A review on current research trends in electrical discharge machining (EDM)

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(BQ) This paper reviews the research trends in EDM on ultrasonic vibration, dry EDM machining, EDM with powder additives, EDM in water and modeling technique in predicting EDM performances

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Nội dung Text: A review on current research trends in electrical discharge machining (EDM)

ARTICLE IN PRESS International Journal of Machine Tools & Manufacture 47 (2007) 1214–1228 www.elsevier.com/locate/ijmactool A review on current research trends in electrical discharge machining (EDM) Norliana Mohd AbbasÃ, Darius G. Solomon, Md. Fuad Bahari Faculty of Mechanical Engineering, Universiti Teknologi MARA, 40450 Shah Alam, Selangor Darul Ehsan, Malaysia Received 15 November 2005; received in revised form 20 May 2006; accepted 17 August 2006 Available online 17 November 2006 Abstract Electrical discharge machining (EDM) is one of the earliest non-traditional machining processes. EDM process is based on thermoelectric energy between the work piece and an electrode. A pulse discharge occurs in a small gap between the work piece and the electrode and removes the unwanted material from the parent metal through melting and vaporising. The electrode and the work piece must have electrical conductivity in order to generate the spark. There are various types of products which can be produced using EDM such as dies and moulds. Parts of aerospace, automotive industry and surgical components can be ﬁnished by EDM. This paper reviews the research trends in EDM on ultrasonic vibration, dry EDM machining, EDM with powder additives, EDM in water and modeling technique in predicting EDM performances. r 2006 Elsevier Ltd. All rights reserved. Keywords: EDM; Ultrasonic vibration; Dry EDM; Powder additives; Dielectric; Modeling 1. Introduction Electrical discharge machining (EDM) is a non-traditional concept of machining which has been widely used to produce dies and molds. It is also used for ﬁnishing parts for aerospace and automotive industry and surgical components [1]. This technique has been developed in the late 1940s [2] where the process is based on removing material from a part by means of a series of repeated electrical discharges between tool called the electrode and the work piece in the presence of a dielectric ﬂuid [3]. The electrode is moved toward the work piece until the gap is small enough so that the impressed voltage is great enough to ionize the dielectric [4]. Short duration discharges are generated in a liquid dielectric gap, which separates tool and work piece. The material is removed with the erosive effect of the electrical discharges from tool and work piece [5]. EDM does not make direct contact between the electrode and the work piece where it can eliminate mechanical stresses, chatter and vibration problems during ÃCorresponding author. Tel.: +60 16 218 6521; fax: +60 3 5543 5160. E-mail address: chelorot@lycos.com (N. Mohd Abbas). 0890-6955/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2006.08.026 machining [1]. Materials of any hardness can be cut as long as the material can conduct electricity [6]. EDM techniques have developed in many areas. Trends on activities carried out by researchers depend on the interest of the researchers and the availability of the technology. In a book published in 1994, Rajurkar [7] has indicated some future trends activities in EDM: machining advanced materials, mirror surface ﬁnish using powder additives, ultrasonic-assisted EDM and control and automation. However, the review presented in this paper is on current EDM research trends carried out by researchers on machining techniques viz. ultrasonic vibration, dry EDM machining, EDM with powder additives and EDM in water and modeling techniques in predicting EDM performances. The areas are selected because of the novel techniques employed (ultrasonic vibration and powder additives), the environmental aspect (dry machining and EDM in water) and effort towards validating and predicting EDM performance (modeling technique). Each topic will present the activities carried out by the researchers and the development of the area that brings it to the current trends. Wire EDM is also discussed in each topic. ARTICLE IN PRESS N. Mohd Abbas et al. / International Journal of Machine Tools & Manufacture 47 (2007) 1214–1228 2. Ultrasonic vibration Introduction of ultrasonic vibration to the electrode is one of the methods used to expand the application of EDM and to improve the machining performance on difﬁcult to machine materials. The study of the effects on ultrasonic vibration of the electrode on EDM has been undertaken since mid 1980s. The higher efﬁciency gained by the employment of ultrasonic vibration is mainly attributed to the improvement in dielectric circulation which facilitates the debris removal and the creation of a large pressure change between the electrode and the work piece, as an enhancement of molten metal ejection from the surface of the work piece [8]. Zhang et al. [9] proposed spark erosion with ultrasonic frequency using a DC power supply instead of the usual pulse power supply. The pulse discharge is produced by the relative motion between the tool and work piece simplifying the equipment and reducing its cost. They have indicated that it is easy to produce a combined technology which beneﬁts from the virtues of ultrasonic machining and EDM. 2.1. Machining of microholes In 1995 Zhixin et al. [10] has developed an ultrasonic vibration pulse electro-discharge machining (UVPEDM) technique to produce holes in engineering ceramics material. They have conﬁrmed by experiment that this new technique is effective in obtaining a high material removal rate (MRR). Ogawa et al. [11] proved that the depth of microholes by EDM with ultrasonic vibration becomes as about two times as that without ultrasonic vibration and machining rate increased. Thoe et al. [12] dealt with combined ultrasonic and electrical discharge machining of ceramic coated nickel alloy. They found the following: when drilling 1 mm diameter single hole using various tool materials (tungsten, silver steel, mild steel and copper) with boron carbide abrasive slurry on ceramic coated nickel alloy work piece, mild steel is found to be the most resilient whereas the other materials fail as a result of fatigue fracture or deformation. Using ultrasonic vibration during EDM greatly increased the MRR of the work piece. Wansheng et al. [13] introduces ultrasonic vibration into micro-EDM: Ti–6Al–4 V as work piece material with 32 mm thickness, carbide YG6X electrode, 20 kHz ultrasonic vibration and 2 mm amplitude; holes with diameter of 0.2 mm and depth/diameter ratio of more then 15 can be drilled. Yan et al. [14] adopted a machining method that combined micro electrical discharge machining (MEDM) and micro ultrasonic vibration machining (MUSM). They showed that the diameter variation between the entrance and exit (DVEE) could reach a value of about 2 mm in microholes with diameters of about 150 mm and depth of 500 mm. They obtained microholes with a roundness value of about 2 mm (the maximum radius minus the minimum radius). A study on the effects of ultrasonic vibration on 1215 the EDM performance for fabricating microholes in Nitinol was done by Huang et al. [15]. Fabricating microholes in nitinol increased the machining efﬁciency more than 60 times without signiﬁcantly increasing the electrode wear. Yeo et al. [16] investigated the limitations of microholes machining capabilities as well as current, attempts to improve micrhole drilling in UEDM. Based on theories in ﬂuidization engineering and ultrasonic digassing, a method of introducing ultrasonic vibrations into MEDM processes was conceptualized and developed. 2.2. Vibration, rotary and vibro-rotary Ghoreishi and Atkinson [17] compared the effects of high and low frequency forced axial vibration of the electrode, rotation of the electrode and combinations of the methods (vibro-rotary) in respect of MRR, tool wear ratio (TWR) and surface quality (SQ) in EDM die sinking and found that vibro-rotary increases MRR by up to 35% compared with vibration EDM and by up to 100% compared with rotary EDM in semi ﬁnishing. 2.3. Theoretical model and fuzzy logic An adaptive fuzzy control system of servomechanism for electro-discharge machining with ultrasonic was studied by Zhang et al. [18] to adjust discharge pulse parameter in a timely manner and machining gap to optimize the machining state and to improve the machining efﬁciency. 2.4. Workpiece vibration A new method for micro ultrasonic machining has been developed by Egashira and Masuzawa [19]. The work piece was vibrated during machining and they have succeeded in machining microholes as small as 5 m in diameter in quartz glass and silicon. In the machining range, high tool wear posed a problem. To solve the problem, a sintered diamond tool was tested and was proven to be effective. Gao and Liu [20] found that the efﬁciency of the ultrasonic microEDM is eight times greater than micro-EDM when work piece material is stainless steel with 0.5 mm thickness and the electrode is tungsten with 43 mm diameter. Prihandana et al. [21] have studied the effect of vibrated work piece. They have shown that when the vibration was introduced on the work piece the ﬂushing effect increased. They have found that high amplitude combined with high frequency increase the MRR. 2.5. Tool wear Yu et al. [22] have studied the tool wear during the 3D micro ultrasonic machining. They showed that the tool shape remain unchanged and the tool wear has been compensated by applying the uniform wear method developed for micro EDM and its integration with CAD/ ARTICLE IN PRESS N. Mohd Abbas et al. / International Journal of Machine Tools & Manufacture 47 (2007) 1214–1228 1216 CAM to micro ultrasonic vibration process for generating accurate three-dimensional (3D) micro cavities. 30% and the roughness of the machined surface reduced from 1.95Ra to 1.7Ra. 2.6. Ultrasonic vibration in gas 2.8. Remarks Zhang et al. [23] studied the ultrasonic EDM in gas. The gas is applied through the internal hole of a thin-walled pipe electrode. The result shows that the MRR increased with respect to the increase of open voltage, pulse duration, amplitude of ultrasonic actuation, discharge current and the decrease of the wall thickness of electrode pipe while the surface roughness is increased with respect to the increase of open voltage, pulse duration and the discharge current. Zhang et al. [24] developed a theoretical model to estimate the roughness of ﬁnished surface. Before that, Zhang et al. [25] found that heat generation by oxidation of the molten and evaporated steel enhances the machining efﬁciency and surface roughness is found to be affected by gas medium. For air and oxygen gas the corresponding values of Ra was measured to be 0.032 and 0.046, respectively. Ultrasonic vibration EDM is suitable to produce deep and small holes products. Cu is most frequently selected as the tool electrode either in gas machining or in dielectric machining. This is maybe due to the characteristics of the material which stable under sparking condition [27]. The range of the ultrasonic frequency used during the experiment is between 17 and 25 kHz. Most of the experiments are evaluating on the performance of steel based work piece materials since these materials are widely used in industries. However the harder material such as alumina ceramic is also evaluated. Fig. 1 shows the progress of method in combining ultrasonic vibration with EDM from year 1995 to 2006. The method starts with vibrating the electrode followed by vibrating the work piece in year 1999, which gains popularity from year 2003 and continues until year 2006. 2.7. Wire EDM 3. Dry machining Guo et al. [26] studied the machining mechanism of wire EDM (WEDM) with ultrasonic vibration of the wire and found that the combined technology of WEDM and ultrasonic facilitates the form of multiple-channel discharge and raise the utilization ratio of the energy that leads to the improvement in cutting rate and surface roughness. High frequency vibration of wire improves the discharge concentration and reduces the probability of rupture wire. Guo et al. [8] concluded that with ultrasonic aid the cutting efﬁciency of WEDM can be increased by In dry EDM, tool electrode is formed to be thin walled pipe. High-pressure gas or air is supplied through the pipe. The role of the gas is to remove the debris from the gap and to cool the inter electrode gap. Fig. 2 shows the principle of dry EDM. The technique was developed to decrease the pollution caused by the use of liquid dielectric which leads to production of vapour during machining and the cost to manage the waste. Yu et al. [28] investigated the capability of the technique in machining cemented carbide material and compared the Workpiece vibrates [14] Tool: rotates vibrates & vibro-rotary [17] Tool rotates Wire vibrates vibrates up&down [26] [13] Wire W'piece Tool vibrates vibrates vibrates [8] [12] [18] Tool vibrates Tool vibrates Tool vibrates Tool vibrates W'piece vibrates W'piece vibrates W'piece vibrates [10] [9] [19] [25] [20] [22] [21] 2002 2003 2004 1995 1996 1997 1998 1999 2000 2001 2005 Fig. 1. Progress of method in combining ultrasonic vibration with EDM from 1995 to 2006. 2006 ARTICLE IN PRESS N. Mohd Abbas et al. / International Journal of Machine Tools & Manufacture 47 (2007) 1214–1228 1217 Fig. 5. Machining time [28]. Fig. 2. The principle of dry EDM [25]. Fig. 6. Electrode wear ratio [28]. Fig. 3. Work removal rate [28]. milling third. The lowest electrode wear ratio machining was dry EDM milling (see Fig. 6). The following sections present the progress in dry EDM based on topics: MRR and tool wear, polarity and surface roughness and improvement techniques. 3.1. MRR and tool wear Fig. 4. Electrode wear [28]. machining characteristics between oil EDM milling and oil die sinking EDM. They found that for machining the same shape oil die sinking EDM shows shorter machining time. But because oil die sinking requires time for producing electrodes, dry EDM should be more useful in actual production. The information given in this paper is interesting and they are reproduced here for better clarity. Figs. 3 and 4 show the work removal rate and electrode wear ratio in groove machining. According to the results, work removal rate of dry EDM milling is about six times larger than that of oil EDM milling, and electrode wear ratio one-third lower. In Fig. 5, it is shown that the EDM method with the shortest machining time was oil die sinking EDM, dry EDM milling was second, and oil EDM In 1991 Kunieda et al. [29] has revealed a new method to improve EDM efﬁciency by supplying oxygen gas into gap. They found that the stock removal rate is increased due to the enlarged volume of discharged crater and more frequent occurrence of discharge. Then in 1997 Kunieda et al. [30] discovered a 3D shape can be machined very precisely using a special NC tool path which can supply a uniform high-velocity air ﬂow over the working gap and MRR is improved as the concentration of oxygen in air is increased. The mechanism for minute tool electrode wear in dry EDM was studied by Yoshida and Kunieda [31]. The tool electrode wear is almost negligible for any pulse duration because the attached molten work piece material protects the tool electrode surface against wear. From observation of the cross-section of the tool electrode surface, it was found that the tool electrode wore by the depth of only 2 mm during the early stage of successive pulse discharges since the initial surface of the tool electrode was not covered with the steel layer. ZhanBo et al. [32] studied the feasibility of 3D surface machining by dry EDM to investigate the inﬂuence of depth of cut and gas pressure, pulse duration and pulse interval and the rotational speed of the tool electrode. The result shows that optimum combination between depth of ARTICLE IN PRESS 1218 N. Mohd Abbas et al. / International Journal of Machine Tools & Manufacture 47 (2007) 1214–1228 cut and gas pressure and when pulse duration 25 mm it is leads to maximum MRR and minimum tool wear. As the rotational speed increases the tool wear increases moderately. 3.2. Polarity and surface roughness A paper entitled ‘Discussion of electrical discharge machining in gas’ written by Li et al. [33] recommend positive polarity to be employed in dry EDM because electrodes play main roles in collision and ionization and in order to ensure machining process stable at the spark discharge state, a certain gas pressure is necessary to strengthen deionization in dry EDM and to keep discharge points dispersed in gap. Zhang et al. [34] developed a theoretical model to estimate the roughness of the ﬁnished surface. Experiments which have been carried out using AISI 1045 steel as work piece material and copper as the electrode show that the roughness of ﬁnished surface increases with an increase in the discharge voltage, discharge current and pulse duration. Curodeau et al. [35] proposed a new EDM process involving the usage of a thermoplastic composite electrode and air as dielectric in order to perform automated polishing of tool steel cavity. The process can reduce 44 mm Ra surface ﬁnish to 36 mm Ra. 3.3. Improvement of the technique 3.3.1. Dry ultrasonic vibration electrical discharge (dry UEDM) Zhang et al. [25] initiated a new method which is ultrasonic vibration electrical discharge (UEDM) machining in gas. The experimental result shows that the increase in open voltage, pulse duration, amplitude of ultrasonic vibration and decrease of wall thickness of the pipe can give an increase of the MRR. He also found that oxygen gas can produced greater MRR than air. Then theories of ultrasonic vibration in increasing MRR were introduced [36]. On further investigation, Zhang et al. [23] found that MRR with the same surface roughness UEDM in gas is nearly twice as much as EDM in gas but less than the conventional EDM. 3.3.2. Dry EDM milling The dry EDM technique is improved by Kunieda et al. [37] when they introduced the high speed 3D milling by dry EDM. The MRR increased when the discharge power density on the working surface exceeds a certain threshold due to thermally activated chemical reaction between the gas and work piece material. The maximum removal rate obtained was almost equal to that of high speed milling of quenched steel by a milling machine. Most researchers deal with steel as work piece when investigating the performance of dry EDM [30,37,38]. 3.3.3. Using piezoelectric actuator Kunieda et al. [38] introduced an improvement of dry EDM characteristics using piezoelectric actuator to help in controlling the gap length. To elucidate the effects of the piezoelectric actuator an EDM performance simulator was developed to evaluate the machining stability and MRR of dry EDM. 3.4. Wire EDM Furudate and Kunieda [39] conducted studies in dry WEDM. The process reaction force is negligibly small, the vibration of the wire electrode is minute and the gap distance in dry WEDM is narrower than in conventional WEDM using dielectric liquid which enables the dry WEDM to realize high accuracy in ﬁnish cutting. No corrosion of the work piece gives an advantage to dry WEDM in manufacturing high precision dies and molds. Wang and Kunieda [40] agreed that WEDM is applicable for ﬁnish cut especially for improving the straightness of the machined surface. Traveling tool electrode can remove debris from the working gap even in atmosphere and by utilizing this process as ﬁnish-cut the straightness obtained along the work’s thickness direction is better than that machined in water [41]. Kunieda and Furudate [42] found some drawbacks of dry WEDM which include lower MRR compared to conventional WEDM and streaks generated over the ﬁnished surface during the studies in high precision ﬁnish cutting by dry EDM. The drawbacks can be resolved by increasing the wire winding speed and decreasing the actual depth of cut. 3.5. Remarks To the best of our knowledge, the earliest paper mentioning about the technique was published in 1991. However, paper entitled ‘Electrical discharge machining in gas’ which was published in CIRP Annals in 1997 was referred for 18 times [43]. Researchers from Japan and China give major contribution for the progressing research in this area. The characteristics of dry EDM list by Kunieda [44] are: (1) Tool electrode wear is negligible for any pulse duration. (2) The processing reaction force is much smaller that in conventional EDM. (3) It is possible to change supplying gas according to different applications. (4) The residual stress is small since the melting resolidiﬁcation layer is thin. (5) Working gap is narrower than in conventional EDM. (6) The process is possible in vacuum condition as long as there is a gas ﬂow. (7) The machine structure can be made compact since no working basin, ﬂuid tank and ﬂuid circulation system needed.