investigation of electrochemical parameters

Technical Papers of NAMRI/SME Volume XXVI, 199863

INVESTIGATION OF ELECTROCHEMICAL PARAMETERS INTO ANELECTROCHEMICAL MACHINING PROCESS

Jenny J. Sun E. Jennings Taylor

Lawrence E. Gebhart Chengdong D. Zhou

Jeffrey M. Eagleton Robert P. Renz

Faraday Technology, Inc.315 Huls Drive

Clayton, Ohio 45315-8983

ABSTRACT

Electrochemical machining (ECM) is a process where thesurface of a part is removed via an anodic charge transfer.Electropolishing, electrochemical deburring, or radiusing areelectrochemical machining processes where the metal removalmust be uniform or localized, respectively. ECM is affected bymany parameters such as machining gap, electric fieldmodulation, electrolyte properties, and flow rate. This paperfocuses on the effect of electrochemical parameters on theECM machining rate, machining accuracy, and surface finish.Since ECM is an electrochemical technique, the basic tool ofthe electrochemist, the polarization behavior, is investigated toestablish a relationship between electrolyte properties and ECMmachining quality for a given material. From the polarizationbehavior, the ECM process may be optimized for specificapplication, such as electropolishing, deburring, or radiusing.

INTRODUCTION

The electrochemical machining process is based on theprinciple of electrochemical metal removal. It selectivelyremoves the surface metal of a workpiece by the conversion ofthe metal into its ions by means of an electrical field applied ina conductive electrolyte. In principle, ECM is capable ofmachining any electrically-conductive material regardless ofthe 1) hardness, toughness and other material properties, or 2)contours or surface profiles, with high removal rates and notool wear. Therefore, ECM is used in industries such as theaerospace, automotive, and electronics industries, for a variety

of tasks, including some that would be difficult,impossible, or time consuming by mechanicalmachining, such as machining hard passive materials(i.e. hardened steels, titanium alloys, nickel basesuperalloys, and heat resistant alloys).

The development of a control system for the ECMprocess to achieve a higher metal machining rate,better dimensional accuracy and better surfacequality, has been the main goal of ECM machinemanufacturers and user industries. Recent research onECM processes has focused on the pulse current ECMprocess (PECM), interelectrode gap, electrolyte flowrate, tool feeding rate, and tool design. It is alsoknown that the PECM can improve electrolyte flowconditions in the interelectrode gap, enhancelocalization of anodic dissolution, and decrease theinterelectrode gap to increase machining dimensionalaccuracy. Experimental results indicated that thePECM parameters (such as the pulse on-time, pulseoff-time, and pulse frequency) could change the masstransport conditions between the tool (cathode) andworkpiece (anode) to affect current distribution [1],but the current efficiency for metal dissolution (suchas nickel dissolution) is independent of theseparameters [2]. In most cases, the current efficiency isdetermined by electrolyte composition and machinedmaterial. The electrolyte composition directlyinfluences the metal removal rate and the surfacecharacteristics, so that the ECM parameters used for agiven application may not yield the same results in a

Technical Papers of NAMRI/SME 64 Volume XXVI, 1998

different electrolyte. The reported research has indicated thatthe dimensional accuracy in the ECM of superalloys is differentfrom that of carbon steels [3-5]. For instance, NaNO3

electrolyte does not provide better accuracy over NaClelectrolytes for some titanium alloys, which is opposite to theECM of carbon steels [4]. So far, research in this area is mainlyfocused on the electrolyte effects. The development of an ECMprocess usually begins with the selection of the correctelectrolyte. In this study, a simple test process to investigate thepolarization behavior is developed to select the optimalelectrolyte for various ECM processes. Using the polarizationbehavior to estimate the relationship between the electrolyteand the material is the first step towards developing a particularECM process.

BACKGROUND

The electrochemical machining process is based on anodicmetal dissolution occurring under specific conditions (such assmall gap between anode and cathode, high current density, anhigh flow rate of electrolyte through the gap). The total cellvoltage for maintain the electrochemical reaction in the smallgap should be:

Ecell = EC – EA + Vg

where EC and EA are the equilibrium potentials of the cathodeand anode reactions at the operating temperature, respectively.Vg is the voltage drop in the gap. It is given by:

Vg = VAa + Vp + VAc + VR + VCa + VCd + VCc

where VAa is the anode activation overpotential, Vp is thepassive film breakdown potential, VAc is the anodeconcentration overpotential, VR is the ohmic overpotential, VCa

is the cathode activation overpotential, VCd is the depositionproducts potential on the cathode surface, and VCc is thecathode concentration overpotential. The cell voltage should behigh enough that the workpiece surface can break down toinitiate the ECM process. The breakdown potential varies indifferent electrolytes for the same material. The machiningsurface also behaves differently in different electrolytes.

Electrolyte SelectionAn extremely useful tool for optimization of the ECM processis the polarization curve. The polarization curve is a plot of theanode potential, Ea, as a function of anodic current, Ia, andprovide information on active and passive regions of materialsin the different electrolytes. Two typical polarization curves areshown in Figure 1 to illustrate the anodic behavior of a metal inECM electrolytes. Curve 1 shows the metal in an activeelectrolyte (such as NaCl), and curve 2 is the metal in a passiveelectrolyte (such as Na2SO4). Before the electric field isapplied, the metal anode immersed in the electrolyte has a

steady-state potential (Ess). When the power is appliedto the anode, the electrode potential will shift in thepositive direction from Ess to Eab (the breakdownpotential), although this potential change does notcause metal dissolution. The range between Ess andEab depends on the different metals and electrolytes.The range of Ess - Eab on the polarization curve isabout 0.5 V for steel in a NaCl electrolyte, around 1.3V for steel in a NaNO3 electrolyte, and about 6 V forTi-6Al-4V in 10% NaCl. Above Eab, the current risesabruptly owing to the dissolution reactions occurringon the anode (region AB). The dissolution rate of theanode metal ceases to increase when a limiting currentIlim is reached (BC region). In this range, the metalatoms form ions and compounds with the activatinganions and pass into the electrolyte. The limitingcurrent Ilim and the ratio of ∆I to ∆E (the slope of ABon the polarization curve) can be defined as the metaldissolution rate and current efficiency in theelectrolyte, respectively. The greater the ratio of∆I/∆E, the less the voltage drop (Vg) between anodeand cathode, because

∆I /∆E α 1/Rg

where Rg is the resistance of the electrochemicalreactions in the interelectrode gap. The electrolyte,which has a high ratio of ∆I/∆E indicates a fast andefficient metal dissolution process. (Note: in manyactive electrolytes, there is no limiting current, andthe metal dissolution rate increases as a function ofthe applied potential). In region BC of the polarizationcurve, the current remains constant (curve 1) or dropsto a lower value (curve 2) indicating that masstransport phenomena limit the rate of metal ionremoval. The products of metal dissolution reach theirsolubility limit and form a loose deposit or passivefilm on the electrode surface. If the metal dissolutionis conducted in a passive electrolyte, the passive filmcan grow faster than metal ions pass into theelectrolyte, with the result that the current falls tolower values (curve 2). Generally, the limiting currentdecreases with increasing electrolyte concentration,due to the decrease in the solubility of the reactionproducts. Since the limiting current is strongly relatedto diffusion, it can be increased in the ECM processby increasing the electrolyte flow rate. When theanode potential increases to region CD of thepolarization curve, the higher potential can breakdown or remove the passive film and deposits, andincrease the ionization rate of the metal to increase thecurrent.


FIGURE 1. POLARIZATION CURVE FOR METAL INDIFFERENT ELECTROLYTES.

The metal surface quality (i.e. brightness and smoothness) indifferent electrolytes can be directly observed from polarizationtests. Consequently, the polarization curve of the optimalelectrolyte for a metal ECM process should 1) provide a lowbreakdown potential (Eab) of the machining metal, 2) have ahigh ratio of ∆I to ∆E (a small change in potential (∆E) with anabrupt increase in current (∆I)), so that a higher and moreefficient ECM machining rate can be obtained, and 3) have asmooth and shiny surface. As a simple, efficient, andeconomical study method, the polarization curve can be used toselect the correct electrolyte for the ECM process.

EXPERIMENTAL STUDY

An experimental investigation was conducted usingpolarization curves, by applying a linear potential scan to theelectrode. Figure 2 schematically shows the experimental set upfor the polarization study. A Model 273 potentiostat (EG&GPrinceton Applied Research) and M352 corrosion analysissoftware were used to conduct the polarization tests to selectthe electrolyte for 1) titanium alloy machining/polishing, 2)molybdenum ECM polishing, and 3) steel core removal from atitanium alloy casting. In the polarization studies, a piece of Ti-6Al-4V plate, molybdenum die material plate, and steel platewith 1cm2 exposure area were used as a working electrode,respectively. A stainless steel rod (d=6 mm, the same materialas the ECM tool) was used as a counter electrode and Ag/AgClwas used as a reference electrode. The electrolytes used in thepolarization tests contained 5-20% NaCl, 10-30% Na2SO4, 5-35% NaNO3, 2-5M NaOH, 1-6M KOH, 10-20% Na2S2O3, 10-48% NaClO4, 10-50% (NH4)2S2O8, 1%-10%KBr, and 5-10%H2SO4. The parameters for the polarization scans for the steeland molybdenum were: initial potential = 0 V; final potential =4 V; scan rate = 8 mV/s, and for the Ti-6Al-4V were initialpotential = 0 V; final potential = 10 V; scan rate = 8 mV/s.

FIGURE 2. SCHEMATIC OF POLARIZATIONEXPERIMENT SETUP.

After the electrolytes were selected from thepolarization study, the ECM tests were conducted ineach selected electrolyte by using DC and PC in ECMprocesses (DECM and PECM). Figure 3 shows thesetup of the ECM experimental apparatus. The systemincludes: 1) electrolyte holding tank with filter, 2)pumping system, 3) machining station, 4) tool feedingcontroller, and 5) rectifier which can output DC andPC waveforms.

In the ECM experiments, solutions of 250 g/L ofNaNO3 and 150 g/L NaCl were used as theelectrolytes to drill a blind hole in the 1018 steelcoupon. A stainless steel tool (outside dia. 11.3 mm,and inside dia. 3.1 mm,) was used in the ECM testsand the process was operated at room temperature.The ECM tests were conducted with DC and PC (asshown in Table 1) to study the effects of differentelectrolytes on the electric field waveform (EFW) forsteel core removal without attack on the base titaniumalloy casting. The electrolyte was pumped from theholding tank to the machining station at the desiredflow rate, at least 9 L/min (2.7 atmos) for the PECMruns and at least 11 L/min (3.4 atmos) in the DECMruns, because a higher electrolyte flow was needed toremove heat and machining products in DECM. Thecell voltage was controlled at 5V during the DECMtest to avoid titanium dissolving around the steel core.

For PECM runs, an average cell voltage of 5 V wascontrolled during the tests, to maintain the samecharge through the cell in each ECM test. The testslasted for 40 minutes. The initial machining gap (gapbetween tool and workpiece) was 0.25 mm. The toolfeed rate was 0.24 mm/min in NaCl, and the feedingrate was manually adjusted in NaNO3. After each

WorkingelectrodeReference

electrode

CWR

Potentiostat∆I(t)

∆Ε(t)

Computersoftware

Printer

Cell

Counterelectrode

Cu

rr

en

t

E a b 2E a b 1

∆ I 1

∆ E 1

∆ I 2

∆ E 2

I ( A )

E ( V )

I l i m 1

I i l m 1 > I l i m 2

(∆ I / ∆ E ) 1 > ( ∆ Ι /∆ E ) 2

A

B C

D

A

B

C

D

I l i m 2

C u r v e 1

C u r v e 2

V o l t a g e E a p 1E ss


experiment, the 1018 steel workpiece (50 x 25 x 75 mm) wasmeasured for volume loss (DV) and cross-sectioned to inspectthe machining shape during the ECM process. The machiningrate and dimension accuracy were estimated to compare theeffect of the electrolyte on the DECM and PECM processes.

TABLE 1. EXPERIMENTAL MATRIX.

TestNo.

Freq(Hz)

Anodic DutyCycle (%)

Ton-time

(ms)Toff-time

(ms)DC 0 100 - 0PC1 5 25 50 150PC2 5 50 100 100PC3 5 80 160 40PC4 50 25 5 15PC5 50 50 10 10PC6 50 80 16 4PC7 100 25 2.5 7.5PC8 100 50 5 5PC9 100 80 8 2

RESULTS AND DISCUSSION

Polarization StudyTable 2 summarizes the polarization results for steel coreremoval from a titanium alloy casting. The optimal electrolytefor a metal ECM process has to satisfy three requirements: 1)low breakdown potential, 2) high ratio of ∆I/∆E, and 3) surfacequality. In addition, the ECM process for the steel core removalfrom a titanium alloy has to provide separated breakdownpotentials between steel and titanium alloy, so that the cellvoltage can be easily controlled to selectively remove steelwithout dissolution of the Ti-6Al-4V.

TABLE 2. POLARIZATION TEST RESULTS FOR STEELREMOVAL.

Steel Ti-6Al-4VElectrolyte

Eab

(V)∆I/∆E Eab

(V)∆I/∆E

20% NaCl 0.50 2,10 6.00 1.7335%NaNO3 1.75 0.65 9.40 0.0125%NaNO3 1.30 0.93 >10 -15%NaNO3 1.30 0.48 >10 -5%NaNO3 1.75 0.30 9.50 0.2235%NaNO3+3%NaCl*

1.75 0.53 8.50 1.63

25%NaNO3+ 2.10 1.30 9.00 1.63

3%NaCl*15%NaNO3+3%NaCl*

2.25 0.65 9.20 1.63

5%NaNO3+3%NaCl*

2.75 0.80 7.00 1.30

30%Na2SO4* 1.25 0.25 >10 -20%Na2SO4* 1.25 0.22 >10 -10%Na2SO4* 1.50 0.18 >10 -30%Na2SO4+3%NaCl*

0.50 0.03 >10 -

20%Na2SO4+3%NaCl*

0.75 0.13 >10 -

10%Na2SO4+3%NaCl*

1.75 0.20 >10 -

*polarization curves similar to curve 2 (as shown in Figure3), the limiting current at activity region (AB region) islower than 0.3 A.

The breakdown potential for steel in 20% NaClelectrolyte is 0.5 V and ∆I/∆E is high, therefore steelcan be efficiently removed at high rate. However, thebreakdown potential gap between steel and Ti-6Al-4Vis only 5.5 V and the ∆I/∆E is also high for Ti-6Al-4V. Therefore, the Ti-6Al-4V is easily dissolved in20% NaCl. In 5% - 35% NaNO3 electrolytes, thebreakdown potential for steel is between 1.3 V to 1.75V. The breakdown potential gap between steel and Ti-6Al-4V is greater than 7.6 V for the NaNO3

electrolyte. With the addition of NaCl in NaNO3 , thebreakdown potential for steel increases. Therefore, themixed electrolyte of NaNO3 + NaCl and a NaClelectrolyte were not chosen for steel core removalfrom a titanium alloy casting. The Na2SO4 electrolytewas passive for both steel and Ti-6Al-4V, and there isa large window between the steel breakdown potentialand the Ti-6Al-4V breakdown potential. The steelbreakdown potential decreases with the addition ofNaCl in Na2SO4. Considering the breakdown potentialand the gap between steel and Ti-6Al-4V, it seemsthat NaSO4 or NaSO4/NaCl electrolytes are betterthan NaNO3. However, the ratio of ∆I/∆E for NaSO4

or NaSO4/NaCl are smaller (about 0.25) than NaNO3

(about 0.93). Thus, the removal rate for steel inNaNO3 electrolyte can be greater than in Na2SO4. Forexample, when the cell voltage increases to 10 V, thecurrent for steel dissolution is greater than 9.3 A inNaNO3 and 2.5 A in Na2SO4. Consequently,~25%NaNO3 was selected as the electrolyte for steelcore removal from titanium.


FIGURE 3. PHOTOGRAPH OF EXPERIMENTAL SETUP FORECM.

FIGURE 4. SCHEMATIC OF THE MACHINED SHAPE OFSTEEL IN ECM PROCESS.

ECM StudyECM tests were conducted to drill a blind hole in the 1018 steelworkpiece with different electric field waveforms (see Table 1).Figure 4 schematically shows the ECM profile for steel coreremoval. Usually, the dimension of the machined hole on theworkpiece is different from the tool. The current distribution onthe side of the tool and at the bottom of the tool are different foreach electric field waveform. For the best current distribution,the diameter on the top of the machined hole is the same as thediameter at the bottom, as shown in Figure 4a. However, inmost cases, the diameter of the machined hole on the top islarger than that at bottom. The dimension error can beestimated by calculating the ratio of hole diameter differencebetween the top and bottom to the hole depth:

Dimension error = (Dtop – Dbot)/H

where, Dtop is the diameter on the top of machined hole and Dbot

is the diameter at the bottom of the machined hole, and H is thehole depth. The larger the dimension error, the poorer themachining accuracy.

TABLE 3. TEST RESULTS.

Test # RemovalRate(g/h)

HoleDepth(mm)

DimensionError

LateralThrowing

Power

NaNO3 ElectrolyteDC 4.44 8.20 0.089 11%PC1 5.43 10.64 0.057 9.8%PC2 5.69 10.59 0.069 9.8%PC3 4.73 8.00 0.065 9.6%PC4 4.5 8.74 0.087 9.2%PC5 3 6.22 0.065 4.8%PC6 3.25 6.86 0.085 7.4%PC7 4.14 8.74 0.032 6.6%PC8 2.95 6.40 0.048 3.6%PC9 2.95 6.10 0.054 5.4%

NaCl ElectrolyteDC 5.69 9.65 0.164 19.6%PC1 5.92 9.66 0.110 14.4%PC2 5.92 8.05 0.161 22.2%PC3 5.03 8.46 0.099 15%PC4 6.38 10.57 0.072 14%PC5 6.40 10.18 0.127 23.8%PC6 6.50 9.78 0.158 27.8%PC7 6.33 9.83 0.137 18.6%PC8 6.50 9.47 0.193 28.4%PC9 7.69 10.29 0.197 34.4%

The lateral throwing power (LTP) can be calculatedas:

LTP = (Dave –Dtool)/Dtool

where, Dave is the average diameter of machined holeand Dtool is the tool diameter. Table 3 summarizes theECM test results. As shown in Table 3, 1) the lowactivity electrolyte, such as NaNO3, provided goodmachining accuracy; however, the machining rate islow compared to NaCl; 2) the machining accuracy ismore sensitive to the electric field waveformparameters in NaCl; and 3) at low frequency and lowduty cycle, the machining rate of steel is higher inNaNO3.

The higher machining rate in NaCl compared toNaNO3 can be explained from the polarization study,which shows higher ∆Ι/∆E in NaCl. Since the straycurrent on the side is lower in a low conductivityelectrolyte (NaNO3) that a high conductivityelectrolyte (NaCl), the machining accuracy is better inNaNO3 compared to NaCl. The current distribution inNaNO3 is close to primary current distribution at lowvoltage. The current distribution in NaCl is close tosecondary current distribution at low voltage. Due tothe change of Tafel slope (and therefore the

(3) & (4)

(5)

1&2

T Tsteel

Tool

T T

steel

Tool


polarization resistance) in PECM [7-9], the secondary currentdistribution becomes more sensitive to the PECM waveform,which makes the machining accuracy more sensitive to thePECM waveform in NaCl.

For the same average voltage, a lower duty cycle and a higherpeak voltage results in a higher metal removal rate during theon-time. The mass transfer rate is also higher at a low dutycycle, which enhances the replenishment of the electrolyte andimproves the machining rate. At the same duty cycle, the lowerthe frequency, the longer the on-time period, and therefore thelonger the machining period, which also improves themachining rate.

CONCLUSIONS

Polarization studies can provide important and usefulinformation on the electrolyte properties for the ECM process.Based on polarization studies and ECM tests, the followingconclusions were obtained:1. The NaNO3 electrolyte will be selected to remove steel

core from titanium alloy casting due to a) the large windowof the breakdown potential between the steel and titaniumalloys, which will protect titanium alloy from dissolution,and b) a reasonable steel removal rate.

2. The higher the ∆I/∆E of the polarization curve, the highermachining rate that can be obtained in the ECM process,due to the less voltage drop between the tool and theworkpiece.

3. The machining rate and machining accuracy are moresensitive to PECM waveform parameters in the electrolytewith a high ∆I/∆E.

ACKNOWLEDGMENTS

Financial support for this work was provided under Air ForceContract No. F33615-97-C-5275. The support andencouragement of Mr. Carl M. Lombard of Wright PattersonAir Force Base is gratefully acknowledged.

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3. Davydov, A.D., and Kozak, J. (1990), High RateElectrochemical Shaping , Ed. Nauka, Moscow.

4. Bannard, J. (1976), J. Appl. Electrochem., Vol. 6, 477.5. Davydov, A.D. (1985), Soviet Surface Engineering and

Applied Electrochemistry, Vol. 2, 103.6. Chin, D-T. and Venkaatesh, S. (1981), J. Electrochem.

Soc., Vol. 128, 1439.

7. Chin, D-T. and Sethi, R. (1982), J. Electrochem.Soc., Vol. 129, 2677.

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investigation of electrochemical parameters

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