Aviation-oriented Micromachining Technology-Micro-ECM in Pure Water

Available online at www.sciencedirect.comChineseScienceDirectJournal ofELSEVIERAeronauticsChinese Journal of Aeronautics 21(2008) 455-461www.elsevier.com/locate/cjaAviation-oriented Micromachining Technology- -Micro-ECM in Pure WaterBao Huaiqianab, Xu Jiawen"*, Li Ying",c"Jiangsu Key Laboratory of Precision and Micro-Manufacturing Technology Nanjing University of Aeronautics andAstronautics, Nanjing 210016, China"College of Mechanical and Electric Engineering, Shandong University of Science and Technology, Qingdao 266510, China'School of Mechanical and Electrical Engineering, Jinling Institute of Technology, Nanjing 210016, ChinaReceived 14 November 2007; accepted 5 May 2008AbstractThis article proposes a precise and ecofriendly micromachining technology for aerospace application called electrochemical ma-chining in pure water (PW-ECM). On the basis of the principles of water dissociation, a series of test setups and tests are devised andperformed under different conditions. These tests explain the need for technological conditions realizing PW-_ECM, and further explorethe technological principles. The results from the tests demonstrate a successful removal of electrolytic slime by means of ulrasonicvibration of the workpiece. To ensure the stability and reliability of PW- ECM process, a new combined machining method of PW. ECMassisted with ultrasonic vibration (PW-ECM/USV) is devised. Trilateral and square cavities and holes as well as a group of English al-phabets are worked out on a stainless steel plate. It is confirmed that PW-ECM will be probably an efficient new aviation precision ma-chining method.Keywords: electrochemical machining in pure water (PW-ECM); cation exchange membrane; water dissociation1 Introductionsidual stresses, and tool wear, on its use. Undersome given conditions, for instance, lower suppliedAs a kind of nontraditional technique, the mi-voltages, specified electrolytes and/or ultra- shortcromachining technology based on different physi-voltage pulses, micro-ECM can be used to turn outcal or chemical reaction principles has been playingsome two-dimensional (2D) or three-dimensionalan active and important role in production in vari-(3D) geometric structures in the μm or nm orderl-s!.ous industries, such as, aviation, aerospace, preci-To realize micro-ECM with high accuracy,sion instrument and microelectronics, and so on.Electrochemical machining (ECM), a sort of mi-some Japanese researchers, through their experi-cromachining, works on the basis of electrochemicalments, proposed a new ECM in pure water (PW-anodic dissolution, to remove the tiniest bits, asECM) idea that ultra-pure water be used to replacesmall as atom- or ion-size, from the surface of thethe hitherto widely applied electrolytes, to preventthe parts from corrosion and the environment fromworkpiece. ECM appeals to researchers on thcontamination!6-7! However, their experimentalstrength of the absence of mechanical damages, re-equipments failed to fit for the practical technologi-cal application中国煤化工。hange fiberE-mail address: xjw502@nuaa edu.cncatalyst couldYHcNMHGmallECMFoundation item: Aeronautical Science Foundation of China (02H52049)..456.Bao Huaigian et al. / Chinese Journal of Aeronautics 21(2008) 455-461gap region. In their experiments, ECM etching op-is characterized by the constant current density as Uerated at the current density of less than 2 A/cm2 .increases. This is because the membrane resistanceThis indicates the feasibility of their proposal in theincreases with the rise in electrical potential gradi-industrial application on one side, and on the otherent. The third region implies the increase in sup-side further experiments are needed to show theplied voltage or the electrical potential gradient willpracticality of machining holes and cavities of vari-cause a rapid increase in both the water dissociationous shapes. This article performs a set of PW-ECMrate and the ion transferring rate, and as such atechnological experiments to machine out holes andsurge of current density. .cavities of different shapes by using a kind of cationi/(A.cm-)exchange membrane instead of a fiber catalyst in-stalled in the ECM gap region, and a current densityof 3.7 A/cm2. To ensure the stability and reliabilityof the process, a new combined machining methodof PW-ECM assisted with ultrasonic vibration(PW-ECM/USV) is devised. All the experimentalresults have demonstrated that the proposed tech-Nnology is bound to have a brilliant future.2 Characteristics of Water DissociationUvIt is well known that, in pure water, the currentFig.1 Voltage-current density characteristic curve of cationdensity through the ECM gap is too low to dissolveexchange membranes.the workpiece, but installation of an ion exchangeThe curve in question shows that it is necessarycatalyst in the ECM gap, generally at the side ofworkpiece， will rapidly increase the electrolyticto install a certain kind of cation exchange mem-brane and to supply a certain high electrical poten-current density to the magnitude necessary for mi-cro-ECM. According to the characteristics of the iontial gradient between anode and cathode to make thecurrent density change in a similar manner as in theexchange membrane[8-10), the current density thr-third region of the curve, and enable realization ofough the ECM gap is directly proportional to thequantity of the transferred ion, which depends onthe micro-ECM. Theretically, the current densitypassing through the ion exchange membrane can be :the membrane resistance and the electrical potentialcalculated as followslgradient. The current density increases with thequantity of the transferred ion and the electrical po-z;D;F(Cb,-Cm)(1)tential gradient increasing. Fig. 1 shows a nonlinear8(m,j-t.,;)relationship between the current density and thewhere tm,j, tb,j are the cationic number of the ionelectrical potential gradient. It must be pointed outexchange membrane and the boundary layer, δ isthat, in this study, a cation exchange membrane isthe boundary layer thickness, Zj the cationic chemi-installed in the ECM gap instead of a fiber catalystcal valence, D; the diffusion coefficient, F the Fara-because of its ease of arrangement in the tiny ECMday constant, Cb,j the cationic concentration in solu-gap region.tion, Cm,j the cationic concentration on the mem-In Fig.1, the curve can be divided into threebrane surface. When the ion concentration on theregions. The first is called the Ohm region, becausemembrane surface is reduced to zero, namely Cm,j" >the relationship of voltage U against current density0, i=lim i,中国煤化工can be de-i almost accords with the Ohm's law. The secondtermined by"TYHCNMH Gregion, which is referred to as an ordinary condition,.Bao Huaiqian et al. / Chinese Journal of Aeronautics 21(2008) 455-461.457 .z;D;FC,j(2)mental results show that the greater the flow rate8(tm,-to,)becomes, the greater the effects on the water disso-ciation will be exerted. In addition, the purity of3 Theoretical Analysis of Waterwater also affects water dissociation. Therefore, aDissociationgroup of technological parameters should be prede-During water dissociation, the water moleculetermined prior to disposing the cation exchangeis dissociated into H+ and OH. However, the H+ ionmembrane, which functions to accelerate water dis-must exist in the form of the hydronium ion H3O*.sociation and raise the current density in the thirdregion of Fig. 1. Then the micro-ECM can be carriedThat is .out smoothly.2H2O= ? H3O++OHT(3)Cation exchange membraneAt a certain temperature, the transfer reactionBoundary layerof protons may reach an equilibrium state and theeqilibrium constant of water dissociation is givenbyK =CoH- XCH,o+(4)田where K is the water ion product constant, whichAnodeCathodechanges along with temperature, and CoH，CH,0.Ca,are the concentrations of OH and H;O", respec-tively. According to the Donnan balance theoryl"),on the interface of the cation exchange membrane,Cmjthere are many factors affecting water dissociation,of which the electric potential gradient on the inter-Cation flow directionface of the cation exchange membrane is the impor-tant one that accelerates the water dissociation.Fig.2 Geometric and physical model on the interface ofcation exchange membrane.Fig.2 shows the geometric structure of bound-ary layers, concentration difference, and electric4 Experimental Investigation of PW-ECMpotential distribution on the interface between thecation exchange membrane and pure water in theA type of cation exchange membrane was cho-ECM process. When the electric potential gradientsen in this experiment. This membrane was made ofon the boundary layer increases to the order of 10*a compound of aromatic hydrocarbon and diolefine,109 V/m, the water dissociation rate is higher thancharacterized by acid-, alkali-, and oxidation-proofthe recomposing rate of H+ and OH into water, towith some other required electrochemical properties.such a point that the recomposing rate may be ig-Table 1 lists the technical parameters of the cationnored. With the electric potential gradient increasing,exchange membrane. Fig.3 shows the experimentalthe reaction that makes the cation exchange mem-device of PW-ECM, where the cathode (tool) isbrane accelerate water dissociation will be so rapidmade of a stainless steel wire, and the anode (work-that the enhanced allowable current density passingpiece) is made of a sheet of indissoluble platinumthrough the ECM gap can raise the ECM rate to the(P), to ensure that the ECM gap is unchangeable.point that micro-ECM can be realized.At room temperature, with motionless electro-In the ECM process, apart from the installedlyte filled and a cation exchange membrane in-cation exchange membrane, the flow rate of purestalled, a chat中国煤化工c potentialwater also affects water dissociation. The experi-gradient E agatMHCNMHGplottedin..458.Bao Huaiqian et al. / Chinese Journal of Aeronautics 21(2008) 455-461Fig.4 from the experiment.From Fig.4, it is found that the changing trendTable 1 Technical parameters of cation exchange mem-of current density i as a function of the electric po-branetential gradient E, nearly coincides with the theo-ParametersValuesretical analytical results shown in Fig.1. When E≥Moisture membrance thickness/mm0.11-0.132.5x106 V/m, then i≥2 A/cm2, and with the electricpotential gradient rising, the current density con-Exchange capacity(mgg |)≥1.8Water content/%24tinuously increases. Fig.4 serves as an importantArea resistance/(Q.cm2)ssbasis to select ECM parameters, including the ap-Transport number/%95plied voltage and the machining gap between cath-Burst strengh/(10fPa)21.5ode (tool) and anode (workpiece).From the experiments and the theoretic analy-sis of water dissociation, the technological experi-ment parameters were optimized to machine a hole.The tool (cathode) is made of a stainless steel(1Cr18Ni9Ti) wire, 3 mm diameter, and the work-piece (anode) is made of a stainless steelE(1Cr18Ni9Ti) sheet, 0.18 mm thick. The appliedvoltage is 28 V and the initial gap 10 um. In themachining process, the current gradually increased(a) Photograph of seupuntil its density finally reached 2.3 Acm. The resultant hole is shown in Fig.5. Also a six-part al-phabetical signal“PW-ECM" has been worked out,因as shown in Fig.6, on an numerical controlled ECM1- -Anode (workpiece)2- -Electrolyte inlet(NC-ECM) machine tool. The test pieces thus fab-3- Cation exchange membranericated by PW-ECM, demonstrate the applicability4- -Gap adjustor5- -Insulating casing pipe ofof PW-ECM to realize micro-ECM on the micron1scale.6- - - Electrolyte outlet7- Cathode (too)8- - -Microposition feeder ofpiezoclectrie ceramic(b) Schematic diagram of setupFig.3 Experimental equipment of PW-ECM.100 um→- Before oxidation+ After oxidaion(a) View ftom enter side(b) View from exit sideFig.5 Micrograph of hole machined by PW-ECM.PWECR11000 umE(10*V.m-)Fig.4 Characteristics of ion exchange membrane in PW-Fig.6 Microgr:中国煤化工W-ECM"ECM.with PW:MYHCNMHG.Bao Huaiqian et al. / Chinese Journal of Aeronautics 21(2008) 455-461.459.5 Analysis of Experimental ResultAs mentioned earlier, under the ordinary con-ditions, the density of OH and H+ in pure water isabout 10 ? mol/L, in which the allowable density ofpassing current is only in the order of 10- A/cm2,Fig.8 A square tool (cathode) with a 0.508 mm side length.which proves to be too low to produce the ECMscrap-removing rate, required by engineering prac-tices. Therefore, the means to increase the currentdensity becomes a key problem confronted by re-searchers within this field. According to the princi-ple of water dissociation, the basic idea is to use| Short-circuitsome kind of catalyst to promote the dissociation ofdamagedwater into Ht and OH . Next, at a certain high elec-100 umtric potential gradient, the electrolytic current den-sity is increased about 10’ times over that of purePure water conductvity: 2-20 us/cm; Side length of cathode: 484 um;Applied voltage: 24 V; Machining current: 4-8 mA;water without the catalyst. If this succeeds, the cur-Initial machining gap: 20 um; Feed-rate of cathode: 3 um/minrent density will reach the level of 1-10 A/cm2, andFig.9 Trilateral cavity made by PW-ECM.the ECM scrap-removing rate will reach the order of1-10 um/min, which meets the practical demand bythe micro-ECM.To test the possibility to fabricate shaped cavi-ties/holes by the PW-ECM, trilateral and squarestainless steel tools (cathodes) were prepared byNC-EDM for the following experiments, as shownin Fig.7 and Fig.8. Fig.9 and Fig.10 show the twostainless steel workpieces machined by PW-ECMwith their pertaining parameters.From Fig.9 and Fig. 10, some localized areas ordamaged points, by the short circuit and/or sparksPure water conductivity: 2-20 us/cm; Side length of cathode: 508 um;can be observed. As the electrolyte is kept almost atApplied voltage: 24 V; Machining current: 6-8 mA;rest during the ECM process in the PW-ECM deviceshown in Fig.3, it will be very difficult to drive theFig.10 Square ceavity made by PW-ECM.ECM byproducts out of the ECM gap, which resultsthe electrolyte cannot flow at a certain speedin possible damage by short-circuit and sparks whenthrough the ECM gap, the depth that the ECM canECM reaches a certain depthl12-13]. In other words, ifsuccessfully work out will be limited to a narrowrange, such as up to 10 um, as shown in Fig.9 andFig.10.6 PW-ECM/USVTo solve the problem in PW-ECM mentionedFig.7 A trilateral tool (cathode) with a 0.484 mm sideearlier, this article pronnses, a new combined tech-length.中国煤化工，nology of PW--bration canbe applied tolYHCNMH(Sde) or the.. 460.Bao Huaigian et al. / Chinese Journal of Aeronautics 21(2008) 455-461tool (cathode)l4-15], but in the article, it is used onthe workpieces because it is easy to install the ul-trasonic vibration device on a worktable. Threestainless steel test pieces were prepared by PW-ECM/USV- each with a trilateral cavity, a squarecavity, and a trilateral hole, as shown in Fig.1l,Fig.12, and Fig.l3, respectively. All this demon-100 umstrates that not only does the PW-ECM/USV avoidthe damages caused by short-circuit and sparks, thePure water conductivity: 2-20 us/cm; Side length of cathode: 484 um;Applied voltage: 18 V; Machining current: 15 mA;machining process is improved as well. This is at-USV amplitude≈10 pum; Initial machining gap: 40 pum;tributed to the fact that the ultra-high frequency vi-Feed-rate of cathode: 6 umn/minbration of the workpiece also induces an ultra-highFig.13 Trilateral hole made by PW-ECM/USV.frequency disturbance in the electrolyte in the ECMthe strong disturbing pressure waves generated ingap, resulting in driving the ECM byproducts out ofthe PW-ECM/USV process could easily destroy thethe ECM gap.passive layer of the workpiece surface, and the ma-By comparison, the PW-ECM/USV is superiorchining current density could reach higher level, notto the normal PW-ECM process, without ultrasoniconly the scrap-removing rate and the quality of thevibration, and with higher machining accuracy. Asmachined surface could be improved, but also themachining process could be stabilized. The encour-aging results of the study imply that the proposedprocess, PW-ECM/USV, is worth further investi-gating systematically and experimentally.7 Conclusions(1) By installing a cation exchange membranein a suitable position and enhancing the electricalpotential gradient between anode and cathode, thePure water conductivity: 2-20 us/em; Applied voltage: 18 V;Side length of cathode: 484 um; Machining current: 15 mA;current density is able to reach 3.7 A/cm, aboutUSV amplitude=10 um; Initial machining gap: 40 um;Feed-rate of cathode: 8 um/mintwice that recommended in Ref.[7], thereby realiz-ing the micro-ECM at a higher scrap-removing rate.Fig.11 Trilateral cavity made by PW-ECM/USV.(2) Damages caused by short circuit and sparkscan sometimes be observed in the PW-ECM process.This emphasizes the need for some technologicalmeans to obviate the short circuit and sparks, to en-sure a stable and reliable PW-ECM process. Amongthem, ultra-short pulses and PW-ECM/USV are con-sidered to be promising.(3) Comparing the experimental results of thecombined technology, PW-ECM/USV, with those onPure water conductivity: 2-20 us/cm; Applied voltage: 18 V;the normal PW-ECM, attests the superiority of theSide length of cathode: 508 um; Machining current: 20 mA;PW-ECM/USV over the PW-ECM in both machin-Feed-rate of cathode: 8 un/mining accuracy ;中国煤化工t is worth-Fig.12 Square cavity made by PW-ECMUSV.while to invesYHCNMHGhiningme-.Bao. Huaiqian et al. /Chinee Journal of Aeronautics 212008) 45-.461.461.chanism and the production specification of the PW-chemical machining in ultrapure water. Acrospace Materials andECM/USV in the future.Technology 2006(S): 58-60. [in Chinese][10] Strathmann H, Krol J J, Rapp H J, et al. Limiting currentt densityReferencesand water dssociation in bipolar membranes. Joumal of Mem-[1] Schuster R, Kirchner V, Allongue P, et al. Electrochemicalbrane Science 1997; 125(1): 123-142.micromachining. Science 200; 289(7): 98-101.[1] Mulder M. Basic principles of membrane technology. Bejing;[2] da Silva Neto J C, da Silva E M, da Silva M B. Intervening vari-Tsinghua University Press, 199 [ in Chinese]ables in electrochemical machining. Journal of Materials Process-[12] Jain v K, Adhikary s. On the mechanism of matcrial removal ining Technology 2006; 179(-3): 92-96.electrochemical spark machining of quartz under diferenet polarity[3] Kock M, Kirchner V, Schuster R. lctrochemiconditions. Journal of Materials Processing Technology 2008;ing with ultrashort voltage pulse- a veratile method with litho-200(1-3): 467-470.graphical preision. Elecrochimica Acta 2003; 48(20-22):[13] Munda J, Malapati M, Bhattacharyya B. Control of micro-spark3213-3219.nt efeet during EMM process. Jounal of Materials[4] WangJ Y, Xu J W. Principles and aplication on electrochemicalProcessing Technology 2007; 194(1-3): 151-158.machining. Beiing:; National Defence Industry Presss 2001. [in[14] Hewidy M s, Ebeid S J, EI-Taweel T A, et al. Modelling the per-Chinese]formance of ECM ssisted by low frequency vibrations. Jourmal of[5] Bhattacharyya B, Munda J. Experimental investigation into elec-Materials Processing Technology 2007; 189(1-3): 466-472.trochemical micromachining (EMM) process. Journal of Materials[15] Batacharya B, Malapati M, Munda J, et al. Infuence of toolPocessing Technology 2003; 140(1-3): 287-291.vibration on machining performance in electrochemical mi-Mori y, Yamamura K, Endo K, et al. Creation of perfeet suraces.cro-machining of copper. International Journal of Machine ToolsJournal of Crystal Growth 2005; 275(1-2): 39-50.and Manufacture 2007; 472);335-342.[7] Mori y, Goto H, Hirose K, et al. A study on etrochemical ma-Biography:chining method in ultra-pure water- increaece of hydroxy1 ion inBao Huaiqian Bor in 1977, he received a Ph.D. degreeutra- pure water by catalytic reaction. Journal of the Japan Societyfrom Nanjing University of Acronautics and Astronautics infor Precision Engineering 2001; 67(6); 932-936.2007, and then became a teacher there. His main academic8] Bao H Q, Xu J W. Mechanism and technological foundation ofinterest lies in nontraditional machining including mi-ECM in utrapure water. Journal of Chemnical Industry and Engi-cro-manufacturing.neering 2006; 573): 626-629. [in Chinese]E-mail:bhqian@163.com9] Bao H Q, Xu J W. Study of water dssociation based on electro-中国煤化工MHCNMH G