Manufacturing Rev. 7, 15 (2020)© S. Kunar et al., Published by EDP Sciences 2020https://doi.org/10.1051/mfreview/2020012

Available online at:https://mfr.edp-open.org

RESEARCH ARTICLE

Fabrication of array of micro circular impressions using differentelectrolytes by maskless electrochemical micromachiningS. Kunar1,*, E. Rajkeerthi2, K. Mandal3, and B. Bhattacharyya3

1 Mechanical Engineering Department, Durgapur Institute of Advanced Technology and Management, Durgapur, India2 Manufacturing Engineering Department, College of Engineering, Guindy, India3 Production Engineering Department, Jadavpur University, Kolkata, India

* e-mail: s

This is anO

Received: 31 January 2020 / Accepted: 17 March 2020

Abstract.Maskless electrochemical micromachining (EMM) is a prominent technique for producing the arrayof micro circular impressions. A method for producing the array of micro circular impressions on stainless steelworkpiece applying maskless electrochemical micromachining process is presented. The experimental setupconsists of maskless EMM cell, electrode holding devices, electrical connections of electrodes and constrictedvertical cross flow electrolyte system to carry out the experimental investigation. One non-conductive maskedpatterned tool can produce more than twenty six textured samples with high quality. A mathematical model isdeveloped to estimate theoretically the radial overcut and machining depth of the generated array of microcircular impressions by this process and corroborate the experimental results. This study provides an elementaryperceptive about maskless EMM process based on the effects of EMM process variables i.e. pulse frequency andduty ratio on surface characteristics including overcut and machining depth for NaCl, NaNO3 andNaNO3+NaCl electrolytes. From the experimental investigation, it is observed that the combined effect oflower duty ratio and higher frequency generates the best array of micro circular impressions using the mixedelectrolyte of NaNO3+NaCl with mean radial overcut of 23.31mm and mean machining depth of 14.1mm.

Keywords: Array of micro circular impressions / Maskless EMM / Electrolytes / Reused masked tool /Overcut / Machining depth

1 Introduction

Maskless EMM process is a prominent advanced micro-machining technique for many advanced applicationsinvolving microsurface texturing containing array of microcircular impressions in the manufacturing of ultra-preci-sion parts. Functional textures on stainless steel exploitsignificant chemical and physical phenomena on thesurfaces at the micro and nano scale in the improvementof many sophisticated fields like information technology,tribology, biology, electronics, energy, biomimetics andoptics [1]. However, microsurface structuring in manyadvanced fields still face two major problems viz. properdesign of textured tools for microtexturing to be beneficialand requirement of economical microsurface texturingmethod to be designed on inexpensive components for massproduction. But, maskless EMM is an economical methodfor fabricating the array of micro circular impressions withhigh quality using reusable masked patterned tool.

andip.sandip.kunar@gmail.com

penAccess article distributed under the terms of the CreativeComwhich permits unrestricted use, distribution, and reproduction

Many micromachining technologies involve fabricatingthe microsurface textures with controlled dimensionalgeometry. Bulk 3D microstructures have generated oncylindrical objects using lithography process. Then,through mask EMM (TMEMM) is applied to fabricatethe controlledmicrostructures over large surface area of thecylinder using 10wt.% NaCl+10wt.% NaNO3 electrolyte[2]. In this method, lithography is utilized to fabricate thepatterned samples on non-planner surfaces, which is costlyprocess for fabrication of many patterned samples. Effectsof structured electrode is investigated on surface pattern-ing by TMEMM method mathematically and experimen-tally using 10wt.% NaCl+10wt.% NaNO3 electrolyte [3].TMEMM is utilized to fabricate well-defined surfacetextures using 3M sulfuric acid. The shape change fromelectrochemical etching is numerically simulated using theboundary elementmethod and it is satisfied experimentallywith simulated etched profiles [4]. The array of microimpressions is produced by TMEMM process using 10%NaNO3 electrolyte and investigational outcome demon-strates the significance of electrical conductivity andpulse power for machining consistency over large area [5].

monsAttribution License (https://creativecommons.org/licenses/by/4.0),in any medium, provided the original work is properly cited.

2 S. Kunar et al.: Manufacturing Rev. 7, 15 (2020)

The micro dimple array is generated by TMEMM processusing 1, 2 and 3M NaCl+1M glycerin electrolyte [6].TMEMMmethod is proposed for the production of array ofmicro impressions using very thin masks. Experiments arecarried out to study the effect of duty ratio on the undercut,surface roughness and etching factor of dimple pattern [7].Through mask electrochemical micromachining is expen-sive and lengthy process for production of the array ofmicro impressions because every workpiece needs individ-ual masking before machining. Higher concentration ofelectrolyte is uneconomical for the geometric accuracy ofmachined profiles because it increases the stray currenteffect during anodic dissolution. Sandwich like EMM isutilized to manufacture the array of micro impressionsusing 100g/l NaNO3 electrolyte [8]. This method deteri-orates the quality of the fabricated micro dimples due tolack of flow of electrolyte. Higher concentration ofelectrolyte lowers the dimensional accuracy of the arrayof micro impressions due to higher electrolyte conductivity.Electrochemical jet processing method is employed forlarge area microtexturing and micro-milling using higherelectrolyte concentration of NaCl, NaNO3 andNaI [9]. Thismethod is expensive process because it fabricates one byone structure. Higher concentration of electrolyte dimin-ishes the quality of surface structures due to higherconcentration of electrolyte which is responsible for higherstray current effect. Electrochemical process is used toproduce the textured micropatterns on an unpatternedworkpiece using 0.5M H2SO4 and 0.5M CuSO4 electrolyte[10]. The machining depth of generated micropatterns isvery less. The conductive mask jet electrochemicalmachining method is employed to reduce the undercutof the array of micro impressions and improve themachining localization [11]. But, this method is prolongedand expensive due to fabrication of one by one circularimpression in the micro dimple array. Electrochemicaltexturing is used for microtexturing using electrolyteconcentration of 200 g/l NaCl. Electrical discharge ma-chining (EDM) technique is used to fabricate the texturedtools [12]. EDM process may deteriorate the geometricprofiles of the fabricated tools due to heat affected zone ofEDM process. Maskless EMM is used to generate thevaractor micropattern using the mixed electrolyte of NaCl(5.8M) and NaNO3 (8.5M) and the effect of EMM processparameters are investigated on machining depth andmaterial removal rate during fabrication of varactormicropattern [13]. The generated varactor micropatternis very complex. Higher concentration of electrolyte is notappropriate because it deteriorates the dimensionalaccuracy of the generated varactor micropattern due tohigher stray current effect. The influence of major EMMparameters, i.e. machining voltage, inter-electrode gap,flow rate and machining time are investigated on widthovercut and machining depth of linear and cascademicropatterns [14]. The influences of major processparameters are explored on major and minor axis overcuts,machining depth and surface roughness (Ra) usinghydrostatic and three different electrolyte flow methodsduring micro ellipse pattern generation [15]. The influenceof inter electrode gap, applied voltage and flow rate isinvestigated on textured characteristics i.e. machining

depth, overcut and surface roughness (Ra) using six typesof masked patterned tools during maskless micro-electro-chemical texturing process [16].

Various techniques are also offered for the generation ofmicrosurface textures such as LIGA technology, electronbeam machining, through mask electrochemical micro-machining, laser beam machining, ion beam machining,etc. However, these methods are incapable of the demandsof economical microsurface structures in industrialmanufacturing that can be generated in mass production[17]. These methods have several limitations such as heataffected zone, lower machining accuracy, lower surfacefinish, etc.

Therefore, maskless EMM is a significant alternativemethod due to its important benefits such as no heataffected zone, high machining rate, good surface finish,machinability of intricate shapes regardless of theirhardness and toughness, reusability of a masked patternedtool, forceless machining, etc. It offers also no maskremoval process from machined workpiece, no chance ofdegradation of surface quality and higher productivity forthe reusable single masked patterned tool, etc. which areinvolved in through mask EMM.

An alternative method of maskless EMM is applied forthe generation of array of micro circular impressions. Inthis research study, the maskless EMM setup is indige-nously developed for conducting the experiments. Onepatterned masked tool can generate a lot of machinedsamples with high quality. The influences of EMM processvariables i.e. duty ratio and pulse frequency on microcircular patterned geometrics namely, overcut and depthfor three different electrolytes i.e. NaNO3, NaCl andNaNO3+NaCl are investigated. A numerical model isdeveloped to calculate the machining depth and radialovercut of the generated array of micro circular impressionstheoretically using EMM process parameters and validatedwith the experimental results. An effort has also been doneto analysis the textured responses of generated dimplepattern for getting the best parametric setting.

2 Experimental procedure

A well planned maskless EMM setup has been developedfor conducting the experimental investigation. Theexperimental setup consists of maskless EMM cell,constricted vertical cross flow electrolyte system, tooland workpiece fixtures, and electrical connections of tooland workpiece as shown in Figure 1. Perspex material isused to generate the maskless EMM cell as shown inFigure 2. Perspex is suitable material for this cell due to itstransparent, corrosion resistance, higher strength proper-ties, etc. The cell material has also higher strength andendures the higher electrolyte flow rate. Stainless steelmaterial is utilized to produce the outlet and inlet ports ofthe cell to avoid the corrosion from environment. Perspex isalso used to make the electrode fixtures. These fixtureshave higher strength to hold the tool and workpieceproperly during machining operation. The gear pump isused to supply the electrolyte through the constricted pathbetween workpiece and tool from downward to upward

Fig. 1. Maskless EMM setup.

Fig. 2. Developed maskless EMM cell with tool and workpiecefixtures.

S. Kunar et al.: Manufacturing Rev. 7, 15 (2020) 3

directions. Electrolyte flow is controlled by flow controlvalves. Extra electrolyte flow is bypassed to the tank. Inthis flow system, vertical cross flow electrolyte circulationsystem is the most important feature in which electrolyteflow is parallel to the workpiece and tool and feedsvertically. This flow produces extra back pressure, whichaids to eradicate the electrolysis products and suppliesalways fresh electrolyte in the micromachining zone. Thegenerated geometric profiles and accuracy of surfacetextures are also better than other electrolyte flowmethods. The DC pulsed power supply is employed tosupply the pulse current during machining, which isprepared with protection functions and in-built functiongenerator.

SU-8 2150 mask (MicroChem, USA) is employed toproduce the array of micro circular impressions on SS-304sheets using UV exposure system for conducting theexperiments. The average diameter of micro circularpattern is 385mm with mask thickness of 220mm. Thegap between two successive impressions across the margin

is 800mm.A small copper is attached with the textured toolusing conductive adhesive silver paste and the arrange-ments of tool is fastened with precession micrometer tocontrol the inter electrode gaps in this process. The maskedtool having array of micro holes are used in for a variety ofelectronic applications including information display,electronic paper and radio frequency identification.

The fabricated tool is utilized to produce the array ofmicro circular impressions in the developed setup usingNaNO3, NaCl and NaNO3+NaCl electrolytes. Beforeconducting the final experimental investigation, theextensive trial experiments are performed to choose theimportant EMM parameters for the generation of array ofmicro circular impressions by NaNO3, NaCl and NaNO3+NaCl electrolytes. Based on extensive trial experiments,two most significant EMM parameters i.e. duty ratio andpulse frequency are considered for experimentation andfinal experiments are conducted by changing one parame-ter at a time with other fixed parameters on the basis ofgood investigational results. NaNO3, NaCl and NaNO3+NaCl have been preferred to study the performance ofelectrolytes on microsurface textured responses and anodicdissolution capability of stainless steel (SS-304) has beeninvestigated during generation of array of micro circularimpressions using NaNO3 (0.17M), NaCl (0.25M), andNaNO3 (0.17M)+NaCl (0.25M) electrolytes. Three typesof electrolytes are employed for methodical investigation ofthe suitability of electrolytes for better machined responsesof micro circular patterns by maskless electrochemicalmicromachining. The ranges of duty ratio and pulsefrequency are 30% to 60% and 5 kHz to 20 kHz respectivelyand other fixed parameters are inter electrode gap of50mm, applied voltage of 8V, flow rate of 6.12 m3/hr andmachining time of 2 minutes. The generated micro circularimpressions are not uniform and most of circularimpressions are not formed below the above mentionedparametric ranges. The dimensional accuracy of microcircular impressions deteriorates beyond the above men-tioned parametric ranges. Electrical resistance is measuredby the conductivity meter. The standard deviations ofradial overcut and depth are also computed to find outthe quality of array of micro circular impressions. Thetextured characteristics are measured with Atomic ForceMicroscope (Nanosurf Easyscan 2, Switzerland), OpticalMicroscope (Leica DM2500, Germany) and 3D Non-Contact Profilometer.

3 Development of mathematical model

Two important EMM process variables i.e. pulse frequencyand duty ratio directly affect the radial overcut andmachining depth. These process parameters can controlsuitably to obtain the required machining depth andovercut of the array of micro circular impressions. Themachining depth and radial overcut can be determinedsuccessfully by the developed mathematical model.

Some assumptions are considered to calculate thetheoretical radial overcut of micro circular pattern:

– Micro circular impression is regarded as a sphericalsegment of a sphere.

Fig. 3. Schematic diagram of a spherical cap.

4 S. Kunar et al.: Manufacturing Rev. 7, 15 (2020)

–

Micro circular impression is homogeneous in terms ofdepth and radial overcut and machining rate is uniformthroughout the micro circular pattern.

The spherical cap of a circular impression is shown inFigure 3 and the radius of sphere is b. The depth and radiusand cap angle of circular impression are d, r and urespectively.

The volume of spherical cap,

v ¼ pd2

3ð3b� dÞ: ð1Þ

Here,

b ¼ d

1� sinu0 � u <

p

2

h i: ð2Þ

From equations (1) and (2), the volume of cap,

v ¼ pd3

3

2þ sinu

1� sinu

� �: ð3Þ

So, the mass of material of removed from micro circularpattern

n � r � pd3

3

2þ sinu

1� sinu

� �ð4Þ

where, total number of circular impressions in microcircular pattern is n and density of material is r.

As per Faraday’s law, the total mass of materialremoved

M ¼ EIT ð5Þwhere

E ¼ A

zF¼ Electrochemical equivalent

where, the atomic weight is A, the machining current is I,the machining time is T, the valency is z and Faraday’sconstant is F.

As per Ohm’s law, V= IR,where, the potential difference is V and the resistance isR.

R ¼ sL

a

where, the resistivity of electrolyte is s, the inter electrodegap is L, the machining area is a, through which electrolyteis passed with flow velocity of S and Q is the flow rate. So,Q= a X S

Now, from equation (5),

M ¼ EVTQ

sSL: ð6Þ

Therefore, the material removal during on-time,

Mon ¼Z ton

0

EVTQ

sSLdt ¼ EVQton

sSL: ð7Þ

From equations (4) and (7), the depth of micro circularpattern,

d ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi3EVQtonð1� sinuÞpnrsSLð2þ sinuÞ

3

s: ð8Þ

Now,

s ¼ 1

K; K ¼ AmC; D ¼ ton

TandT ¼ 1

F

where, the conductivity of electrolyte is K, the molarconductivity is Am, the concentration of electrolyte is C,duty ratio isD, the pulse period isT and the frequency is F.

So, equation (8) will be

d ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi3EVDQCAmð1� sinuÞ

pnrSLFð2þ sinuÞ3

s: ð9Þ

Equation (9) provides the relationship of the depth ofmicro circular pattern between pulse frequency and dutyratio, where the depth increases with increasing duty ratioand the depth decreases with increasing pulse frequency.

Again, for estimation of theoretical overcut of microcircular pattern:

The volume of spherical cap,

v ¼ pd2

3ð3b� dÞ ð10Þ

Here,

b ¼ r

cosu¼ P þR

cosu0 � u <

p

2

h ið11Þ

where, P is the radius of circular impression of tool andR isradial overcut of machined circular impression.

So, the total material removed =

nrpd2

3ð3b� dÞ: ð12Þ

From equation (7), the total mass of material removedas per Faraday’s law

Mon ¼Z ton

0

EVTQ

sSLdt ¼ EVQton

sSL: ð13Þ

Fig. 4. Influence of duty ratio on overcut.

S. Kunar et al.: Manufacturing Rev. 7, 15 (2020) 5

Now,

s ¼ 1

K;K ¼ AmC;D ¼ ton

Tand T ¼ 1

F:

From equations (11), (12) and (13), the required radialoverall overcut

R ¼ EVQDCAm cosu

pnrd2SLF� P þ d cosu

3

� �ð14Þ

Equation (14) provides that if the radial overcutincreases with increasing duty ratio and the radial overcutdecreases with increasing pulse frequency.

The developed mathematical model presents theestimation value of depth and radial overcut of microcircular pattern by co-relating the influence of importantprocess parameters during micro circular pattern genera-tion. This numerical model has been theoretically evaluat-ed and validated with experimental results in this study.

4 Experimental results and discussion

The characteristics that describe the quality of a micro-texture are radial overcut and machining depth.Experimental results are plotted in terms of various graphsand investigated keeping in mind the objectives of shapeand size control with less overcut and controlled depth ofmicro circular pattern using suitable electrolyte.

4.1 Influence of process parameters on micro circularpattern

Experiments are performed to show the effects of duty ratioon radial overcut with constant pulse frequency of 20 kHzusing reusable patterned tool. The comparision betweenexperimental and theoretical results for three differentelectrolytes with respect to duty ratio as shown in Figure 4.

With increase in duty ratio the experimental overcutincreases for NaNO3, NaCl and NaNO3+NaCl electrolytesowing to higher stray current effect in higher machiningtime for higher duty ratio. For NaNO3, the overcut liesbetween NaCl and NaNO3+NaCl except duty ratio of 40%because of lower stray current effect for lower electricalconductivity compared to NaCl. At duty ratio of 40%, thelowest radial overcut is observed for NaNO3 due tocontrolled ectching. In case of NaCl, The overcut is higherthan NaNO3 and NaNO3+NaCl due to higher electricalconductivity. In case of NaCl+NaNO3, lower overcut isachieved than NaCl and NaNO3 without duty ratio of 40%because of higher machining rate of NaCl and highermachining accuracy of NaNO3. At duty ratio of 40%, theovercut locates in-between NaNO3 and NaCl due to non-uniform machining. The standard deviations of radialovercut are lower at 30% duty ratio for three differentelectrolytes. Among these three electrolytes, the loweststandard deviation has been observed for NaCl+NaNO3electrolyte.

The theoretical overcut increases with increasing dutyratio for three different electrolytes as per developedequation (14). The experimental overcut also increaseswith increasing duty ratio for three electrolytes as perdeveloped equation (14). For NaCl, the experimentalovercut is deviated equal or less than 2.7mm from thetheoretical graph. For NaNO3, the experimental overcut isdeviated equal or less than 4.1mm from the theoreticalgraph. For NaCl+NaNO3, the experimental overcut isdeviated equal or less than 3.1mm from the theoreticalgraph. The developed equation (14) is used for the initialestimation of theoretical radial overcut of the array ofmicro circular impressions for various duty ratios andvalidates the experimental results. The overcut andmachining depth of three electrolytes for different dutyratios have been shown in Tables 1–3.

Influence of duty ratio on the machining depth is shownin Figure 5 and it reveals the evaluation between theexperimental and theoretical depths of the micro circularpattern for three different electrolytes with other constantparameter of pulse frequency of 20 kHz. From the graph, itis clear that with increasing duty ratio the experimentaldepth increases. It is because higher pulse on time i.e.higher machining time is acquired than pulse off-time inhigher duty ratio and controlled machining localizationtakes place for higher current density. The localizationeffect increases because the electrolysis products can beflushed away by the flow of electrolyte during the pulse off-time and makes the ready for machining during the pulse-on time. In case of NaNO3, the depth occupies the middlezone in-between NaCl and NaCl+NaNO3 due to con-trolled electrical conductivity compared to NaCl. In case ofNaCl, the machining depth is lower than NaCl+NaNO3and NaNO3 electrolytes due to the higher metallic ions. Incase of NaCl+NaNO3, higher machining depth is observedthan NaCl and NaNO3 owing to controlled etching. Thestandard deviations of machining depth is lower at 30%duty ratio for three different electrolytes. Aomng them, thelowest standard deviation and uniformity of micro circularpattern have been shown at duty ratio of 30% forNaCl+NaNO3 electrolyte.

Table 3. Overcut and machining depth for NaNO3 electrolyte.

Dutyratio

Overcut(Theoretical)NaNO3

Overcut(Experimental)NaNO3

Depth(Theoretical)NaNO3

Depth(Experimental)NaNO3

30 27.9 26.68 14.3 12.840 37.2 33.148 18.8 18.350 46.5 50.251 23.5 21.560 55.8 57.201 28.2 28.7

Table 1. Overcut and machining depth for NaCl+NaNO3 electrolyte.

Dutyratio

Overcut(Theoretical)NaCl + NaNO3

Overcut(Experimental)NaCl + NaNO3

Depth(Theoretical)NaCl + NaNO3

Depth(Experimental)NaCl + NaNO3

30 24.9 23.31 15.4 14.140 33.2 36.16 20.4 22.650 41.5 43.15 25.5 24.260 49.8 46.62 30.6 31.5

Table 2. Overcut and machining depth for NaCl electrolyte.

Dutyratio

Overcut(Theoretical)NaCl

Overcut(Experimental)NaCl

Depth(Theoretical)NaCl

Depth(Experimental)NaCl

30 30.6 29.12 13.1 11.640 40.8 37.1 17.2 16.850 51 53.15 21.5 19.660 61.2 59.07 25.8 21.7

Fig. 5. Influence of duty ratio on machining depth.

6 S. Kunar et al.: Manufacturing Rev. 7, 15 (2020)

The theoretical and experimental depths increase as perdeveloped equation (9). For NaCl, the experimental depthis deviated equal or less than 4.1mm from the theoreticalgraph. For NaNO3, the experimental depth is deviatedequal or less than 2mm from the theoretical graph. ForNaCl+NaNO3, the experimental depth is deviated equalor less than 2.2mm from the theoretical graph. Thedeveloped equation (9) is suitable for the preliminarydetermination of theoretical depth for various duty ratiosand confirms the validation of experimental results.

Effect of frequency on experimental and theoreticalovercuts is shown in Figure 6 and represents thecomparisons between them. The experimental overcutdecreases with increased pulse frequency for variouselectrolytes because the rate of repetition of pulse on timei.e. machining time decreases with increasing pulsefrequency. The stray current effect also decreases forlower pulse on time in higher frequency. In case of NaNO3,the overcut locates in-between NaNO3+NaCl and NaCldue to controlled machining for lower metallic ions. In caseof NaCl, higher overcut is obtained than NaNO3+NaCl

Fig. 6. Influence of pulse frequency on overcut. Fig. 7. Influence of pulse frequency on machining depth.

Table 4. Overcut and machining depth for NaCl+NaNO3 electrolyte.

Frequency Overcut(Theoretical)NaCl + NaNO3

Overcut(Experimental)NaCl + NaNO3

Depth(Theoretical)NaCl + NaNO3

Depth(Experimental)NaCl + NaNO3

5 51.38 53.1 33.6 31.810 42.82 38.8 28 26.315 34.26 35.1 22.4 24.720 25.7 23.31 16.8 14.1

Table 5. Overcut and machining depth for NaCl electrolyte.

Frequency Overcut(Theoretical)NaCl

Overcut(Experimental)NaCl

Depth(Theoretical)NaCl

Depth(Experimental)NaCl

5 58.2 56.4 26.8 24.710 48.5 51.6 22.5 23.715 38.8 35.8 17.9 15.220 31.6 29.12 12.9 11.6

S. Kunar et al.: Manufacturing Rev. 7, 15 (2020) 7

and NaNO3 because of uncontrolled anodic dissolution forhigher metallic ions. For NaCl+NaNO3, lower overcut isacquired than NaNO3 and NaCl owing to uniform etchingfor controlled current distribution. The standard devia-tions of radial overcut are lower at pulse frequency of20 kHz for various electrolytes. Among them, the loweststandard deviation and regularity of micro circular patternare shown at frequency of 20 kHz for NaNO3+NaClelectrolyte.

Here, the theoretical and experimental overcutsdecrease with increase in pulse frequency as per developedequation (14) for three different electrolytes. For NaCl, theexperimental overcut is deviated equal or less than 5.1mm

from the theoretical overcut. For NaNO3, the experimentalovercut is deviated equal or less than 5.2mm from thetheoretical overcut. For NaCl+NaNO3, the experimentalovercut is deviated equal or less 4mm than the theoreticalgraph. The developed equation (14) is suitable for theprimary determination of radial overcut of array of microcircular impressions for various pulse frequencies andcorroborates the experimental results. The overcut andmachining depth of three electrolytes for different pulsedfrequencies have been shown in Tables 4–6.

The effect of frequency on themachining depth is shownin Figure 7 and represents the evaluation between thetheoretical and experimental depths with fixed duty ratio

Table 6. Overcut and machining depth for NaNO3 electrolyte.

Frequency Overcut(Theoretical)NaNO3

Overcut(Experimental)NaNO3

Depth(Theoretical)NaNO3

Depth(Experimental)NaNO3

5 50.4 52.8 29.2 32.110 43 46.2 22.7 23.815 35.6 32.9 18.2 21.220 28.2 26.68 13.7 12.8

Fig. 8. (a) Unused masked pattern before machining; (b) usedmasked pattern after machining.

Fig. 9. Regular array of micro circular impressions generated byNaCl+NaNO3.

8 S. Kunar et al.: Manufacturing Rev. 7, 15 (2020)

of 30%. With increasing frequency the experimental depthdecreases. The machining depth reduces owing to theavailability of lower pulse on time in higher frequency thanlower pulse frequency. Higher machining depth is observedat lower pulse frequency because the anodic dissolutiontakes place in controlled manner. In case of NaNO3, themachining depth occupies the middle zone in-betweenNaCl and NaNO3+NaCl because of controlled dissolutionfor lower electrical conductivity. In case of NaCl, lowermachining depth is achieved than NaNO3 and NaNO3+NaCl because of uncontrolled dissolution for highermetallic ions. In case of NaNO3+NaCl, higher depth isobtained than NaNO3 and NaCl because of highermachining localization for uniform current flux distribu-tion. The standard deviations of machining depth are lowerat frequency of 20 kHz for NaNO3, NaCl and NaNO3+NaCl. Only NaNO3+NaCl electrolyte maintains thelowest standard deviation than other two electrolytes.

Here, the experimental and theoretical depthsdecrease with increasing pulse frequency as per developedequation (9) for three different electrolytes. For NaCl, theexperimental depth is deviated equal or less than 2.7mmfrom the theoretical overcut. For NaNO3, the experimentalovercut is deviated equal or less than 3mm from thetheoretical overcut. For NaCl+NaNO3, the experimentalovercut is deviated equal or less 2.7mm than the theoreticalgraph. Hence, the developed equation (9) is used for thepreliminary evaluation of theoretical depth of the array ofmicro circular impressions for various pulse frequencies.Thus, it correlates with the equation (9) and experimentalresults.

4.2 Analysis of micrographs

Effects of process parameters on microtextured character-istics are studied for NaNO3, NaCl and NaNO3+NaCl

electrolytes using SU-8 2150 photoresist tool. Thisphotoresist has enough strength to fabricate more thantwenty six machined samples. This mask does not warpafter producing multiple machined samples because it hasfast photospeeds for mass production, ability to endure thechemical reactions for long time, capability to adhere withsubstrates, etc. The array of micro circular impressions onSU-8 2150 mask is shown in Figure 8a before machining.The used mask after machining of twenty six samples isshown in Figure 8b. So, SU-8 2150 photoresist has goodefficacy for producing multiple high quality micro circularimpressions economically using maskless EMM technique.Maskless EMM method is considered an alternativemethod instead of TMEMM process because throughmask EMM process needs individual masking for eachworkpiece before machining.

Figure 9 depicts the micrograph of micro dimplepattern and SEM image of machined micro circularimpression and it is fabricated with the specifiedparametric setting i.e. electrolyte of NaCl (0.17M)+NaNO3 (0.11M), frequency of 20 kHz, voltage of 8V, flowrate of 6.12 m3/hr, duty ratio of 30%, inter electrode gap of50mm and machining time of 2 minutes. This array ofmicro circular impressions upholds more or less identicalgeometrical shape and size throughout the pattern becausethe combined electrolyte performs the uniform anodicdissolution for controlled current flux distribution. Thedepth is also homogeneous across the pattern due to higheranodic dissolution.

Three-dimensional view and two-dimensional profile ofa small segment of micro circular impression are shown inFigure 10. The machining depth of micro circularimpression is 24mm. Figure 11 shows two-directional

Fig. 11. 2D surface roughness profile of a micro circular impression generated by NaCl+NaNO3.

Fig. 10. 3D view and 2D depth profile of a micro circular impression generated by NaCl+NaNO3.

Fig. 12. 3D view and 2D profile of a micro circular impression generated by NaCl+NaNO3.

S. Kunar et al.: Manufacturing Rev. 7, 15 (2020) 9

surface roughness profile of a small segment ofmicro circularimpression. The value of surface roughness of a smallsegment of micro circular impression is 0.0405mm.

Three dimensional view and two-dimensional profile ofa small segment of micro circular impression are shown inFigure 12. These figures are taken by Atomic ForceMicroscope (AFM). The values of root mean square (Rq),maximumprofile peak height (Rp), maximumprofile valleydepth (Rv) and surface roughness (Ra) are 0.068mm,0.213mm, �0.072mm and 0.052mm.

5 Conclusions

A simple and unique texturing method of maskless EMMhas been presented for the production of high qualityarray of micro circular impressions. In this process, singletextured tool fabricates many machined micro circularpatterns economically without masking of every indi-vidual workpiece before machining, which is moresignificant matter in industrial context. Effects offrequency and duty ratio are discussed on dimensional

10 S. Kunar et al.: Manufacturing Rev. 7, 15 (2020)

overcut and depth of micro circular pattern. Thefollowing conclusions have been summarized fromexperimental investigations:

– Maskless EMM is an alternative technique for thegeneration of multiple high quality arrays of microcircular impressions economically. A well plannedexperimental setup with vertical cross flow electrolytesystem has been fabricated for producing good array ofmicro circular impressions with higher productivity.One masked patterned tool can produce more thantwenty six textured samples using SU-8 2150 mask.

–

The developed mathematical model is suitable for initialestimation of theoretical radial overcut and depth ofmicro circular pattern and validates the experimentalresults for three different electrolytes. The loweststandard deviation has been observed for NaCl+NaNO3electrolyte and this electrolyte maintains proper unifor-mity of the micro circular pattern than other twoelectrolytes.

–

Machining with higher pulse frequency and lower dutyratio are suggested for better dimensional uniformity andcontrolled depth using NaNO3, NaCl and NaNO3+NaClelectrolytes. Only NaNO3+NaCl electrolyte is moreappropriate to produce good array of micro circularimpressions.

–

Form the analysis of micrograph, the best machiningparameters are pulse frequency of 20 kHz, duty ratio of30% and electrolyte of NaCl (0.17M)+NaNO3 (0.11M)and this parameter setting generates the uniform array ofmicro circular impressions having radial overcut of23.31mm and machining depth of 14.1mm.

Finally, this research study in the area of micro-electrochemical texturing provides an alternative re-placement and will accomplish different vital needs in thearea of microsurface texturing. However, this researcharea still needs further investigation for better control ofthe tool and workpiece movement strategies in masklessEMM.

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Cite this article as: S. Kunar, E. Rajkeerthi, K. Mandal, B. Bhattacharyya, Fabrication of array of micro circular impressionsusing different electrolytes by maskless electrochemical micromachining, Manufacturing Rev. 7, 15 (2020)