electro chemical discharge machining

Netaji Subhas Institute Of Technology

Seminar Report

Advanced Manufacturing Methods Electro-chemical Discharge Machining

Subject code- MA-418

Submitted by Lokesh kr. Gahlot 651/MP/12

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CONTENTS

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Abstract Introduction History Comparison between ECM, EDM and ECDM ECDM setup and principle Discharge mechanism Chemical reactions Machining mechanism Subsystems Present status Advantages and limitations Applications Future scope References

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ABSTRACT Electrochemical discharge machining is a very recent technique in the field of non-conventional machining to machine electrically nonconducting materials using the electrochemical discharge phenomenon. If a beyond-critical voltage is applied to an electrochemical cell, discharge initiates between one tool of the electrodes and the surrounding electrolyte, which is termed here ‘electrochemical discharge’. This report contain the description of the process its applications and different theories reported by researchers.

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INTRODUCTION

Electrochemical discharge machining (ECDM) is a hybrid non-conventional manufacturing process which combines the features of electrochemical machining (ECM) and electro discharge machining (EDM). ECDM is a very recent technique in the field of non-conventional machining to machine electrically non-conducting materials using the electrochemical discharge (KID) phenomenon. If a beyond-critical voltage is applied to an electrochemical cell, discharge initiates between one tool of the electrodes and the surrounding electrolyte, which is termed here ‘electrochemical discharge’. It can be successfully used for machining electrically non-conductive advanced engineering materials such as glass and ceramics has shown the possibility of drilling micro - holes by smaller electrodes efficiently and economically. It has been found that the advanced materials are difficult to machine by the conventional machining processes. It is no longer possible to produce parts with better surface finish, close tolerances and complex shapes in advanced materials by conventional machining methods. So far, it is still necessary to provide more study for machining of non-conductive brittle materials since they have become key materials in the MEMS field. For example, the glass or quartz is usually bonded with the semi- conductive material due to their transparency, properties and so on. Likewise, the engineering ceramic is also used often in the high-tech apparatus [1]. ECDM is known by different names such as: ECSM- Electro Chemical Spark Machining (by V.K. Jain et al.)  SACE- Spark Assisted Chemical Engraving (by Fascio et al.)  ECAM- Electro Chemical Anode Machining (by Kubota).  DMNC - Discharge machining of Non-Conductors (by Cook et al.)  SAE- Spark Assisted Etching (by Daridon et al.)

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HISTORY

1968 1973 1985 1990 1997 2000 2004 2008 2009 2011 2014

First report by Kurafuji and Suda First characterization by Cook et al. Extension to travelling wire-ECDM by Tsuchiya et al. First functional devices First models by Ghosh et al. and Jain et al. Study of SACE in light if electrochemistry SACE and nanotechnology Systematic studies on SACE 2D machining Machining structures less than 100um Study of various tool geometries First commercial machine by Posalux SA

[7] The first work on electrode effect, anode or cathode was first reported by Fizeau and his group of researchers and Foucault and his co researchers in the year 1844, thus as a principal. So, this principles are very old that is of nineteenth century. The basic mechanism of the process is not yet completely understood and is still a matter of research investigations. Various researchers have put forth explanations of ECDM phenomenon based on their experimental studies. Crichton and McGough [6] performed streak photography to get insight into the various stages of discharge by applying a 85 V pulse for a duration of 200 us. They concluded that electrical discharge between cathode tool and electrolyte interface occurs due to: (i)

generation of electrolytic gas at the surface of electrodes;

(ii)

the growth of layers of low ionic concentration near the electrodes and formation of oxide films on the anode surface;

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(iii)

the local variations in the electrolyte flow pattern caused by flow stagnation and eddy.

Basak and Ghosh [7] treat the discharge phenomenon as a switching off process due to bubble bridges. According to these authors, when hydrogen bubbles become sufficiently large in number, the resistance at the tool–electrode interface increases substantially due to constriction effect. This leads to ohmic heating of electrolyte in that region, causing generation of vapour bubbles. When the nucleation site density of bubbles increases, the bubbles cover the maximum possible area of the tool, a blanketing of tool occurs, creating a switching off situation for a short time with zero current. Jain et al. [1] have considered each gas bubble as a valve, which after its breakdown due to high electric field produces discharge in the form of arc. In proposing their ‘valve theory’ several assumptions regarding diameter of the bubble, electric field intensity, frequency of spark, etc. have been made. It seems that with overall increase in hydrogen bubble number, the equivalent parallel resistance offered by these valves reduces, which is in contrast with the statement that the overall resistance increases [1].

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ECDM SETUP AND WORKING PRINCIPLE

In electrochemical discharge phenomenon, two electrodes are dipped inside an aqueous electrolyte. The cathode is chosen with a much smaller surface than the anode. When the D.C. voltage is applied electrolysis happens and Hydrogen gas bubbles are formed at the tool-electrode (cathode) and oxygen bubbles at the counter electrode (anode). When the voltage is increased, the current density increases too and more and more bubbles grow forming a bubble layer around the electrodes. When the voltage is increased above the critical voltage, bubbles coalesce into a gas film around the tool-electrode. Sparking phenomena is observed in the film where electrical discharges happen between the tool-electrode and the surrounding electrolyte. Similar behaviour can be obtained by inverting the polarity of the electrodes and by changing the electrolytes. Fig 1 explains the ECDM phenomenon [3].

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Discharge Mechanism The discharge in ECDM process can be compared to the ‘arc discharge’ in gases. When a DC voltage greater than the threshold value that is needed to produce the discharge is applied to the ECDM cell, electrolysis reactions take place.

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Reduction of electrolyte at cathode results in hydrogen gas bubbles. These bubbles get accumulated at the cathode tip immersed in electrolyte. Bubble generation goes on increasing, leading to combining of bubbles into a single large bubble which isolates the tip completely from the electrolyte. This causes the local electric field gradient between the tool and the electrolyte interface to go beyond the breakdown limit of 25 V/μm leading to an arc discharge [2].

Based on the time dependence of current, fig.-2 describes the discharge mechanism pictorially.  Column 1 in the figure shows the physical situation existing in the ECDM cell.  Column 2 shows the voltage distribution or potential gradient in the ECDM cell under respective conditions indicated in column one.  Column 3 shows portions of the experimentally obtained current versus time plot from which the situations in columns 1 and 2 are inferred. The time sequence of the events occurring can now be described as follows. 1. When a DC voltage V is applied, the potential drop across the cathode– electrolyte interface appears (Fig.2 (a)) and a small ionic current flows (see column 3, current versus time). 2. Hydrogen gas bubbles evolve causing the cathode– electrolyte interface resistance to increase and hence current is reduced as seen in Fig. 2(b) columns 1, 2 and 3. 3. The number and size of bubbles within the interface region grow over time as shown in Fig. 2(c). 4. The cathode–electrolyte interface resistance further increases with resulting voltage distribution as seen in column 2 of Fig. 2(c). This results in further decrease in current as seen in the column 3. 5. Bubble generation goes on increasing, leading to combining of bubbles into a single large bubble which isolates the tip completely from the electrolyte as shown in column 1 of Fig. 2(d).

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When an isolating film of hydrogen gas bubble covers the cathode tip portion in the electrolyte, the tip is covered by a gaseous layer. At this time a large dynamic resistance is present and the current through the circuit becomes almost zero ( column 3, Fig. 2(d)). 6. A high electric field of the order of 10 V/μm gets generated causing an arc discharge within the gas layers covering the tip (column 3, Fig. 2(d)). The arc should occur between the tip of the tool and the inner surface of the electrolyte. At the instant when discharge occurs, a huge number of electrons caused by ionization of gas flow towards anode. Thus a large current spike flows through the highly conducting spark channel (Fig. 2(e)) for a very short duration of time (of the order of few milliseconds).

CHEMICAL REACTIONS At larger electrode and electrolyte interface (anode) are shown below: Mz+ + Ze-

M Mz+ + Z(OH)-

M(OH)z (This insoluble metal hydroxide precipitates)

2H2O

O2 + 4H+ +4e-

(acidic)

4(OH)-

2H2o O2 4e-

(alkaline)

The chemical reactions at the smaller electrode & electrolyte interface are as shown: 2H2O + 2e-

H2

2H+ + 2e-

H2

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(hydrogen bubble generation)

Illustration of the electrochemical discharge phenomena. Two electrodes are dipped into an electrolyte. The voltage is progressively increased from 0 to 40 V. At around 25 V the cathode effect starts and at around 30 V the discharges are clearly visible. (a) 0 V (b) 7.5 V (c) 15 V (d) 30 V

.MACHINING MECHANISM

(i) Melting and vaporisation due to electrochemical discharges (ii) High temperature dissolution (iii) Differential expansion of constituents. (iv) Random thermal stresses and micro-cracking. (v) Mechanical shock due to expanding gases and electrolyte movement

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MAIN SUBSYSTEMS The Machine: usually it is a table top fabricated machine or a specialized set-up comprising of all the necessary features. The Power Supply : Consisting of AC to DC converter and Voltage modulator or pulsed modulator. The tool must be continuously dipped in the electrolyte so as to complete the circuit. The current required is generally, in the range of 5 to 20 ampere and voltage is varied in the range of 25 to 150 volts. The electrolyte circulation system can have a filter sludge removal system and treatment units

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PRESENT STATUS Studies on gas film in ECDM. Some researchers Cheng and his group and all have shown that, the quality of the gas film is the dominant factor that determines the geometric accuracy, in ECDM surface finish and repeatability in this ECDM process. Whereas, Bhattacharya and his co researchers have reported that the critical voltage depends on the concentration of the electrolyte, the conductivity of the electrolyte and the tool geometry. Cao Xuan and his team of researchers have worked on stabilizing these gas films, necessary to control the sparking by reducing the required voltage. They used load cells and small immersion tool depths, this helped to generate high aspect ratio structures along with high resolution. Now, there are one or two variant processes also in ECDM one is wire ECDM also called WECDM. This is a combination of wire EDM and ECDM. This is the hybridization of one hybridized process already and the basic EDM. Wherein a continuous moving wire is controlled by a spool feeder, which moves over a non conducting work-piece, dipped in an electrolyte the sparking causes the material removal. Basak and Ghosh added extra inductance in the circuit and found that, there was a substantial increase in material removal rate on normal glass slides. Yang and his group added silicon carbide abrasives in the electrolyte, and found increased performance in terms of improvement in the overcut quality. In traveling wire ECDM it was also found that stronger pulse current has better machining affects, while slicing glass and quartz. Some experiments on ceramics using gas filled process reported saving, an amount of electrochemical energy and increase in material removal rate. Kurita and Hattori they have developed a combined EDM, ECM lapping technique to improve the surface finish, in which they have achieved surface finish of 0.2 micrometer of average roughness, which was improved from 1 micrometer to 0.2 micrometer.

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Zain and his group have used alumina glass composite ceramic material, and found that the machining rate is greatly affected by porosity of the samples. The results also show that the material removal occurs by attacking, the green boundaries most probably due to the etching process. Liu has reported that the craters formed in the ECDM process, are almost same as those formed in the EDM process along with some recast effects, which is mainly due to the sparking action.

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ADVANTAGES ■ ECDM can produce significantly smoother surfaces due to the presence of high-rate ECD. ■ The depth of the heat-affected layer can be significantly reduced or eliminated. ■ High machining rates are also possible thereby increasing the productivity and reducing the unit production cost. ■ The erosion of tool electrodes is reduced by a factor of 4 to 5 percent compared to that of pure EDM. ■ Burrs at the edges are particularly absent due to the existence of the ECD phase.

DISADVANTAGES ■ The produced accuracy is low. ■ There is difficulty in handling the electrolyte. ■ The process cannot produce internal and external sharp edges.[8]

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APPLICATIONS

ECDM is hybrid micro-machining technology for production of:            

Through and blind micro-holes Micro grooves Micro-slots Micro-channels Complex shaped micro-contours in electrically non-conductive materials. Miniature features for turbine blades(micro- cooling holes) Filters for food and textile industries Micro fluidic channels in non conducting materials (quartz, glass and ceramics) Micro-electro-seam welding of copper plates and foils Micro-fabrication of an array of holes in micro filters Fabrication of electrodes for pressure micro-sensors and resonance detection micro-sensors Turning and bracing of metal mounded grinding tools. In micro-pumps, micro-accelerometers and 3 dimensional fissures in glass substrate.

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FUTURE SCOPE  Micro-holes are widely used, and their machining process is vital to modern manufacturing, especially for materials that are difficult to machine. Tube electrode high-speed electrochemical discharge machining (ECDM) combines electrochemical machining (ECM) and electrical discharge machining (EDM) and is considered a promising hybrid machining method for the fabrication of micro-holes in difficult-to-machine materials. However, because of the characteristics of the hybrid process, it produces not only debris and heat but also hydroxides and bubbles. The removal of these complex machining by-products constitutes a challenge. To address this issue, super-high-pressure interior flushing, which utilises more than twice the pressure traditionally used for interior flushing, was applied to ECDM.  The micro-electro-mechanical-system (MEMS) field is growing constantly. MEMS emerged in the late 1980s with the development of integrated circuits fabrication processes. If silicon remains the most widely used material, glass becomes more and more important. Machining high-aspect ratio micro-holes in glass would open several new possibilities in the MEMS field. 

Combining with other technologies and practical realisations it is possible to combine ECDM with other micromachining technologies. Esashi et al. used ECDM in combination with clean room technologies for micro-fabricating an absolute pressure sensor. Gue´rin et al. used ECDM together with excimer laser micro-machining and clean room technologies to manufacture a miniature oneshot .

 Travelling wire electrochemical discharge machining TW-ECDM An extension to wire machining, called travelling wire electrochemical discharge machining (TW-ECDM), was proposed by Tsuchiya et al. and studied further on by Jain et al. [1] and Peng et al. TW-ECDM may be used as well for 2D contours cutting . 18

Few researchers suggest that, the tool positioning control system is a key factor and discussed its control, stability and robustness. Thus we have seen in most of the aspects, in this process are at the research stage only, and the things are to be yet to be concretized and different groups and different countries are working in this process.

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REFERENCES

[1]. V.K. Jain, S. Adhikary “On the mechanism of material removal in electrochemical spark machining of quartz under different polarity conditions” journal of materials processing technology vol.200 (2008), pp 460–470. [2] H.W. Ott, Noise reduction techniques, Wiley, New York, 1976 [3]. R. Wuthricha, V. Fasciob, “Machining of non-conducting materials using electrochemical dischargephenomenon An overview” International Journal of Machine Tools & Manufacture, vol.45 (2005), pp 1095–1108. [4] An experimental study of discharge mechanism in electrochemical discharge machining A. Kulkarni, R. Sharan, G.K. Lal ∗ Indian Institute of Technology, Kanpur, UP, India. [5] I.M. Crichton, J.A. McGough, Studies of the discharge mechanisms in electrochemical arc machining, Journal of Applied Electrochemistry 115 (1985) 113–119. [6] I. Basak, A. Ghosh, Mechanism of spark generation during electrochemical discharge machining: a theoretical model and experimental investigation, Journal of Materials Processing Technology 62 (1996) 46–53. [7]Micromaching using Electrochemical Discharge Phenomenon by Rolf wuthrich , Jana D Abou Ziki.(google books) [8] Advanced Machining Processes Nontraditional and Hybrid Machining Processes by Hassan El-Hofy Production Engineering Department Alexandria University, Egypt Module - 3 Advanced Machining Processes Lecture - 11 Electro Chemical Discharge Machining (ECDM) NPTEL lectures.

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