1International Journal of Machine Tools accepted 29 April 2004AbstractWire electrical discharge machining (WEDM) is a specialised thermal machining process capable of accurately machining parts with varying hardness or complex shapes, which have sharp edges that are very difficult to be machined by the main stream machining processes. This practical technology of the WEDM process is based on the conventional EDM sparking phenomenon utilising the widely accepted non-contact technique of material removal. Since the introduction of the process, WEDM has evolved from a simple means of making tools and dies to the best alternative of producing micro-scale parts with the highest degree of dimensional accuracy and surface finish quality.Over the years, the WEDM process has remained as a competitive and economical machining option fulfilling the demanding machining requirements imposed by the short product development cycles and the growing cost pressures. However, the risk of wire breakage and bending has undermined the full potential of the process drastically reducing the efficiency and accuracy of the WEDM operation. A significant amount of research has explored the different methodologies of achieving the ultimate WEDM goals of optimising the numerous process parameters analytically with the total elimination of the wire breakages thereby also improving the overall machining reliability.This paper reviews the vast array of research work carried out fromthe spin-off fromthe EDM process to the development of the WEDM. It reports on the WEDM research involving the optimisation of the process parameters surveying the influence of the various factors affecting the machining performance and productivity. The paper also highlights the adaptive monitoring and control of the process investigating the feasibility of the different control strategies of obtaining the optimal machining conditions. A wide range of WEDM industrial applications are reported together with the development of the hybrid machining processes.The final part of the paper discusses these developments and outlines the possible trends for future WEDM research.2Keywords: Wire electrical discharge machining (WEDM); Hybrid machining process; Process optimisation; Cutting rate; Matenal removal rate;Surface finish1. IntroductionWire electrical discharge machining (WEDM) is a widely accepted non-traditional material removal process used to manufacture components with intricate shapes and profiles. It is considered as a unique adaptation of the conventional EDM process, which uses an electrode to initialise the sparking process. However,WEDM utilises a continuously travelling wire electrode made of thin copper, brass or tungsten of diameter 0.05–0.3 mm, which is capable of achieving very small corner radii. The wire is kept in tension using a mechanical tensioning device reducing the tendency of producing inaccurate parts. During the WEDM process,the material is eroded ahead of the wire and there is no direct contact between the workpiece and the wire,eliminating the mechanical stresses during machining. In addition, the WEDM process is able to machine exotic and high strength and temperature resistive (HSTR) materials and eliminate the geometrical changes occurring in the machining of heat-treated steels.WEDM was first introduced to the manufacturing industry in the late 1960s. The development of the process was the result of seeking a technique to replace the machined electrode used in EDM. In 1974, D.H. Dulebohn applied the optical-line follower systemto automatically control the shape of the component to be machined by the WEDM process [1]. By 1975, its popularity was rapidly increasing, as the process and its capabilities were better understood by the industry[2]. It was only towards the end of the 1970s, when computer numerical control (CNC) system was initiated into WEDM that brought about a major evolution of the machining process. As a result, the broad capabilities of the WEDM process were extensively exploited for any through-hole machining owing to the wire, which has to pass through the part to be machined. The common applications of WEDM include the fabrication of the stamping and extrusion tools and dies, fixtures and gauges, prototypes, aircraft and medical parts, and grinding wheel form tools.This paper provides a review on the various academic research areas involving the WEDM process, and is the sister paper to a review by Ho and Newman [3] on die-sinking EDM. It first presents the process overview based on the widely accepted principle of thermal conduction3and highlights some of its applications. The main section of the paper focuses on the majorWEDM research activities, which include the WEDM process optimisation together with the WEDM process monitoring and control. The final part of the paper discusses these topics and suggests the future WEDM research direction.2. WEDMThis section provides the basic principle of the WEDM process and the variations of the process combining other material removal techniques.2.1. WEDM processThe material removal mechanism of WEDM is very similar to the conventional EDM process involving the erosion effect produced by the electrical discharges (sparks). In WEDM, material is eroded from the workpiece by a series of discrete sparks occurring between the workpiece and the wire separated by a streamof dielectric fluid, which is continuously fed to the machining zone [4]. However, today’s WEDM process is commonly conducted on workpieces that are totally submerged in a tank filled with dielectric fluid. Such a submerged method of WEDM promotes temperature stabilisation and efficient flushing especially in cases where the workpiece has varying thickness. The WEDM process makes use of electrical energy generatinga channel of plasma between the cathode and anode [5], and turns it into thermal energy [6] at a temperature in the range of 8000–12,000 vC [7] or as high as 20,000 vC [8] initialising a substantial amount of heating and melting of material on the surface of each pole. When the pulsating direct current power supply occurring between 20,000 and 30,000 Hz [9] is turned off, the plasma channel breaks down. This causes a sudden reduction in the temperature allowing the circulating dielectric fluid to implore the plasma channel and flush the molten particles from the pole surfaces in the formof microscopic debris.While the material removal mechanisms of EDM and WEDM are similar, their functional characteristics are not identical. WEDM uses a thin wire continuously feeding through the workpiece by a microprocessor, which enable parts of complex shapes to be machined with exceptional high accuracy. A varying degree of taper ranging from15 v for a 100 mm thick to 30v for a 400 mm thick workpiece can also be obtained on the cut surface. The microprocessor also constantly maintains the gap between the wire and the workpiece, which varies from0.025 to 0.05 mm [2]. WEDM eliminates the need for elaborate pre-shaped electrodes,which are commonly required in EDM to perform the roughing and finishing operations. In the case of WEDM, the wire has to make several machining passesalong the profile to be machined to attain the required dimensional accuracy and surface 4finish (SF) quality. Kunieda and Furudate [10] tested the feasibility of conducting dry WEDM to improve the accuracy of the finishing operations, which was conducted in a gas atmosphere without using dielectric fluid. The typical WEDM cutting rates (CRs) are 300 mm2/min for a50 mm thick D2 tool steel and 750 mm2/min for a 150 mm thick aluminium [11], and SF quality is as fine as 0.04–0.25 lRa. In addition, WEDM uses deionised water instead of hydrocarbon oil as the dielectric fluid and contains it within the sparking zone. The deionised water is not suitable for conventional EDM as it causes rapid electrode wear, but its low viscosity and rapid cooling rate make it ideal for WEDM [12].2.2. Hybrid machining processesThere are a number of hybrid machining processes (HMPs) seeking the combined advantage of WEDM with other machining techniques. One such combination is wire electrical discharge grinding (WEDG),which is commonly used for the micro-machining of fine rods utilized in the electronic circuitry. WEDG employs a single wire guide to confine the wire tension within the discharge area between the rod and the front edge of the wire and to minimise the wire vibration.Therefore, it is possible to grind a rod that is as small as 5 lmin diameter [13] with high accuracy, good repeatability and satisfactory straightness [14]. Other advantages of WEDG include the ability to machine a rod with a large aspect ratio, maintaining the concentricity of the rod and providing a wider choice of complex shapes such as tapered and stepped shapes at 1248 K.H. Ho et al. / International Journal of Machine Tools & Manufacture 44 (2004) 1247–1259 various sections [15]. Several authors [16–19] have employed the WEDG process in the micro-machining of fine electrodes or pins with a large aspect-ratio, which are difficult to be machined by traditional precision micro-machining methods such as Micro-EDM,LIGA and excimer laser drilling.Some of the HMPs seek to improve the WEDM performance measures such as the surface integrity and the CR. For example, the ultrasonic vibration is applied to the wire electrode to improve the SF quality together with the CR and to reduce the residual stress on the machined surface [20]. On the other hand, the wire electrochemical grinding (WECG) process replaces the electrical discharge used in WEDG with an electrochemical solution to produce high SF quality part for a wide range of machining condition [15]. Masuzawa et al. [13,15] compared the SF quality obtained from the WECG with WEDG, which is suitable for finishing micro-parts. A rotary axis is also added to WEDM to achieve higher material removal rate (MRR) and to enable the generation of free-formcylind rical geometries [21,22]. The effects of the various process parameters such as part rotational speed, wire feed rate and pulse on-time on the surface integrity and roundness of the part produced have been 5investigated in the same feasibility study [23].3. WEDM applicationsThis section discusses the viability of the WEDM process in the machining of the various materials used particularly in tooling applications.3.1. Modern tooling applications WEDM has been gaining wide acceptance in the machining of the various materials used in modern tooling applications. Several authors [24,25] have investigated the machining performance of WEDM in the wafering of silicon and machining of compacting dies made of sintered carbide. The feasibility of using cylindrical WEDM for dressing a rotating metal bond diamond wheel used for the precision form grinding of ceramics has also been studied [22]. The results show that the WEDM process is capable of generating precise and intricate profiles with small corner radii but a high wear rate is observed on the diamond wheel during the first grinding pass. Such an initial high wheel wear rate is due to the over-protruding diamond grains, which do not bond strongly to the wheel after the WEDM process [26]. The WEDM of permanent NdFeB and ‘soft’ MnZn ferrite magnetic materials used in miniature systems, which requires small magnetic parts, was studied by comparing it with the laser-cutting process [27]. It was found that the WEDM process yields better dimensional accuracy and SF quality but has a slow CR, 5.5 mm/min for NdFeB and 0.17 mm/min for MnZn ferrite. A study was also done to investigate the machining performance of micro-WEDM used to machine a high aspect ratio meso-scale part using a variety of metals including stainless steel, nitronic austentic stainless, beryllium copper and titanium [28].3.2. Advanced ceramic materialsThe WEDM process has also evolved as one of the most promising alternatives for the machining of the advanced ceramics. Sanchez et al. [29] provided a literature survey on the EDM of advanced ceramics,which have been commonly machined by diamond grinding and lapping. In the same paper, they studied the feasibility of machining boron carbide (B4C) and silicon infiltrated silicon carbide (SiSiC) using EDM and WEDM. Cheng et al. [30] also evaluated the possibility of machining ZrB2 based materials using EDM and WEDM, whereas Matsuo and Oshima [31] examined the effects of conductive carbide content, namely niobiumcarbide (NbC) and titaniumcarbide (TiC), on the CR and surface roughness of zirconia ceramics (ZrO2) during WEDM. Lok and Lee [32] have successfully WEDMed sialon 501 and aluminium oxide–titaniumcarbide (Al2O3–TiC). However, they realized that the MRR is very low as compared to the cutting of metals such as alloy steel SKD-11 and the surface roughness is generally inferior to the one obtained with the EDM process. Dauw et al. [33] explained that the MRR and surface roughness are not only dependent on the machining 6parameters but also on the material of the part.An innovative method of overcoming the technological limitation of the EDM and WEDM processes requiring the electrical resistivity of the material with threshold values of approximately 100 X/cm [34] or 300 X/cm [35] has recently been explored. There are different grades of engineering ceramics, which Konig et al. [34] classified as non-conductor, natural-conductor and conductor, which is a result of doping nonconductors with conductive elements. Mohri et al. [36] brought a new perspective to the traditional EDM phenomenon by using an assisting electrode to facilitate the sparking of highly electrical-resistive ceramics.Both the EDM and WEDM processes have been successfully tested diffusing conductive particles from assisting electrodes onto the surface of sialon ceramics assisting the feeding the electrode through the insulating material. The same technique has also been experimented on other types of insulating ceramic materials including oxide ceramics such as ZrO2 and Al2O3, which have very limiting electrical conductive properties [37].3.3. Modern composite materialsAmong the different material removal processes, WEDM is considered as an effective and economical tool in the machining of modern composite materials.Several comparative studies [38,39] have been made between WEDM and laser cutting in the processing of metal matrix composites (MMC), carbon fibre and reinforced liquid crystal polymer composites. These studies showed that WEDM yields better cutting edge quality and has better control of the process parameters with fewer workpiece surface damages. However,it has a slower MRR for all the tested composite materials. Gadalla and Tsai [40] compared WEDM with conventional diamond sawing and discovered that it produces a roughness and hardness that is comparable to a low speed diamond saw but with a higher MRR. Yan et al. [41] surveyed the various machining processes performed on the MMC and experimented with the machining of Al2O3/6061Al composite using rotary EDM coupled with a disk-like electrode. Other studies [42,43] have been conducted on the WEDM of Al2O3 particulate reinforced composites investigating the effect of the process parameters on the WEDM performance measures. It was found that the process parameters have little influence on the surface roughness but have an adverse effect on CR.4. Major areas of WEDM researchThe authors have organised the various WEDM research into two major areas namely WEDM process optimisation together with WEDM process monitoring and control.4.1. WEDM process optimisationToday, the most effective machining strategy is determined by identifying the different factors affecting the WEDM process and seeking the different ways of obtaining the optimal 7machining condition and performance.This section provides a study on the numerous machining strategies involving the design of the process parameter and the modelling of the process.4.1.1. Process parameters designThe settings for the various process parameters required in the WEDM process play a crucial role in producing an optimal machining performance. This section shows some of the analytical and statistical methods used to study the effects of the parameters onthe typical WEDM performance measures such as CR,MRR and SF.4.1.1.1. Factors affecting the performance measures.WEDM is a complex machining process controlled by a large number of process parameters such as the pulse duration, discharge frequency and discharge current intensity. Any slight variations in the process parameters can affect the machining performance measures such as surface roughness and CR, which are two of the most significant aspects of the WEDM operation [44]. Suziki and Kishi [45] studied the reduction of discharge energy to yield a better surface roughness, while Luo [46] discovered the additional need for a highenergy efficiency to maintain a high machining rate without damaging the wire. Several authors [47] have also studied the evolution of the wire tool performance affecting the machining accuracy, costs and performance measures.The selection of appropriate machining conditions for the WEDM process is based on the analysis relating the various process parameters to different performance measures namely the CR, MRR and SF.Traditionally, this was carried out by relying heavily on the operator’s experience or conservative technological data provided by the WEDM equipment manufacturers, which produced inconsistent machining performance. Levy and Maggi [48] demonstrated that theparameter settings given by the manufacturers are only applicable for the common steel grades. The settings for machining new materials such as advanced ceramics and MMCs have to be further optimised experimentally.4.1.1.2. Effects of the process parameters on the cutting rate.Many different types of problem-solving quality tools have been used to investigate the significant factors and its inter-relationships with the other variables in obtaining an optimal WEDM CR. Konda et al. [49] classified the various potential factors affecting the WEDM performance measures into five major categories namely the different properties of the workpiece material and dielectric fluid, machine characteristics, adjustable machining parameters, and component geometry. In addition, they applied the design of experiments 8(DOE) technique to study and optimize the possible effects of variables during process design and development, and validated the experimental results using noise-to-signal (S/N) ratio analysis. Tarng et al. [50] employed a neural network system with the application of a simulated annealing algorithm for solving the multi-response optimisation problem. It was found that the machining parameters such as the pulse on/off duration, peak current, open circuit voltage,servo reference voltage, electrical capacitance and table speed are the critical parameters for the estimation of the CR and SF. Huang et al. [51] argued that several published works [50,52,53] are concerned mostly with the optimisation of parameters for the roughing cutting operations and proposed a practical strategy of process planning fromroughing to finishing operations. The experimental results showed that the pulse on-time and the distance between the wire peripheryand the workpiece surface affect the CR and SF significantly. The effects of the discharge energy on the CR and SF of a MMC have also been investigated [54].4.1.1.3. Effects of the machining parameters on the material removal rate. The effects of the machining parameters on the volumetric MRR have also been considered as a measure of the machining performance. Scott et al. [52] used a factorial design requiring a number of experiments to determine the most favourable combination of the WEDM parameter. They found that the discharge current, pulse duration and pulse frequency bare the significant control factors affecting the