Keywords

1 Introduction

Machining is an essential manufacturing process toward the development of technology and its applications in various fields. Conventional machining ruled the field of manufacturing since stone from stone weapons to the development of CNC and artificial intelligence. Due to the presence of new and advanced materials, the development of machining technologies is also improving. In the age of non-traditional machining, there are a lot of processes, which have been used for machining hard, brittle, and difficult to machine materials like glass, ceramics, metal matrix, etc. Further, the development of a micro-electro-mechanical system (MEMS) pushed forward the micro-machining as well as non-traditional machining processes. Among various kinds of non-traditional machining, there are some thermal, chemical, electrochemical, and mechanical machining processes depending on their material removal mechanisms. Every non-traditional machining process consists of advantages and limitations. The electrical discharge machining process can machine electrically conducting materials with a less hard tool than a workpiece. This process can be used to machine hard materials through the formation of recast layer, HAZ, and other thermal effects on the machined surface is the major drawbacks of this process. The electrically non-conducting materials cannot be machined by the EDM process [1]. Electrochemical machining processes are effective during machining of electrically conductive materials and surface finishing, deburring as well as micropattern can be developed through this process but only in electrically conductive materials [2]. The hybrid machining processes were introduced to overcome the limitations of the separate process. The addition of ultrasonic vibration, powder additives in the EDM process improved the surface finish and material removal rate in the EDM process [3]. Electrochemical discharge machining overcame the limitations present in ECM and EDM separately and was able to machine electrically non-conductive materials. ECDM process proved its effectiveness for machining alumina, borosilicate glass, etc. [4]. The process of electrochemical discharge was first discovered in Japan by Kurafuji and Suda though they did not use the term ECDM rather they proposed it as electrical discharge drilling [5]. After the 1990s lot of works were carried out to understand the process and its usability in the machining of electrically non-conductive materials like glass, ceramics, quartz, etc. [6]. Jain et al. used the term electrochemical spark machining (ECSM) in 1990 and developed a second-order model of responses using the CADEAG-1 software package [7]. In 1996, Singh et al. used cathode wire to machine partially conductive materials and found that material removal rate increased with the increase of supply voltage and electrolyte concentration within the range of machine conditions [8]. Research on the mechanism of spark generation was first carried out by Basak and Ghosh in 1996 [9, 10]. They developed a theoretical model and experimented to verify the model. Jain et al. in 1999 prepared a model for material removal rate by finite element modeling using 3D unsteady state heat conduction problem and observed that the computed MRR was higher than the experimental results [11].

2 Fundamentals of ECDM

Since the first invention of electrochemical discharge machining, researchers around the world have tried to understand the fundamental concept or behavior of the process. Till now it has not been fully understood phenomenon as the removal of materials, gas film formation, or like such things are used to happen during machining which is uncontrollable. It has been observed from the development of ECDM that material removal took place due to the combined effects of electrochemical reaction and electrical spark discharge [12]. During the investigation on non-conducting ceramics materials, Bhattacharyya et al. analyzed the electrochemical reaction that occurred when voltage was applied between the two electrodes immersed into the electrolyte [13]. In ECDM, two electrodes are used which are immersed into the electrolyte and among them, one electrode is a cathode generally used as a tool and another electrode is an anode used as an auxiliary electrode. The size of the auxiliary electrode is much bigger than the tool electrode. The electrochemical reaction that happens in the system can be divided into two parts like reaction on cathode where the evolution of hydrogen gas occurs due to evaporation of electrolytes and reaction on anode where the evolution of oxygen gas occurs.

Reaction at cathode

$$2{\text{H}}^{ + } + 2{\text{e}}^{ - } \to {\text{H}}_{2} \uparrow \,\left( {\text{in acidic electrolytic solution}} \right)$$
(1)
$$2{\text{H}}_{{2}} {\text{O}} + 2{\text{e}}^{ - } \to 2\left( {{\text{OH}}} \right)^{ - } + {\text{H}}_{2} \uparrow \,\left( {\text{in alkaline solution}} \right)$$
(2)

Reaction at anode

$$2{\text{H}}_{2} \to {\text{O}} \to {\text{O}}_{2} \uparrow + 4{\text{H}}^{ + } + 4{\text{e}}^{ - } \,\left( {\text{in acidic solution}} \right)$$
(3)
$$4\left( {{\text{OH}}} \right)^{ - } \to 2{\text{H}}_{2} {\text{O}} + {\text{O}}_{2} \uparrow + 4{\text{e}}^{ - } \,\left( {\text{in alkaline solution}} \right)$$
(4)

Figure 1 shows the schematic diagram of the experimental setup of the electrochemical discharge machining process.

Fig. 1
An illustration depicts an experimental setup of E C D M with parameters labeled.

Schematic diagram of the experimental setup of ECDM process

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2.1 Fishbone Diagram

The size of the auxiliary electrode has to be around 100 times larger than the tool electrode. During the application of the voltage between two electrodes, the hydrogen gas bubbles generated by the evaporation of electrolytes accumulate around the tool electrode or cathode. These hydrogen gas bubbles are used to form gas film to cover the open area to the electrode with the increase of voltage. At a certain critical voltage, this gas film collapsed and discharge can be seen in the form of light emission. This discharge did not occur between the two electrodes rather it occurred between cathode and electrolyte [14]. Any electrically non-conductive materials placed in that discharge area are exposed to high thermal energy which melts and vaporized the materials from the workpiece. During the drilling of glass, Wüthrich et al. have increased material removal rate by twice than without vibration [15]. Some researchers have claimed that many phenomena can be monitored like local electrolyte temperature, presence, and absence of gas film by analyzing the current signal during the process [16]. Gas film thickness can be changed by improving the wettability of the tool in the electrolyte [17]. In direct polarity, material removal takes place by melting and vaporization, whereas in reverse polarity chemical reactions also happen with melting and vaporization. Although in reverse polarity overcut, tool wear and surface roughness are higher than direct polarity [18]. The thermal conductivity of the tool can also affect material removal. Mousa et al. [19] found that a tool with higher thermal conductivity can increase the material removal process due to its ability to vaporize more electrolytes in the discharge regime whether hydrodynamic regime material removal decreases due to higher viscosity of molten workpiece. Besides those, many research works have been done and till now development of this process is going on to understand the actual phenomenon that happens during electrochemical discharge and to stabilize it for commercial applications. Figure 2 shows the different effective factors which can influence the ECDM process, and Fig. 3 shows the application of ECDM in different fields.

Fig. 2
An illustration depicts the factors that influence E C D M.

Cause and effect of different factors in ECDM

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Fig. 3
An 8-textbox flowchart depicts the development of E C D M.

Development of ECDM

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3 Advancement in ECDM

3.1 Control of Gas Bubbles and Gas Film

Surface active agent or surfactant has the property to reduce the surface tension of the system. Surfactant SDS and CTAB with 25 wt% NaOH and 25 wt% KOH solution can decrease surface tension up to Critical Micelle Concentration (CMC). Due to decreased value of surface tension, the thin gas film produced as a large bubble is unable to be stuck with the tool electrode [20]. Lin et al. have added ethanol to the electrolyte which helped to reduce the angle between electrodes and as bubble, and magnetic field organized the charged particle due to Lorentz force and stabilized the generated flow around the electrode. Besides that minimum level of electrolyte, the minimum concentration of electrolyte and maximum possible distance between electrodes is suitable for stable gas film formation. Figure 4 shows the generated gas film under extreme and optimum conditions [21].

Fig. 4
Two images represent the generated gas film. The first image is under extreme parameters and the second, optimum parameters. The labels include electrolyte, tool, and gas film.

Generated gas film in extreme and optimum condition [21]

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As the buoyancy force is a problem in a high level of electrolyte and more concentration of electrolyte accelerates the electrochemical process which enhanced the instability of gas film due to increased buoyant force. Arab et al. have documented that higher surface roughness tools are responsible for thicker gas bubbles and gas film thickness, whereas lower surface roughness tools produce thin gas film and lower HAZ [22]. Higher surface roughness tool can capture bigger gas bubbles due to the presence of dipper valley which produced the thicker gas film. Behroozfar et al. have reported that in 30 wt% NaOH solution with 35 V applied voltage and using 500 µm tungsten tool an average diameter of 260 µm of plasma channel can be obtained [12]. Assisted ultrasonic vibration of 10 µm amplitude to the tool electrode has reduced gas film thickness by 65%. Figure 5a shows the craters which were formed during electrochemical discharge machining, and Fig. 5b shows the nature of single discharge during ECDM [23, 24].

Fig. 5
The images represent the craters. Image a represents the formation during electrochemical discharge, and image b represents the nature of a single discharge.

Craters on a workpiece (a) and single discharge (b) [23]

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Gas film behavior is not only influenced by electrolyte, tool, or vibration in tool electrodes but also closely related to the gas bubble evaluation. The balance force acting on the gas bubble can be manipulated by applying extra Lorentz force which resulted in thinner gas film formation [25]. Appalanaidu et al. have controlled the gas film shape with the help of generated electrostatic force on hydrogen gas bubbles [26].

3.2 Enhancement of MRR

The application of ultrasonic vibration of 10 µm amplitude has resulted in 5.48 µg of material removal which is greater than without vibration. It has also been observed that a large amount of amplitude destroyed the effect of vibration [24]. Due to the reduction of gas film thickness through ultrasonic vibration, the discharge activities affect more than without vibration which leads to improvement of MRR [23]. Square waveform voltage to the actuator has increased MRR by 40% in the case of a cylindrical rod and 20% increase of MRR found when sinusoidal waveform applied [27]. Bellubbi et al. have observed that higher electrolyte concentration and machining time can increase MRR [28]. The electrolyte is a very important factor of ECDM because its properties like conductivity, viscosity, density, and temperature influence the gas film making process which leads to proper machining. A mixer of NaOH and KOH has provided more electrical conductivity than separate NaOH and KOH. The depth of the microchannel increased by 19% in the case of 25 wt% mixed electrolyte (NaOH + KOH) than 25 wt% KOH and increased by 31% than 25 wt% NaOH. Figure 6 shows the comparison of current signal among KOH, NaOH, and mixed electrolyte and visible that in case of mixed electrolyte of NaOH, KOH provides more current than other two [29].

Fig. 6
3 line graphs depict the comparison of individual and mixed electrolytes of the given current signal.

Comparison of current signal between individual and mixed electrolytes [29]

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By dispersing nanoparticles, Elhami et al. have improved hole depth improved by 21.1% for Al2O3 and 18.7% for Cu nanoparticles [30]. Application of electrolyte injection provides sufficient electrolyte even at a deeper hole to maintain the material removal mechanism. It also provides drag forces to detach smaller diameter bubbles from the cathode surface. It has been observed that the use of electrolyte injection increases the hole depth by 70% with a 1.7 mm diameter tool electrode [31]. Researchers also reported that continuous replenishment of vaporized electrolytes at the machining zone stabilized the discharging process. With this stable process, a noticeable amount of increase in hole depth was found [32]. The spherical head tool electrode made less contact with the workpiece and allows the electrolyte to flow in the machining zone which produce stable gas film and lead to an increase in the machining depth by increasing MRR [33]. Ratan et al. have examined and observed that the presence of a magnetic field in the ECDM process generates Lorentz force onto the ion and helps them to move which creates magnetohydrodynamic convection. Due to MHD, convection electrolytes are used to flow through the small distance between electrodes and workpiece and more frequent sparking occurs which increases the MRR [34].

3.3 Improvement of Surface Quality and Dimensional Accuracy

The optimum tool-workpiece gap has a great impact on the surface quality and dimensional accuracy [35]. Experiments have shown that lower viscosity and higher ion mobility are the two major factors for a better quality of through-hole. In the case of KOH, the quality of the through-hole improved a lot [35]. Chenxiang et al. have found more tapering effect and bigger size hole diameter at higher conductivity solution of NaNO3 [36]. To get more energy from electrochemical discharge, the concentration of electrolyte needs to be increased but at higher concentrations, the machined surface is damaged due to higher thermal conductivity. The mobility and thermal conductivities of potassium ions are greater than sodium ions, but the mixture of KOH and NaOH at 15 and 25% give higher thermal conductivity than separate KOH and NaOH which provide higher depth and better-machined quality [29]. A greater number of (OH)− ion in the electrolyte have reduced the surface tension and produced the thinner gas film. The presence of (OH)− ion also enhanced the chemical etching process and improves the quality of machined surfaces [37]. The mobility and electrical conductivities of potassium ions are larger than sodium ions which helps the potassium ion to generate more power than sodium ion and consequently heat-affected zone is higher in KOH solution. Figure 7 shows different surface qualities at different electrolyte concentrations [38].

Fig. 7
4 images depict different surface qualities observed for different electrolytic conditions of 10–30 percent Na O H.

Different machined surfaces at different electrolyte concentrations of 10–30% NaOH [38]

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Lower tool electrode feed rate provides more interaction with the workpiece and time for the flow of electrolyte at the machining zone. It has been found by researchers that tool feed rates higher than 5 µm/s produced nonuniform electrochemical discharge. If the tool feed rate is lower than 5 µm/s, then uniform electrochemical discharge can be obtained which resulted in a smaller size of top hole and bottom hole diameter which was 300 µm and 70 µm, respectively at 3–4 µm/s feed rate [39].

3.4 MicroChannel and Milling with Electrochemical Discharge

The research work in the field of channeling and milling is very less compared to the drilling process. Some researchers experimented to find out the capability of ECDM in channeling and milling. Using surfactant in electrolytes, the surface quality of microchannels was improved due to the increase in the viscosity of electrolyte and decrease in the thermal conductivity [20]. Sabahi et al. have increased the depth of microchannel in case of a mixed electrolyte by 19% than KOH and 31% than NaOH solution, and the overcut values for NaOH, KOH, and mixed electrolyte are 655 µm, 638 µm, and 541 µm, respectively [29]. Electrolyte concentration has a prominent effect on the channel and milling process done by electrochemical discharge machining. Jana et al. observed that at 10wt% NaOH the surface of the machining zone became featherly like texture while by increasing the electrolyte concentration from 10 to 20 to 30 wt%; the featherly like texture became spongy-like texture. Figure 8a shows featherly like texture at 10 wt% NaOH, and Fig. 8b shows the spongy-like texture at 30 wt% NaOH solution [40].

Fig. 8
2 images represent the surface texture for different concentrations of electrolyte. Images a and b represent feathery texture for 10 percent and spongy texture for 30 percent Na O H respectively.

Texture of machined surfaces at different electrolyte concentrations. a Featherly like texture at 10 wt% NaOH and b spongy-like texture at 30% NaOH [40]

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Electrolyte temperature increases the energy of ions at the electrolyte surface which leads to more powerful discharge and a faster channeling process [41]. Higher electrolyte concentration increases the MRR by decreasing inter-electrode resistance but at higher electrolyte concentration, the sludge gets accumulated and side sparking increases [42]. The textured tool has resulted in an increase in MRR and depth of microchannel by 19.27% and 64.81%, respectively. In the case of microchannel, if the tool feed rate increased, then the channel depth decreased such as at lower feed rate channel depth was obtained 23 µm which was 11 µm in case of higher speed [40]. Low feed rate leads to longer interaction time between tool and workpiece which opens up the opportunity for chemical action and increased MRR [42, 43]. The large distance between tool and workpiece increased the kinetic energy as it travels more distance to collide with one another which deteriorates surface quality and reduces the area of groove [44, 45]. The use of the magnetic field in the micro-milling of glass by electrochemical discharge machining was very helpful as stated by Hajian et al. [46]. They published that at lower electrolyte concentration, the viscosity was lower which helped to remove the etched glass from the machining zone. The Lorentz force generated by the magnet controls the motion of gas bubbles at the inter-electrolyte area which improved the MRR. It has been observed that with the increase in applied voltage and electrolyte concentration, the tool feed rate was greatly increased [46, 47]. Another study has shown that by increasing electrolyte concentration, the channel depth can be increased but the surface quality will deteriorate in case of higher electrolyte concentration [47].

4 Significance

Machining a material with the help of non-conventional machining processes like electric discharge machining (EDM), electrochemical machining (ECM), abrasive water jet machining (AWJM), laser beam machining (LBM), etc., can be done but with certain limitations. Electrically non-conducting materials cannot be machined with ECM and EDM. Miniature product is difficult to fabricate with WAJM as overcut is a major problem here and skillful operators are required to operate the machine with the available setup. Transparent materials cannot be easily machined with the help of LBM. These disadvantages of the machining process enhance the demand for micro-electrochemical discharge machining process since conductive materials as well as electrically non-conductive, which are very difficult to machine can be easily machined by ECDM. No isolated chamber is required for the setup. No mask is needed and just as in traditional machining, the desired structure can be machined directly. Electrochemical discharge machining (ECDM) can machine materials irrespective of the electrical and mechanical properties with small thermal effect, i.e., formation of heat-affected zone of workpiece materials, and the area of machining zone can be varied from several millimeters to microns without large force as found in all common conventional machining processes.

5 Summary

After reviewing the research papers on the ECDM process, it has been observed that the process is very complex and uncertain. Lots of phenomena happen during the machining process, and most of the researchers agreed that electrochemical and electrical discharge machining process both occurs in the ECDM process. In this machining process, if a high-temperature etching process happens during machining, then the quality of machining or the machined surface became smooth surface. During the milling tool-workpiece distance and tool-workpiece interaction, time plays a vital role in the smooth milling surface. The addition of surfactant to the electrolyte reduces the surface tension which provides thinner gas film besides that adding vibration to the tool electrode also provides thinner gas film. Different types of tool electrodes, electrolytes, and the addition of magnetic field, vibration may improve the total machining efficiency. From the above discussions, the following conclusions can be made.

  • The use of surfactant reduces the surface tension of the medium and the addition of vibration to the tool electrode leads to thinner gas film, and thinner gas film can produce a smooth surface. Using surfactant in electrolyte, the surface quality of microchannels was improved due to the increase in the viscosity of electrolyte and decrease in the thermal conductivity.

  • The evaluation of gas bubbles is also an important factor that can be triggered by using a magnetic field to the system where the Lorentz force helps to organize the generated gas bubbles.

  • Higher surface roughness tools produce bigger bubbles due to the presence of peak and valley, and the minimum electrolyte concentration and maximum inter-electrode distance may produce the suitable gas film.

  • A mixed alkaline solution can produce a better microchannel in glass materials.

  • Lower tool electrode feeding provides more time for tool-workpiece interaction which leads to high-temperature etching and smooth machined surface.

6 Future Scope

The complex phenomenon of ECDM is not still well understood. Drilling of non-conductive materials like glass, ceramics, quartz, etc., has been carried out by applying different techniques. Still lot of research is required to make the process commercialized. Very few research papers are available regarding channeling or milling. The process is highly dependent on various factors like workpiece materials, tool electrodes, power supply, electrolytes properties, tool feed rate, etc. So, many factors need to optimize properly to get smooth and accurate machining.

Control of gas film:

  • Analysis of electrolyte properties like concentration, viscosity, surface tension by addition of abrasives, mixed electrolyte, surfactant, etc.

  • Application of vibration and use of a lower surface textured tool to control the size of the gas bubble.

  • Application of magnetic field to organize the gas bubble.

Enhancement of MRR:

  • The continuous flow of electrolytes or application of proper flushing techniques.

  • Use of appropriate tool electrodes so that availability of electrolyte can make at the machining zone.

  • Analysis of voltage application needed at the different conditions to increase MRR without compromising the machining quality.

Improvement of machined quality:

  • Generation of the lower gas film reduced the HAZ and tapering effect and the gas film can be reduced by different combinations of effective parameters.

Continuous maintaining of machining gap and tool rotation can also provide a smooth machined surface.