ORIGINAL ARTICLE

The effect of electrolyte current density on the electrochemicalmachining S-03 material

Lin Tang & Bo Li & Sen Yang & Qiuli Duan & Baoyin Kang

Received: 28 September 2013 /Accepted: 7 January 2014 /Published online: 25 January 2014# Springer-Verlag London 2014

Abstract Selecting an appropriate electrolyte is veryimportant for high-efficiency electrochemical machiningnovel S-03 special stainless steel aerospace component.A series of experiments were conducted with NaCl,NaNO3, and their admixture solutions. This researchfocused on the relationship between current efficiencyand current density. The current density effects onsurface roughness, machining velocity, and grainboundary corrosion were analyzed. The results showedthat: the current efficiency in NaCl electrolyte was100 % with different concentrations. Under the condi-tions of 24 V voltage, 30 °C electrolyte, and 0.8 MPaelectrolyte pressure, the 10 % NaCl electrolyte canobtain 3.6 mm/min cathode feed speed; the surfaceroughness is Ra 0.08 μm; and the material removalrate is 411.4 mm3/min. Comparing forward flow toforward flow with added backpressure, we found that:the surface roughness value decreased sharply at3.6 mm/min in NaCl electrolyte.

Keywords Electrochemical machining (ECM) . Electrolyteconcentration . Current density . Current efficiency . Surfaceroughness .Material removal rate . Forward flow

1 Introduction

Electrochemical machining (ECM) has the merit of highefficiency, good surface quality, no stress, and no cath-ode wear, and it has become one of the best methodsfor manufacturing the complex parts in aerospace, mil-itary equipment, and automobile industry [1–8]. Muchresearch has focused on how to improve machiningefficiency and precision, reduce surface roughness, op-timize machining parameters, and combine ECM withother technologies to heighten machining performance.Ultrasonic vibration-assisted ECM [9] and magnetic field-assisted ECM [10, 11] can both improve machining precisionand quality. Electrochemical discharge machining (ECDM)technology was used to machine steel difficult to cut [12].Solar cell material was processed by electrochemical mechan-ical polishing [13]. Electrochemical drilling machining, pulseECM, and electrochemical micromachining are all used in themanufacturing industry [14–19]. AsWu [20] found, flow fieldis one of the important factors in ECM. Using the finiteelement method in ECM can reduce costs, shorten the cathodedesign circle, and improve machining efficiency [21]. Theinterelectrode gap has been studied by many scholars becauseit is also important in achieving ECM precision machining[22–24]. Titanium alloy, nickel-based material, and tungstencarbide alloy can be processed using ECM and have a highefficiency, but using other machining method can be hard ordifficult to process [25–29]. All kinds of complex-shapedaerospace component were manufactured by ECM method[2, 7, 23, 30]. Electrolyte type, electrolyte flow field, andelectrolyte concentration are important factors for precision,

L. Tang (*) : B. Li :Q. Duan : B. KangShaanxi Key Laboratory of Non-Traditional Machining, Xi’anTechnological University, 2 Xuefu Road, Weiyang District,Xi’an 710021, People’s Republic of Chinae-mail: tang_lin168@yahoo.com

L. Tange-mail: tang_lin168@126.com

L. TangChangzhou Institute of Technology, Jiangsu Key Laboratory ofDigital Electrochemical Machining, Tongjiang SouthRoad 299,Changzhou 213002, People’s Republic of China

S. YangAVIC XI’AN AERO-ENGINE (GROUP) LTD., Weibin Road,Weiyang District, Xi’an 710021, People’s Republic of Chinae-mail: ys6616@126.com

e-mail: steven791025@126.com

Q. L. Duan · B. Y. Kang

Int J Adv Manuf Technol (2014) 71:1825–1833DOI 10.1007/s00170-014-5617-x

material removal rate, and surface topography [15, 20,31–36]. To choose appropriate process parameters, scholarshave conducted parameters optimization research [37–40].Nickel, iron, and other metals were studied in ECM aboutdissolution efficiency [41–44], current efficiency at differentgaps [45], pulsed current [46], electrolyte concentration, pH,and temperature on surface brightening [47–51] by manyscholars. Oxide film and microfeatures were also researchedin ECM [52, 53].

Although many scholars have performed machiningmaterials using ECM, few papers are concerned withECM electrolyte characteristics in novel S-03 stainlesssteel material. More studies are therefore needed tomake ECM S-03 technology applicable to actual prod-ucts. The purpose of this research is focused on therelationship between current efficiency and current den-sity. Furthermore, we have done a series of experimentsto find the relationship between cathode feed speed andmaterial removal rate, research the surface topographiesin different electrolytes, and study the effects of currentdensity on surface roughness, machining velocity, andgrain boundary corrosion. Finally, we research the rela-tionship between surface roughness and electrolyte flowpattern. We found that under the conditions of 24 V,30 °C electrolyte, and 0.8 MPa electrolyte pressure, the10 % NaCl electrolyte with forward flow-addedbackpressure can achieve the surface roughness valuedecreased sharply at 3.6 mm/min cathode feed speed.The surface roughness can reach Ra 0.08 μm, and thematerial removal rate was 411.4 mm3/min.

2 Experiment

2.1 Experimental material

S-03 material is widely used in gas generators, liquidoxygen pumps, and other significant aerospace parts.

The composition of S-03 stainless steel is shown inTable 1. Its composition and structure satisfy the re-quirements of both high and low temperatures. BecauseS-03 stainless steel contains titanium (Ti), molybdenum(Mo), nickel (Ni), and silicon (Si), it requires differentdissolution voltages for its electrochemical machining.Under the same voltage, these four elements have dif-ferent dissolution velocities; this causes short circuitsand creates sparks during the little machining gap.Therefore, choosing the appropriate electrolyte iscritical.

2.2 Electrochemical machining electrolyte

NaNO3, NaClO3, and NaCl are used as electrolyte so-lution. The NaCl electrolyte has stray corrode; it is alinear electrolyte. NaClO3 and NaNO3 are blunt nonlin-ear electrolytes. NaClO3 is more expensive than NaNO3

and NaCl. Therefore, NaCl and NaNO3 and their mix-ture in aqueous solutions with different concentrationswere used in machining S-03; the experimental resultsare shown in Table 2.

2.3 Electrochemical machining equipment

The characteristics of ECM are low voltage, high current,and high-speed electrolyte flow. The ECM system includesan ECM machine tool, which is designed and manufacturedby Xi’an Technology University; electrolyte pumps (China,Xi’an DaXing company); and the DC/pulse ECM powersupply voltage range from 0 to 30 voltage, the pulsefrequency is from 0 to 2,000 Hz, and the biggest currentcan reach 3,000 A (China, YangZhou, ShuangHong Co.,Ltd.). The filter was used to prevent impurity substancefrom entering into the machining zone in order to make theclean electrolyte into the machine tool; we select the freshfilter that is 20 μm, which is manufactured by ShanghaiHengtai Filter Equipment Co., Ltd. The electrolyte poolvolume is 2,000×1,500×1,200 mm, which is made of316 stainless steel because 316 stainless steel can protectthe pool from electrolyte to corrode, especially in NaClelectrolyte. The heat exchanger control system was usedto heat the electrolyte to a certain temperature; in ourexperiment, we keep the electrolyte temperature at 35 °C.The whole ECM system is shown in Fig. 1.

During the whole process, the work piece was sta-tionary. The cathodes moved up and down along theprincipal axis. The designed electrochemical machining

Table 1 Chemical compositionof S-03 stainless steel (weightpercent)

Element C Si Mn S P Al Cr Ni Mo Ti Fe

Percentages 3 0.15 0.15 0.6 0.8 0.2 0.11 9 0.5 0.15 Balance

Table 2 Electrolytes’ composition and their machining speed reached

Electrolyte composition Cathode feed speed

10 % NaNO3 0.1–1.2 mm/min

5 %NaCl+5 %NaNO3 0.1–2.0 mm/min

10 %NaCl+5 %NaNO3 0.1–2.4 mm/min

10 %NaCl 0.1–3.6 mm/min

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cathodes, which are depicted in Fig. 2, are made of redcopper or stainless steel.

Under the same conditions of 24 V voltage application,30 °C electrolyte temperature, and 0.8 MPa electrolyte pres-sure, four types of electrolytes with different concentrationswere carried out in machining S-03 material from a low feedspeed to the highest speed reached. The machined holes aredisplayed in Fig. 3.

3 Results and discussion

From Table 1, we can see that S-03 material containsTi, Mo, Ni, Cr, Fe, and Mn elements, and differentelements’ dissolution need a different electrode poten-tial. At the same application of voltage, these elementshave different dissolve speeds, therefore, selecting ap-propriate electrolyte and concentration is significant. Wecan see this from the following equations. E0 denotesstandard electrode potential. If the metal has a lowerstandard electrode potential, it can more easily loseelectrons and oxidation. Conversely, if the metal has ahigher standard electrode potential, it can more easilyobtain electrons and be reduced. That is to say, thestandard electrode potential level determines the orderof the element electrode reactions. In the ECM process,

we can see that the Mn element is the first reaction tooccur in the negative electrode, and H+ is the firstreaction to occur in the anode.

Ni→Ni2þ þ 2e;E0 ¼ −0:23V ;Fe→Fe2þ þ 2e;E0 ¼ −0:44V ;Fe→Fe3þ þ 3e;E0 ¼ −0:036V ;Fe2þ→Fe3þ þ e;E0 ¼ þ0:771V ;Cr→Cr2þ þ 2e;E0 ¼ −0:86V ;Cr→Cr3þ þ 3e;E0 ¼ −0:74V ;Cr2þ→Cr3þ þ e;E0 ¼ −0:41V ;2Cr3þ þ 7H2O→Cr2O7

2− þ 14Hþ þ 6e;E0 ¼ þ1:33V ;Mo→Mo3þ þ 3e;E0 ¼ −0:2V ;Tiþ 2H2O→TiO2 þ 4Hþ þ 4e;E0 ¼ −0:86V ;Mn→Mn2þ þ 2e;E0 ¼ −1:05V ;2Hþ þ 2e→H2;E

0 ¼ 0V ;4OH−−4e→2H2Oþ O2;E

0 ¼ þ0:401V ;2Cl−−2e→Cl2;E

0 ¼ þ1:3583V ;

Thus, different concentrations of electrolyte machin-ing S-03 have different dissolution speeds and materialremoval rates. The relationship between cathode feed

Fig. 2 Electrochemical machining cathode physical map

Fig. 1 Electrochemical machining system physical map

Fig. 3 Workpiece machined holes physical map

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speed and material removal rate is shown in Fig. 4. Wecan see that 10 % NaCl electrolyte has the fastestmachining velocity; the cathode feed speed can reach3.6 mm/min, which is three times that of the 10 %NaNO3 electrolyte. 5 % NaCl and 5 % NaNO3 elec-trolyte composition machining S-03 can reach 2.0 mm/min; 10 % NaCl and 5 % NaNO3 electrolyte compo-sition can reach 2.4 mm/min. Comparing these twoelectrolyte compositions, we can see that with the in-crease of NaCl concentration, the cathode feed rate of10 % NaCl and 5 % NaNO3 electrolyte composition isfaster than the 5 % NaCl and 5 % NaNO3 electrolytecomposition. That is because NaCl is a linear electro-lyte; its current efficiency is 100 %. When adding 5 %NaNO3 in 10 % NaCl, the electrolyte composite currentefficiency is less than 100 %. This reason can explainwhy the 10 % NaCl machining speed is faster than10 % NaCl and 5 % NaNO3 electrolyte composition.

Different elements have diverse dissolution velocities.During the machining period, flow mark, short-circuiting, and sparks phenomena occurred, as shownin Fig. 5. From Fig. 5a, we can clearly see the flowmark in the side wall of the machined work piece. Thestray corrosion is also clearly visible. This is because

NaCl has corrosive characteristics. Using the NaNO3

electrolyte process, S-03 can easily cause short-circuiting and sparks, as shown in Fig. 5b, c. Thefastest machining speed is 1.2 mm/min in 10 % NaNO3

electrolyte. When adding a moderate amount of NaCl tothe NaNO3 solution, the machining speed can greatlyimprove. Now, we will analyze the reason and explainthis phenomenon from the perspectives of current den-sity and efficiency.

According to Faraday’s law, m represents the chemicalelement quality changes which is gained by:

m ¼ A

nFIt ð1Þ

where F denotes the constant of Faraday’s 96,500 C, I is theelectric current (A), t is the dissolution time, A is the element’satomic weight, n denotes the element’s ion valence, ŋ denotescurrent efficiency, i is the current density, ω is the volumeelectrochemical equivalent, and v indicates cathode feedspeed. M expresses S-03 material removal quality, ρ showsthe density of S-03 material, and S is the workpiece machinedarea; here, it is 2 cm3.

v ¼ ηωi ð2Þ

η ¼ M

ωρItð3Þ

i ¼ I

Sð4Þ

V ¼ M

ρð5Þ

From the experimental data, current efficiency anddensity values are calculated using Eqs. 3 and 4, re-spectively. The relation between current efficiency anddensity is depicted in Fig. 6. We can see that the NaClelectrolyte has the highest current efficiency and that

Fig. 4 The relationship between cathode feed rate and material removalrate

Fig. 5 Flow mark, short-circuiting, and sparks

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the value is almost 100 %, and sometimes it canexceed 100 %. It almost has no change with differentcurrent densities. That is to say, the current density hasalmost no effect on current efficiency in the NaClelectrolyte. Because NaCl is a linear electrolyte, itcan be completely dissolved in a water solution. Butfor the other three electrolytes, the results are different.

With the increasing of current density, the order ofcurrent efficiency value from high to low is 10 %NaCl and 5 % NaNO3 electrolyte composition, 5 %NaCl and 5 % NaNO3 electrolyte composition, and10 % NaNO3 electrolyte. When the current density isup to a certain value for these three electrolytes, thecurrent efficiency will remain steady and have nochange. We also found that as the increasing of cath-ode feed speed, the current density of these threeelectrolytes are also improved. The reason can be ex-plained by Eq. 2. When the current density is 120 A/cm2, 0.45, 0.75, 0.9, and 1.0 are the current efficiencyof 10 % NaNO3 electrolyte, 5 % NaCl and 5 %NaNO3 electrolyte composition, 10 % NaCl and 5 %NaNO3 electrolyte composition, and 10 % NaCl elec-trolytes, respectively. According to Eq. 2, we can cal-culate the machining velocities of these four electro-lytes. Why did the 10 % NaCl electrolyte currentefficiency exceed 100 %? The most likely reason isthat some metals or nonmetal materials are droppedfrom the workpiece material for high-speed flowingelectrolyte, which makes the actual material erosionmass larger than the theoretical value. The results arein agreement with [54, 55].

Fig. 6 The relationship between current efficiency and density

Fig. 7 Surface topographies of different electrolytes. a 10 %NaCl, b 5 % NaCl and 5 % NaNO3, c 10 % NaNO3, and d 10 % NaCl and 5 % NaNO3

Int J Adv Manuf Technol (2014) 71:1825–1833 1829

The machined workpiece surface topographies in theabove four electrolytes are shown in Fig. 7. We can seethat different electrolytes and concentrations have di-verse topographies. These are obtained under the condi-tion of 24 V voltage, 0.9 mm/min machining speed, and0.8 MPa electrolyte pressure. There are significant dif-ferences between panels a and c of Fig. 7. ComparingFig. 7b, d, we can see that as the NaCl quality fractionin the electrolyte composition increases, the grainboundary corrosion becomes more obvious in the workpiece surface.

Using NaNO3 electrolyte machining S-03, the surfacetopographies with different current densities are shown

in Fig. 8. From Fig. 8a, we can see that the surfaceroughness is poor at the current density of 72.5 A/cm2.That is because different metal elements have differentdissolution velocities under the condition of low currentdensity. As the current density increases, the surfaceroughness becomes better. The pitting and intergranularcorrosion also tend to disappear. When the current den-sity is 113 A/cm2, however, the surface roughness valueincreases. From Eqs. 6, 7, and 8, we can see that theelectrode gap is small when the cathode feed speedreaches a high value. It will lead the flow field

Fig. 8 The relationship betweencathode feed rate and materialremoval rate. a i=72.5 A/cm2, bi=88.5 A/cm2, c i=101 A/cm2,and d i=113 A/cm2

Fig. 9 Schematic diagram of forward flow with added backpressure. 1workpiece, 2 cathode, 3 piezometer, 4 pressure-regulating valve, 5 workbox, 6 electrolyte cell, and 7 heat exchanger

Fig. 10 The relationship between surface roughness and electrolyte flowpattern

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instability in forward flow and make the surface rough-ness value improved, which is shown in Fig. 8d.

i ¼ ηωσΔ

UR ð6Þ

ηωσUR ¼ C ð7Þ

Va ¼ C

Δð8Þ

where UR is ohm voltage of the electrolyte, σ is the electrolyteconductivity, C is a constant, and Δ is the interelectrode gap.

From the viewpoint of machining efficiency, the 10 %NaCl electrolyte is the best choice. But when the machiningspeed is 3.6 mm/min, surface roughness gradually increases.We adopted an electrolyte flow pattern of forward flow withadded backpressure to make the flow field even. The sche-matic diagram is shown in Fig. 9. We mainly adjusted theelectrolyte outlet pressure to improve the flow field steady.

Through this simple design, we can obtain good surfaceroughness at a high material removal rate. Comparing forwardflow to forward flow with added backpressure, we can betterunderstand the similarities and differences. As the cathode feedvelocity increases, the surface roughness value gradually de-creases. However, when the speed exceeds 3.2 mm/min, thesurface roughness gradually increases in forward flow. But inthe pattern of forward flow with added backpressure, the sur-face roughness value still decreased. The results are shown inFig. 10. Since the machining speed is increasing, the electrodegap becomes narrow. Under the condition of forward flow, theflow field is unstable, which makes the electrolyte unevenbetween the cathode and the workpiece. The flow field is radialwith a constantly decelerating electrolyte flow as it movesoutward. So the flow marks make the surface roughness valuerise slightly. However, with the electrolyte flow pattern offorward flow with added backpressure, the electrolyte flowfield is so stable and even the interelectrode gap becomessmaller and smaller. The machining precision and the surfaceroughness become better and better. The experimental valuesagree with the results depicted by R Chin et al. [56, 57].

4 Conclusion

The novel S-03 stainless steel material is widely used in allkinds of aerospace engines for its performance and cost. Tomaintain a highmaterial removal rate, electrolytes of 10%NaCl,10 % NaNO3, 5 % NaNO3 and 5 % NaCl, and 10%NaCl and5%NaNO3were used to study the processing characteristics ofS-03 material. In order to improve the surface roughness of themachined work piece, the forward flow with added

backpressure pattern was also discussed. The following con-clusions can be obtained from the research outcomes:

(1) S-03 material contains complex elements that have di-verse dissolution velocities; dissolving these elementstherefore requires diverse electrode potential.

(2) The NaCl electrolyte has the highest current efficiency.The current density has almost no effect on the currentefficiency of the NaCl electrolyte. As the current densityincreases, the current efficiency gradually increases forother electrolytes in a certain range.

(3) For a certain current density, the order of current effi-ciency value from high to low is 10 % NaCl electrolyte,10%NaCl and 5%NaNO3 electrolyte composition, 5 %NaCl and 5 %NaNO3 electrolyte composition, and 10%NaNO3 electrolyte.

(4) As the cathode feed velocity increases, the surfaceroughness value gradually decreases. In adopting anelectrolyte flow pattern of forward flow with addedbackpressure when machining the S-03 work piece, thebest surface roughness can reach Ra 0.08 μm at a speedof 3.6 mm/min.

Some limitations of this paper are that we could not studythe effect of electrolyte temperature on current efficiency andelectrolyte conductance. In the future, we will purchase aprecision temperature transducer and heat exchanger to keepthe electrolyte temperature stable during the process. ButNaCl electrolyte will affect the non-machined zone and ma-chine tool and how to protect them needs to do further re-search. Despite its limitations, this paper does indicate thatusing the 10 % NaCl electrolyte with an electrolyte flowpattern of forward flow with added backpressure is a simpleand feasible method to increase the material removal rate andsurface roughness. It is helpful to promote this technology,which is widely used in actual products.

Acknowledgments The work is partially supported by a grant fromShaanxi Provincial Department of Education Fund (grant no.2013JK1014) and Supported by Open Research Fund Program ofShaanxi Key Laboratory of Non-Traditional Machining (grant no.ST-11001). Jiangsu Key Laboratory of Digital Electrochemical Machin-ing, Changzhou Institute of Technology, (grant no. KFJJ2004009). Theauthors would like to thank professor Fan of Xi’an TechnologicalUniversity for her helpful suggestions as well as Mr. Zhao for supplyingthe materials for the workpieces. We would also like to thankMs Mu andMr. Li for their continuous support for this research. Thank you thoseanonymous reviewers and the magazine people.

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