Developments in electrochemical discharge machining: A review on electrochemical discharge machining, process variants and their hybrid methods

Author’s Accepted Manuscript Developments in electrochemical discharge machining: A review on electrochemical discharge machining, process variants and their hybrid methods Tarlochan Singh, Akshay Dvivedi www.elsevier.com/locate/ijmactool

PII: DOI: Reference:

S0890-6955(16)30012-8 http://dx.doi.org/10.1016/j.ijmachtools.2016.03.004 MTM3140

To appear in: International Journal of Machine Tools and Manufacture Received date: 24 September 2015 Revised date: 3 March 2016 Accepted date: 7 March 2016 Cite this article as: Tarlochan Singh and Akshay Dvivedi, Developments in electrochemical discharge machining: A review on electrochemical discharge machining, process variants and their hybrid methods, International Journal of Machine Tools and Manufacture, http://dx.doi.org/10.1016/j.ijmachtools.2016.03.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Developments in electrochemical discharge machining: a review on electrochemical discharge machining, process variants and their hybrid methods Tarlochan Singh* and Akshay Dvivedi1 Mechanical and Industrial Engineering Department, Indian Institute of Technology Roorkee, Roorkee, India- 247667 Email address: [email protected] Email address: [email protected] *

Corresponding author: Tarlochan Singh. Tel: +91 70602 17355.

Abstract Electrochemical discharge machining (ECDM) is a hybrid non-conventional machining process, used to machine electrically conductive and non-conductive materials. It is a preferred process to fabricate micro scale features like micro holes, micro channels, microwaves and 3-dimensional intricate shapes on variety of materials. In order to improve the efficacy of ECDM process, certain technical augmentations are provided with basic configuration of ECDM. These augmentations result in developments of ECDM process variants. Further, research community has developed ECDM based triplex hybrid methods for further process enrichment. This review article presents a comprehensive review of these recent developments in ECDM process, its variants and their triplex hybrid methods. The future research possibilities are identified and presented as research potentials.

1

Tel.: +91 1332 285428. Fax: +91 1332 285665

Keywords Electrochemical discharge machining; micromachining; ECDM variants; triplex hybridization 1. Introduction Exponential evolution of micro level products has grabbed great attention, because of their tremendous applications in various fields like micro electro mechanical systems, micro fluidics, bio medical testing systems etc. [1-3]. Manufacturing of these products has put huge pressure on industries to come out with efficient and economical solutions. In order to facilitate the real goals of industries, researchers have been using either of non-conventional or lithography based micro machining techniques. For small lot sizes, non-conventional machining processes are economical over lithographic techniques [4]. These non-conventional machining processes use either of, thermal, mechanical and chemical energies or their combinations for material removal. The thermal energy based processes include laser beam machining (LBM) and electric discharge machining (EDM). LBM uses thermal energy for machining of ceramics, polymers and metals. But it requires expensive equipment and has a high maintenance cost. Further, the existence of heat affected zone (HAZ) limits its industrial usages [5]. The thermal energy has also been used in the form of discharges in micro electric discharge machining (μ-EDM) [6] and wire-cut electric discharge machining (W-EDM) [7]. However, these processes can be used for machining of micro features on conductive materials only. By using mechanical energy, micro USM and AJM can produce complex micro features on hard and brittle materials, irrespective of their electrical conductivity [8, 9]. Their incapability to machine ductile materials results in limited applications. The chemical energy based machining methods develop intricate micro profiles on a variety of materials with smooth surface finish. But low processing speed, low dimensional

accuracy and low aspect ratio structures are the limitations of chemical process [10]. There was a need for a machining process that can machine micro features over a wide variety of work materials, irrespective of material hardness, strength and conductivity. Thus electrochemical discharge machining (ECDM) was developed by Kura Fuji in 1968s [11]. Electro chemical discharge machining (ECDM) is a combination of two non-conventional machining processes, namely electrochemical machining (ECM) and electric discharge machining (EDM) [12]. Simplicity and small sized set-up make ECDM a very attractive technology for micromachining. In literature, the ECDM process has been presented through several names: viz. spark assisted chemical engraving [13], electro chemical spark machining [14], micro electro chemical discharge machining [15], electro chemical discharge machining [12], electro chemical arc machining [16], discharge machining of non-conductors [17] and electro erosion dissolution machining [18].The advantage of ECDM, compared with other processes, is that it can machine conductive/non-conductive, hard/brittle and ductile materials. Initially, this method was attempted to drill on glass substrates [11], subsequently this method was employed in the drilling of ceramics [19], composites [14] and stainless steel materials [18]. In order to explore the capabilities of ECDM process, the electrochemical discharge energy was manipulated for the development of ECDM variants. By using these variants researchers had been performing various operations like the fabrication of complex and intricate micro profiles [20], micro dies [18], deep drilling [21], machining of cylindrical parts [22], dressing of micro grinding tools [23] and slicing of glass rods [24]. In general, ECDM variants have proved their usefulness for micro fabrication. However, ECDM is also associated with limitations like low aspect ratio structures, low accuracy etc. To overcome these limitations several external energies have been applied to the ECDM. This resulted in development of several ECDM process

variants. Recently, the simultaneous involvement of these external energies with ECDM process, further developed new triplex hybrid methods. These triplex hybrid methods enhanced the performance meaningfully. Nonetheless research is still continuing to get the better results. The present paper attempts to report the developments in variants of ECDM process and their triplex hybrid methods. This paper also includes the findings and the future scope for further research work. 1.1. Electrochemical discharge machining (ECDM) Electrochemical discharge machining (ECDM) is one of the widely accepted hybrid nonconventional micromachining techniques, which is used to fabricate miniaturized products on electrically conductive [25] and non-conductive materials [26]. ECDM set-up consists of two electrodes immersed in an electrolytic solution, namely, tool electrode (cathode) and auxiliary electrode (anode). The work piece is placed below the tool electrode as shown in Figure 1. A DC power source is used to provide the voltage across these two electrodes. Thus, an electrochemical cell (ECC) is formed. The electrolysis occurs in ECC due to the potential difference across the two electrodes and results in the formation of hydrogen and oxygen gas bubbles at the tool electrode and at the auxiliary electrode respectively. Beyond the critical voltage, the generation rate of hydrogen gas bubbles near the tool electrode exceeds the generation rate of bubbles floating on the electrolyte surface that ensures the staying of hydrogen gas bubbles around the tool electrode. The physical contact of hydrogen gas bubbles with each other formed big sized single gas bubble and gets converted into hydrogen gas film around the tool electrode [27]. The hydrogen gas film behaves like a dielectric medium between the cathode tool and the electrolyte and acts as an insulator around the tool electrode. This insulation of tool electrode, almost ceases the flow of current and develops high electric field (10 V/µm) across the dielectric film and that

results in arc discharge. The existence of high current densities at the sharp edges of tool results in spark initiation at these tool edges [28]. Subsequently, discharge location changes over entire face of tool electrode [26]. During the discharge period, tool electrode bombards a large number of electrons on the workpiece surface kept close to the tool electrode. Bombardment of these electrons rise the temperature of the work material that finally results in melting of work piece and material removal [29]. Figure 2 represents the schematic view of the discharge mechanism that includes the following steps (i) electrolysis, (ii) generation and accumulation of hydrogen gas bubbles, (iii) bubble coalesce and gas film formation , (iv) sparking . Many other researchers have explained this discharge mechanism in different ways. Basak and Ghosh observe that there are some narrow conducting bridges inside the closed-packed gas film over the tool electrode. The existence of high current densities across these conducting bridges caused instant boiling. The boiling of these bridges results arc discharge [30]. Jain et al. consider each gas bubble as a valve. Under the influence of high electric field, the breakdown of each valve produces their identical arc discharge [31]. Behroozfar et al. report the shape of the arc discharge by taking the simultaneous images of current signals and the signatures engraved on the workpiece surface. The circular form of the signatures on the work piece surface reflects the cylindrical form of the arc discharge [32]. The impingement of these continuous discharges over the workpiece surface raises the temperature of the electrolyte. Increased temperature of the electrolyte promotes the chemical etching over the machined surfaces. Consequently, surface finish is achieved [33]. The simultaneous involvement of discharge melting and chemical etching actions for material removal makes the process mechanisms very complex. Literature does not contain any report regarding quantification of the contribution of each material removal action.

But still the

research is in progress to extract such information that may contribute in the improvement of

process performance. Meanwhile, experimental results revealed simultaneous and direct involvement of various process parameters that affect process performance remarkably. These process parameters are broadly categorized into six groups as shown in Figure 3. These groups are tool electrode, workpiece, electrolyte, auxiliary electrode, power source and the presence of gaps between two electrodes and tool-workpiece. The cause and effect diagram is shown in Figure 3. It identifies the root causes of the above discussed process parameters that governs the process performance of the ECDM method. 2. ECDM variants Basic principle of electrochemical discharge machining process can be used to perform various operations such as drilling, milling, cutting, die-sinking, dressing and turning (Figure 4). These operations are very effective to fabricate different profiles on hard and brittle materials, irrespective of their electric conductivity. The next section discusses these ECDM process variants. 2.1. Electrochemical discharge drilling (ECDD) Electrochemical discharge drilling (ECDD) is being driven by the demand of high aspect ratio precise holes on thick and thin substrates. To fulfill the demands of micro fabrication, many researchers had drilled through and blind micro holes by using ECDD method. This process had been used to drill conductive materials like cobalt, low-alloy steels, chrome, titanium and nimonic alloys. Machined surface of these materials exhibit, quite smooth surface finish similar to electrochemically machined surfaces [34]. Beside these conductive materials, ECDD process has also been used to drill silicon nitride ceramics [19], steels [35], borosilicate glass [36-38], Pyrex wafer [39], soda lime glass [40], other glass wafers [41], silicon wafers [25] and e-glassfibre-epoxy composite [42]. The ECDD operation is performed through the controlled and

progressive movements of the tool electrode along the z-axis. The simultaneous interaction of tool electrode movements with discharge energy makes the process complicated. There are various process parameters related to the electrolyte, tool electrode and power supply that effectively control the discharge energy. The subsequent section details on some of significant investigations related with ECDD process parameters. In ECDM, the machining voltage and drilling depth are important process parameters that directly controls the discharge energy over the machining zone. Thus, the accuracy of machined micro holes is characterized as a function of machining voltage and drilling depth as shown in Figure 5 [40]. The resultant mean diameter obtained from the various combinations of machining voltage and drilling depth is identified into three zones, namely, zone A, B and C. In zone A, the micro holes are drilled at 28-37V with 100μm drilling depth. Under these circumstances, the mean diameter of the machined hole is independent of the machining voltage. Under these circumstances, the drilling takes place in only discharge regime. This result in drilling of welldefined cylindrical contours with smooth surface. In zone B, the drilled micro holes have jagged outline contours. The inputs for zone B are 30V at depths of 200μm and 300μm. The drilling takes place in transition zones, i.e., between discharge and hydrodynamic regime. The zone C represents, those holes, which are drilled at a voltage higher than 30V with depths greater than 100μm. Here, holes are surrounded by heat affected zone. For the fabrication of deep microholes (as in zone C), the supply of continuous electrolyte supply at the tool tip is becomes difficult. Consequently, the machining speed is not controlled by the number of discharges occurring in the machining zone. Thus, the drilling speed only depends on drilling depth and becomes independent of the machining voltage [40].

In ECDD process, it is necessary to reduce the gas film thickness for better repeatability [43]. Thus literature has advised, use of surfactants into the electrolytic bath. The addition of surfactant (liquid soap) into the electrolytic bath reduces gas film thickness. Thin gas films results in lower differences amongst energy contained by intermittent discharges. Consequently ECDD process provides consistent results [44]. Tool electrode based parameters also plays significant role during electrochemical discharge drilling operation. Tool electrode materials with high thermal conductivity provide higher machining rates in discharge regime, while low machining rates in hydrodynamic regime. In discharge regime, the material removal action takes place due to the presence of heat energy at tool surface. But in hydrodynamic regime the availability of heat energy at machining zone determines the machining status. Tool electrode with high thermal conductivity transfers maximum heat energy at tool surface as compared to the machining zone. So this presence of heat energy at tool surface results in higher machining rates in the discharge regime as compared to the hydrodynamic regime [45]. The surface roughness of tool electrode also effect machining rates in ECDM. The surface roughness of the tool electrode determines its wettability, which finally decides the coalescence status of the gas film and processing stability in terms of hole diameter and machining rate [46]. It is advisable to use coarser surfaces of tool electrode for higher material removal rate (MRR) [46]. A tool electrode with spherical tip exhibits higher machining rates and accuracy as compared to the conventional cylindrical tool electrode. It is observed that the curved surface of the tool electrode reduce the contact area between tool electrode and work piece, which facilitate better electrolyte flow at the tool electrode end [47]. Consequently, formation of gas film is rapid, that increases discharging frequency and MRR. Finally the formation of rapid gas film result in better machining rates. As compared to conventional cylindrical electrode, a tool electrode with

spherical tip exhibit 83% reduction in machining time and 65% reduction in hole diameter during fabrication of 500 µm deep holes [47]. Tool wear is characterized as a potential candidate that greatly influence the machining accuracy. In order to preserve the machining accuracy in ECDD, Behroozfar et al. presented tool wear behaviour for different tool materials at various voltage levels. It is observed that the steel and tungsten carbide (WC) tools result in better performance over brass tool [48]. Apart from high aspect ratio holes, it is possible to drill holes with conical and spherical cross-section using ECDD. These holes can be drilled by opting for tool electrode polarity either as cathodic or anodic. The connection of tool electrode with negative or positive terminal of the power supply is generally referred to as the cathodic (straight) or anodic (reverse) polarity respectively. A change in polarity results in change in electrochemical action, i.e., chemical etching. It is accepted, that the presence of hydroxyl (OH-) radicals near the machining zone promote the chemical etching. In case of cathodic polarity, the concentration of hydroxyl radicals decreases along the hole depth and results in hole with conical cross-section. A representative image of these holes is shown in Figure 6 [36]. The Hickling-Ingram mechanism produces uniform concentration of hydroxyl radicals along the hole depth with anodic polarity. Consequently, hole with spherical cross-section can be drilled. It is interesting to note that chemical etching action over the spark discharged machined surface, results in quite smooth surface finish. [36]. ECDD, being such a versatile process has been used to develop the rapid prototyping fused silica micro devices with high aspect ratio, micro-crack free and nearly polished surfaces [49]. It is reported that conical holes with 300μm diameter and 450μm depth are drilled in only 30 seconds [49]. A DC pulsed power supply is advisable for drilling of nearly straight holes. Additionally,

investigations have reported use of offset voltage (10 V) for nearly 60% reduction in machining time with similar in hole accuracy as shown in Figure 7 [50]. 2.2. Electrochemical discharge milling Electrochemical discharge milling process is demonstrated as a potential method for the fabrication of complex 3D microstructures on glass and quartz materials. Many instigations are revealed for surface texturing of the micro channels [54], and fabrication of micro channels [20, 51, 52], micro grooves [53] etc. In this process, a rotary cylindrical wheel acts as a cutting tool (cathode electrode). This tool travels across a pre-defined path. Amongst the process parameters of electrochemical discharge milling process, the tool rotation rate and tool travel rate are observed as influential process parameters. Higher tool rotation rates assists in elimination of electrolyte replenishment problems and results in micro grooves with lower width and sharp edges. Interestingly, the groove depth is independent of tool rotation rate. Figure 8 shows the individual effect of tool rotational rate on groove width and depth [55]. A higher tool travel rate can produce shallower micro grooves with larger width as shown in Figure 9 [55]. In order to develop the deep micro grooves, layer-by-layer material removal approach with small depth of cut was introduced in electrochemical discharge milling process. This layer-by-layer approach facilitates electrolyte flushing at deeper and narrower gaps. Additionally, deeper micro grooves with smooth surface finish can be machined. For the fabrication of precise shaped micro grooves on glass substrates, Zheng et al. attribute the following parameter settings: 40 V DC supply, pulse on:off = 2ms: 2ms, tool rotation rate = 1500 rpm and tool travel rate = 1000µm per minute [55]. Figure 10 shows process potential of electrochemical discharge milling process. The machining of these micro structures (micro-grooves, micro-pillars and micro-pyramids) is quite appreciable [56].

2.3. Electrochemical discharge turning (ECDT) Electrochemical discharge machining with continuous rotation of work-piece is a versatile realization of ECDM process to machine the cylindrical parts. A schematic diagram of ECDT is shown in Figure 11. It consists of rotary work-piece, immersed in an electrolytic bath. The rotary motion of work-piece, during machining, facilitates feeding of fresh electrolyte across the narrow gap between tool and work piece. The rotation rate of the work piece is a dominant process parameter that effect process performance. At an optimum level of rotation rate machining of deep, narrow and grooves with sharp edges is observed [22]. It is imperative to note that very high rotation speed lowers MRR, due to difficulties in film formation at higher rotation speeds. 2.4. Electrochemical discharge dressing The dressing of worn micro grinding tool by using the principles of ECDM develop an interesting process variant known as electrochemical discharge dressing [23]. Electrochemical discharge dressing consists of an electrolyte dipped worn grinding tool (cathode) and auxiliary electrode (anode). In this process, spark energy helps to erode the metallic bonds as well as work-piece debris bonds from the surface of the worn micro grinding tool and finally results the protrusion of fresh grains over the periphery of the micro grinding tool. In this method, electrolyte plays multiple roles such as a dresser, dielectric medium, cooling agent and a flushing agent to avoid the burs from the machining areas. The performance of the process is evaluated in terms of surface morphology of the grinding tools and the acting of grinding forces during the use of grinding tools and the roughness of the machined surfaces. Through this method, protruded grains on the grinding wheel are observed to be damage free. The use of these grinding tools exhibit 50% reduction in normal grinding forces and consequent surface roughness [57]. 2.5. Wire electrochemical discharge machining (WECDM)

Many researchers have combined wire electric discharge machining concept with electrochemical discharge machining process to slice/cut hard and brittle materials [62]. In WECDM, electrolyte dipped travelling wire serves as a cutting tool (cathode) as shown in Figure 12 [24, 58]. To ensure the physical contact between work piece and tool wire, Yang et al. propose two work piece feed mechanisms, namely, weight-loading mechanism, and reciprocating mechanism. Amongst these two mechanisms, reciprocating mechanism display better surface quality and accuracy of micro grooves as shown in Figure 13 [59]. In weightloading mechanism, tool wire and the workpiece is kept close to each other by using mechanical force. This made flushing of debris from the machined surfaces very difficult. This unflushed debris promotes secondary discharges and results in poor surface quality with inaccurate micro groove profiles. The applications of reciprocating (to-and-fro) movements of the workpiece across the tool wire facilitate debris removal. In order to enhance the machining rates as well as geometrical accuracy of the micro grooves, SiC abrasive particles are added into the electrolytic bath (Yang et al.). The presence of abrasive particles in electrolytic bath disrupts the formation of insulating layer around the tool wire, which leads to increase in the critical voltage. SiC abrasives also acts as a finishing tool. They assists in removal of the micro cracks and melted zones from the machined surfaces [59]. During the fabrication of micro grooves, feed rate is most important process parameter. It significantly affects the slit depth as shown in Figure 14 [60]. A maximum value of slit depth is measured as 2000μm at feed rate of 350μm/min. Higher feed rates (beyond 350μm/min) cause, continuous disruption of gas film around the tool and result in lower slit depths. The mean slit width is independent of feed rate [60]. In WECDM, the occurrence of electric discharges within the electrolytic bath generates toxic fumes, which pollute environment, especially around the machining chamber. In order to

reduce the pollution around the machining chamber, Chand et al. developed titrated electrolyte flow method. In titrated flow method electrolyte flows in the form of droplets. Thus, the quantity of electrolyte available at machining zone is reduced. This provide lesser environmental pollution [60]. It is interesting to know that material removal in titrated electrolyte flow method is more, less or same as that of earlier set-up. The optimum wire breakage ranges are identified at 10 % electrolyte concentrations and less than 60 V. The W-ECDM requires lower processing cost [61]. As per the concern of electric conductivity of work materials, WECDM can process conductive as well as non-conductive materials like glass [58], ceramics [63], quartz [60], glassepoxy, Kevlar-epoxy [64] and metal matrix composites [65]. 2.6. Die-sinking electrochemical discharge machining (DS-ECDM) The die-sinking variant of ECDM process has been in use for fabrication of small and shallow dies on conductive [18] and non-conductive materials [66]. Investigations on DS-ECDM process reveal higher MRR is possible than individual ECM and EDM actions [18]. Further, the dimensional tolerances of machined parts are close to the EDM process and interestingly better than the ECM process. Furthermore, surface integrity of machined parts is effected due to both ECM and EDM. High sinking rates are achieved by using hollow bronze tool at 3 to 18 mm/min feed rate with pulsed DC voltage of range 20 to 30V in 120 g/l NaClO3 electrolyte [18]. In this investigation tool electrode is made of bronze piping with inner diameter of 3.7 mm and outer diameter of 9.4 mm. 2.7. Electrochemical discharge trepanning Electrochemical discharge trepanning method was attempted successfully to fabricate deep holes on hard and brittle materials. In electrochemical discharge trepanning action, an orbital motion is provided to tool electrode by offsetting the tool axis from spindle axis. This process is observed

as an effective and economical method for fabrication of deep holes. Jain et al. relaxed the limited depth constrained of ECDM process by fabricating 1.35 mm and 2.35 mm deep through holes on alumina and quartz materials, respectively [21]. For the sake of improvement in terms of surface quality and hole depth, Chak et al. replaced gravity fed tool with spring fed abrasive particle embedded tool. These abrasive particles and rougher surfaces impart high frequency electrical discharges and improved cutting action respectively. Consequently, machined surface with higher MRR is possible. Abrasive electrode also contribute to reduce the average value of taper from 13.411 to 2.051. Experimental results reveal that the pulsed DC power supply provide better process efficiency [67]. 3. ECDM based triplex hybrid methods ECDM is a hybrid machining method that combines the constituents of electrochemical and electro discharge. The combination of these two energies had been used to perform various operations like drilling, milling, turning etc. In order to improve the performance of ECDM based operations, researchers have combined different energies for simultaneous and controlled interaction with ECDM configuration and developed ECDM based triplex hybrid methods. On the basis of the nature of third energy source or interaction, these ECDM based triplex hybrid processes are classified as primary and secondary hybrid processes (Figure 15). The mixing of third energy with ECDM via tool rotations or addition of powders in electrolyte is termed as primary hybridization. Further, the material removal caused by the assistance of mechanical or magnetic forces are termed as the secondary hybrid processes. It is noteworthy to mention that in these processes, the primary energy for material removal is electrochemical discharge. Following section of this paper details on ECDM based triplex hybrid methods. 3.1. Rotary electrochemical discharge machining (R-ECDM)

The rotary motion of tool electrode during ECDM has developed a primary triplex hybrid method known as rotary electrochemical discharge machining (R-ECDM) [68-72]. The advantage of this process is that rotary motion of rotary tool electrode can drill straight holes with small entrance diameter. Under the application of tool rotation, the spark energy distributes uniformly over the machining regions instead of single point. Consequently, the drilled holes are straight, crack-free and smooth. But a higher tool rotation rate generates serious agitation in the electrolytic bath that leads to destabilization of discharge and results in low machining rates. During the machining of Pyrex glass with cylindrical tool (diameter 200μm), the variation in hole entrance diameter with respect to tool rotation is divided into two distinct regions. In region I, from 500 to 1500 rpm of tool electrode, the entrance diameter of hole progressively decreases with an increase in tool rotations. In region II (beyond the 1500 rpm), with lesser sparking frequency and prolonged machining time results in larger hole entrance diameter. As per the concern of form and dimensional accuracy of the holes structures, the gap between hole periphery and tool sidewall plays an important role. So to preserve the form accuracy of the hole structures, Zheng et al. have designed a tool with flat sidewalls and flat front as shown in Figure 16. In this tool design, there is a provision to increase or decrease the distance between hole periphery and tool sidewall by changing the tool thickness. The maximum distance between hole periphery and tool sidewall reduce the discharge influence on the hole side walls. This design also facilitate supply of electrolyte in narrow gap between tool and hole walls. Consequently deep micro holes are drilled with less taper [68]. Tungsten carbide based rotary tool has been used to develop the micro holes on glass substrates with aspect ratio of 11:1 [69]. J. Kozak et al. have presented physical and mathematical models for the determination of performance characteristics of R-ECDM. It is recommended to use shorter pulse on-times for better surface

quality. Also, this mathematical model was suggested for the use in practical applications [70]. R-ECDM has been used to fabricate micro holes on various materials such as borosilicate glass [69], Pyrex glass [68], difficult to cut steel [71] and stainless steel [72]. 3.2. Powder mixed ECDM (PM-ECDM) The mixing of abrasive particles in electrolytic media results in an ECDM based triplex hybrid method known as powder mixed electrochemical discharge machining (PM-ECDM). This powder mixed electrolytic media improves the surface integrity and process repeatability. The presence of abrasive particles (graphite) within the gas film reduces the direct impact of spark energy on the work surface that results in smooth surface finish. The spark energy reduction phenomenon depends upon the behavior of conductive abrasive particles in-between the tool electrode and the work surface. Han et al. have described the behavior of conductive particles in two ways: (1) tool electrode attached conductive particles cause intensification of electric field at local areas that result in stable discharges, (2) electrostatic force caused dynamic movements of the abrasive particles results the constant charge transfer between the tool electrode and work surface. During the machining of borosilicate glass material, the mixing of graphite abrasive particles of 10 µm diameters in sodium hydroxide electrolytic solution exhibit improved surface roughness from 4.86 to 1.44 µm [73]. 3.3. Electrochemical discharge grinding (ECDG) Electrochemical discharge grinding (ECDG) is a triplex hybrid machining method that incorporates the combined action of three different processes namely mechanical abrasive cutting, electrochemical dissolution action and electric discharge erosion [74]. In ECDG, abrasive particles embedded rotary tool (cathode) serves as a grinding wheel. The periphery of this grinding wheel contains abrasive particle layers and metallic bonding systems in between

these abrasive particle layers. The electrically conductive metallic bond material over the abrasive coated tool generates the spark. Further, this spark energy removes the material from the work surface through melting. The abrasive cutting action also removes the material from work surface through mechanical abrasion [75]. Figure 17 shows the schematic view of material removal mechanism for ECDG process [76]. In order to achieve the better surface quality and higher material removal rates, experimental observations have revealed that there should be low gap and maximum contact area between abrasive tool and work surface. This offers thin gas film with maximum cutting edges. In order to make the appropriate contact between the abrasive particles and work surface, tool feed rate is mentioned as a critical parameter. ECDG has been used successfully to develop micro holes and to finish cylindrical surfaces (through center less grinding module of ECDM) [23]. Researchers have proposed conical and cylindrical shaped grinding wheels for better machining of borosilicate glass and alumina materials, respectively [76]. This process has been attempted successfully to machine metal matrix composites [77, 78], alumina and glass [76] materials. 3.4. Magnetic field assisted electrochemical discharge machining (MAECDM) In order to enhance the machining efficiency and to develop the accurate micro holes, Cheng et al. have placed a magnetic unit inside the tool chuck. Thus, magnetic-field-assisted electrochemical discharge machining method (MAECDM) came into existence. MAECDM consists of a special magnetic tool chuck to hold the tool electrode as shown in Figure 18. A provision to apply the magnetic field in upward and downward directions is also built in the setup. Under the application of magnetic field, a magneto hydrodynamic (MHD) convection is induced in the electrolyte. Further, MHD convection enhance the circulation of electrolyte within the narrow gaps. Owing to effective circulation of electrolyte, hydrogen gas bubbles break away

more easily from the tool surface. So to maintain the stable gas film over the tool electrode higher voltage is required. This higher voltage contributes toward higher machining efficiency. Further, effective circulation of electrolyte prevents the debilitation of gas film quality during the fabrication of deep holes. A better gas film quality generates stable discharges, which finally contributes in better geometrical accuracy. Further, it is assessed that with an upward magnetic field, process exhibited 57.4% improvement in machining time while 23.8% reduction in overcut [79]. 3.5. Vibration assisted electrochemical discharge machining (VAECDM) In ECDM, inappropriate electrolyte supply between the tool tip and the workpiece top surface is a limitation in fabrication of deep micro holes. In order to overcome this limitation, some researchers apply vibrations either of tool electrode [80], electrolyte [81] or work-piece [82]. It is interesting to note that Wuthrich et al. drilled 300 µm deep micro holes in 10 seconds by applying low frequency vibrations (0-30 Hz) to tool electrode having 0.4 mm diameter. These tool vibrations increase the MRR by a factor of two [80]. Further, to modify the gas film geometry, ultrasonic vibrations of 1.7 MHz frequency is applied to the electrolyte solution. This resulted in uniform sparks around the walls and at the bottom face of the tool. The features machined through uniform sparks reveal more depth with less taper as shown in Figure 19. The hole depth increases from 320μm to 550μm [81]. The application of ultrasonic vibrations facilitates electrolyte supply at machining zone. However, amplitude of these vibrations is an important process parameter. The ultrasonic vibrations with low amplitude (less than or equal 2µm) resulted in generation of consecutive pulse discharges and consequently higher MRR in comparison with ECDM without any vibration. These consecutive pulses over a longer duration results in deeper holes. It is interesting to note that an increase in amplitude from 2µm to 3.5µm,

wide and dense pulse discharges occur. These discharges result in lower material removal rate but improved surface integrity. As per the percolation theory, wide current pulses are due to faradic currents. These pulses are produced due to electrolysis on the tool surface when then gas bubble density is low on tool electrode surface [82]. 4. Research potential This review article presents an insight into the research work carried out in the broad area of electrochemical discharge machining (ECDM), its process variants and triplex hybrid methods. There are numerous possibilities that exist for further exploration of the ECDM process or its variants. Literature review reveals that the focus of past research was almost laid on the development of micro features on non-conductive materials. An overview of published research work in ECDM variants is shown in Figure 20. It can easily be deducted that the electrochemical energy had been used to drill micro holes by using electrochemical discharge drilling (ECDD) process variant. ECDD has capability to drill micro holes that may have conical or spherical profiles [36]. It also has a potential to fabricate deep micro holes of aspect ratio 11:1 [69]. The electrochemical discharge milling variant has the capability to engrave micro channels of serpentine shape having a width of 30μm and depth of 70μm with surface roughness of 0.099μm [56]. The possibility to generate the surface textures on microchannel surfaces through electrochemical discharge milling [54] reveals an emerging scope for suitable micro fluidic applications. Thus, surface textures generated by electrochemical discharge milling needs further investigation to achieve repeatable and desired texture forms. Additionally there are many more process variants that have been used to perform the variety of operations on several materials. The capabilities of these process variants and their respective research opportunities are compiled in Table 1.

Electrolytic media is an important process parameter of the ECDM method that affects the process performance. The search for environmental friendly, economical and efficient electrolyte is still going on. A water based emulsions can be used as an environment friendly electrolyte. The prime requirements for industry are high precision products with good surface integrity at relatively low processing cost. Needless to mention that each investigation on ECDM process or its variant focus on attainment of above mentioned requirements. However, a possibility still exists that minor modifications in existing process (not requiring extreme changes) can be integrated to improve process efficiency. Thus, with small process variation the vibration assisted ECDM evolves [80]. This process can drill 300μm holes in less than 10 seconds. It would be imperative to comprehensively study the vibration assisted ECDM process for improving its potential applicability. The manufacturing world is moving towards the development of micro level products especially on high strength advanced materials. In order to fullfill the current market demands regarding the development of micro products, the possibility for the fabrication of multiple micro features (holes, channels and texturing of micro channel surfaces) on a single substrate by ECDM may open research avenues for various fields like lab-on-a-chips, MEMS etc. Composite materials are widely accepted as future materials because of their numerous features like high strength to weight ratio, corrosion resistance, chemical resistance, and good durability [83]. Limited research is revealed on machining of composite materials (metal matrix and polymer matrix composites). A possibility exists to fabricate the complicated geometries on the composite materials. The constructive use of research in electrochemical discharge machining will be justified, when it is incorporated by the industry. 5. Conclusion

After inclusive analysis of presented work on developments in variants of electrochemical discharge machining (ECDM) and its triplex hybrid methods, the major conclusions can be drawn as below. 1. Electrochemical discharge machining (ECDM) is prominent non-conventional hybrid machining process for machining of both conductive and non-conductive materials. 2. The influential advancement of electrochemical discharge machining (ECDM) variants is fabrication of micro scaled intricate profiles on difficult to machine materials with good surface quality and higher machining rates. 3. Electrochemical discharge dressing is emerging method that is quite suitable for dressing of worn micro-grinding tools. 4. Wire- Electrochemical discharge machining (W-ECDM) is very effective option to slice the materials, irrespective of their conductivity. 5. Electrochemical discharge turning (ECDT) is flexible attainment of electrochemical discharge machining (ECDM) method to machine cylindrical parts. 6. Rotary

electrochemical

discharge

machining

(R-ECDM),

Vibration

assisted

electrochemical discharge machining (VAECDM) and magnetic field- assisted electrochemical discharge machining (MAECDM) improve the drilling performance in terms of material removal rate, hole depth and accuracy, while electrochemical discharge trepanning method is well suited for drilling deep holes of large diameter. 7. Electrochemical discharge grinding (ECDG) is very effective approach to finish the micro holes and cylindrical surfaces on conductive and non-conductive materials.

Acknowledgment This work is supported under research project by Science & Engineering Research Board (SERB), Department of Science & Technology, India. The authors acknowledge the assistance and support. Grant Code: SR/S3/MERC/0070/2012. References 1.

J. W. Judy, Microelectromechanical systems (MEMS): fabrication, design and applications, Smart Materials and Structures 10 (2001) 1115-1134.

2.

S. Haeberle, R. Zengerle, Microfluidic platforms for lab-on-a-chip applications, Lab on a Chip 7 (2007) 1094-1110.

3.

P. Dario, M.C. Carrozza, A. Benvenuto, A. Menciassi, Micro-systems in biomedical applications, Journal of Micromechanics and Microengineering 10 (2000) 235–244.

4.

K. P. Rajurkar, G. Levy, A. Malshe, M. M. Sundaram, J. McGeough, X. Hu, R. Resnick, A. DeSilva, Micro and nano machining by electro-physical and chemical processes, CIRP Annals-Manufacturing Technology 55 (2006) 643-666.

5.

M. D. Shirk, P. A. Molian, A review of ultrashort pulsed laser ablation of materials, Journal of Laser Applications 10 (1998) 18-28.

6.

Q. Liu, Q. Zhang, G. Zhu, K. Wang, J. Zhang, C. Dong, Effect of Electrode Size on the Performances of Micro EDM, Materials and Manufacturing Processes 31 (2016) 391-396.

7.

X. Geng, G. Chi, Y. Wang, Z. Wang, Study on Micro rotating Structure Using Micro wire Electrical Discharge Machining. Materials and Manufacturing Processes 29 (2014) 274–280.

8.

M. S. Cheema, A. Dvivedi, A. K. Sharma, Tool wear studies in fabrication of microchannels in ultrasonic micromachining, Ultrasonics 57 (2015) 57-64.

9.

N. Yuvaraj, M.P. Kumar, Multiresponse Optimization of Abrasive Water Jet Cutting Process Parameters Using TOPSIS Approach, Materials and Manufacturing Processes 30 (2015) 882–889.

10. O. Çakir, A. Yardimeden, T. Ozben, Chemical machining, Archives of Materials Science and Engineering 28 (2007) 499-502. 11. H. Kurafuji, K. Suda, Electrical discharge drilling of glass, Annals of the CIRP 16 (1968) 415- 419. 12. K. Allesu, A. Ghosh, M. K. Muju, Preliminary qualitative approach of a proposed mechanism of material removal in electrical machining of glass, European Journal of Mechanical Engineering 36 (1992) 202–207. 13. V. Fascio, R. Wüthrich, H. Bleuler, Spark assisted chemical engraving in the light of electrochemistry, Electrochimica Acta 49 (2004) 3997–4003. 14. S. Tandon, V. K. Jain, P. Kumar, K. P. Rajurkar, Investigations into machining of composites, Precision Engineering 12 (1990) 227–238. 15. H. Langen, J. M. Breguet, H. Bleuler, Ph. Renaud, T. Masuzawa, Micro electrochemical discharge machining of glass, International Journal of Electrical Machines 3 (1998) 65–69. 16. I. M. Crichton, J. A. Mcgeough, Studies of the discharge mechanisms in electrochemical arc machining, Journal of Applied Electrochemistry 15 (1985) 113-119. 17. N. H. Cook, G. B. Foote, P. Jordan, B. N. Kalyani, Experimental studies in electromachining, Journal of Engineering for Industry 95 (1973) 945–950. 18. A. B. E. Khairy, J. A. Mcgeough, Die-sinking by electro erosion-dissolution machining, CIRP Annals - Manufacturing Technology 39 (1990) 191-195.

19. B. R. Sarkar, B. Doloi, B. Bhattacharyya, Parametric analysis on electrochemical discharge machining of silicon nitride ceramics, International Journal of Advance Manufacturing Technology 28 (2006) 873-881. 20. T. F. Didar, A. Dolatabadi, R. Wüthrich, Characterization and modelling of 2D-glass micromachining by spark-assisted chemical engraving (SACE) with constant velocity, Journal of Micromechanics and Micro engineering 18 (2008) 9. 21. V. K. Jain, S. K. Chak, Electrochemical spark trepanning of alumina and quartz, Machining Science and Technology 4 (2000) 277-290. 22. K. Furutani, H. Maeda, Machining a glass rod with a lathe-type electro-chemical discharge machine, Journal of Micromechanics and Microengineering 18 (2008) 8. 23. M. Schöpf, I. Beltram, M. Boccadoro, D. Kramer, ECDM (Electrochemical discharge machining) An new method for trueing and dressing of metal bonded diamond grinding tools, CIRP Annals - Manufacturing Technology 50 (2001) 125–128. 24. W. Y. Peng, Y. S. Liao, Study of electrochemical discharge machining technology for slicing non-conductive brittle materials, Journal of Material Processing Technology 149 (2004) 363–369. 25. L. Paul, S.S. Hiremath, J. Ranganayakulu, Experimental investigation and parametric analysis of electro chemical discharge machining, International Journal of Manufacturing Technology and Management 28 (2014) 57-79. 26. P. K. Gupta, A. Dvivedi, P. Kumar, Effect of Pulse Duration on Quality Characteristics of Blind Hole Drilled in Glass by ECDM, Materials and Manufacturing Processes (2015) Oct 16 (just-accepted). DOI: 10.1080/10426914.2015.1103857

27. B.

Bhattacharyya,

B.N.

Doloi,

S.K.

Sorkhel,

Experimental

investigations

into

electrochemical discharge machining (ECDM) of non-conductive ceramic materials, Journal of Material Processing Technology 95 (1999) 145-154. 28. R. Wuthrich, C.H. Comninellis, H. Bleuler, Bubble evolution on vertical electrodes under extreme current densities, Electrochimica Acta 50 (2005) 242-246. 29. A. Kulkarni, R. Sharan, G.K. Lal, An experimental study of discharge mechanism in electrochemical discharge machining, International Journal of Machine Tools and Manufacturing 42 (2002) 1121-1127. 30. I. Basak, A. Ghosh, Mechanism of spark generation during electrochemical discharge machining: a theoretical model and experimental verification, Journal of Material Processing Technology 62 (1996) 46-53. 31. V.K. Jain, P.M. Dixit, P.M. Pandey, On the analysis of the electrochemical spark machining process, International Journal of Machine Tools and Manufacture 39 (1999)165–186. 32. A. Behroozfar, M.R. Razfar, Experimental and numerical study of material removal in electrochemical discharge machining (ECDM), Materials and Manufacturing Processes 31(2016) 495-503. 33. C.T. Yang, S.S. Ho, B.H. Yan, Micro Hole Machining of Borosilicate Glass through Electrochemical Discharge Machining (ECDM), Key Engineering Materials196 (2001) 149166. 34. A. de Silva, J. A. McGeough, Surface effects on alloys drilled by electrochemical arc machining, Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture 200 (1986) 237-246.

35. M. Coteaţă, L. Slătineanu, O. Dodun, C. Ciofu, Electro chemical discharge machining of small diameter holes, International Journal of Material Forming 1 (2008) 1327 –1330. 36. J. West, A. Jadhav, ECDM methods for fluidic interfacing through thin glass substrates and the formation of spherical micro cavities, Journal of Micromechanics and Micro engineering 17 (2007) 403–409. 37. J. D. A. Ziki, R. Wüthrich, Forces exerted on the tool-electrode during constant-feed glass micro-drilling by spark assisted chemical engraving, International Journal of Machine Tools and Manufacture 73 (2013) 47–54. 38. L. Paul, S. S. Hiremath, Response surface modelling of micro holes in electrochemical discharge machining process, Procedia Engineering 64 (2013) 1395-1404. 39. Y. S. Liao, W. Y. Peng, Study of hole-machining on pyrex wafer by electrochemical discharge machining (ECDM), Material Science Forum 505 (2006) 1207-1212. 40. P. Maillard, B. Despont, H. Bleuler, R. W¨uthrich, Geometrical characterization of microholes drilled in glass by gravity-feed with spark assisted chemical engraving (SACE), Journal of Micromechanics and Micro engineering 17 (2007) 1343-1349. 41. P. K. Gupta, A. Dvivedi, P. Kumar, A study on the phenomenon of hole overcut with working gap in ECDM, Journal of Production Engineering 17 (2014) 30-34. 42. A. Manna, V. Narang, A study on micro machining of e-glass fibre–epoxy composite by ECSM process, International Journal of Advance Manufacturing Technology 61 (2012) 1191-1197. 43. R. Wüthrich, L. A. Hof, The gas film in spark assisted chemical engraving (SACE) - A key element for micro-machining applications, International Journal of Machine Tools and Manufacture 46 (2006) 828–835.

44. Y. S. Laio, L. C. Wu, W. Y. Peng, A study to improve drilling quality of electrochemical discharge machining (ECDM) process, Procedia CIRP 6 (2013) 609-614. 45. M. Mousa, A. Allagui, H. D. Ng, R. Wüthrich, The effect of thermal conductivity of the tool electrode in spark-assisted chemical engraving gravity-feed micro-drilling, Journal of Micromechanics and Micro engineering 19 (2009) 7. 46. C. K. Yang, C. P. Cheng, C. C. Mai, Y. B. Hwa, Effect of surface roughness of tool electrode materials in ECDM performance, International Journal of Machine Tools and Manufacture 50 (2010) 1088-1096. 47. C. K. Yang, K. L. Wu, J. C. Hung, S.M. Lee, J.C. Lin, B.H. Yan, Enhancement of ECDM efficiency and accuracy by spherical tool electrode, International Journal of Machine Tools and Manufacture 51 (2011) 528-535. 48. A. Behroozfar, M. R. Razfar, Experimental Study of the Tool Wear during the Electrochemical Discharge Machining, Materials and Manufacturing Processes 31 (2016) 574-580. 49. R. Wüthrich, K. Fujisaki, P. Couthy, L. A. Hof, H. Bleuler, Spark assisted chemical engraving (SACE) in microfactory, Journal of Micromechanics and Micro engineering 15 (2005) 276-280. 50. Z. P. Zheng, J. K. Lin, F. Y. Huang, B. H. Yan, Improving the machining efficiency in electrochemical discharge machining (ECDM) micro hole drilling by offset pulse Voltage, Journal of Micromechanics and Micro engineering 18 (2008) 6. 51. L. Paul, S. S. Hiremath, Characterisation of micro channels in electrochemical discharge machining process, Applied Mechanics and Materials 490 (2014) 238-242.

52. K. H. Nguyen, P. N. Lee, B. H. Kim, Experimental investigation of ECDM for fabricating micro structures of quartz, International Journal of Precision Engineering and Manufacturing 16 (2015) 5-12. 53. L. Changjian, G. An, L. Meng, Y. Shengyi, The micro-milling machining of pyrex glass using the electrochemical discharge machining process, Advanced Material Research 403 (2012) 738-742. 54. J. D. A. Ziki, T. F. Didar, R. Wüthrich, Micro texturing channel surfaces on glass with spark assisted chemical engraving, International Journal of Machine Tools and Manufacture 57 (2012) 66-72. 55. Z. P. Zheng, W. H. Cheng, F. Y. Huang, B. H. Yan, 3D micro structuring of Pyrex glass using the electrochemical discharge machining process, Journal of Micromechanics and Micro engineering 17 (2007) 960-966. 56. X. D. Caoa, B. H. Kimb, C. N. Chua, Micro-structuring of glass with features less than 100 µm by electrochemical discharge machining, Precision Engineering 33 (2009) 459-465. 57. C. Wei, D. Hua, K. Xu, J. Ni, Electro chemical discharge dressing of metal bond microgrinding tools, International Journal of Machine Tools and Manufacture 51 (2011) 165-168. 58. B.K. Bhuyan, V. Yadava, Experimental Study of Traveling Wire Electrochemical Spark Machining of Borosilicate Glass, Materials and Manufacturing Processes 29 (2014) 298304. 59. C. T. Yang, S. L. Song, B. H. Yan, F. Y. Huang, Improving machining performance of wire electrochemical discharge machining by adding SiC abrasive to electrolyte, International Journal of Machine Tools and Manufacture 46 (2006) 2044-2050.

60. K. Y. Kuo, K. L. Wu, C. K. Yang, B. H. Yan, Wire electrochemical discharge machining (WECDM) of quartz glass with titrated electrolyte flow, International Journal of Machine Tools and Manufacture 72 (2013) 50-57. 61. A. Singh, C. S. Jawalkar, R. Vaishya, A. K. Sharma, A study on wire breakage and parametric efficiency of the wire

electro chemical discharge machining process, in:

Proceedings of the Fifth International and twenty sixth All India Manufacturing Technology, Design and Research Conference, IIT Guwahati Assam, India, 2014, pp. 272.1-272.6 62. W.Y. Peng, Y.S. Liao, Study of electrochemical discharge machining technology for slicing non-conductive brittle materials, Journal of Materials Processing Technology 149 (2004) 363–369. 63. H. Tsuchya, T. Inoue, M. Miyazaki, Wire electrochemical discharge machining of glass and ceramics, Bulletin of the Japan Society of Precision Engineering 19 (1985) 73-74. 64. V. K. Jain, P. S. Rao, S. K. Choudhury, K.P. Rajurkar, Experimental investigations into travelling wire electrochemical spark machining (TW-ECSM) of composites, Journal of Engineering for Industry 113(1991) 75-84. 65. J. W. Liu, T. M. Yue, Z. N. Guo, Wire electrochemical discharge machining of Al2O3 particle reinforced Aluminium alloy 6061, Materials and Manufacturing Processes 24 (2009) 446-453. 66. M. C. Panda, V. Yadava, Intelligent modelling and multi objective optimization of die sinking electrochemical spark machining process, Materials and Manufacturing Processes 27(1) (2011) 10-25.

67. K. Sanjay, P. Chak, V. Rao, Trepanning of Al2O3 by electro-chemical discharge machining (ECDM) process using abrasive electrode with pulsed DC supply, International Journal of Machine Tools and Manufacture 47 (2007) 2061–2070. 68. Z. P. Zheng, H. C. Su, F. Y. Huang, B. H. Yan, The tool geometrical shape and pulse-off time of pulse voltage effects in a Pyrex glass electrochemical discharge micro drilling process, Journal of Micromechanics and Micro engineering 17 (2007) 265–272. 69. S. K. Jui, A. B. Kamaraj, M. M. Sundaram, High aspect ratio micromachining of glass by electrochemical discharge machining (ECDM), Journal of Manufacturing Processes 15 (2013) 460–466. 70. J. Kozak, M. Zybura, Mathematical modelling of rotary electrochemical discharge machining (RECDM), in: Proceedings of the world congress on engineering and computer science (WCECS 2012), San Francisco, USA, 2012. 71. M. Coteata, H. P. Schulze, L. Slatineanu, Drilling of difficult-to-cut steel by electrochemical discharge machining, Materials and Manufacturing Processes 26 (2011) 1466-1472. 72. S. F. Huang, Y. Liu, J. Li, H. X. Hu, L.Y. Sun, Electrochemical discharge machining microhole in stainless steel with tool electrode high-speed rotating, Materials and Manufacturing Processes 29 (2014) 634-637. 73. M. S. Han, B. K. Min, S. J. Lee, Improvement of surface integrity of electro-chemical discharge machining process using powder-mixed electrolyte, Journal of Material Processing Technology 191 (2007) 224–227. 74. H. El-Hofy, Advanced machining processes, McGraw-Hill: United State of America (2005).

75. K. C. Sanjay, P. V. Rao, The drilling of Al2O3 using a pulsed DC supply with a rotary abrasive electrode by the electrochemical discharge process, International Journal of Advance Manufacturing Technology 39 (2008) 633–641. 76. V. K. Jain, S. K. Choudhury, K. M. Ramesh, On the machining of alumina and glass, International Journal of Machine Tools and Manufacture 42 (2002) 1269–1276. 77. L. J. Wen, Grinding-aided electrochemical discharge machining of metal matrix composites, Dissertation: The Hong Kong polytechnic university (2009). 78. J. W. Liu, T. M. Yue, Z. N. Guo, Grinding-aided electrochemical discharge machining of particulate reinforced metal matrix composites, International Journal of Advance Manufacturing Technology 68 (2013) 2349–2357. 79. C. P. Cheng, K. L. Wu, C. C. Mai, Y. S. Hsu, B. H. Yan, Magnetic field-assisted electrochemical discharge machining, Journal of Micromechanics and Micro engineering 20 (2010) 7. 80. R. Wuthrich, B. Despont, P. Maillard, H. Bleuler, Improving the material removal rate in spark-assisted chemical engraving (SACE) gravity-feed micro-hole drilling by tool vibration, Journal of Micromechanics and Micro engineering 16 (2006) N28–N31. 81. M.S. Han, B.K. Min, S.J. Lee, Geometric improvement of electrochemical discharge microdrilling using an ultrasonic vibrated electrolyte, Journal of Micromechanics and Micro engineering 19 (2009) 8. 82. M. Rusli, K. Furutani, Performance of Micro-hole drilling by Ultrasonic-assisted ElectroChemical Discharge Machining, Advanced Materials Research 445 (2012) 865-870. 83. A. Dvivedi, P. Kumar, I. Singh, Development of a New Stir Caster Design for the Production of Metal Matrix Composite, Indian Foundry Journal 54 (2008) 21-27.

List of Figure captions Figure 1 Schematic of ECDM set-up Figure 2 Process mechanism of ECDM Figure 3 Cause and effect diagram showing ECDM parameter Figure 4 ECDM process variants Figure 5 Influence of machining voltage and drilling depth on mean diameter of the micro hole [40] Figure 6 ECDD developed micro holes of (a) conical and (b) spherical shapes [36] Figure 7 Micro holes developed by using (a) DC voltage (b) pulse voltage (c) offset pulse voltage [50] Figure 8 Influence of tool rotational rate on groove width and depth [55] Figure 9 Influence of tool travel rate on groove width and depth [55] Figure 10 (a) Micro-grooves, (b) enlarged figure of micro-grooves, (c) micro-pillar, (d) microwall, and (e and f) micro-pyramid machined on glass [56] Figure 11 Schematic of ECDT set-up Figure 12 Schematic of W-ECDM set-up Figure 13 Straight grooves developed through (a) Weight-loading mechanism (b) reciprocating mechanism [59] Figure 14 Influence of feed rate on slit mean width and depth [60] Figure 15 ECDM based triplex hybrid methods Figure 16 Schematic of (a) flat side-wall flat front tool and (b) discharge influenced zone Figure 17 Schematic of electrochemical discharge grinding mechanism Figure 18 Schematic of MAECDM set-up

Figure 19 Cross-section view of holes (a) without ultrasonic (b) with ultrasonic [81] Figure 20 Published research work in the area of electrochemical discharge machining variants

List of Tables Table: 1 Capabilities and Research opportunities of the ECDM variants ECDM Variants

Capabilities of the ECDM process variants Geometrical features Work materials

Electrochemical Feature

shapes:

Research opportunities

conical, Cobalt, low-alloy steels, Improvement in process

discharge

spherical

[36]

and chrome,

drilling

cylindrical [40] hole forms.

nimonic

titanium, repeatability alloys

and

[34], accuracy can increase

Maximum aspect ratio = silicon nitride ceramics industrial applicability 11:1 [69]

[19], cutting steel [35], borosilicate glass [3638], e-glass–fibre–epoxy composites [42], soda lime glass [40], silicon wafers [25] and Pyrex wafer [39].

Electrochemical Feature shapes: Complex Pyrex glass [53, 55],

Investigations can be

discharge

3D

conducted on

milling

micro grooves [53], micro and quartz [52].

electrochemical

channels (straight, U-shaped

discharge milling for

and

fabrication of complex

micro-structures

serpentine),

like borosilicate glass [54]

micro-

pillars, micro-pyramids [56]

micro features with high

and surface texturing of the

aspect ratios.

micro channels [54]. Dimensions: width of 30μm and depth of 70μm with surface

roughness

of

0.099μm [56]. Electrochemical Feature shapes: micro discharge

grooves on cylindrical rods

Soda lime glass [22].

Possibilities exist to machine other materials.

turning

of diameter 5 mm [22]. Dimensions: Width .35 mm and depth .6mm [22].

Electrochemical Dressing of 850μm grinding

Applied only for

In depth parametric

discharge

metallic bonded

analysis can provide

tool [57].

dressing

diamond coated grinding better results as well as wheels [23, 57].

wider scope.

Wire-

Feature shapes: Micro

Glass [58], ceramics

Possibilities exist for

electrochemical

grooves, micro slots [61, 59,

[63], quartz [60] glass-

addition of additional

discharge

and 60] and cutting of glass

epoxy, Kevlar-epoxy

axes in manipulator for

machining

rods [62].

[64] and metal matrix

machining of complex

Dimensions: 8.8 mm long

composites [65].

3D profiles.

slots [61] and minimum width of the slots 160μm [60]. Die sinking

Feature shapes: Cylindrical

316 Stainless steel alloy

Research related to the

electrochemical

dies [19].

[19].

processing of different

discharge

Dimensions: Depth of blind

materials and complex

machining

sink = 30 mm [19].

die shapes demands

Conicity in cylindrical die

more investigation.

shape (die-shaping factor) = 0.05 to 0.19 mm/mm [19]. Electrochemical Feature shapes: cylindrical

Alumina and quartz

Modifications in tool

discharge

holes [21].

[21].

design can further

trepanning

Dimensions: 2.35 mm depth

enhance the dimensional

of hole [21].

accuracy of the process.

Highlights    

A comprehensive review on these recent developments in ECDM process, its process variants and their triplex hybrid methods. Machined features with its complexity and accuracy level have been also presented. Information regarding the materials that can machine has been also presented. Research opportunities.

Figure 1

DC power source

I. Electrolysis

Hydrogen ions Conductive ions Hydroxyl ions

Tool

Anode

Work-piece

IV. Sparking

III. Bubbles coalesce & layer formation

Tool Spark initiation point

Energy interaction behaviour between tool & workpiece (II to IV)

II. Generation & accumulation of hydrogen bubbles

Tool Bubble layer

High electric field (10 V/μm )

Tool Conductive ions channel

Discharge Work-piece

Figure 2

Work-piece

Work-piece

Hydrogen bubbles

Figure 3

Figure 4

330 300 200 100

Figure 5

µm µm µm µm

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

Figure 15

Figure 16

Figure 17

Figure 18

Figure 19

Figure 20