Int J Adv Manuf Technol DOI 10.1007/s00170-012-4210-4

ORIGINAL ARTICLE

Sinking EDM in water-in-oil emulsion Yanzhen Zhang & Yonghong Liu & Renjie Ji & Baoping Cai & Yang Shen

Received: 17 November 2010 / Accepted: 26 April 2012 # Springer-Verlag London Limited 2012

Abstract In this paper, a new type of sinking electric discharge machining (EDM) dielectric–water-in-oil (W/O) emulsion is proposed, and the machining characteristics of W/O emulsion are investigated by comparing with that of kerosene. In the experiments, machining parameters such as the dielectric type, peak current, and pulse duration are changed to explore their effects on machining performance, including the material removal rate (MRR), relative electrode wear rate (REWR), and surface roughness. Experimental results revealed that W/O emulsion could be used as the dielectric fluid of sinking EDM and adopting long pulse duration and large peak current could lead to obtaining higher MRR than kerosene. Compared with kerosene, W/O emulsion is observed to cause lower carbon adhered to the electrode surface. Therefore, its REWR is higher. Statistics of the discharge waveform show that more stable discharge processes can be obtained by using W/O emulsion compared with kerosene. Furthermore, W/O emulsion is more economical and more environmentally friendly than kerosene, and it could be an alternative to kerosene in sinking EDM application. Keywords Electric discharge machining . Sinking . Dielectric . Water in oil . Emulsion

Y. Zhang : Y. Liu (*) : R. Ji : B. Cai : Y. Shen School of Mechanical and Electronic Engineering, China University of Petroleum, Dongying, Shandong 257061, China e-mail: [email protected] Y. Liu e-mail: [email protected]

1 Introduction During the sinking EDM processes, the working fluid fulfills an important function regarding productivity, costs, and quality of the machined parts; however, it is one of the main roots of environmental pollution. An optimal selection of the cutting fluid for green manufacturing is an important path to reduce the environmental pollution. The objective factors of decision-making problems in the traditional selection of working fluid are usually two: quality and cost; but from the viewpoint of green manufacturing, environmental impact should also be considered. In electrical discharge machining (EDM), a process utilizing the removal phenomenon of electrical discharge in dielectric, the working fluid plays an important role in affecting the material removal rate (MRR) and the properties of the machined surface. Most sinking EDM processes use hydrogen–carbon oil, usually kerosene-based oil, as the working fluids. However, when the kerosene-based working fluids are used, other factors, such as health, safety, and environment, must be taken into account. Kerosene, however, is inflammable and therefore undesirable as the possibility of fire hazard has always been of great concern in sinking EDM. Tönshoff et al. reported that, when using a dielectric such as mineral oil or an organic fluid for sinking EDM, hazardous fumes like polycyclic aromatic hydrocarbons, benzene, vapor of mineral oil, mineral aerosols and various by-products are generated by the dissociation of oil and its additives [1]. How to minimize the environmental impact of sinking EDM aroused by working fluid has become one of the important research issues in the field of green manufacture [2]. To alleviate the environmental impact, other dielectrics

Int J Adv Manuf Technol

such as deionized water, water mixed with organic compounds, and oil-in-water emulsions have been studied by many researchers [3–8]. Water is introduced to solve the problem of inflammability, but it has not so high productivity as EDM oil. This has been attributed by Koenig and Joerres to the lower viscosity of water, which produces less restriction of the discharge channel, thus reducing the energy density and, as a consequence, decreasing the MRR. Moreover, large amounts of energy required to heat and vaporize water compared with oil result 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. Currently, water-based dielectric is only used in the case of micro EDM [9, 10]. In an attempt to improve the performance of deionized water, Koenig and Joerres have studied the feasibility of adding organic compounds with large molecular structures. Besides raising the viscosity of the working fluid (leading to more effective restriction of the discharge channel), 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 MRR. It was confirmed that solutions containing organic compounds with larger molecular weights were more efficient for MRR. Kunieda et al. reported that the performance of EDM using gas (air and O2) can be better than that with a dielectric liquid under some special situations, i.e., the use of a tubular electrode with very thin wall (