Effect of Anodic Behavior on Electrochemical Machining of...

Electrochimica Acta 233 (2017) 190–200

Effect of Anodic Behavior on Electrochemical Machining of TB6Titanium Alloy

Weidong Liua, Sansan Aoa, Yang Lia, Zuming Liua, Hui Zhanga,Sunusi Marwana Manladanb, Zhen Luoa,c,*, Zhiping Wangd

a School of Materials Science and Engineering, Tianjin University, Tianjin 300350, ChinabDepartment of Mechanical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur 50603, MalaysiacCollaborative Innovation Center of Advanced Ship and Deep-Sea Exploration, Shanghai 200240, Chinad School of Aviation Engineering, Civil Aviation University of China, Tianjin 300300, China

A R T I C L E I N F O

Article history:Received 9 November 2016Received in revised form 3 March 2017Accepted 4 March 2017Available online 6 March 2017

Keywords:Jet electrochemical machiningAnodic behaviorNovel simulation modelNozzle travel rate optimizationTB6 titanium alloy

A B S T R A C T

Jet electrochemical machining is a promising shaping method that has the potential to replace traditionalsinking ECM in the industry owing to its flexibility and stability. However, its machining quality isimpaired by the occurrence of a unique phenomenon (i.e., the anode cannot be uniformly dissolved withhigh nozzle travel rate). This paper aims to analyze this unique phenomenon from the viewpoint ofanodic behavior and to select the optimum travel rate parameter window. The anodic behavior of TB6titanium alloy in sodium chloride solution was investigated by linear sweep voltammetry, cyclicvoltammetry, electrochemical impedance spectroscopy and chronoamperometry. A time-dependentmechanism of pitting/large localized corrosion/polishing for the anodic interface structure wasdeveloped. The full electricity quantity of the equivalent oxide layer capacitor, Q, was proposed as acriterion for the initiation of uniform dissolution state. Subsequently, a novel model that incorporates theparameter Q was developed to account for the effect of anodic behavior. This model is able to predict themachined profile as well as the localized corrosion range. Finally, the effect of nozzle travel rate on jetECM performance was investigated, and a nozzle travel rate of 10 to 25 mm s�1 was selected as theoptimum parameter window in the present work.

© 2017 Elsevier Ltd. All rights reserved.

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1. Introduction

Electrochemical machining (ECM) is a widely used shapingtechnology in various industries. In ECM, a controlled anodicdissolution is used to locally remove materials from the workpieceto obtain the desired shape [1]. Therefore, it has several inherentadvantages, such as no thermal-mechanical effect, relatively highmachining rate irrespective of material hardness, almost no toolwear, and excellent machined surface quality [2–4]. GeneratingECM is an important variant of traditional ECM [5]. It greatlyimproves the process flexibility and stability by combining the

* Corresponding author at: No.31 Building, Room 291, School of Materials Scienceand Engineering, Tianjin University-Peiyang Park Campus, Tianjin 300350, China.Tel.: +86 02227406602.

E-mail addresses: [email protected] (W. Liu), [email protected] (S. Ao), [email protected] (Y. Li), [email protected] (Z. Liu), [email protected](H. Zhang), [email protected] (S.M. Manladan), [email protected] (Z. Luo), [email protected] (Z. Wang).

http://dx.doi.org/10.1016/j.electacta.2017.03.0250013-4686/© 2017 Elsevier Ltd. All rights reserved.

technology features of computer numerical control (CNC) and ECM[4,5].

In practical industrial application, ECM is usually applied tomachine difficult-to-cut materials. Among them, titanium alloy isan excellent material for diverse applications [6,7]. Therefore,many studies have been published on the ECM of different titaniumalloys, including pure Ti [8,9], TC4 [10–12] and g-TiAl [11,13,14].TB6 (ASTM Ti1023) is a novel near b titanium alloy that has thepotential to replace conventional titanium alloys in the aircraft andaerospace industry owing to its lower density but equal strength[15]. However, unfortunately, little is known about the ECM of TB6titanium alloy.

As the basic mechanism of ECM, anodic behavior is the keyfactor governing ECM machinability [1–4,16]. Numerous studieshave focused on the anodic mechanism in ECM of differentmaterials, including Fe [17–19], Al [20], Cu [21,22], Ni [23], Co[24,25], and their alloys [26,27]. However, very little work has beenconducted on titanium and its alloys [11,28–30]. Some of theseresults can be used as hints for the investigation of the anodicbehavior of TB6 titanium alloy.

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Fig. 1. Schematic illustration of electrolytic cell for electrochemical measurements.

Table 2

W. Liu et al. / Electrochimica Acta 233 (2017) 190–200 191

In the generating ECM process, the current density at any pointof the anodic surface dramatically changes within a short timeowing to the quick movement of the electrodes [5,31–34]. Thiseffect results in inferior surface quality or even machining failureas observed in our previous research [15], even though the currentdensity meets the ECM conditions (30�100 A cm�2) [1]. Thisunique phenomenon has also been observed in rotate-print ECM[29], vibration assisted ECM [30], and jet ECM [31]. A possiblereason for this phenomenon is the time-dependent anodicdissolution process [18]. However, almost all the previous studiesof the ECM mechanism only focus on the final stable dissolutionstate, regardless of the effect of transient behavior in the earlystage. Thus, this mechanism cannot adequately explain the uniquephenomenon found in generating ECM.

This paper aims to analyze this unique phenomenon from theviewpoint of transient anodic behavior. Jet electrochemicalmachining is a typical generating ECM method, in which anelectrolyte jet is used as the tool to achieve the desired materialremoval [32]. With about thirty-years of development, jet ECM hasproven to be a promising shaping method. Compared with othergenerating ECM methods, it offers improved machining accuracy,process stability, and flexibility [33,34]. Therefore, in this work jetECM was used to investigate the unique phenomenon. Electro-chemical characterization methods, including linear sweepvoltammetry, cyclic voltammetry, electrochemical impedancespectroscopy, and chronoamperometry were used to investigatethe anodic behavior. Based on the obtained results, a three-dimensional simulation that takes the anodic behavior intoaccount was developed to predict the effect of the nozzle travelrate on the machining performance. Experiments were performedto verify the proposed simulation model and select the optimumnozzle travel rate parameter window.

2. Experimental details

All the anodic materials used in the present study were made oftraditionally forged TB6 (ASTM Ti1023) titanium alloy. Table 1 liststhe chemical composition of this alloy. Bulk TB6 materials wereprovided by the research group of Chen et al. [35] at BeijingUniversity of Aeronautics and Astronautics, and were cut intosquare-shaped samples with a length of 15 mm and thickness of3 mm by wire-electrical discharge machining.

Electrochemical properties were measured using an electro-chemical workstation (LK3200A, Lanlike Chemical ElectronicHigh-Tech Co. Ltd., Tianjin, China) with a modified three-electrodesystem, as schematically shown in Fig. 1. Considering the quickflowing electrolyte in ECM, an electrolyte circulation system wasadded to the electrolytic cell used. A platinum sheet (10 � 10 � 0.1mm3) and a saturated calomel electrode (SCE) acted as counterelectrode (CE) and reference electrode (RE), respectively, while theTB6 titanium alloy sample was the working electrode (WE). Toreflect the high current density in ECM [1], the WE samples werefixed 600 mm away from the CE using an electrode clamp. Toguarantee a safe working current range for the electrochemicalworkstation under high current density, most surfaces of theworking electrode were insulated with epoxy resin and only asurface area of 1 mm2 was exposed to the electrolyte. Based on ourprevious research [15], 15 wt% NaCl was used as the electrolyte

Table 1Chemical composition of TB6 titanium alloy.

Composition Main Impurity

Fe Al V Ti H O N

Content (wt %) 1.93 2.95 10.15 Bal. 0.01 0.03 0.03

(pH = 7) in the present experiments. The NaCl solution wasprepared using analytical grade NaCl and deionized waterprovided by Tianli Chemical Reagents Co. Ltd., Tianjin, China.

The jet ECM tests were conducted on a commercial ECMmachine (PHPECM-2000HSP, Huidie Automatic Technology Co.Ltd., Dongguan, China), consisting of a XYZ axis mechanicaldisplacement system, an electrolyte circulation system, and adirect current supply and monitor system. More details of the ECMmachine are available in the literature [15]. Table 2 summarizes theexperimental conditions used for the jet ECM.

After the experiments, the surface topography of the samplewas observed using scanning electron microscopy (SEM; S4800,Hitachi, Japan). The chemical composition of the corroded surfacewas analyzed with energy dispersive spectrometry (EDS; GenesisXM2, EDAX, US). The cross-sectional profiles of the jet ECMmachined grooves were measured with a 3D laser microscope(OLS4100, Olympus, Japan).

3. Simulation considering the effect of anodic behavior

Although there are some difficulties in modeling ECM processowing to its complex multi-physical fields and chemical reactions,simulation is considerably important for such a non-contactmachining technology to predict and control the machined shape[36]. In the present study, the anodic behavior has a significantimpact on the machining performance. For this reason, a three-dimensional simulation model that takes the effect of anodicbehavior on jet ECM into account is proposed in this work.

3.1. Mathematical model

The anodic behavior is closely dependent on the current densityand its acting time [16,18]. Therefore, the determination of currentdensity distribution over time is of primary interest in this work.The generation, transfer and removal of ions, heat, and bubblesmay be neglected. The electrolyte conductivity is considered to be

Machining conditions for jet ECM tests.

Process parameters Value

Applied voltage /V 24Machining gap /mm 0.6Nozzle inner diameter /mm 2Electrolyte flow rate /L min�1 2.1Nozzle travel rate /mm s�1 250, 100, 50, 25, 10

192 W. Liu et al. / Electrochimica Acta 233 (2017) 190–200

constant and equal in both the interface layer and bulk electrolyte.Also, for the sake of model simplicity, real-time anodic shapeevolution has not been taken into account.

The potential distribution in the electrolyte domain isexpressed using Laplace's equation,

r2’ ¼ @2’

@2xþ @

2’

@2yþ @

2’

@2z¼ 0 ð1Þ

where ’ represents the potential at any point in the electrolytedomain. The electrode surfaces are considered as equipotentialsurfaces. Therefore, constant potentials Ua and 0 V are set as theanode boundary (’a = Ua= 24 V) and cathode boundary (’c = 0 V)conditions, respectively. All other surfaces are virtual boundaries,taken as electrically insulated interfaces, through which no currentdensity passes (n�j = @’/@n = 0).

Based on this potential model, the current density can becalculated by solving Ohm's law. Especially, the normal currentdensity passing through the anode interface is of critical interest,and can be written as,

jn ¼ �ðkr’Þ � n ð2Þwhere k is the electric conductivity of the electrolyte, which isconsidered uniform, and n represents the unit normal vector onthe electrode surfaces. It is known that the normal current densityconsists of two different parts with different effects, namely, afaradaic current density, jf, and a non-faradaic current density, jnf.Thus,

jn ¼ jf þ jnf ð3ÞNon-faradaic current density is associated with the breakdown

of the oxide layer and establishment of a desirable anodic interfacestructure for the subsequent dissolution of the base metal [18].Weinmann et al. [11] characterized the oxide layer of titaniumalloy as a conventional RC circuit, based on the single-layer modeldeveloped by Gonzalez et al [29]. Although the impedance of anequivalent RC circuit slightly varies during polarization, thespecific capacity, C, can be regarded as constant [20]. Therefore,the non-faradaic current density can be expressed using thecharging current density of a capacitor with unit area,

jnf ¼dQdt

¼ CdVpoldt

ð4Þ

where Q is the full electricity quantity of the oxide layer capacitor,which is also taken as a constant, and Vpol is the over potential.

Fig. 2. Geometry model; (a) Schematic illustration of cell structu

Faradaic current density has a close relationship with basemetal dissolution. According to Faraday's law, the rate of anodicmaterial removal is directly proportional to the faradaic currentdensity. Thus, the amount of local anodic material removed at anytime, t0 can be calculated by time integration of Faraday's lawequation,

ha ¼Z t00

hvjf dt ð5Þ

where ha is the local materials removal (i.e., the machined depth)of any small element on the anodic surface, h is the currentefficiency, and v is the volumetric electrochemical equivalent ofTB6 titanium alloy. Using Eq. (5), the evolution of the anodic shapewith time can be predicted.

3.2. Preset conditions

Based on the typical geometric structure of the cell in jet ECM, ageometry model with necessary domain and boundaries (as shownin Fig. 2) is built up. This geometry model consists of an electrolytedomain V, two electrode boundaries (cathode boundary, G2 andanode boundary, G10) and eight virtual boundaries (G1, G3, G4, G5,G6, G7, G8, and G9). The corresponding mathematical modeldescription for these domain and boundaries is given in Section 3.1.

As a generating ECM process, the cathode tool (i.e., the nozzle)in jet ECM moves along a pre-designed orbit over the anodeworkpiece and achieves specific local electrochemical dissolution[33,34]. To track the movement of the nozzle and electrolyte jet,the corresponding cathode boundaries (G1) and virtual boundaries(G3, G4, G5) need to be adjusted timely. Therefore, the Lagrangian-Eulerian (ALE) method, which is able to move boundaries withoutmoving the mesh, is applied to track these moving boundaries atspecified nozzle travel rates.

The anodic behavior of titanium alloy is a transition process thatcan be typically divided into two time-continuous stages, i.e.,breakdown of the oxide layer and subsequent dissolution of thebase metal [9–11,15]. As mentioned above, an equivalent capacitormodel can be used to describe this anodic behavior [10]. Thecharged electricity quantity at any specific time t0 can be calculatedby time integration of the non-faradaic current density,

Qt0 ¼Z t00

jntdt ð6Þ

Note that the oxide layer is totally removed in the early stage, astable dissolution with the desired interface structure (i.e., metal/

re for jet ECM experiments; (b) Extracted geometry model.

Fig. 3. Relationship between breakdown time and current density.


supersaturated film/electrolyte [16]) can be obtained. Therefore, itis logical to regard the full charging of the capacitor as the startingsignal of base metal dissolution. In this simulation, Q works asvalve value to control the current efficiency h and thus determinewhether the base metal dissolution process begins,

h ¼ 0; if Qt0 < Q1; if Qt0 � Q� �

ð7Þ

where Q is the full electricity quantity of the oxide layer capacitor.For simplicity, Q is regarded as a material property constant, whichcan be obtained by electrochemical characterization. CombiningEqs. (6) and (7), the relationship between oxide layer breakdowntime, tb, and current density (i.e., the time integration of thecurrent density equals the constant Q) can be obtained. In the caseof constant current density, the relationship is simplified to aninverse proportion function as shown in Fig. 3. It is important tonote that smaller current density requires longer breakdown time,as reported in the literature [31].

For all cases, simulation software (COMSOL 4.4) was applied tocalculate the results for the corresponding experimental con-ditions: v = 2.757 m3C�1, k = 16 S m�1, Q = 492 C cm�2.

Fig. 4. Polarization curve of TB6 titanium alloy in NaCl electrolyte; (a) Linear sweep voltamearly stage of polarization.

4. Results and discussion

4.1. Anodic dissolution behavior

Titanium has strong self-passivation ability and easily forms atitanium oxide layer, preventing the base material from dissolving[11]. However, the breakdown of this oxide layer can result intranspassive dissolving of base metal [37]. It is, therefore, crucial tocharacterize the oxide layer breakdown effect.

4.1.1. Linear sweep voltammetry (LSV) and cyclic voltammetry (CV)As a preliminary investigation of polarization, LSV and CV

studies were conducted on the TB6 alloy in NaCl solution. The LSVcurve obtained upon linear increase of current density is shown inFig. 4(a). The CV plot in Fig. 4(b) exhibits the details of the earlystage of polarization.

It can be observed from Fig. 4(a) that the LSV curve contains tworegions, i.e., a passive region at low current density and atranspassive region at high current density. The forward scan ofcyclic polarization curve in Fig. 4(b) shows an active-passive-transpassive transition behavior. When the voltage is above�0.75 V, the current density suddenly increases and reaches17.03 � 10�6 A cm�2 at �0.12 V, limiting the active region. In thevoltage range of �0.12 to 4.7 V, the current density remains in thepassivation range (i.e. 7.62 to 26.07 � 10�6 A cm�2), which isrelated to the formation of one or more protective films [11,28].When the voltage is increased beyond 4.7 V, the slope of theforward polarization curve increases clearly and shows an ohmicbehavior, indicating the oxide layer breaks down [11,28] and theelectrolyte resistance becomes mainly controlled. The reverse scanof the CV curve comes above the forward scan, forming a positivehysteresis hoop. This indicates the susceptibility of TB6 alloy topitting in NaCl solution [38]. The surfaces of the samples after thepolarization studies indeed exhibit pits, verifying the analysis of CVcurve. Jiang et al. [39], Sazou et al. [40] and some early researchers[41–43] have also reported that in the presence of Cl� titaniumlosses its immunity and localized attack (or pitting) commencesrelatively easily, reversely to the type of behavior widely observedfor the passivity on metals and alloys [44–46].

4.1.2. Chronoamperometry (CA)Chronoamperometry was performed to characterize the

structural evolution of anodic interface and to determine the

metry with linearly increasing current density; (b) Cyclic voltammetry curve of the

Fig. 5. Chronoamperometry plots obtained at different voltages.


significant parameter Q for simulation. Because the interfacestructure under ECM conditions was of interest, only high voltagerange (i.e., current density i > 10 A cm�2) was considered.

Figure 5 shows the current-time curves obtained at appliedvoltages of 15, 20, and 25 V. All the curves exhibit a sharpdecreasing tendency in the early stage. This may be caused by theequivalent capacitor effect [47]. After this stage, all the curvesreach a plateau of current density (i.e., the current density remainssteady with time) and then exhibit a slight decreasing tendency.This behavior corresponds to the formation of an oxide layer on theinitial anodic surface. Owing to the presence of oxide layer, theanodic material dissolves with difficulty and consequently theinter-electrode gap remains almost constant, leading to the plateaustage. Finally, the oxide layer is totally removed, and the interfacestructure for a stable dissolution state forms. Hence, the base metalis quickly dissolved, thereby increasing the inter-electrode gap andso decreasing the current density. It is, therefore, logical to consider

Fig. 6. Surface topography of dissolved samples at different

the time of the curve inflection point as the time of the oxide layerbreakdown. The surface topography of specific areas of the anodeat different times under a voltage of 25 V was observed using SEMand the chemical composition of these areas was measured by EDS,as shown in Figs. 6 and 7.

As shown in Fig. 6(a), severe localized corrosion occurs on theanodic surface in the early stage. Detailed observation reveals thatmany small pits are present on the oxide layer surface, leading to ahoneycomb pit structure. With increased dissolving time, theoxide layer is gradually removed and lager surface of base metal isexposed to the electrolyte. The appearance of the surface is veryrough, as shown in Fig. 6(b). Finally, when the dissolution reaches astable state, the anodic surface become totally clean, even, andsmooth, as shown in Fig. 6(c). To speculate on the interfacestructure, the chemical composition of the locally corroded area(Region A), oxide layer (Region B), and uniformly dissolved surface(Region C) was characterized by EDS. The results of the EDSanalysis, shown in Fig. 7(a), indicate that the amounts of oxygen,sodium, and chlorine present in the locally corroded area areconsiderably high compared with the composition of the as-received TB6 alloy. This suggests that the localized corrosion islikely caused by the attack of aggressive Cl� ions. By comparing theoxygen content in Fig. 7(b) and 7(c), it can be concluded that theoxide layer indeed forms in the early stage and then breaks downwhen the final stable state is reached. Fig. 7(c) shows that someresidual reaction products composed of titanium and Cl� ions areattached to the anodic surface at Region C. This may be related tothe salt precipitation mechanism [48], which involves the ratelimiting diffusion of cations of the dissolving metal from theelectrode to the bulk solution. In this mechanism, a thin salt film ispresent on the interface and the interface concentration is equal tothe saturation concentration. As reported in previous studies,dissolution of Fe into NaCl or NaNO3 electrolyte [17–19] and of Cointo NaOH electrolyte [24,25] follows this mechanism. However, itshould be noticed that the chloride content of Region C is lowerthan that of Regions A and B, implying an inconformity to the above

times under a voltage of 25 V; (a) 7 s; (b) 10 s; (c) 12 s.

Fig. 7. Chemical composition of the areas specified in Fig. 6; (a) Region A; (b) Region B; (c) Region C.


mechanism. However, this comparison is not sufficiently conclu-sive because EDS analysis is conducted after the experiments, andit cannot be used to ascertain the real-time interface structureduring dissolution. Therefore, to confirm the salt precipitationmechanism with solid evidence, an investigation on electrodeinterface by electrochemical impedance spectroscopy was carriedout.

4.1.3. Electrochemical impedance spectroscopy (EIS)EIS is able to suggest the relationship between spectra features

and electrode interface structure. It has been already used to studythe interface structure of titanium alloy under different corrosionstates, such as passivation [11,29,30,38,49], pitting [38], andtranspassivation [50]. In this work, EIS was carried out duringpolarization at various potentials, i.e., 1 V (passivation), 5 V(pitting), 7 and 9 V (transpassivation). Zview software wasemployed for the fitting of impedance results to equivalentcircuits. The Nyquist plot during polarization at 1 V (given inFig. 8(a)) shows two semicircles, indicating that two capacitorshave been detected in the equivalent circuit. According to themodel developed by Gonzalez [29], the two semicircles are relatedto the double layer capacitor and the TiO2 dielectric passive layer,respectively. Increasing the polarization potential to 5 V, as shownin Fig. 8(b), leads to the evident reduction of impedance modulusand the appearance of an inductive loop. These remarkablefeatures can suggest the adsorption on titanium surface of highlyaggressive Cl� [49] and the propagation of the active pits [38].Further increasing the potential to the transpassive range

(Fig. 5(c)) eliminates the feature of inductive loop. A semicircleat high frequencies indicates the capacitive properties of thedissolving interface, while a line with a slope of 1 may correspondto the diffusion process. The fitting values of C1 for the interface,being approximately 0.1 mF cm�2, are sufficiently smaller than thatof titanium oxide layers [51,52]. This means that an extremelysmall capacitive layer forms conceivably at the interface [50],thereby effectively confirming the assumption of salt filmmechanism. On the other hand, the value of C1 slightly decreaseswith increase in potential. In general, the C can be regarded as aparallel flat capacitor with invariable dielectric constant [50]. Thus,increasing the potential causes thicker capacitive element (i.e.,thicker salt film).

As summarized in Fig. 9, a time-dependent mechanism ofpitting/large localized corrosion/polishing was developed todescribe the anodic behavior of TB6 titanium alloy in NaClsolution. At the very beginning, small pits form on the anodicsurface, as shown in Figs. 9(a) and 9(b). In general, pitting corrosioncan be divided into initiation and propagation stages [53,54]. Theexact mechanism of pit initiation has not yet been ascertained, butthree possible mechanisms [55] have been proposed. As thepenetration mechanism suggested, aggressive Cl� ions are trans-ported through the oxide layer to the underlying metal surfacewhere they participate in localized dissolution at the metal/oxideinterface [39–43,54]. Additionally, because pitting caused by largerBr� and I� ions has also been reported [40], film thinningmechanism must be considered. The Cl� ions adsorb on the oxidesurface and then form surface complexes with titanium oxide

Fig. 8. Electrochemical impedance spectroscopy during polarization at different potentials; (a) At passivation potential of 1 V; (b) At breakdown potential of 5 V; (c) Attranspassivation potential of 7 and 9 V.

Fig. 9. Transient evolution of anodic interface of TB6 titanium alloy in NaCl electrolyte.


which lead to local dissolution and thinning of the passive film[17,54–56]. Following the pit initiation stage, a highly corrosiveinternal electrolyte, which is acidic and concentrated in Cl� and Ti4+ ions, forms during pit propagation, as shown in Fig. 9(b). Thislocal condition is capable of sustaining further pit growth

autocatalytically [53,54]. As the process proceeds, the numberand size of pits increase, forming a honeycomb pit structure. Thefurther growing pits may eventually overlap with each other, asshown in Fig. 9(c). This effect enhances the removal of the oxidelayer and causes larger active cell, which is defined as a large

Fig. 10. Simulation based on the present novel model; (a) Typical current density distribution at different times with travel rate of 25 mm s�1; (b) Current density and electriccharge quantity at point A versus time; (c) Total electric charge Q0 versus location along line AB.

Fig. 11. Comparison of cross-sectional profiles simulated by the proposed and


localized corrosion in this work. The local stirring effect of thequick flowing electrolyte also contributes to the mechanicalbreakdown of the oxide layer [26]. Then, as shown in Fig. 9(d),the localized corrosion areas are enlarged by overlapping withother large active cells or small pits, until the passive layer of thewhole anodic surface is removed. Simultaneously, a large amountof reaction products quickly forms on the active surface and cannotbe moved into the bulk electrolyte immediately, causing a very thinsupersaturated salt film [17] that works as a polishing film andlevels the anodic surface [48], as shown in Figs. 9(e) and 9(f). Thethickness of this film is influenced by the current density andelectrolyte flow rate [33]. Once the oxide layer is totally removed, astable interface structure with base metal/polishing film can beobtained [16–27,48]. This stable interface structure is the key factorresponsible for the excellent performance of ECM.

Additionally, an equivalent charging process of the oxide layercapacitor [11] is commonly used to describe the breakdown of theoxide layer. Therefore, the state of the anodic interface structurecan be quantitatively characterized by comparing the chargedelectricity quantity at any time Qt0with the full electricity quantityof the oxide layer Q. The value of Q was evaluated by multiplying ofthe measured breakdown time and the current density obtained inFig. 5. In this experiment, the average value of Q under 15, 20, and25 V was calculated to be 492 C cm�2, which was used for all thesimulations. Additional experiments performed at other voltagesindicate that this value of Q fits well, as shown in Fig. 3.

4.2. Jet ECM simulation and experiments

4.2.1. Comparison of novel and conventional model simulation resultsIn practical jet ECM processes, the cathode nozzle travels over

the workpiece surface, thereby causing a drastic change in theelectric field distribution. Fig. 10(a) shows the typical current

density distribution on the anodic surface (G10) at 35, 80 and 135 swith a travel rate of 25 mm s�1. For any point on the anodic surface,the current density varies obviously with time. For example, thecurrent density-time curve for point A (shown in Fig. 10(b))exhibits a typical “n” type behavior. The time integration curve ofthe current density (i.e., charged electricity quantity at any timeQt0) at point A is calculated, and it indicates that Qt0 tends to aconstant final value, Q0 (i.e., total charged electricity quantity).According to the electrochemical analysis in Section 4.1, comparingof the values of Q0 and Q determines whether the oxide layer isremoved and stable dissolution begins. Thus, it can be concludedthat the current density at point A before 50 s contributes to theoxide layer breakdown while the current density from 50 to 160 s is

conventional models.


responsible for the dissolution of base metal. The machined depthat point A can be calculated using Eq. (6) and the faradaic current.

In jet ECM, the machined cross-sectional profile is commonlyused to evaluate the machining performance. Thus, as shown inFig. 10(c), the value of Q0 for each point along the line AB wascalculated to predict the cross-sectional profile and the localizedcorrosion range using the method applied for point A. For areas inwhich, the value of Q0 is less than the that of Q (492 C cm�2),uniformly dissolving of the base metal cannot occurs, but onlyserious localized corrosion occurs, producing a surface with poorquality. Therefore, it is reasonable to consider the areas with Q0< Qas the localized corrosion range, which is expected to be as small aspossible during jet ECM process. For areas with Q0 > Q, thedissolved profile of the cross-section can be calculated usingEq. (5). Fig. 11 compares the cross-sectional profiles simulated bythe proposed and conventional models at a travel rate of 25 mm s�1.Obviously, the profile simulated using the proposed model is inbetter agreement with the experimental results, indicating the

Fig. 12. Top-view of grooves obtained at different nozzle travel rates; (a) 10 mm s�1; (b) 2quality index measurement.

Fig. 13. Effect of the nozzle travel rate on jet ECM; (a)

superiority of our novel model over the conventional model. Thesmall difference observed at the bottom of curves between theexperimental results and those obtained using the proposedsimulation may be caused by ignoring the real-time anodic shapechange to simplify the model.

4.2.2. Effect of nozzle travel rate on jet ECMTo obtain a suitable parameter window for the nozzle travel rate

in jet ECM process, its effect on the machining quality index wasinvestigated using both simulation and experiments, as shown inFigs. 12 and 13. Fig. 12 exhibits top-view profiles of the groovesobtained using different nozzle travel rates: 250, 100, 50, 25 and10 mm s�1. As illustrated in Fig.12(f), the cross-sectional profile andsome main quality indexes were measured. Together, Figs. 12 and13(a) indicate that higher travel rates led to smaller groove widthand depth. Additionally, the localized corrosion range becomeslarger with increased travel rates. Particularly, when the travel ratereaches 250 mm s�1, the anode could not be uniformly dissolved

5 mm s�1; (c) 50 mm s�1; (d) 100 mm s�1; (e) 250 mm s�1; (f) Schematic illustration of

Cross-section profile; (b) Machining quality index.


and only localized corrosion occurred. This fact is in goodagreement with the simulated results, as shown in Fig. 13(a). Apossible explanation for this unique phenomenon is the time-dependent anodic behavior discussed in Section 4.1. At anexcessive travel rate, when the nozzle moves away, the oxidelayer cannot be totally removed and the transition anodic interfacestructure (as shown in Figs. 9(b), 9(c), and 9(d)) is retained. Thethree main machining quality indexes measured at different travelrates are shown in Fig. 13(b). The machined depth can be used toassess the machining efficiency, while the localized corrosionrange and overcut range are indicators of the machining accuracy.It is observed that at lower travel rate, the localized corrosion rangedecreased while the machined depth and overcut are enlarged.Based on comprehensive consideration of both the machiningefficiency and accuracy, the machining performance obtained at anozzle travel rate of 10 to 25 mm s�1 is found to be acceptable forthe industrial applications.

5. Conclusions

To analyze the unique phenomenon observed during jet ECMand to select the optimum nozzle travel rate window, thequantitative effect of anodic behavior on the ECM of TB6 titaniumalloy was investigated in this work. The anodic behavior of TB6titanium alloy in NaCl solution was analyzed using LSV, CV, EIS andCA experiments. Furthermore, the mechanism of pitting/largelocalized corrosion/polishing was determined to describe theevolution of the anodic interface structure. This time-dependentprocess usually takes several seconds and significantly affects theanodic surface quality.

The anodic behavior is described in terms of an equivalentcharging process of the oxide layer capacitor and thus the fullycharged electricity quantity of the oxide layer, Q, was proposed as avalve value that determines whether uniform dissolution of theanodic material begins. The value of Q for TB6 titanium alloy wasmeasured to be 492 C cm�2. A novel three-dimensional modelincorporating Q was proposed to account for the quantitative effectof the anodic behavior. Compared with those of the conventionalmodel, the simulated results using the novel model bettercorrespond to the experimental results.

The effect of nozzle travel rate was also investigated bothexperimentally and by simulation. The results revealed that anexcessive travel rate (250 mm s�1 in this work) causes failure of jetECM because the uniform dissolution state cannot form withinsuch a short timescale. It is found that within the workable range oftravel rate, 10 to 100 mm s�1, decreasing the travel rate leads tonarrower localized corrosion range but larger depth and overcut.Based on comprehensive consideration of the machining efficiencyand accuracy, 10 to 25 mm s�1 is considered to be the optimumnozzle travel rate parameter window that is acceptable forindustrial applications.

Acknowledgments

This research was financially supported by the Natural ScienceFoundation of China (grant Nos. 51275342 & 51405335).

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Effect of Anodic Behavior on Electrochemical Machining of TB6 Titanium Alloy1 Introduction2 Experimental details3 Simulation considering the effect of anodic behavior3.1 Mathematical model3.2 Preset conditions

4 Results and discussion4.1 Anodic dissolution behavior4.1.1 Linear sweep voltammetry (LSV) and cyclic voltammetry (CV)4.1.2 Chronoamperometry (CA)4.1.3 Electrochemical impedance spectroscopy (EIS)

4.2 Jet ECM simulation and experiments4.2.1 Comparison of novel and conventional model simulation results4.2.2 Effect of nozzle travel rate on jet ECM

5 ConclusionsAcknowledgmentsReferences

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