On the Simulation of Electrochemistry Aspect of Electrochemical

Spark Micromachining Process

Anjali V. Kulkarni

Centre for Mechatronics, IIT Kanpur, Kanpur, UP, India

Keywords: COMSOL, Electrochemical Phase, Electrochemical Spark Micromachining Process, Multiphysics, Spark

Formation Cycle Ed.

Abstract: Electrochemical spark micromachining (ECSMM) process, an advanced machining process is investigated to

understand process mechanism. The material removal mechanism in ECSMM is a complex phenomenon due

to its multiphysics and transient nature. Experimental measurements of online current and voltage have been

performed simultaneously. Different sequential operational stages in one single spark cycle have been

identified in the light of the transient measurements. The Semiempirical electrical impedances during these

identified operational stages have been formulated and compared with those derived by using measured online

current and voltage data. Only the impedance results during electrochemical phase in a single spark cycle

have been reported here. In case of ECS, considering the kinetics at the electrolyte and electrode interfaces,

the effective equivalent circuit is derived. The charge transfer resistance in the equivalent circuit during the

electrochemical phase is found by performing impedance spectroscopy using COMSOL multiphysics

modeling software. For this 1-d model of the electrochemical process is developed using secondary current

distribution. It is for the first time that COMSOL study has been attempted in analyzing the physics behind

the material removal phenomenon mainly during the electrochemical operational phase of ECSMM. The

modeled and measured impedances show close similarities.

1 INTRODUCTION

Electrochemical spark micromachining (ECSMM)

process is an advanced machining process in which

sparking is responsible for machining of wide variety

of materials. The process is investigated in the holistic

approach to understand process mechanisms.

Material removal mechanism in particular is a

complex phenomenon due to its multiphysics nature.

The sparking during the machining process is not

continuous as it is conventionally identified; rather it

is a repetitively occurring discrete and complex

phenomenon. Complexity arises primarily due to the

presence of various physio-chemical phases involved

in forming the discrete sparking. Moreover

electrochemical systems are known to exhibit

complex non-linear behavior. These nonlinearities

arise due to electro hydro dynamism, ionic reactions,

bubble generation, their growth and their breakdown

phenomena, with material removal over a finite area

as a final result. The spark formation cycle is a series

of such activities. Also sparks are non-thermal in

https://orcid.org/0000-0001-5210-570X

nature as opposed to the theoretical assumption that

spark is a heat source for material removal to take

place. In the existent literature, spark energy is

considered to be of thermal nature. Also the thermal

analysis and material removal are considered to be

due to this thermal source (Basak & Ghosh, 1992;

Basak & Ghosh, 1996; Jain, Dixit & Pandey, 1999).

But in a separate transient temperature study using

pyrometer (Kulkarni, 2009), it has been found

experimentally that the spark is a non thermal

discharge. Through this study, it is established that

the process is similar to a repeated sparking/discharge

similar to that of the breakdown of the hydrogen gas

bubble isolating the tool tip from the surrounding

electrolyte. Consequent to this finding, electrical

impedance in each sequential stage of a single spark

cycle is modeled based on the physics of each stage

respectively. This semiempirical impedance is

compared against the impedance computed by taking

the ratio of the measured online, transient and

synchronized voltage and current data during single

spark formation (Kulkarni, 2017). The modeled and

172

Kulkarni, A.

On the Simulation of Electrochemistry Aspect of Electrochemical Spark Micromachining Process.

DOI: 10.5220/0011122500003274

In Proceedings of the 12th International Conference on Simulation and Modeling Methodologies, Technologies and Applications (SIMULTECH 2022), pages 172-177

ISBN: 978-989-758-578-4; ISSN: 2184-2841

measured impedances show close similarities.

Findings of the impedance results during

electrochemical phase during a single spark cycle

have been reported here.

It is for the first time that COMSOL study has

been attempted in analyzing the physics behind the

mechanical removal phenomenon mainly during the

electrochemical operational phase/stage in working

of ECSMM. Hence no literature is available with the

research findings with which comparison of results

can be performed.

2 EXPERIMENTAL

The time varying online process current is measured

in synchronization with the machining supply voltage

using digital storage oscilloscope (DSO, Hameg

1008) by ‘resistive shunt method’. For this, a 1 Ω

resistance is connected in series with cathode (made

up of copper wire of diameter 100 µm with its tip

partially dipped in the NaOH electrolyte) and ground

of the power supply to the ECS cell. The time varying

voltage across this resistance is the direct measure of

the time varying process current. These synchronized

current and voltage waveforms are saved on the

control PC via RS 232 connectivity module of the

DSO. Each waveform contains 2000 samples of the

voltage and current readings. The current waveform

contains many spark cycles (Kulkarni, 2013;

Kulkarni, 2015).

3 SEQUENTIAL OPERATIONAL

PHASES OF ECSMM PROCESS

IN THE LIGHT OF ONLINE,

TRANSIENT CURRENT

The formation of spark happens through series of

chemical and physical processes. When a voltage

(enough to form spark) is applied across the

electrolyte cell (containing electrolyte with cathode

and electrode dipped partially in it with workpiece on

which the micromachining is intended), mainly

following series of events take place (Kulkarni, 2013;

Kulkarni, 2015):

1. Electrochemical phase giving rise to oxidation

and hydrogen gas generation.

2. Hydrogen and vapor bubbles formation,

coalescence and bubble growth during pool

boiling, slowly covering the tool tip.

3. Tool tip isolation, momentary ‘virtual switch

off’ phase.

4. Instantaneous generation of high electric field

which causes sparking due to hydrogen

breakdown.

5. Drifting of energetic electrons towards the

workpiece due to potential gradient and

subsequent material removal from the

workpiece kept near the tool tip due to this

‘electron gun’.

6. Reestablishment of tool-electrolyte contact

leading to a ‘virtual switch on’ phase.

7. steps 1-6 begin all over again.

Following section establishes the theoretical

background behind electrochemical phase operation.

It also identifies major electrical parameters

responsible for the instantaneous current contribution

in that phase of operation during the single spark

formation.

3.1 Electrochemical Phase

When the machining supply to the electrolyte cell is

applied in the proper polarity, (i.e. positive terminal

connected to graphite anode and negative terminal to

copper cathode) electrochemical action starts.

Electrochemical reactions that occur at the electrode–

electrolyte interface continuously supply electrons

from cathode to solution and solution to anode. This

is called as the ‘migration’ state of the ECSMM

process and causes the electronic current. The anodic

and cathodic reactions occur together with reduction

in electrolyte. These liberated positive ions move

towards cathode and negative ions move towards

anode and causes the ionic current. Ionic and

electronic current together form the average current

and it is of the order of 100 – 200 mA as is measured.

Major other (transient) current contributing circuit

elements during this phase are investigated in the next

section.

3.2 Current Contributing Elements

during Electrochemical Phase

Electrolyte resistance, double layer capacitance,

polarization resistance, and charge transfer resistance

are the elements those contribute to the overall

electrochemical reactions during electrochemical

phase and hence the instantaneous current, as

described below.

On the Simulation of Electrochemistry Aspect of Electrochemical Spark Micromachining Process

173

3.2.1 Electrolyte Resistance

In an electrochemical cell, bulk solution resistance is

a significant factor. In this experimental situation,

estimation of NaOH solution resistance is estimated

by taking the inverse of the conductivity. But in real

situations, it depends on the geometry of the solution

(container), its temperature, etc. It is found that the

effect of electrolyte resistance is dominant during

electrochemical and bubble growth phases only.

3.2.2 Double Layer Capacitance

An electrical double layer exists on the interface

between an electrode and its surrounding electrolyte.

This double layer is formed as ions from the solution

‘stick on’ the electrode surface. The charged

electrode is separated from the charged ions. The

separation is very small, of the order of angstroms.

Charges separated by an insulator form a capacitor.

Practically the capacitance such formed is of the order

of 20 to 60 µF per cm

area of the electrode. Figure 1

shows the process of double layer formation. All the

electrolyte processes take place at the electrode-

electrolyte interface. The structure of the double layer

is similar to an electrical capacitor formed by a

dielectric of thickness of about an ionic radius, i.e. 50

nm. Figure 1 A shows the formation of inner

Helmholtz plane (IHP), outer Helmholtz plane (OHP)

and the electrode with an excess of negative charge.

Figure 1 B shows the localization of the excess

charges and Figure 1 C shows the potential gradient

formed.

3.2.3 Polarization Resistance

Whenever the potential of an electrode is forced away

from its value at ‘open-circuit’, it is referred to as

polarizing the electrode. When an electrode is

polarized, it causes current to flow through

electrochemical reactions that occur at the electrode

surface. The amount of current is controlled by the

kinetics of the reactions and the diffusion of reactants

both towards and away from the electrode. In the case

of ECS cell, the diffusion of reactants is absent.

Hence only the kinetics of the reactions plays the

major role.

3.2.4 Charge Transfer Resistance

Another resistance is formed by a single kinetically

controlled electrochemical reaction. In the case of

ECS, the reactions occurring at the cathode-

electrolyte interface are of concern and form the basis

of the analysis. The kinetics of the charge transfer

depends on the kind of reaction, the concentration of

reactants, the potential, and many other factors.

Figure 1: This Process of the double layer formation

(Kulkarni, 2013).

Figure 2 shows the various potential drops formed

during a general electrochemical cell with an applied

supply voltage. The supply voltage overcomes the

following potentials:

• The electrode potential,

• The activation over potential,

• Ohmic potential drop,

• Concentration over potential ( not of much

relevance in ECS) and

• The potential due to resistance of electrolyte.

SIMULTECH 2022 - 12th International Conference on Simulation and Modeling Methodologies, Technologies and Applications

174

Figure 2: Various potential drops during a general electrochemical cell (Kulkarni, 2013).

4 EQUIVALENT CIRCUIT

DURING ELECTROCHEMICAL

PHASE

Figure 3 shows the circuit parameters for a general

electrochemical cell. In the case of ECS, the kinetics

at the anode electrode is neglected due to the non

consuming electrode material.

Figure 3: Circuit parameters for a general electrochemical

cell (Kulkarni, 2013).

4.1 Time Varying Impedance during

Electrochemical Phase

Neglecting the wiring inductance, bulk contact

resistance, Z

, the effective equivalent circuit during

electrochemical phase is as shown in Figure 4.

Figure 4: Electrical equivalent circuit in the electrochemical

stage of ECS process.

The time varying impedance Z(t) during

electrochemical phase (neglecting R

ext

) is shown by

equation (1) below, where, Rsol is Electrolyte

resistance, Cdl is Double layer capacitance, and Rct

is Charge transfer resistance

Zt  𝑅𝑠𝑜𝑙 

𝑅𝑐𝑡

1

𝑗

𝜔𝑅𝑐𝑡𝐶𝑑𝑙

)

Butler-Volmer and Tafel (an equation in

electrochemical kinetics relating the rate of an

electrochemical reaction to the overpotential)

expressions (https://www.gamry.com/application-no

tes/EIS/basics-of-electrochemical-impedance-spectr

oscopy/, retrieved 26 May 2012) are used to derive

electrode kinetics for the charge transfer reactions.

In the Secondary Current Distribution interface,

the electrochemical reactions are described as a

function of the overpotential. The interface uses

several relations for the charge transfer current

density and the overpotential. The most general

expression is of Butler-Volmer type:

)

where i

loc,m

denotes the local charge transfer current

density for reaction m, i

the exchange current

density, α

the anodic transfer coefficient, α

the

cathodic charge transfer coefficient, F the Faraday

constant, and R the universal gas constant. On a single

electrode the Tafel equation can be stated as:

η = ± A × log

(i/i

)

where the plus sign under the exponent refers to an

anodic reaction, and a minus sign to a cathodic

reaction, and η is overpotential (V), A is "Tafel

slope", (V), i is current density (A/m

) and I

"exchange current density" (A/m

On the Simulation of Electrochemistry Aspect of Electrochemical Spark Micromachining Process

175

Butler-Volmer (B-V) equation has been used to

estimate the charge transfer resistance with

appropriate boundary conditions. The interfaces

define two dependent variables of potentials. The

conduction of current in the electrolyte is assumed to

take place through transport of ions while electrons

conduct the current in the electrode.

4.2

Impedance

Spectroscopy using

COMSOL 4.2 Multiphysics

The ‘electrochemical model’ in COMSOL 4.2

software is used to solve 1-d simulation of the

electrode kinematics in the ECS cell using the

‘secondary current distribution’ physics built-in in

COMSOL (https://doc.comsol.com/5.3/doc/com.com

sol.help.echem/ElectrochemistryModuleUsersGuide.

pdf, retrived on 26 May 2012). Figure 5 shows the

simulated electrolyte potential distribution in the cell.

The electrolyte voltage in the cell is seen around 75 x

-6

V at a distance of 22 µm away from the cathode.

Figure 5: Electrolyte potential distribution in the cell

(Kulkarni, 2013).

Figure 6 gives the Nyquist plot generated by

COMSOL software by using the perturbation current

of 5 mA. It gives Rct around 14.8 x 10

-6

Ω. The Rct x

Cdl time constant is very small and is of the order of

few hundreds of ns. That means the electrochemical

reactions are very fast. The hydrogen bubble growth

due to pool boiling phenomenon soon takes over

(which is not the scope of this paper).

Figure 6: Nyquist plot generated by the COMSOL 4.2

software (Kulkarni, 2013).

5 CONCLUSIONS

• The important intermittent sequential phases of

the process have been identified and analyzed.

Repeated formations of spark happen through

series of chemical and physical processes in

phases. These are: electrochemical action,

bubble formation, coalescence and growth of

bubble during pool boiling slowly covering the

tool tip, tool tip isolation leading to a momentary

‘virtual switch off situation, instantaneous

application of high electric field causes sparking

of hydrogen breakdown, reestablishment of tool-

electrolyte contact leading to ‘virtual switch on’

situation, drifting of energetic electrons towards

the workpiece due to potential gradient.

• The instantaneous process current comprises of

current due to these sequential events. The

electrical impedance during each phase of

operation varies. Of these phases, the

electrochemical action phase is the fastest

process.

• Charge transfer kinetics at anode electrode is

neglected. This is minimized by the design of the

anode electrode.

• The charge transfer resistance in the equivalent

circuit during the electrochemical phase is found

by performing the impedance spectroscopy. It is

done using COMSOL Multiphysics modeling

software. For this 1-d model of the

electrochemical process is developed using

secondary current distribution. The time constant

due to the parallel combination of the double

layer capacitor and the charge transfer resistance

is of the order of thousands of ns. That means

the formation of the double layer capacitance and

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charge transfer resistance happens within

thousands of ns time.

ACKNOWLEDGEMENTS

I am indebted to my guide Prof. V. K. Jain (retired,

ME Dept. IIT Kanpur) for his guidance throughout

my academic carrier.

Thanks are due to the staff at Manufacturing

Science Lab and the Centre for Mechatronics at IIT

Kanpur in setting up the experiments and carrying out

the experimental work.

The grant (SR / S3 / MERC-079/2004) from

Department of Science and Technology, New Delhi

is gratefully acknowledged.

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