International Journal of Machine Tools & Manufacture 44 (2004) 1577–1589 www.elsevier.com/locate/ijmactool Advancement in electrochemical micro-machining B. Bhattacharyya , J. Munda, M. Malapati Department of Production Engineering, Jadavpur University, Kolkata 700032, India Received 7 April 2004; accepted 14 June 2004 Abstract Electrochemical micro-machining (EMM) appears to be very promising as a future micro-machining technique, since in many areas of applications it offers several advantages, which include higher machining rate, better precision and control, and a wide range of materials that can be machined. In this paper, a review is presented on current research, development and industrial practice in micro-ECM. This paper highlights the influence of various predominant factors of EMM such as controlled material removal, machining accuracy, power supply, design and development of microtool, role of inter-electrode gap and electrolyte, etc. EMM can be effectively used for high precision machining operations, that is, for accuracies of the order of 1 lm on 50 lm. Some industrial applications of EMM have also been reported. Further research into EMM will open up many challenging opportunities of improvement towards greater machining accuracy, new materials machining and generation of complex shapes for effective utilization of ECM in the micro-machining domain. # 2004 Elsevier Ltd. All rights reserved. 1. Introduction The anodic dissolution of metals was already known in the previous century. But it was not until the 1960s that it came into use as a practical machining method. In non-traditional machining processes, electrochemical machining (ECM) has tremendous potential on account of versatility of its applications, and it is expected that it will be a promising, successful and commercially utilized machining process in the modern manufacturing industries. The ECM process was first patented by Gusseff in 1929. Significant advances dur- ing the 1950s and 1960s developed ECM into a major technology in the aircraft and aerospace industries for shaping, finishing, deburring and milling operations of large parts [1,2]. All these processes of ECM now plays an important role in the manufacture of a variety of parts, ranging from machining complicated, shaped large metallic pieces to opening windows in silicon that are a few micrometers in diameter. ECM has seen a resurgence of industrial interest in the last decade due to its various advantages over other machining pro- cesses such as no tool wear, absence of stress/burr, high MRR, bright surface finish and the ability to machine complex shapes in materials regardless of their hardness. ECM is an anodic dissolution process where workpiece and tool are respectively anode and cathode, which are separated by an electrolyte. When an electric current is passed through the electrolyte, the anode workpiece dissolves locally, so that the shape of the generated workpiece is approximately a negative mirror image of that of the tool. The electrolyte, which is gen- erally a concentrated salt solution, is pumped at high velocities through the machining gap in order to remove the reaction products and to dissipate the heat generated. Machining performance in ECM is gov- erned by the anodic behavior of the workpiece material in a given electrolyte [3]. In recent years, ECM has received much attention in the fabrication of micro- parts [4–9]. Fig. 1 shows a schematic view of an elec- trochemical micro-machining (EMM) system set-up, which consists of pulsed DC power supply, machine controller, microtool drive unit, mechanical machining unit, electrolyte flow system, etc. A few research attempts have been made by the semiconductor and the electromechanical consumer product industries as well as the T.J. Watson research center of IBM apart from Corresponding author. Tel.: +91-33-241-461-53. E-mail address: bb13@rediffmail.com (B. Bhattacharyya). 0890-6955/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2004.06.006
Transcript
� Corresponding author. Te
E-mail address: bb13@red
0890-6955/$ - see front matte
doi:10.1016/j.ijmachtools.200
l.: +91-33-241-461-53.
iffmail.com (B. Bhattacharyya).
r # 2004 Elsevier Ltd. All rights reserved.
4.06.006
International Journal of Machine Tools & Manufacture 44 (2004) 1577–1589
www.elsevier.com/locate/ijmactool
Advancement in electrochemical micro-machining
B. Bhattacharyya �, J. Munda, M. Malapati
Department of Production Engineering, Jadavpur University, Kolkata 700032, India
Received 7 April 2004; accepted 14 June 2004
Abstract
Electrochemical micro-machining (EMM) appears to be very promising as a future micro-machining technique, since in manyareas of applications it offers several advantages, which include higher machining rate, better precision and control, and a widerange of materials that can be machined. In this paper, a review is presented on current research, development and industrialpractice in micro-ECM. This paper highlights the influence of various predominant factors of EMM such as controlled materialremoval, machining accuracy, power supply, design and development of microtool, role of inter-electrode gap and electrolyte, etc.EMM can be effectively used for high precision machining operations, that is, for accuracies of the order of �1 lm on 50 lm.Some industrial applications of EMM have also been reported. Further research into EMM will open up many challengingopportunities of improvement towards greater machining accuracy, new materials machining and generation of complex shapesfor effective utilization of ECM in the micro-machining domain.# 2004 Elsevier Ltd. All rights reserved.
1. Introduction
The anodic dissolution of metals was already known
in the previous century. But it was not until the 1960s
that it came into use as a practical machining method.
In non-traditional machining processes, electrochemical
machining (ECM) has tremendous potential on
account of versatility of its applications, and it is
expected that it will be a promising, successful and
commercially utilized machining process in the modern
manufacturing industries. The ECM process was first
patented by Gusseff in 1929. Significant advances dur-
ing the 1950s and 1960s developed ECM into a major
technology in the aircraft and aerospace industries for
shaping, finishing, deburring and milling operations of
large parts [1,2]. All these processes of ECM now plays
an important role in the manufacture of a variety of
parts, ranging from machining complicated, shaped
large metallic pieces to opening windows in silicon that
are a few micrometers in diameter. ECM has seen a
resurgence of industrial interest in the last decade due
to its various advantages over other machining pro-
cesses such as no tool wear, absence of stress/burr,
high MRR, bright surface finish and the ability to
machine complex shapes in materials regardless of their
hardness. ECM is an anodic dissolution process where
workpiece and tool are respectively anode and cathode,
which are separated by an electrolyte. When an electric
current is passed through the electrolyte, the anode
workpiece dissolves locally, so that the shape of the
generated workpiece is approximately a negative mirror
image of that of the tool. The electrolyte, which is gen-
erally a concentrated salt solution, is pumped at high
velocities through the machining gap in order to
remove the reaction products and to dissipate the heat
generated. Machining performance in ECM is gov-
erned by the anodic behavior of the workpiece material
in a given electrolyte [3]. In recent years, ECM has
received much attention in the fabrication of micro-
parts [4–9]. Fig. 1 shows a schematic view of an elec-
trochemical micro-machining (EMM) system set-up,
which consists of pulsed DC power supply, machine
controller, microtool drive unit, mechanical machining
unit, electrolyte flow system, etc. A few research
attempts have been made by the semiconductor and the
electromechanical consumer product industries as well
as the T.J. Watson research center of IBM apart from
1578 B. Bhattacharyya et al. / International Journal of Machine Tools & Manufacture 44 (2004) 1577–1589
other research institutions [6,10]. However, to exploitfull the potential of EMM, research is still needed toimprove its accuracy and compactness.
In this paper, a review is presented on currentresearch, development and industrial practice in micro-ECM. The importance of EMM in the area of micro-fabrication and the recent improvements in accuracyand material removal in the micro-machining domainhave been highlighted. Basic principles and recentdevelopments in power supply for EMM are also dis-cussed. Tool shape plays a vital role in achieving accu-rate shape, and the recent developments of improvingtool shape are addressed. The influence and methods ofprediction of the inter-electrode gap are also discussed.In addition, the role of the electrolyte and electrolyteflow has been presented for a better understanding ofthe EMM process. Some striking applications of EMMfor microfabrication are exhibited.
2. Need for EMM
Recent changes in demand from society have forcedthe introduction of more and more microparts in vari-ous types of industrial products. For example, in thecase of fuel injection nozzles for automobiles, severalregulations arising from environmental problems haveforced manufacturers to improve their design, makingthem smaller and more compact, with high accuracy.Inspection of the internal organs of the human bodyand surgery without pain are universally desired. Min-iaturization of medical tools is an effective approach toarrive at this target. Micro-machining technology playsan increasingly decisive role in the miniaturization ofcomponents ranging from biomedical applications tochemical micro-reactors and sensors [11–13]. Micro-machining is the key technology in micro-electromech-anical systems (MEMS) [14,15]. Since miniaturization
will continue as long as people require effective spaceutilization with more efficient and better-accuracy pro-ducts, micro-machining technology will be still moreimportant in the future.The term micro-machining refers to material removal
of small dimensions ranging from 1–999 lm. Advancedmicro-machining may consist various ultraprecisionactivities to be performed on very small and thin work-pieces; small and micro-sized holes, slots and complexsurfaces are needed in large numbers. Sometimes, whenthese shapes are produced with conventional machiningtechniques, the problem usually encountered is hightool wear, lack of rigidity of the process and heat gen-eration at the tool and workpiece interface. Inaddition, it becomes troublesome to machine three-dimensional micro-shapes [16]. Non-conventionalmachining is now receiving acknowledgement of itsimportance because of some of its specific advantages,which can be exploited during the micro-machiningoperation [17]. Most non-conventional machining pro-cesses are thermal oriented, e.g. laser beam machining(LBM) and electron beam machining (EBM), whichmay cause thermal distortion of the machined surface[18]. Chemical machining and ECM are heat-free pro-cesses, but chemical machining cannot be controlledproperly in this micro-machining domain. ECMmachining is applied to the micro-machining range ofapplications for manufacturing ultraprecision shapes; itis then called electrochemical micro-machining (EMM).EMM appears to be a very promising micro-machiningtechnology due to its advantages, which include highMRR, better precision and control, short machiningtime, reliability, process flexibility, and environmentalacceptability, and it also permits the machining of che-mically resistant materials like titanium, copper alloysand stainless steel, which are widely used in biomedical,electronic and MEMS applications [19–21]. EMM canbe advantageously employed in most applications
Fig. 1. Schematic view of EMM system.
B. Bhattacharyya et al. / International Journal of Machine Tools & Manufacture 44 (2004) 1577–1589 1579
related to the micro-machining of metallic parts due to
its cost effectiveness and the high precision achievable;
these parts were previously fabricated by chemical
micro-machining. A general comparison between ECM
and EMM is presented in Table 1.
3. Material removal and machining accuracy
in EMM
3.1. Material removal in EMM
In the machining region where the workpiece directly
faces the cathode tool, the anodic reaction rate is con-
stant for a constant inter-electrode Gap (IEG) and
electrolyte conductivity. The machining performance is
influenced by various predominant process parameters,
such as current density, IEG, electrolyte flow rate, con-
centration and type of electrolyte, and also the anode
reactions [22,23]. If the gap (IEG) is reduced, the resol-
ution of machining shape becomes better and the possi-
bility of applying ECM for micro-machining increases.
Material removal is maximum for small IEG. Experi-
mental results have proved that the addition of a mag-
netic field causes increase in material removal rate and
accuracy. In a particular instance, when the inter-elec-
trode electric current was 6 A, the material removal
rate was 37 mg/min, but by introducing a magnetic
field, the material removal rate increased to 55 mg/min
[24]. The material removal rate is expressed in terms of
unit removal (UR) in the micro-machining domain
[17]. UR is defined as a unit of workpiece removed
during one cycle/pulse of machining. The UR basically
depends on the following factors [4,14,20].
3.1.1. Anodic reaction and current efficiencyDepending on the operating conditions and metal
electrolyte combinations, different anodic reactionstake place at high current densities. The rates of thesereactions are dependent on the ability of the system toremove the reaction product as soon as they areformed and supply fresh electrolyte to the anode sur-face. All of these factors influence the machining per-formance, namely dissolution rate, shape control andsurface finish of the workpiece. The current efficiencyof metal dissolution, g is related to weight loss, rw by
g ¼ rwvF
Itað1Þ
where I is the applied current, t is the machining time,F is Faraday’s constant, v is the valence of metal dis-solution and a is the molecular weight of the metal.
Actually, the dissolution valence v will influence themetal removal and is related to the UR as follows:
UR ¼ IagpvFAq
ð2Þ
where q is the density in g/m3, A is the surface area inm2 and p is the number of pulse/unit time.
Current efficiency for metal dissolution, which is afunction of current density and local flow conditions,varies as a function of distance from the tool.
3.1.2. Mass transport effectsMass transport plays an important role in the anodic
dissolution process for shaping and surface finishing.Mass transport rates depend on the hydrodynamic con-ditions for a given metal–electrolyte combination [20].Current distribution and accuracy of the job can beaffected by the mass transport conditions. An increasein current density leads to an increase in the rate of
00–600 lm 5–50 lm Operation M askless Mask/maskless
Machining rate 0
.2–10 mm/min 5 lm/min
Side gap >
20 lm <10 lm Accuracy � 0.1 mm �0.02–0.1 mm
Surface finish G
ood, 0.1–1.5 lm Excellent, 0.05–0.4 lm Problems due to waste disposal/toxicity L ow Low to moderate
1580 B. Bhattacharyya et al. / International Journal of Machine Tools & Manufacture 44 (2004) 1577–1589
metal dissolution at the anode. In anodic dissolution,salt film mechanism and acceptor mechanism havebeen identified for mass transport. In the salt filmmechanism, the rate of transport of dissolution pro-ducts from the anode surface is rate limiting. In highrate anodic dissolution in neutral electrolyte, the trans-port of reaction products away from the anode is ratelimiting and salt film precipitation occurs at the surfaceof the anode [14,20]. In the acceptor mechanism, thelimiting factor is the rate of transport towards theanode of acceptor types such as complexing ions; thesevarieties react with the dissolved metal ions to formhydrated complex ions. In EMM, a smooth surface fin-ish can be achieved only at limiting current density. Ifthe current density is too high, it may cause the forma-tion of heat-affected zones, and it finally results inimproper surface finish and low accuracy [20]. The lim-iting current density is controlled by convective masstransport; the anodic limiting current density J is givenby
J ¼ vFDCsat
dð3Þ
where D is the effective diffusion coefficient, whichtakes into account the contribution from transport bymigration, Csat is the surface concentration and d is thediffusion layer thickness.
3.1.3. Current distribution and shape evolutionThe current distribution pattern will also influence
the shape generation and degree of leveling in EMM.In through-mask EMM, three different scales must beconsidered with respect to current distribution, viz.workpiece scale (cell scale), pattern scale and featurescale. At the workpiece/cell scale, the geometry of theworkpiece and tool can be controlled by the currentdistribution. At the pattern scale, current distribution isachieved by carrying out dissolution under mass trans-port control. Current distribution also depends on thespacing of the features and on their geometry [14,20].At the feature scale, shape is evaluated through thecurrent distribution. The prediction of shape evolutionduring high rate anodic dissolution requires solving thecurrent distribution at the anode along with a movingboundary algorithm. The current distribution at theanode depends on geometry, anode reaction kinetics,electrolyte conductivity and hydrodynamic conditions[4]. Current distribution is calculated for a given timeusing appropriate finite difference, finite and boundaryelement codes. The finite element method (FEM) andboundary element method (BEM) have been used forthe simulation of tool shape evolution during anodicleveling of model surface profiles. The results of thesesimulation techniques agreed with the experimentalresults of anodic leveling of model profiles [2,4,25,26].Non-uniformity of current density along the profile
surface led to a higher rate of metal dissolution at thepeaks than valleys. The surface roughness, defined asthe profile height to profile amplitude ratio, decreasedas anodic leveling proceeded.
3.2. Machining accuracy in EMM
Machining accuracy can be influenced by all para-meters, such as power supply, electrolyte selection andflow, selection of tool, IEG, UR, etc. For achievinghigher accuracy, pulsed power supply with small IEG,passivating electrolyte, e.g. sodium nitrate, balanceelectrode and dual pole tool are preferred, but theselection of all machining parameters depends onthe shape of the final product [27]. Temperature of theelectrolyte will also influence the machining accuracyand surface finish in EMM. The estimation methods ofelectrolyte temperature have already been developed,which is helpful to judge the accuracy and surface fin-ish [28]. Experimental investigation of the influence ofEMM parameters on material removal rate and accu-racy recommends a machining voltage range of 6–10 V,which gives an appreciable MRR at moderate accuracy[29]. With a pulsed supply, it is possible to producecomplex shapes such as dies, turbine blades and alsoprecision electronic micro-components, with an accu-racy of 0.02–0.10 mm [2,30]. For further improvementof micro-machining accuracy, a piezoelectric transduceris used to vibrate the microtool so as to retract it fromthe workpiece during pulse off-time to enlarge the endgap to intensify the electrolyte flushing [31]. Furtherinvestigations are still needed in the area of EMM forbetter understanding of the process mechanism, para-metric optimization, current density and local removalrate.
4. Power supply for EMM
Basic theoretical and fundamental research work andpreliminary industrial practice have indicated thatEMM using pulsed current offers considerable poten-tial for enhancement of the ECM process [32]. Thehigh current density required for proper operation ofthe EMM process may give a high concentration ofreaction products, which can only be partly removedby the electrolyte, especially if the inter-electrode gap isnarrow. Increasing contamination can cause depositionon the microtool, so the workpiece material no longerdissolves uniformly. Furthermore, changes in the elec-trolyte composition may cause a raise in temperature,and hence the electrical resistivity can also make theaccuracy worse. These problems can be largely avoidedby applying a pulsed voltage instead of a continuousvoltage. Pulsating current has three parameters: pulseon-time, pulse off-time and peak current density, which
B. Bhattacharyya et al. / International Journal of Machine Tools & Manufacture 44 (2004) 1577–1589 1581
can be varied independently in order to achieve thedesired machining rate. It can also help to control thedimensional accuracy and surface finish. Fig. 2 showscurrent efficiency against current density for continuousand pulsed voltage for an interval of 10 ms with apulse duration of 1 ms. For continuous voltage, theefficiency decreases gradually when the current densityis reduced, whereas with the pulsed voltage, thedecrease is much more rapid [5].
The steep fall in efficiency with decreasing currentimproves the accuracy of form of the workpiece. Thisimprovement depends on the pulse duration and theinterval time. In pulsed supply micro-machining, per-iodic replacement of electrolyte makes it possible toapply a higher instant current density during the pulseon-time, leading to significant improvement of surfacequality. Pulsed current enables recovery of the gap dur-ing pulse off-time, giving improved dissolution and
accuracy compared to continuous current [33]. Pulsat-ing current is particularly suitable for high precisionmicro-machining of delicate workpieces, where highelectrolyte flow rate may initiate vibration and disturb-ance in the IEG and therefore cannot be tolerated. Pul-sating current has been used for the newly developedelectrochemical saw [6]. In the pulsed EMM process, apulse generator is used to supply the voltage pulsesacross the electrodes. Anodic electrochemical dissol-ution occurs during the short pulse on-time, each ran-ging from 0.1 to 5 ms. Pulse on-time reduction tomicroseconds along with an increase in current densityresults in the improvement of accuracy and localizationof anodic dissolution [34]. The dissolution products canbe flushed away from the inter-electrode gap by theflow of the electrolyte during the pulse off-times.
Fig. 3 shows a block diagram of a typical power sup-ply system utilized for the EMM system. Informationon machining voltage, machining current, and micro-tool electrode position are monitored through a per-sonal computer. The personal computer controls themovements of the microtool by a linear actuator andthe piezoelectric transducer. Gap checking and toolrepositioning can also be conducted during these pulseoff-times. The on- and off-time period is in the range of1–5 ms. A pulsed power supply unit usually consists offour parts as shown in Fig. 4.
An AC/230 V supply is converted to the requiredDC voltage by the use of a step-down transformer anda rectifier. Decreasing the current density can increasethe accuracy of the workpiece. A low current densityand an operating voltage of the order of 4–8 V arerequired for proper operation of EMM. A pulse gener-ator is used to pulse the voltage. A power switch device
Fig. 2. Current efficiency vs current density [5].
Fig. 3. Power supply system of EMM [31].
1582 B. Bhattacharyya et al. / International Journal of Machine Tools & Manufacture 44 (2004) 1577–1589
unit, an essential element in the power supply, is used
to create the pulse. The device is mainly evaluated
according to switch time, control mode and cost. The
metal oxide semiconductor field effective transistor
(MOSFET) is appropriate for high-speed switching in
pulsed ECM [33]. According to the experimental inves-
tigation on EMM [29], the most effective zone of pulse
on-time can be considered as 10–15 ms, which gives an
appreciable MRR as well as less overcut.However, short duration pulses have great potential
to achieve high dimensional accuracy due to the small
amount of material removed per pulse [5,15,27,35,36].
The pulse duration of nanoseconds provides direct con-
trol of the accuracy of the machining component, i.e.
machining precision is increased with shorter pulses.
Fig. 5 exhibits the SEM micrograph of two holes
machined on a copper sheet, using a 50 lm diameter
platinum tool with a voltage of 1.5 V, 5 ls pulse dur-
ation causes poor surface finish and inaccuracy of the
hole, but 100 ns pulses significantly improves the sur-
face finish and accuracy [12]. 3D micro-machining is
achieved by the application of ultra-short voltage pul-
ses in the range of duration of nanoseconds. EMM
with ultra-short pulses can be applied to all electro-
chemically active materials including semiconductors.
Further improvement of machining precision should be
possible with even shorter pulses [37]. Fig. 6 shows the
effect of pulse period on machining time and hole
diameter. From the graph it shows that machining witha pulse period of 500 ns (460 ns off-time) leads to thespecified diameter and accuracy being reached morerapidly than with a 2 ls (1960 ns off-time) pulse period[27].Recent research reports indicate improvements in
machining rates and surface finish for some materials,by utilizing a pulse reverse current, which consists ofan anodic pulse (forward), followed by a cathodic pulse(reverse) and a relaxation period [2,38]. For full accept-ance of ultra-short pulsed machining system by indus-try, a low cost power supply with high reliableperformance needs to be developed. For most of theexisting micro-machining systems, a trial and errormethod is used to formulate planning and strategy fordesigning power parameters to achieve accuracy con-trol and process optimization. A lot of research isgoing on in this area and is urgently required.
5. Design and development of microtools
Attention was paid to tool design on the introduc-tion of ECM into the manufacturing industries. Tooldesign remains a major challenge for manufacturingmicroparts [39]. Tool design mainly deals with thedetermination of tool shape, which will produce aworkpiece with proper dimensions and accuracy. It isnot yet successful for practical applications due to thecomplex gap configuration [2]. The tool shape is a per-fect negative mirror image of the workpiece to be pro-duced; prediction of the tool shape is a formidableinverse boundary problem involving Laplace equations[25,40,41]. The difficulties in designing ECM tools stemfrom a lack of adequate understanding of the process,dissolution phenomena and mathematical complexities,which makes the present methods of tool design allowcalculation of only a first approximation of the finaltool shape [2, 42]. The iterative procedure of tooldesign is time consuming, resulting in costly machine
Fig. 4. Pulsed power supply unit [2].
5. SEM micrograph of machined micro-holes on a Cu sh
Fig. eet.
(a) 5 ls pulse period; (b) 100 ns pulse period [12].
6. Hole diameter vs machining time (6 V, 40 ns pulse on-ti
Fig. me)
[27].
B. Bhattacharyya et al. / International Journal of Machine Tools & Manufacture 44 (2004) 1577–1589 1583
downtime and long lead times. Early investigationsinto tool design procedures were mainly limited to‘‘simplified methods’’, such as analytical solution, andgraphical, geometrical and complex variable techni-ques. Recent studies have primarily been directed atnumerical solutions to the inverse boundary probleminstead of simple geometrical approximations[2,25,26,39,40]. These studies have primarily appliednumerical techniques such as the FEM and the BEM.These two methods show flexibility in the treatment ofthe boundary than others. A ‘correction factor’ concepthas been adopted to modify the tool shape generatedby the FEM. The modification is repeated until it pro-duces the desired workpiece shape. Recently, the Galer-kin FEM was applied to solutions of the workpieceprediction problem [26].
In addition to tool design, suitable development ofthe microtool is required for successful machining.Microtools are fabricated using electrochemical etchingand wire electro-discharge grinding (WEDG) [2,43,44].An accurate tool profile is accomplished by controllingthe current density and voltage [45]. Micro-ECM issuitable for surface finishing of micro-pins and micro-spindles. These micro-pins can be used as microtools inEMM, which can enhance accuracy [42]. In general,the material for microtools should consist of a chemi-cally inert material, which has good electrical conduc-tivity and which is easily machinable. For reducing theeffect of stray current, the microtool is also insulated sothat current flows only through the front face [46]. Aninsulating cover of SiC/Si3N4 may be coated onto thecathode tool by means of chemical vapour deposition(CVD), which can increase the accuracy and surfacefinish. Giving an orbital movement to the cathode toolelectrode can also increase accuracy [5,23,47,48]. Adual pole tool is also preferred for increasing themachining accuracy [49]. In a dual pole tool, the insu-lated negative charged tool is covered by a positivecharged insoluble metal bush, which reduces the chan-ces of overcut due to stray material removal due to theflow of stray current flux in the micro-machining zone.Fig. 7 shows the effect of uninsulated, insulated anddual pole tools on machining accuracy.
In microtool manufacture, the tool cathode is ametal wire, which slides slowly along a groove in a wireguide as shown in Fig. 8 [2,44]. In this, a very smallgap is essential to localize dissolution, and an electro-lyte with very low conductivity, such as deionizedwater, is used in order to realize a current density inthe normal range. Microtools can be manufactured byapplying small hall ECM and electrochemical broach-ing, alternately changing the polarity of the machiningcurrent in such a way that dissolution takes place fromthe tool. Finally, micron tool dimension is achieved.Fig. 9 shows microtools in the range of 15–20 lmdiameter, which were produced on a special precision
grinding machine [13]. A recent report describes themicrofabrication of gold and platinum single crystalultra-microelectrodes (SCUME), which can be effec-tively utilized as microtools [50].
A practical tool design solution should provide notonly the cathode dimensions but also a suitable electro-lyte path and an appropriate insulating pattern to pre-vent undesired overcut. There is no method availablefor perfect prediction of tool shape, especially in thearea of micro-ECM, so far. So in-depth research com-bining theory and supporting experimental investi-gation is still needed in the area of microtool designand development.
6. Role of inter-electrode gap (IEG)
The IEG should be as small as possible. If the elec-trode gap is kept small, the resolution of the machinedshape becomes better and the possibility of applyingECM to micro-machining increases. Localization of the
7. Influence of (a) uninsulated, (b) insulated, and (c) dual p
Fig. ole
tools on machining accuracy.
Fig. 8. Fabrication of microtool.
1584 B. Bhattacharyya et al. / International Journal of Machine Tools & Manufacture 44 (2004) 1577–1589
dissolution process can be increased by reducing thegap width [2]. The gap width is only one parametercharacterizing the micro-machining precision [12]. Itcan also increase accuracy and unit removal. In theEMM process, the IEG is kept in the order of 10–50lm. Maintaining the specific range of IEG, i.e. 15–20lm uniformly is an important requirement to achievehigh accuracy and surface finish. The electrical conduc-tion method for maintaining the electrode gap distancebetween the tool electrode and workpiece can beapplied. Around 1 V can be applied between the toolelectrode and the workpiece for measuring the currentso that the electrical contact of the microtool electrodewith the workpiece can be checked [37,48]. In a pulsedEMM system, the IEG and tool position monitoringcan be conducted during pulse off-time, leading to asignificant reduction in the indeterminacy of the gap.Machining inaccuracy is directly proportional to theIEG size.
Equation for IEG with respect to electrolyte flowvelocity [7] is
IEG ¼ VgVsp
Vfð4Þ
where Vf is flow velocity, Vsp is specific soluble volume,i.e. a=vFq and V is potential difference.
The need for on-line monitoring arises due to numer-ous complex, transient and stochastic processes occur-ring in the gap. From the on-line monitoring, linearcorrelation between the pulse signal variance and thegap size has been found [36]. This relationship becomes
significant when a shorter pulse on-time (<1 ms) isused and more importantly, it exists within the mostgap range (0.1–0.2 mm) commonly used by industry.For on-line monitoring of the IEG, application of eddycurrents is a technique aimed at the implementation ofnon-intrusive measurement system [51]. This methodachieves measurement by determining the magnitude ofan induced eddy current circulating in the workpiece,resulting from an emitting coil embedded in the tool.Some success is reported in the application of thismethod, but it leads to experimental ambiguities, i.e.the composition and density of the workpiece/tool,conductivity of electrolyte and size of microtool canaffect the measurement. These factors would be parti-cularly problematic in the implementation of a broadrange of measurement systems in EMM. Radio-frequency emission has also been used for the IEGmonitoring and control [33]. In ultrasonic measurementsystems, ultrasound is used for fully passive (inde-pendent of process conditions) non-intrusive, on-linegap measurement [52]. Such an approach has becomepractical due to recent developments in the resolutionof ultrasonic sizing technologies. Using this, the IEGcan be measured by direct and indirect means. Directmeasurement determines the gap based on the propa-gation delay through the electrolyte between the toolsurface/electrolyte interface and workpiece surface/electrolyte interface. Indirect gap measurement is doneby subtracting the tool face position from the work-piece height referenced to the same origin.Most of the methods discussed for monitoring and
measuring the IEG are actually applicable in the nor-mal ECM process. However, these principles and meth-ods can also be applied to the EMM process. Further,researchers’ attention is needed in the area of dynamicgap measurement, aimed at a more general character-ization of EMM machinability.
7. Electrolyte for EMM
The electrolyte not only completes the electric circuitbetween the tool and workpiece, but also allows thedesired machining reactions to occur. Generally, ano-dic films are allowed to form on the workpiece surfaceas this helps to achieve anodic smoothing; however,sometimes it may cause short circuiting during EMMdue to the smaller IEG. Although the precipitate hasno direct effect on the process, it definitely increases thepossibility of damage of the microtool from short cir-cuiting. Hence, it is advisable to use fresh and cleanelectrolyte for micro-machining instead of re-circu-lation. The electrolyte carries away heat and reactionproducts from the zone of machining. The electrolytemust possess less throwing power, apart from its basicproperties, to increase accuracy [1]. ECM electrolytes
Fig. 9. High aspect ratio microtools [13].
B. Bhattacharyya et al. / International Journal of Machine Tools & Manufacture 44 (2004) 1577–1589 1585
are classified into two categories: passivating electro-lytes containing oxidizing anions, e.g. sodium nitrate,sodium chlorate, and non-passivating electrolytes con-taining relatively aggressive anions such as sodiumchloride. Passivating electrolytes are known to give bet-ter machining precision [5,6,42]. The pH value of theelectrolyte solution is chosen to ensure good dissol-ution of the workpiece material during the ECM pro-cess without affecting the microtool. Acidic electrolytesare preferable for EMM because they do not createany insoluble reaction products [27]. It is usual to workwith sodium nitrate electrolyte solution (pH 7) [53]. Inthe case of EMM, the material removal is very low.The gap between the microtool and workpiece can bedecreased to a still lower value by decreasing the elec-trolyte concentration. Maximum feed rate can beachieved with sodium nitrate electrolyte. Sodiumnitrate is more advantageous than the other electrolytesdue to its lower throwing power, and high and con-trolled metal removal, which leads to high-speed andaccuracy in machining [54]. Jet EMM, where an elec-trolyte jet plays a vital role during machining, asshown in Fig. 10 is preferred for generating micro-holes with high aspect ratio to achieve higher accuracyand high material removal rate [21,55].
The improvement of machining accuracy in ECMcan be achieved through modifying electrolyte flow dis-tribution. ECM with an eccentric orbital movement isproposed to enhance the uniformity of electrolyte flowand to eliminate flow field disruption [47]. Experi-mental results show improvement in accuracy usingorbital ECM. After machining, the electrolyte is passedthrough a settling tank and a filter, which removes theforeign material present in the electrolyte. The electro-lyte is circulated to the machining zone by means ofcentrifugal action.
Regarding the micro-holes machined on the metallicfoils by EMM, it may be observed that a lower electro-lyte concentration with higher machining voltage andmoderate value of pulse on-time will produce more
accurate shape with less overcut at moderate MRR [7].Some research studies have been carried out to increasethe accuracy by selecting a proper electrolyte; it stillrequires research in the area of fluid kinematics, andtype and concentration of electrolyte that can be effec-tively utilized for EMM.
8. Applications
Research and development in the area of EMM willbe important and will fulfill the various urgent needs ofthe electronics and precision industry in the area ofultraprecision microfabrication. Fig. 11 shows adeveloped EMM laboratory set-up with detailed sub-units, which comprises of a power supply unit, machin-ing unit and electrolyte flow system. In this section,typical applications of EMM technologies for themicrofabrication of components are introduced.
8.1. Surface finishing of print bands
The print bands used in high-speed impact printersare fabricated from sheets of hardened ferrite stainlesssteel. The print band system consists of a group offormed characters. Precise location of all the characters
Fig. 10. Jet electrolyte flow for machining high aspect ratio holes
[55].
Fig. 11. Developed electrochemical micro-machining laboratory set-up [29].
1586 B. Bhattacharyya et al. / International Journal of Machine Tools & Manufacture 44 (2004) 1577–1589
on a band is achieved through timing marks. The char-
acters and timing marks on the print bands must have
special characteristics to meet the desired trade-off
between ribbon life and print quality. Bands with
round-edged characters increase ribbon life. To provide
a high degree of character rounding, the EMM should
involve a high rate of dissolution. Surface finishing of
print bands is most important in the print band manu-
facturing process. An electropolishing process has been
developed which gives micro-smooth surfaces for print
bands. Fig. 12 shows a print band (a) before finishing
and (b) after electropolishing and character rounding.
Electropolishing gives better surface finish and higher
throughput. A wide range of metals can be used for
print bands through the use of electropolishing tech-
nology [4,14].
8.2. Nozzle plate for ink-jet printer head
Electroformed nozzles are currently used in a num-
ber of commercially produced ink-jet printers. Electro-
formed nozzles are produced by plating nickel on to
a mandrel (mold), which defines the pattern of the
nozzle, and then removing the finished product [4,56].
Pulsating current/voltage permits better control over
EMM of thin films and foils for applications in micro-
fabrication. Through-mask EMM was used to fabricate
a series of flat-bottomed conical nozzles in a metal foil
as shown in Fig. 13. The process is applicable to vari-
ous materials including high strength corrosion resist-
ant materials such as conducting ceramics [57,58]. The
final shape of the nozzle depends on dissolution time
and conditions. Pulsed current can improve the accu-
racy of finish of the nozzles.
8.3. Electrochemical saw for maskless metal cutting
The workpiece anode is fed at a constant rate
towards a thin blade cathode, which is insulated on all
sides except that facing the anode. Electrolyte is admit-
ted to the inter-electrode gap through capillaries such
that electrolyte contact is maintained only in the
machining gap. Supply of fresh electrolyte to the IEG
and removal of reaction products and heat is
accomplished by the to and fro movement of the cath-
ode blade. Varieties of metals, alloys and conducting
ceramics have been cut using the electrochemical saw.
The apparatus should find important application in
single crystal cutting where metal cutting should be
achieved without introducing thermal or mechanical
defects and should involve minimum tool removal [5].
8.4. Deburring
The ECM method is widely used for removing burrs
left by other operations. ECM is a useful method for
deburring such products, since it can meet the increas-
ing requirements for accuracy of dimension and form
[5,6]. Electrochemical deburring is very rapid, with vir-
tually no damage to the workpiece. This was observed
during removal of burr that is 25 lm above the U 400
lm punched hole. The burr was completely removed in
about 0.5 s with a flat tool about 100 lm above the
workpiece. During the process maximum sheet
removed is 2 lm without changing the shape of the
punched die [5].
Fig. 12. SEM micrograph of a print band character [4].
Fig. 13. Group of flat-bottomed conical nozzles [56].
B. Bhattacharyya et al. / International Journal of Machine Tools & Manufacture 44 (2004) 1577–1589 1587
8.5. Production of high accuracy holes
EMM can be used for the accurate production ofholes [6]. An example is the making of a diaphragm foran electro optical system. Metal is removed from thetop of a hollow cone, producing a truncated conewhose flat top is the diaphragm. The production ofsuch a diaphragm requires careful control of the holediameter. The operation is performed with a stationaryflat tool about 0.1 mm above the top of the cone.From the other side, a pin of diameter 48 lm is placedagainst the inside of the top of the cone. The pin, madeof an insoluble material, is connected to a generator bya micro switch. During the process the top dissolvesfirst, and then the opening becomes steadily larger. Assoon as the required diameter has been reached, the pingauge starts to move through the hole and the currentis switched off. Any burrs formed as a result of themovement of the pin can be removed by applying extrapulses. The SEM micrograph of a micro-hole machinedby EMM under particular machining conditions, i.e.machining voltage 10 V, electrolyte concentration 15 g/l, pulse on-time 12.5 ms and frequency of pulsed powersupply 50 Hz, is shown in Fig. 14. Under these para-metric conditions, the overcut of the machined holeproduced by the EMM is comparatively low. Through-mask EMM of Invar alloy films was investigated andmathematical modeling was reported to predict thewall profile development during EMM [59].
A typical application is the production of micro-holes in turbine blades for generating a cooling effect,where EMM’s advantages are fully exploited, includingits applicability regardless of material hardness for gen-erating complex geometry, high surface quality withstress and burr free surfaces and economic large scaleproduction [60]. It also produces many parts for aero-space and aircraft applications, like rocket engineparts, and jet engine rings more efficiently. It has also
found many applications in other industries, like the
automobile, medical and defense industries. The pro-
duction of artificial hip joints of titanium and cobalt
alloys and valve parts are now established industrial
processes. It has also been used to produce micro
grooves for self-acting fluid film bearings, which can be
controlled precisely without distorting the other sur-
faces.
8.6. 3D micro-machining
3D EMM is shown in Fig. 15, an SEM micrograph
of a machined component of an electronic circuit
board in which a platinum wire of 10 lm diameter was
used as a tool on a copper sheet with the application of
50 ns, 1.6 V pulses of 2 MHz frequency, to obtain a
delicate 3D copper structure, i.e. 5 lm� 10 lm�12 lm in the middle of a hole on a base, i.e.
Fig. 14. SEM micrograph of machined micro-hole [29].
Fig. 15. SEM micrograph of a 3D electronic circuit board compo-
nent [37].
Fig. 16. SEM micrograph of a 3D thin component [37].
1588 B. Bhattacharyya et al. / International Journal of Machine Tools & Manufacture 44 (2004) 1577–1589
15 lm� 15 lm� 10 lm. The microtool was first fedvertically 12 lm deep into the workpiece. After thisvertical machining, the microtool is moved laterallyalong the prescribed path in the copper sheet. Theouter rectangular trough was dissolved to a dimensionof 22 lm� 14 lm. During the process, the microtoolfeed rate was adjusted to 0.5 lm by monitoring thepeak current transient of the inter-electrode gap. Fig. 16shows a 3D image of a very thin tongue with a thick-ness of 2.5 lm under the same conditions [37].
9. Summary
This paper highlights the recent developments andfuture trends of EMM. The micro-ECM (EMM)method can be effectively used for high precisionmachining operations such as removal of burrs, mak-ing patterns in foils, and 3D micro-machining, and alsoin various applications. Results of recent research indi-cate that the applications of Electrochemical metalremoval in micro-machining offer many opportunitiesthat have been unexplored till now. Further researchinto EMM will open up many challenging possibilitiesfor effective utilization of ECM in the micro-machiningdomain. Extensive research efforts and continuingadvancements in the area of EMM for effective utiliza-tion in microfabrication require improvements inmicrotool design and development, monitoring andcontrol of the IEG, control of material removal andaccuracy, power supply, and elimination of micro-sparks generation in IEG and selection of electrolyte,are expected to enhance the application of EMM tech-nology in modern industries. The increasing demandsfor precision manufacturing of microparts for biomedi-cal components, automotive components and IT appli-cations will lead modern manufacturing engineers toutilizing EMM technique more successfully consideringits advantages, i.e. quality, productivity and ultimatelycost efficiency, which are still vital for success in a com-petitive environment.
References
[1] J.A. McGeough, Principles of Electrochemical Machining,
Chapman and Hall, London, 1974.
[2] K.P. Rajurkar, D. Zhu, J.A. McGeough, J. Kozak, A. De Silva,
New developments in electrochemical machining, Annals of the
CIRP 48 (2) (1999) 567–579.
[3] S.K. Sorkhel, B. Bhattacharyya, Parametric control for optimal
quality of the work piece surface in ECM, International Journal
of Material Processing Technology 40 (1994) 271–286.
[4] M. Datta, R.V. Shenoy, L.T. Rominkiw, Recent advances in the
study of electrochemical micromachining, Transactions of the
ASME 118 (1996) 29–36.
[5] C. Van Osenbrugger, C. de Regt, Electrochemical micromachin-
ing, Philips Technical Review 42 (1985) 22–32.
[6] M. Datta, L.T. Romankiw, Applications of chemical and elec-
trochemical micromachining in the electronic industry, Journal
of the Electrochemical Society 136 (1989) 285c.
[7] B. Bhattacharyya, J. Munda, Experimental investigation into
electrochemical micromachining (EMM) process, International
Journal of Materials Processing Technology 140 (2003) 287–291.
[8] R.V. Shenoy, M. Datta, L.T. Romankiw, Investigation of island
formation during through mask electrochemical micromachining,
Journal of the Electrochemical Society 143 (7) (1996) 2305–2309.
[9] Datta M, Sheppard K, Snyder D. Electrochemical microfabrica-
tion. The Electrochemical Society Proceedings, NJ, 1978.
[10] M. Datta, Microfabrication by electrochemical metal removal,
IBM Journal of Research and Development 42 (5) (1998) 655–669.
[11] R. Kurita, H. Jabei, Z. Liu, T. Hariuchi, O. Niwa, Fabrication
and electrochemical properties of an interdigitated array elec-
trode in a microfabricated water jet cell, Sensors and Actua-
tors—‘B’ Chemical B71 (2000) 82–89.
[12] M. Kock, V. Kirchner, R. Schuster, Electrochemical micro-
machining with ultra short voltage pulses—a versatile method
with lithographical precision, Electrochimica Acta 48 (1) (2003)
3213–3219.
[13] H. Ohmori, K. Katahira, Y. Vehara, Y. Watanabe, W. Liu,
Improvement of mechanical strength of microtools by control-
ling surface characteristics, Annals of the CIRP 52 (1) (2003)
467–470.
[14] M. Datta, D. Landolt, Fundamental aspects and applications
