thyssen Edelstahl Technische Berichte (1990) 100–105. [18] I. Ogata, Y. Mukoyama, Carburizing and decarburizing phenomena in EDM’d surface, Int. J. Jpn. Soc. Precision Eng. 27 (1993) 197–202. [19] M. Kunieda, S. Furuoya, Improvement of EDM efficiency by supplying oxygen gas into gap, Ann. CIRP 40 (1991) 215–218. [20] M. Kunieda, M. Yoshida, Electrical discharge machining in gas, Ann. CIRP 46 (1997) 143–146. [21] Q.H. Zhang, J.H. Zhang, J.X. Deng, Y. Qin, Z.W. Niu, Ultrasonic vibration electrical discharge machining in gas, J. Mater. Process. Technol. 129 (2002) 135–138. [22] X.C. Tan, F. Liu, H.J. Cao, H. Zhang, A decision-making framework model of cutting fluid selection for green manufacturing and a case study, J. Mater. Process. Technol. 129 (2002) 467–470. [23] A.A. Munoz, P. Sheng, An analytical approach for determining the environmental impact of machining process, J. Mater. Process. Technol. 53 (1995) 736–758. [24] G. Byrne, E. Scholta, Environmentally clean machining process—a strategic approach, Ann. CIRP 42 (1993) 471–474. [25] C.L. Goh, S.F. Ho, Contact dermatitis from dielectric fluids in electrodischarge machining, Contact Dermatitis 28 (1993) 134–138.
[26] S.H. Yeo, H.C. Tan, A.K. New, Assessment of waste streams in electric-discharge machining for environmental impact analysis, Proc. Inst. Mech. Eng. B: J. Eng. Manuf. 212 (1998) 393–401. [27] G.N. Levy, Environmentally friendly and high capacity dielectric regeneration for wire EDM, Ann. CIRP 42 (1993) 227–230. [28] Z. Wansheng, H. Yonghui, G. Liming, L. Jinchun, A measuring and evaluating system of the utilization ratio of electrical energy in EDM, in: Proceedings of the 11th International Symposium on Electromachining (ISEM XI), Lausanne, Switzerland, 1995, pp. 253– 259. [29] S. Evertz, A. Eisentraeger, W. Dotti, F. Klocke, A. Karden, G. Antonoglou, Environmental and industrial hygiene in connection with electrical discharge machining at high discharge energies, in: Proceedings of the 13th International Symposium on Electromachining (ISEM XIII), vol. I, 2001, pp. 193–210. [30] B. Bommeli, Study of the harmful emanations resulting from the machining by electro-erosion, in: Proceedings of the Seventh International Symposium on Electromachining (ISEM VII), 1983, pp. 469– 478. [31] T.S. Kokhanovskaya, EDM working fluids, in: Proceedings of the Seventh International Symposium on Electromachining (ISEM VII), 1983, pp. 251–264.
A review on the use of environmentally-friendly dielectric fluids in electrical discharge machining Fábio N. Leão∗ , Ian R. Pashby School of Mechanical, Materials, Manufacturing Engineering and Management, University of Nottingham, University Park, Nottingham NG7 2RD, UK Accepted 31 October 2003
Abstract Electrical discharge machining became a commercial process after the discovery of the importance of the dielectric fluid, which affects factors such as productivity and quality. Health, safety and environment are also important factors, particularly when oil-based fluids are used. This paper presents a literature survey on the use of dielectric fluids that provide an alternative to hydrocarbon oil. It has been reported that water-based dielectrics may replace oil-based fluids in die sink applications. Gaseous dielectrics such as oxygen may also be the alternative. Nonetheless, these need further research in order to be commercially viable. © 2004 Elsevier B.V. All rights reserved. Keywords: Electrical discharge machining; Dielectric fluids; Die sink; Environment
1. Introduction Electrical discharge machining (EDM) is a highly developed technology which accounts for about 7% of all machine tool sales in the world [1]. EDM can be applied in a very wide range of operations including the manufacture of moulds and dies, surface texturing of steel rolls, surface alloying, production of aero engine components, production of components for electronic industries and manufacture of metallic prosthesis. The bases of EDM were probably established already in 1694, when Sir Robert Boyle described the phenomena of electric discharges in a gap [2]. However, it was only in early 1940 that electrical discharge machining started to become a well-know manufacturing process when Boris and Natalie I. Lazarenko discovered the decisive role of the dielectric fluid [3]. Since then, EDM has experienced a dramatic evolution. There are different variations of EDM processes; these can be classified according to the type of dielectric fluid used. Die sink EDM generally operates with hydrocarbon oil, while wire, micro-EDM and fast hole drilling usually work with deionised water. The dielectric fluid fulfils an extremely important function regarding productivity, costs and quality of the machined parts. Health, safety and environment are also important as∗ Corresponding author. Tel.: +44-115-9513738. E-mail address: [email protected] (F.N. Leão).
0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2003.10.043
pects, particularly when hydrocarbon oil is used. This work will cover a review on the performance of water-based dielectrics and gaseous dielectrics for die sink EDM, aiming to have a view on the potential of dielectrics alternatives to hydrocarbon oil.
2. Water-based dielectrics for die sink EDM Research over the last 20 years has involved the use of plain water, water mixed with organic compounds and commercial water-based dielectrics. 2.1. Performance of plain water The performance of plain water in terms of material removal rate and electrode wear is generally lower when compared to that obtained with hydrocarbon oils in die sink EDM. However, the use of deionised water or even tap water may result in higher levels of material removal rate in some special situations such as when a brass electrode at negative polarity is used [4]; pulse durations smaller than 500 s are employed [5] and machining of Ti–6A1–4V with a copper electrode [6]. Erden and Temel [4] showed that machining a steel workpiece with a negative brass electrode in deionised water and with pulse time ranging from 400 to 1500 s resulted in improved performance (higher material removal rate and lower
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electrode wear) when compared to performing the same operation in a hydrocarbon oil. For a pulse time of 800 s, material removal rate was approximately 60% higher and electrode wear 25% lower. Jilani and Pandey [5] reported that for pulse durations less than 500 s, the performance of tap water was better than distilled water and hydrocarbon oil, for surface roughness ranging from 40 to 60 m. Pulses of longer duration were associated with extensive gas accumulation in the gap, which affected the breakdown characteristics of the dielectric, decreasing the material removal rate. Distilled water was reported [6] as superior to hydrocarbon oil with a Ti–6Al–4V workpiece and copper electrode. When hydrocarbon oil was used, the decomposed carbon adhered to the surface of the electrode and titanium carbide (TiC) was formed on the workpiece surface. It was suggested that since the carbide has a higher melting point, the impulsive force of discharge is unstable, thus reducing the material removal rate. When distilled water was used, no carbon adhered to the surface of the electrode and titanium oxide (TiO2 ) was formed on the workpiece surface. As the oxide has a lower melting point than the carbide, the impulsive force of discharge was much more stable and the material removal rate was higher. Although the use of plain water results in a better performance in some situations, hydrocarbon oils are superior in a wider range of machining conditions. This has been attributed to the lower viscosity of water, which produces less restriction of the discharge channel, thus reducing the energy density and, as consequence, decreasing the material removal rate [7]. Moreover, the large amount of energy required to heat and vaporise water compared with oil results in lower gas pressure in the gap. Consequently the molten metal is not removed properly in every discharge, because of the insufficient pressure produced by the burst of water [8]. 2.2. Performance of water mixed with organic compounds In an attempt to improve the performance of deionised water, some authors [7,9,10] have studied the feasibility of adding organic compounds with large molecular structures such as ethylene glycol, glycerine, polyethylene glycol 200, polyethylene glycol 400, polyethylene glycol 600, dextrose and sucrose. Besides raising the viscosity of the working fluid (leading to more effective restriction of the discharge channel) [7], such compounds would be decomposed by the sparks producing gases with higher pressure than those produced by the decomposition of pure water. This would improve the removal of the molten metal out from the craters, increasing the material removal rate [9]. Masuzawa et al. [9] argued that solutions containing organic compounds with larger molecular weights were more efficient for material removal rate. A solution with high concentration of polyethylene glycol 600 had a performance compatible with Mitsubishi EDF oil.
König and Jörres [7] found that the best performance in terms of productivity/costs was achieved with a solution of glycerine with 87% of concentration. Rough machining of tool steel 56 NiCrMoV 7 with graphite electrodes resulted in higher material removal (40%) and lower relative wear (90%) when compared with hydrocarbon oil. In a more recent work, König et al. [10] reported that water-based dielectrics with a glycerine concentration ranging from 50 to 60% are suitable both for roughing and finishing machining. Moreover, an increase in the removal rate of up to 100% can be achieved in intensive machining of large areas such as forging dies. 2.3. Performance of commercial water-based dielectrics Commercial water-based fluids, whose performance have already been investigated include Elbolub (manufactured by Elotherm, Germany), Vitol QL (manufactured by Sodick, Japan) and Ionorex 500 plus. The latter has been recently developed by Oel-Held (Germany) and a consortium formed with other companies and universities, aiming to improve EDM technology (in economic and ecological terms) for production of moulds and dies. Dünnebacke [11] investigated the material removal rate and electrode wear achieved with Elbolub during the machining of parts such as crankshaft dies, forging dies for reversing ratchet handle and steering knuckle upper dies. In roughing operations, material removal rate achieved with Elbolub has been reported to be 2–3 times higher than that achieved with hydrocarbon oil. On the other hand, the high material removal rate was only achieved with graphite electrodes, which had higher wear than that seen when machining with oil. Dewes et al. [12] compared the performance of Ionorex 500 with hydrocarbon oil BP180 and deionised water mixed with a corrosion inhibitor when die sink machining Inconel 718. The lowest material removal rate was achieved with deionised water. Removal rates obtained with Ionorex were similar to those obtained with BP180; however, the highest values of relative electrode wear were achieved with BP180. In die sink EDM, it has been usual to keep the tool electrode and the workpiece immersed in the dielectric fluid. Sometimes EDM users apply the dielectric directly into the working gap by means of a jet, without submerging the electrode and the workpiece in the dielectric tank. Karasawa and Kunieda [13] have compared the machining performance of water-based dielectric Sodick VITOL-QL using a side jet, and submerging the workpiece and the electrode into the dielectric tank. The authors reported that the material removal rate achieved with the side jet was 20% higher than that of the submerged method, due to better flushing conditions. The improvement of performance using side jets may also be achieved in oil-based EDM, but fire risk is considerably higher when compared to the submerged method.
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2.4. Surface effects Workpiece surfaces machined with water-based dielectrics and hydrocarbon oil are different both in appearance and in terms of their surface roughness values. Hydrocarbon oil usually results in a dirty appearance with carbon particles inside and around the craters. Deionised water usually results in oxides on the machined surfaces (particularly when the workpiece material is steel) and lower values of surface roughness [4,7,8]. EDM spark temperatures may range from 9000 to 30 000 K during the on time and they will depend on the type of dielectric used, i.e., 20 850 K for water and 17 175 K for hydrocarbon oil [14]. Therefore, metallurgical alterations and, as a consequence, changes in the mechanical properties of the workpieces are expected to occur. Two distinct layers are identified in the surface of parts produced with EDM: the white or recast layer and the heat-affected zone [15]. Since water-based dielectrics and hydrocarbon oils have different chemical compositions and thermal conductivities, it is expected that the formation of the two layers and the associated mechanical properties will depend on the type of the dielectric used. It has been reported [16–18] that a process of carburisation occurs in the white layer of steel workpieces machined with hydrocarbon dielectric. In contrast, decarburisation of the white layer is observed when deionised water is used. Carburisation occurs due to the thermal decomposition of the carbon found in the hydrocarbon oil. Atoms of carbon transfer to the surface of the white layer, increasing the carbon concentration [18]. The increase of carbon content in the white layer may increase the microhardness [16]. Decarburisation is the decrease of carbon content of the white layer due to the binding of carbon (from steel) with hydrogen and oxygen found within water [18]. Kruth et al. [16] observed that the white layers of C 35 steel parts machined in hydrocarbon oil contained about four times more carbon than the base material. When machined in deionised water, the white layer underwent a decrease of nearly 50% in carbon content of the base material. One important aspect that characterises the white layer is the presence of micro-cracks, which decrease the fatigue strength of the machined parts. Kruth et al. [16] found a smaller number of cracks in the white layer of parts of steel machined with water, compared to those machined with oil. In contrast, Chen et al. [6] observed a larger number of cracks on the whiter layer of parts of Ti–6Al–4V machined with deionised water. Kranz et al. [17] studied the microstructure of the surface layer and the corresponding toughness of different types of tool steels machined with commercial water-based dielectric, Elbolub, and a hydrocarbon oil. The authors have demonstrated that the thickness of the heat-affected zone was more pronounced with Elbolub than with oil. This was attributed to the differences of viscosity and thermal conductivity of the dielectrics. The changes in microstructure,
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combined with cracks resulted in severe toughness losses for high-strength steels (X155CrVMo121 and S6-5-2), after machining with water-based dielectrics. On the other hand, the authors found that, for lower-strength steels (56 NiCrMoV 7) there were no significant changes in toughness.
3. EDM with gaseous dielectrics The use of a dielectric liquid as the working medium for EDM has never been questioned since Lazarenko discovered its decisive role in the performance of the process. One of the main functions of the dielectric is the restriction of the spark in order to have a higher density of energy and as consequence a higher performance. This has been reported to occur only when a dielectric liquid is used. If the sparks happened in the air, for example, the erosion effect would be very small because the electrical discharge would spread in the gap, loosing its energy. The bubble of vapour resulted from the spark into the dielectric liquid expands and the dynamic plasma pressure rises because the surrounding liquid dielectric restricts the plasma growth. When the temperature decreases during the off time, the bubble implodes removing the molten metal out of the crater. In 1985, however, a paper from NASA reported that a “dry EDM process” is possible if argon or helium is used as dielectric (cited by Kunieda and Furuoya [19]). Other working media that have been proposed to replace dielectric liquids include air and oxygen [20]. Kunieda and Yoshida [20] observed that the performance of EDM using gas (air and O2 ) can be better than that with a dielectric liquid under some especial situations, i.e., the use of a tubular electrode with very thin wall (<0.3 mm), negative polarity of the electrode, rotation/planetary motion of the electrode and high-speed gas flow. Material removal rate achieved with oxygen was higher than that achieved with air (55%) and EDM oil (21%). The greatest advantage of EDM in gas is the very low level of electrode wear (almost zero), which was reported to be independent on the pulse duration. The material removal rate of EDM with gas can be improved using ultrasonic vibrations of the workpiece, as it helps the flushing of the molten metal from the craters [21]. EDM with gaseous dielectrics could be applied for the production of small cavities (typical value of material removal rate is 8 mm3 /min). This technique, however, needs further developments before becoming commercially available.
4. Environmental aspects The manufacturing industry has been considered as one of the main sources of environmental pollution [22] and machining processes play an important role since it is the most widely used manufacturing process [23]. The minimisation
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Fig. 1. Environmental impact of die sink EDM.
of environmental impact has been an important topic for manufacturers all around the world, especially after the introduction of ISO 14000 environmental management system standards. In addition to maximising quality and cost, it is imperative for manufacturing industries to be concerned with minimising environmental impact of their processes and products. The approach that has been used in order to have a clean manufacturing that is in accordance with the requirements of the ISO 14000 is to identify and eliminate the source of pollution. If the source is inherent to the process itself, then alternative processes have to be considered [24]. One of the main sources of pollution in die sink electrical discharge machining is the dielectric fluid, particularly hydrocarbon oil. At the moment, there is no total clean manufacturing process that could replace EDM. The use of EDM in gas (air, oxygen) could be an alternative as it does not produce any waste and does not cause any adverse effect to health. However, this technique is not developed enough to be employed efficiently. The use of water-based dielectrics can be a solution to minimise the environmental problems in die sink EDM. The environmental impact resulting from the use of die sink EDM is shown in Fig. 1. During operation, the emissions resulting from the dielectric breakdown can be easily inhaled by the operator and may cause an adverse effect to health, especially when hydrocarbon oils are used. More-
over, hydrocarbon oils may cause skin problems such as dermatitis [25]. At the end of the operation, the sludge (materials removed from both the workpiece and the tool), dielectric waste, filter cartridges and deionising resins need to be disposed of appropriately; otherwise, there is a possibility of both the land and water being polluted. Wastes of dielectric oil are very toxic and cannot be recycled [26]; their disposal must follow environmental regulations. In contrast, deionised water wastes may simply be released via municipal sewer pipes, if the debris is effectively removed [27]. Although wastes of deionising resins and water-based dielectrics are less toxic when compared to hydrocarbon oil, they also need to be disposed of following environmental regulations. An approach that could be used to reduce wastes of water-based dielectric is the application of the fluid in the gap by means of a jet, instead of immersing the workpiece and the electrode in the dielectric tank. This would greatly reduce the volume of fluid used. This approach is not recommended for hydrocarbon oils due to the risk of fire. One of the main problems of die sink EDM is the high amount of energy consumed, especially when compared with conventional processes such as milling, for the same levels of material removal rate. This indirectly affects the environment, as more waste is produced in order to produce more electricity by means of thermal and/or nuclear power plants. The energy consumed in the spark gap, which is the effective energy for the erosion of the material, is usually less than 20% of the total input of electrical energy [28]. On the other hand, the energy consumed by the dielectric system may represent 50% of the total input of electrical energy, especially when low values of peak current are used [28]. The use of water as an alternative to hydrocarbon may affect those numbers as they have different values of specific weight, viscosity, dielectric strength and different ionisation mechanisms. The EDM process generates gases and fumes due to the thermal decomposition of the dielectric. Table 1 shows some of the substances which can be found in the emissions when the dielectric used is oil-based, deionised water and commercial water-based fluids. It can be seen that deionised water produces the lowest number of substances, which are much less hazardous to the operator and to the environment. Some of those substances, e.g., benzene, benzopyrene, are classified into norms and are submitted to severe prescriptions in matter of toxicology and limit values of concentration. Benzopyrene and benzene are considered as carcinogenic.
Table 1 Substances generated by die sink EDM with different types of dielectric fluid Oil-based [29–31]
Deionised water [27,30]
Commercial water-based [11,29]
Polycyclic aromatic hydrocarbons (e.g., benzopyrene); parafinnic vapours of hydrocarbons; oil mists (aerosol); metallic particles; nitroaromatic compounds; aldehydes; acetylene; ethylene; hydrogen; carbon dioxide; carbon monoxide; carbon black; xylene; butyl alcohols; butyl acetates
Water vapours; carbon monoxide; nitrogen oxide; ozone; metallic particles; chloride
Fluorite; chlorite; nitrite; bromide; nitrate; phosphate; sulphate; carbon dioxide; ozone; carbon monoxide; xylene; formaldehyde; toluene; benzol
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However, some studies [29,30] have shown that the concentration of both chemicals may be inferior to the MAK values (maximum allowable concentration).
Water-based dielectrics can replace hydrocarbon oils as they have higher performance and are more environmentally suitable.
5. Costs
Acknowledgements
Few publications are available on operating costs of die sink EDM using fluids alternative to hydrocarbon oil. Dünnebacke [11] found that the total operating cost when machining with commercial water-based Elbolub was ∼36% higher than with BP180 hydrocarbon. One of the most important costs was the water deionisation process. Costs that were found to be higher when water is used include equipment depreciation (21%), electrical energy (15%), dielectric losses (78%) and filtering aids (57%). However, the operating cost per part was ∼42% lower, since the material removal rate achieved with Elbolub was 2.33 times higher than that achieved with oil. Costs for waste disposal have been regarded as the same for both water-based dielectric and hydrocarbon oils [10]. Another important aspect that may greatly affect costs is the risk of fire of hydrocarbon oil.
The authors would like to thank CAPES Foundation (Brazil) for supporting this work.
6. Conclusions There is extensive evidence that hydrocarbon oils are more efficient than deionised/distilled water in die sink applications. However, water may result in higher levels of material removal rate in some situations. Some authors have studied the feasibility of adding organic compounds to deionised water and obtained a performance higher than that of hydrocarbon oils for roughing and finishing operations. Increase in material removal rates of up to 100% in roughing operations has been reported. Some commercial water-based fluids have performance similar or higher than that of hydrocarbon oils. A specific product may provide material removal rates 2–3 times higher than that achieved with hydrocarbon oil and lower cost per part (approximately 40%). Workpiece surface roughness/integrity is dependent on the type of dielectric fluid. Surface roughness produced with deionised water is generally lower than that of hydrocarbon oils. Steel parts machined with hydrocarbon have an increase in the carbon content. In contrast, parts machined with water have a decrease in the concentration of carbon. As a result, the microhardness of the white layer is generally higher when hydrocarbon oil is used as the dielectric. A thicker workpiece heat-affected zone with higher concentration of micro-cracks is produced with water-based dielectrics as opposed to hydrocarbon oils. Gaseous dielectrics such as air and oxygen can provide higher material removal rates than that with hydrocarbon oil, under some very special machining conditions. This application needs further research in order to be commercially viable.
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