[机械模具数控自动化专业毕业设计外文文献及翻译]【期刊】脉冲电化学微细加工研究-外文文献

Journal of Manufacturing ProcessesVol. 6/No. 120047Journal of Manufacturing ProcessesVol. 6/No. 12004AbstractThe need for complex and accurate microcomponents isincreasing rapidly for many industrial and consumer prod-ucts. The electrochemical machining (ECM) process has thepotential of generating desired crack-free and stress-freesurfaces of microcomponents. However, it is necessary tomodify the existing ECM process for reduction of theinterelectrode gap (improved accuracy) and higher local-ized electrochemical dissolution (smaller feature size). Thispaper reports a newly developed pulse electrochemicalmicromachining (ECMM) process for generating complex 2and possibly 3-D microcomponents of high accuracy. A math-ematical model based on the first principle of the processmechanism has been developed and experimentally veri-fied using a recently designed and built ECMM system. Theeffect of voltage and feed rate on process performance hasbeen studied. The application of the proposed method hasbeen illustrated by machining complex cavities of 160 mand 180 m slot width with straight edges.Keywords: Electrochemical Machining, Pulse Electrochem-ical Micromachining, Micromachining, Microcomponent Man-ufacturingIntroductionIt is expected that the world market for micropartsand microproducts needed in all areas of industrialand consumer products will reach 60 billion USDby 2005 (Alting et al. 2003; Eversheim et al. 1997).Many existing precision engineering technologiesin modified form are being tried to manufacturemicrocomponents. At the same time, new processesare being developed to produce microscale parts withdesired accuracies (Kirchmer et al. 2001; Kozak,Rajurkar, and Wei 1994; Shuster et al. 2000). Theprocess technologies such as lithography, variousetching techniques, and the deposition of thin filmsare being used mainly to produce two-dimensionalmicrocomponents (Friedrich et al. 1997; Jerman andTerry 1997). However, only recently have attemptsbeen made to investigate processes for producingthree-dimensional microcomponents. Besides tradi-tional machining techniques such as microturningand milling, attention is being focused on nontradi-tional machining techniques such as micro electri-cal discharge machining (EDM) and microelectrochemical machining (ECM) and laser machin-ing because of their unique characteristics (Datta andRomankiw 1989; Datta, Shinoy, and Romankiw1993; Datta 1998; Masuzawa 2000). The ability ofECM in rapidly generating a stress-free and crack-free smooth surface on any electrically conductivematerial (irrespective of hardness) makes it an ex-cellent choice as a microproduction process. Elec-trochemical machining (ECM) is based on acontrolled anodic dissolution process of theworkpiece (anode) with the tool as the cathode in anelectrolyte cell. In the ECM process, a low voltage(830 V) is normally applied between electrodes witha small gap (usually 0.20.8 mm), producing a highcurrent density of the order of 10100 A/cm2, and ametal removal rate ranging from 0.1 mm3/min. to103mm/min. Electrolyte (typically NaCl or NaNO3aqueous solutions) is supplied to flow through thegap with a velocity of 1050 m/s to maintain theelectrochemical dissolution with a high rate to flushaway the reactions products (usually gases and hy-droxides) and the heat generated by the passage ofcurrent and electrochemical reactions.However, to make ECM suitable for microma-chining applications, it is necessary to develop amodified ECM system that will provide as small agap as possible (for higher accuracy) and highlylocalized and controlled electrochemical dissolu-tion (for desired microfeatures). Instead of design-ing and fabricating a tool for a given componentsize and shape, a recently reported electrochemi-cal micromachining (ECMM) approach uses a uni-versal tool electrode (e.g., cylindrical) that isnumerically controlled. Although the feasibility ofsuch a system has been described in Kirchmer etal. (2001) and Shuster et al. (2000), a comprehen-Study of Pulse Electrochemical MicromachiningJ. Kozak, Warsaw University of Technology, Warsaw, PolandK.P. Rajurkar and Y. Makkar, Center for Nontraditional Manufacturing Research,University of NebraskaLincoln, Lincoln, Nebraska, USAThis paper is an original work and has not been previously publishedexcept in the Transactions of NAMRI/SME, Vol. 31, 2003.Journal of Manufacturing ProcessesVol. 6/No. 120048sive study consisting of a mathematical model forthe micromachining process and influence of ma-chining parameters on the process performancehas not been reported. This paper presents such amicro ECM system, related mathematical modelfor microshaping, and experimental verificationand results.ECMM Process and ModelThe proposed numerically controlled ECMM (NC-ECMM) process uses a universal flat-faced wire toolelectrode, which is moved in a manner similar tothat of a miniature milling cutter, along the three axesusing a computer-controlled nanoresolution drivesystem. The proposed NC-ECMM method has nu-merous advantages, such as the elimination of thetool design process, which is complex, time con-suming, and costly. Additionally, with the proposedapproach, the long path of electrolyte flow is elimi-nated, thus maintaining a small interelectrode gap,which is necessary for accurate micromachining.A very small gap of a few micrometers range isachieved through a specially designed circuit con-sisting of two electrodes submerged in electrolyteand resulting in electrochemical double layers (DL)on electrode surfaces. The DL works like a parallelplate capacitor, which gets charged during the pulse-on time and is discharged during the pulse-off time.This phenomenon of DL helps building a specialcircuit, which consists of the DL itself, a variableDC power supply, and a pulse power supply. A verysmall DC voltage reinforced with the high-frequencypulse voltage is supplied to the machining chamberthrough this circuit. As the electrochemical reactionsare exponentially dependent on the potential dropin the DL, the reactions are confined to the polar-ized regions, which are very close to the electrodesurfaces. However, the discharging of DL during thepulse-off time is neutralized by the DC voltage,thereby preventing the reversal of polarity and hencepreventing the tool dissolution.The ECMM process, similar to pulse ECM, uses apulse generator to supply the working voltage pulsesacross the two electrodes, typically in the form ofpulse strings consisting of single pulses or grouppulses (Figure 1). The proposed ECMM approachresults in the localization of the electrochemical dis-solution to sub-micrometer regions, by the applica-tion of ultra short (in microseconds) voltage pulses(Kirchmer et al. 2001). It seems that the localization ofdissolution coupled with the capability of NC drivesystem to obtain a stable and highly accurate feed (ofthe order of nanometer scale) in the direction of depthof cut helps in maintaining a very small and stable gapbetween the workpiece and the tool electrode.The anodic electrochemical dissolution occursduring the short pulse-on time, tp, each ranging from0.005 to 5 ms, for the pulse ECM process and 5 nsto 5000 ns for the ECMM process using ultra shortvoltage pulses (Figure 1). The dissolution products(sludge, gas bubbles, and heat) can be flushed awayfrom the interelectrode gap by the flowing electro-lyte during the pulse-off time, to= T tp, where T isthe cycle time.Existing work on pulse ECM has shown consid-erable improvements in dimensional controllability,shaping accuracy, process stability, and simplifica-tion of tool design (Kozak, Rajurkar, and Wei 1994).These performance characteristics of pulse ECM,along with the abovementioned special circuit forlocalization of electrochemical dissolution, make theproposed ECMM a very desirable alternative for pro-ducing accurate and complex 3-D microcomponents.The purpose of ECMM process modeling is topredict the shape of the workpiece for a given setof machining conditions. To formulate the math-ematical model, a general case is considered thatdescribes the changes in the electrochemicallygenerated shape of a workpiece using a wire toolelectrode. It is assumed that a coordinate systemis attached to the workpiece, which is stationaryduring machining.The workpiece surface at a given point in time(Figure 2) is described as: z = Z (x,y,t).Following are additional assumptions that havebeen made in developing the mathematical modelof the ECMM process:Figure 1Principle Scheme of ECMM ProcessUTttpJournal of Manufacturing ProcessesVol. 6/No. 120049Electrolyte flow rate between the two electrodesis high enough to neglect changes in electricalconductivity of electrolyte.Workpiece material is homogeneous.Reaction products do not affect electrolyteproperties.Workpiece (anode) surface is uniformly cov-ered by electrolyte.Electrical field in the gap is quasi-stationary.Primary distribution of electrical potential in thegap, i.e., electrode polarization is constant andis taken as an average value.A moving boundary simulation is required to pre-dict the final shape where, at each time step, the dis-tribution of dissolution velocity on the workpiecesurface needs to be determined. According to elec-trochemical shaping theory, the evolution of theshape of the workpiece Z (x, y, t) (Kozak 1967) canbe described as follows:221+=yZxZiKtZAv(1)where Kvis the coefficient of electrochemical ma-chinability and iAis the current density on the anodesurface.At the beginning, at time t = 0, the initial space ofthe workpiece surface is given by the following:z = Z0 (x, y)(2)In the region where the concentration gradient canbe ignored, Ohms law in differential form describescurrent density i in the electrolyte, as follows:iu=v(3)where H11612u is the potential gradient and H9010 is the elec-trical conductivity of the electrolyte.Based on electrical neutrality of the electrolyte,the electric field is sourceless and has a quasi-sta-tionary nature. Therefore, the distribution of electri-cal potential, u, in the electrolyte can be describedby the following:div(H9010 H11612u) = 0 (4)(div = divergence)Assuming that heating and gas generation do notaffect electrolyte conductivity in the interelectrodegap, Eq. (4) can be transformed into a Laplace equa-tion, as follows:2222220uxuyuz+=(5)The boundary conditions for Eqs. (1)(5) aregiven by the state of the system on the electrodes.Assuming perfectly conducting electrodes, connectedwith the external source of voltage U, the boundaryconditions can be given as follows:U = Ec(t) on the cathode (TE)u = U(t) Ea(t) on the anode(WP)0=nu on the insulating wallsThe last condition expresses the fact that currentdoes not flow through an insulator. The anodic po-tential, Ea, and the cathodic potential, Ec, depend oncurrent density and are determined by the sum ofboth the concentration and the activation over po-tential for each electrode.Equations (1)(5) give the relative motion ofthe electrode. The governing partial differentialequations have been solved by the finite differ-ence method for simulating the shape generationby ECMM. A small time interval, H9004t, needs to beselected so that the interfaces can be regarded asstationary when calculating the electrical field andcurrent density on the anode during this time in-terval. After solving the corresponding problemFigure 2Scheme for Modeling ECMM(TETool electrode, WPWorkpiece)TEz=Z(x,y,t)zyUXWPJournal of Manufacturing ProcessesVol. 6/No. 1200410for a known boundary, the problem is solved, it-eratively, for a new boundary at the next instant,given by t +H9004t.At known time, t, the partial differential Eq. (1)can be further modified to represent the profile gen-eration of a workpiece during machining (Figure 2)in two sections separately, one describing the fron-tal gap and second describing the side gap.The equation in the differential form for the fron-tal gap is obtained as follows:()1)(1222+=zxEUKVdxdzvf(6)and similarly for the side gap the equation is ob-tained as:()1)(12222+=RyxEUKVdxdyvf(7)where Vfis the feed rate of tool electrode, H9268 is theduty factor, R is the tool electrode radius, E is thepotential drop, and H9010 is the electrolyte conductivity.To verify the theoretical results obtained using Eqs.(6) and (7), the following experimental study hasbeen conducted.Experimental Setup and ProcedureThe ECMM experiment setup designed is shownin Figure 3. The specifications of the designed anddeveloped ECMM system are listed in Table 1.The system mainly consists of a three-axis NCdrive table, a high-frequency pulse power supply, avariable DC power supply, a 20X magnification mi-croscope, and an oscilloscope. The drive table pro-vides a motion along each axis by a micro translationmotor, which in turn can be controlled by a control-ler installed in a computer. Thus, any complex 3-Dmotion of the drive table and hence of the workpieceis obtained by writing a program in the Windows-based software of the controller. An oscilloscope isconnected in parallel to the power supply line of theelectrochemical machining cell to monitor the cur-rent waveforms.A 30X magnification microscope (with two mea-suring micrometer attachments of 1 m least countfor 2-D measurements) is used to measure the lengthand width of the machined microcavity. To measurethe depth of the cavity, a needle probe attached on aMitutoyo dial gauge having a least count of 0.002mm is used.Similar to the conventional ECM process, theECMM process performance is mainly governedby the electrical potential difference between theelectrodes, interelectrode distance, and the toolelectrode feed rate. However the ECMM processFigure 3Schematic Diagram for ECMM ProcessTool electrodeElectrolyte supply lineReturn linePDrive tableOscilloscopeNC control unitTable 1System SpecificationsDrive SystemMotor type DCMaximum travel 15 mmResolution 7 nmNumber of axes 3Maximum velocity 1.4 mm/secOperating system Windows2000Operating software Win MovePower SupplyOperation mode Pulse or DCCurrent output 0 1 AVoltage output 0 15 VPulse frequency 10 Hz 1000 KHzDuty Cycle 20% 80%Journal of Manufacturing ProcessesVol. 6/No. 1200411involves the use of pulse power supply, thereforethe pulse parameters, which are pulse-on time,duty cycle, and pulse frequency, also affect theprocess performance. In this stage of investiga-tion, the effect of pulse frequency, pulse voltage,duty cycle, and the tool electrode feed rate hasbeen studied on the process performance.The performance measures in the present investi-gation are dimensions of the microcavity machined.Two measured dimensions side gap along thewidth (Sx) and side gap along the length (Sy) areshown in Figure 4a. The frontal gap (that is, the dis-tance between the workpiece surface and the flat faceof the tool during steady-state ECM operation) isshown in Figure 4b.Initially, a tool electrode of diameter 280 m(made by the micro WEDM process) is used formachining the slots of 2 mm length on stainlesssteel (SS-440). The electrolyte used is 10% solu-tion of NaNO3. The initial interelectrode gap iskept at 20 m.After setting up control parameters, which arethe voltage, pulse frequency, duty factor, and thefeed rate, an experiment is started by moving theworkpiece relative to a stationary tool electrodein a desired profile in an electrolyte environmentrepeatedly to remove material layer by layer simi-lar to that of removing material by a miniaturemilling cutter. At the end of each iteration, theworkpiece is given a microfeed relative to the toolelectrode in the direction opposite to the depth ofcut. Because ECMM is a self-adjusting process,after certain number of iterations the machiningprocess reaches an equilibrium state such that thedownward feed and the machining rate in eachiteration become equal. It has been determined ex-perimentally that starting with an initial gap of 20m, and for any value of control parameters in therange as specified earlier, the equilibrium state isobtained after approximately 80120 iterations,given the feed is maintained between 0 and 1 m.An extensive experimental investigation wasconducted following a full-factorial experimentaldesign scheme of four factors, two levels each andthree repetitions for each treatment. ANOVA re-sults (including second-order interactions) for eachof the performance measures (side and frontalgaps) indicated a high R2value (95.76%), sug-gesting that all four factors play an important rolein obtaining the small gaps. However, the men-tioned gaps were consistently smaller at the 1 MHzlevel of frequency and the interaction effect of fre-quency with other factors was not found to be sig-nificant. So further investigation was carried outat a frequency level of 1 MHz, and the voltageand feed rate were found to be most significant.The effect of these two factors (at 1 MHz frequencylevel) is shown in Figures 5 and 6.Experimental ResultsThe side gap and frontal gap are calculated as perthe geometry shown in Figures 4a and 4b and bymeasuring the slot width using the measurement sys-tem described earlier.Figure 4(a) Side gap along width and along length,(b) frontal gap from the face of the tool(a)(b)Tool electrodeWorkpiecehSfS0Tool electrodeSxMachinedslotSyL1bdXYJournal of Manufacturing ProcessesVol. 6/No. 1200412The experimental results of the effect of voltageon side gap (Sx) and frontal gap (Sf) are shown inFigure 5. The plot shows that with the increase involtage both the side gap and the frontal gap increase.A sudden increase in the gap can be observed atabout 12 volts, suggesting that it is necessary to keepthe applied voltage below 10 volts. The microscopicexamination shows that the edges of the cavity ma-chined are sharper at the lower voltage values.It can be seen from Figure 6 that both the side gapand the frontal gap decrease with increasing feedrate. However, for the feed rate above 63 mm/min.