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BEHAVIOUR OF BUBBLES

GENERATED IN ELECTRO-CHEMICAL

DISCHARGE MACHINING

Debasish Nandi*, Asit Baran Puri, Indrajit Basak

Department of Mechanical Engineering,

National Institute of Technology, Durgapur-713209, India Email ID*: debasish_nandi1971@yahoo.co.in Abstract:

The present paper emphasized on the behavior of bubbles formed around the electrodes in Electro-Chemical Discharge Machining (ECDM) process as a factor of spark initiation. A finer observation on the voltage-current (V-I) characteristics prior to the discharge is made to investigate the bubble behavior. V-I characteristics are recorded and analyzed experimentally with different combinations of the electrolytes, tool diameter and tool depth. It has been clearly observed that there are three distinct regions in the V-I characteristics for alkaline, neutral and acidic electrolyte. Due to local turbulence at higher current density, the linear nature of V-I characteristic curve changes and decrease in cell resistance is noticed. However this local turbulence has little effect on critical voltage or critical current. The span of the region near to the spark onset is different for different electrolytes.

Keywords: Electrochemical discharge machining, Path resistance, Bubble absorption, Bubble dispersion,

Mechanism of spark initiation. Nucleation site density.

1 Introduction

With the development of technology, more and more challenging problems are being faced by the scientists and technologists in the field of manufacturing science, mainly from the following three basic areas: i) new materials with low machinability, ii) dimensional and accuracy requirement and iii) a desirable production rate and economy. New materials and alloys, particularly developed for the application in aero-space, nuclear engineering and in precision industries, with their high strength-to-weight ratio, hardness and heat resisting quality are the challenges for machining. Producing complicated geometries in such materials becomes extremely difficult with the usual methods.

To overcome these difficulties non-traditional or unconventional processes such as, Electrochemical Machining (ECM) and Electro-discharge Machining (EDM) are successful to some extent, but have limitations in material removal rate and in the fact that only electrically conducting materials can be machined. Hybrid process for electrically conducting materials, termed as Electrochemical Arc Machining (ECAM) and that for non-conducting materials as Electrochemical Discharge Machining (ECDM) now being explored for higher machining capacity. Kubota [1] explained the ECDM process as a combination of ECM and EDM.

1.1 Background of electrochemical discharge (ECD)

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internal inductance and capacitance in the electrolyte were observed with the application of rectified and smooth DC voltages. The introduction of an external inductor into the circuit has an advantage of supplying a constant power input to the cell. Through holes were drilled successfully in quartz glass plates using the total discharge configuration. Basak and Ghosh [7] presented a theoretical model of the discharge phenomenon. With the help of this model, the critical voltage and current required to initiate discharge between the electrode and the electrolyte are estimated. The ECD phenomenon has been analyzed as a switching process between the tool and the electrolyte. Jain et al. [8] modeled the electrochemical discharge phenomenon as similar to that which occurs in arc discharge valves. The spark energy and the approximate order of hydrogen gas bubble diameter are computed by the proposed valve theory. Temperature distribution, material removal per spark, overcut obtained in the machined cavity and attainable maximum penetration depth are computed by the finite element method. Kulkarni et al. [9] explained that the discharge is a discrete phenomenon. The breakdown of the accumulated gas is taken place due to a large electric field which gets developed locally. Fascio et al. [10] presented two theoretical models of spark assisted chemical engraving phenomenon. One model based on percolation theory to predict the critical voltage and current. In the second model a method to estimate the spark’s characteristics (amplitude and duration) is presented and with the help of these data the machining depth was estimated.

1.2 Use of ECD in ECDM

A general ECDM setup is shown in the figure 1, consists of two electrodes, grossly different sizes, dipped in an electrolyte. If a suitable electrolyte is chosen and then beyond a certain value of the applied potential, electric spark appear at the bottom edge of the smaller electrode. The resulting discharge strikes the work piece when the distance between the smaller electrode (tool) and work piece is smaller than 25 m for glass [11]. A part of the energy released by the discharge is conducted to the work piece and raises the temperature to a high value. If the maximum temperature attained is more than the melting temperature of the work piece, some part of the work piece melts. The molten portion is then removed by the shock due to the discharge and it ultimately results in a small crater.

Figure 1:ECDM setup

2 Preview on bubble behavior

Hydrogen gas is liberated in the form of bubble at the cathode due to electrochemical reaction. The bubbles gradually grow in size and after attaining a critical size, they detach from the electrode surface. Venczel [12] noticed that, during the growth their shapes remain hemispherical due to their fast growing nature. Nucleation site density of H2 bubble increases with the applied voltage (i.e., with the current density also) to the cell. When

the nucleation site density of H2 bubble becomes sufficiently high, substantial constriction of the current path

takes place at the interface forming a gas film around the electrode. This causes an increased resistance at that region and the ohmic heating of the electrolyte becomes significant. This causes the onset vapour bubble nucleation on the electrode surface in addition to the H2 bubbles. Beyond this stage the number of the combined

nucleation site increases very rapidly with the applied voltage. It was observed by Janssen and Hoagland [13] that with the increase of current density through an electrochemical cell, more and more nucleation sites for H2

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generation at a particular site depends on the region of the bubble growth. For very small bubbles, whose growth is dynamically controlled, Dd.f2 = constant, where, Dd is the base diameter of the bubble and f is the frequency

of the bubble generation at a particular site. On the other hand for thermally controlled large bubbles, Dd.f1/2 =

constant.

It has been observed by Stralen and Cole [16] that the vapour bubble growth rate at initial stage is in the form of D(t) α t3/2, which changes to asymptotic growth, given by D(t) α t1/2at thelater stage. Where D is the diameter of the bubble and t is the time. Electrolytic gas evolution at atmospheric pressure is completely controlled by mass diffusion and is of the form D(t) α t1/2.

As the nucleation site density reaches a critical value, vapour blanketing of the electrode occurs. This was also noticed by Rohsenow and Griffith [17] and Zuber [18]. Basak and Ghosh [7] suggests that at the critical value of the nucleation site density maximum possible coverage of the electrode surface takes place with the full grown hemispherical bubbles of base diameter Dd. The electrical field in the film of bubble is high enough, typically

106-108 V/m [9, 19], allows electrical discharge between the electrode and the electrolyte. At this stage the points of contact between the electrolyte and the tool electrode, known as Bubble Bridge, blows off instantly due to intense heating. Consequently the current through the circuit drops to zero within a very short time span. Discharge takes place along the locations of the bubble bridge. The bubble dislodges from the electrode surface due to bridge blowing and the contact between the electrode and the electrolyte is reestablished. This cycle repeats continuously.

2.1 Mechanism of hydrogen gas generation

When electric current passes through electrochemical cell electrolysis of the aqueous electrolyte takes place. As a result the hydrogen gas is liberated at cathode. When the liberated hydrogen gas saturates the neighboring electrolyte, it comes out in the form of bubbles. The quantity of H2 liberated by electrochemical reaction is

given by the Faraday’s law of electrolysis. If w be the deposited or liberated quantity of element, then

w = EIt

Where, E is the electrochemical equivalent. I is the current and t is time. Where I is in ampere and t is in second, then the product It is termed as coulomb and the electrochemical equivalent of H2 is 1.045 X 10-5 mg/coulomb.

2.2 Mechanism of water vapour generation

The water vapour forms on the electrode surface due to ohmic heating. If I be the current and R be the interface resistance, then the rate at which heat liberates is I2R. A major part of this heat generated is lost due to conduction in the tool and convection in the electrolyte. Only a small part is utilized in vapour formation. So, the rate of vapour formation is given by:

V = δI2R

Where, δ is the coefficient of vapour formation and expressed in cm3/watts.

3 Experimentation

The survey of the past works indicates that the mechanism of spark initiation is identified to some extent. It is also observed that the general behavior of the path resistance in ECDM with the applied voltage is identified but the complete explanation is still unavailable mostly due to the complex and transient conditions.

There are two possible mechanisms of this discharge as referred by the previous researchers:

i) Discharge between electrodes in ionized gaseous medium, as the electrode surface and electrolyte is separated by a small distance and the gap is filled with ionized hydrogen.

ii) Discharge by switching phenomena due to continuous contact and break between the tool electrode surface and electrolyte at much localized point particularly at the bottom edge of the tool.

However, the main open question is the mechanism of transition between the traditional electrolysis regime and the electrochemical discharge regime. The first one is a bubble production process, whereas second one is an electrical discharge phenomenon in gas.

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present work observations on the voltage-current (V-I) characteristics prior to the discharge is examined critically to investigate the condition up to below critical voltage.

It is clear from the past studies that the electrolyte and its concentrations are the main parameters for the determination of the critical voltage. In addition to this, the area between the tool and the electrolyte is another factor to determine the critical current. The applied voltage, the electrolyte temperature along with the electrolyte and tool diameter and the immersion depth of tool into electrolyte are the main functions of the path resistance in ECDM. To obtain a representative result, some variables were made constant and some treated as parameters. Furthermore, suitable experimental setup is essential to continue with and planned from the previous works.

3.1 Fixing of experimental variables

With the main object to determine the bubble behavior, the common electrolytes like NaCl, NaOH and HCl were chosen. The larger electrode is made of carbon rod and tool material is copper. To maintain comparability, tool diameter of 0.5 mm, 0.8 mm, 1 mm and 1.6 mm and immersion depth 1 mm, 2 mm and 3 mm were selected. To eliminate any effect on the discharge phenomenon by the filter capacitor of the rectifier, it was decided to use only the full wave rectified DC supply. To maintain the electrolyte temperature constant, it was cooled down by keeping it in a tray time to time.

3.2 Experimental setup

To determine the behavior of bubble in ECD, it was required to design and develop a set up in which the applied voltage, the tool diameter and the tool depth (immersion depth) in the electrolyte could be controlled precisely. It was also necessary to read the input voltage and the corresponding current. The set up is schematically shown in Figure 2. It consisted of a controlled power supply, an electrochemical bath with electrodes, a tool holder mechanism and the required measuring facilities. The whole system was isolated from the AC mains by an isolation transformer. The input voltage was controlled by the variac, which was subsequently rectified by a bridge rectifier. Standard voltmeter and ammeter were employed to read the applied voltage and corresponding current respectively. A screw gauge micrometer was used as a screw feed mechanism which was employed to dip the tool in the electrolyte with controlled depth. A glass beaker was used as the electrolyte bath.

1. Isolation transformer 2. Variac

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3.3 Observations

The voltage-current characteristics were obtained by gradually increasing the voltage and recording the corresponding current up to the sparking voltage. Very slow increase could not be done, as the time taken to reach the sparking voltage increases, resulting rise of temperature of the electrolyte. The characteristics for the tool depth of 1 mm, 2 mm and 3 mm for a particular tool diameter and electrolyte was observed. The characteristics for all the electrolytes and tool diameter were identically measured. The observed characteristics are shown in Figure 3 to 8. There are three distinct regions in the characteristics. In region 1, current increases linearly with the applied voltage. Beyond this region the rate of rise of the current with the applied voltage increases (region 2). After this region the rate of rise of the current with the applied voltage falls and discharge initiates.

Figure 3:V-I characteristic for different tool depth Figure 4:V-I characteristic for different tool depth [Electrolyte: NaOH; Tool diameter: 0.5 mm] [Electrolyte: NaOH; Tool diameter: 1 mm]

Figure 5:V-I characteristic for different tool depth Figure 6:V-I characteristic for different tool depth [Electrolyte: NaCl; Tool diameter: 0.8 mm] [Electrolyte: NaCl; Tool diameter: 1.6 mm]

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Figure 7:V-I characteristic for different tool depth Figure 8:V-I characteristic for different tool depth

[Electrolyte: HCl; Tool diameter: 0.5 mm] [Electrolyte: HCl; Tool diameter: 1 mm]

The entire characteristics curve for different input parameters such as electrolyte, tool diameter and tool depth are found similar in nature. The current through the cell initiates at the range of 0.5 to 1 volt of input voltage and varies linearly up to a certain region (region 1). The non-zero value of input voltage to initiate the cell current is due to the cell potential. In this region the cell behaves like a normal electrochemical cell with stable and equilibrium relation between input voltage and cell current.

Beyond the region 1, the rate of rise of cell current increases with applied voltage (region 2). However this increase pattern is different for alkaline, acidic and neutral electrolyte. This increase of rate may be attributed to the increase rate of bubble generation, subsequent bubbles shooting and local heating of the electrolyte. The phenomena of bubble shooting and local heating effectively results in local convection of the electrolyte causing local flow disturbing the stagnant condition. Therefore the resistance at the vicinity of the tool electrode decreases and allows higher current flow. The rate absorption of the H2 bubble are high in acidic electrolyte and

also this region (region 2) attains at lower voltage which results lower steam bubble generation. The combination of these effects makes low magnitude of local disturbance and region 2 is not much prominent in acidic medium. For alkaline and neutral electrolyte, the conditions are different and a sharp change in V-I characteristics is observed.

In the next region (region 3) of the curve, the rate of increase of cell current decreases with input voltage and at the end of this region discharge initiates. With the increase of current, the electrochemical rate increases, local heating increases and more and more bubbles are generated. The detached bubbles (both steam and gas) remain in the vicinity of the tool electrode because the local saturation of electrolyte reduces the rate of bubble absorption and bubble dispersion. This results in constriction in the electrolyte path and increase in path voltage. Actually, this region of the electrolyte is responsible for elevation of electrolyte temperature due to ohmic heating. At the end of this region, the rate of bubble nucleation and generation from the electrode surface becomes so intense that complete isolation between the electrode and electrolyte takes place. The discharge initiates and the cell current drops radically. Beyond this region is the useful region for electrochemical discharge machining.

It has been observed at higher input voltage, the discharge behaves like an arc. As the objective of the present work was to observe the cell resistance characteristics, the analysis of discharge and its pattern was not taken up.

4 Conclusions

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decreases due to increased tool depth in electrolyte and increased tool diameter. It was noticed that the dependency of contact area and total resistance is non-linear.

References

[1] M. Kubota, Characteristics of ECDM, Proceedings of International Conference on Production Engineering, Tokyo, Japan, 1974, pp.51-55.

[2] C.S. Taylor, The anode effect, Transactions of the Electrochemical Society 47 (1925) 301-316.

[3] H.H. Kellog, Anode effect in aqueous electrolysis, Journal of Electrochemical Society 97 (1950) 133-142. [4] H. Kurafuji, K. Suda, Electrical discharge drilling of glass, Annals of CIRP 16 (1968) 415-419.

[5] N.H. Cook, G.B. Foote, P. Jordan, B.N. Kalyani, Experimental studies in electro-machining in ECM, Transactions of ASME, Journal of Engineering for Industries 96 (1973) 945-950.

[6] V. Raghuram, T. Pramila, Y.G. Srinivasa, K. Narayanasamy, Effect of circuit parameters on the electrolytes in the electrochemical discharge phenomenon, Journal of Materials Processing Technology 52 (1995) 301-318.

[7] Basak, A. Ghosh, Mechanism of spark generation during electrochemical discharge machining: a theoretical model and experimental verification, Journal of Materials Processing Technology 62 (1996) 46-53.

[8] V.K. Jain, P.M. Dixit, P.M Pandey, On the analysis of the electrochemical spark machining process, International Journal of Machine tools & Manufacture 39 (1999) 165-186.

[9] Kulkarni, R. Saran, G.K. Lal, An experimental study of discharge mechanism in electrochemical discharge machining, International Journal of Machine tools & Manufacture 42 (2002) 1121-1127.

[10] V. Fascio, R. Wiithrich, H. Bleuler, Spark assisted chemical engraving in the light of electrochemistry, Electrochimia Acta 49 (2004) 3997-4003.

[11] J. Venczel, Über den gasblasen bei elektrochemischen prozessen, Electrochimica Acta 15 (1970) 1909-1920.

[12] L.J.J. Janssen, J.G. Hoogland, The effect of electrolytically evolved gas bubbles on the thickness of the diffusion layer,Electrochimica Acta 15 (1970) 1013-1023.

[13] V. Fasico, R. Wiithrich, D. Viquerat, H. Langen, 3D Microstructuring of glass using electrochemical discharge machining (ECDM), International symposium on Micromechatronics and Human Science (MHS’ 99), 1999, pp. 179-183.

[14] B.E.Staniszewski,Nucleate boiling bubble growth and departure, Technical Report No. 16, MIT, DSR Project No. 7- 7673 (1959). [15] H.J. Ivey, Relationships between bubble frequency, departure diameter and rise velocity in nucleate boiling, International Journal of

Heat and Mass Transfer 10 (1967) 1023-1040.

[16] S.J.D. Van stralen, R. Cole, Boiling phenomenon, vol.1, Hemisphere, Washington, 1979. [17] Rohsenow and Griffith, Chemical Engg. Programme Symp. Series 52 (1956) 47. [18] Zuber,Doctoral Dissertation, University of California, Los Angeles, 1959.

Figure

Figure 1: ECDM setup
Figure 2: Experimental setup
Figure 3:                 [Electrolyte: NaOH; Tool diameter: 0.5 mm] V-I characteristic for different tool depth
Figure 7: V-I characteristic for different tool depth                 Figure 8:                [Electrolyte: HCl; Tool diameter: 0.5 mm]

References

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