Research ArticleStudy on Breakdown Probability of MultimaterialElectrodes in EDM
Y Liu 1 W Wang1 W Zhang 1 F Ma1 Y Wang2 B Rolfe2 and S Zhang 1
1School of Mechanical Engineering Dalian Jiaotong University Dalian 116028 China2School of Engineering Deakin University Geelong VIC 3220 Australia
Correspondence should be addressed to S Zhang zsfdjtueducn
Received 31 August 2017 Accepted 6 February 2018 Published 27 March 2018
Academic Editor Frederic Dumur
Copyright copy 2018 Y Liu et al -is is an open access article distributed under the Creative Commons Attribution License whichpermits unrestricted use distribution and reproduction in any medium provided the original work is properly cited
With the development of EDM technology some multimaterial electrodes which have some special functions are regularly takingthe place of traditional single material electrodes on some machining occasions and becoming widely used In this paper theinfluence of material on discharge breakdown in EDM with multimaterial electrodes is studied A comparison model aboutmaterial influence on discharge breakdown under both single discharge condition and continuous discharge condition isestablished and the material property factors affecting the probability of discharge breakdown are also analyzed Finally a series ofexperiments are carried out to study the effects of different electrode materials on the discharge breakdown of EDM a fittingformula of breakdown probability is presented and the effectiveness of the comparison model is also verified by comparing withexperimental results
1 Introduction
Electrical discharge machining (EDM) is a commonly usednontraditional machining method which can be used toprocess difficult-to-cut materials and parts that feature withcomplex shapes It is widely used in small hole and cavitymachining of mold production With the continuous de-velopment of mold manufacturing technology the die cavityis developing towards the direction of more complexity andprecision and conventional single material EDM electrodeshave been difficult to meet the diverse requirements of cavitystructure and die accuracy [1] In addition for differentprocessing features such as plane and hole different elec-trode materials can meet the requirements of accuracy andsurface quality of the corresponding features in one singleprocessing thus improving the efficiency of forming process-erefore some multimaterial electrodes which have somespecial functions are increasingly replacing traditional singlematerial electrodes in some machining processes
Extensive researches have been carried out on EDMwith multimaterial electrodes Mohri et al presented a newmethod of surface modification by EDM using the composite
structured electrode Copper aluminum tungsten carbideand titanium were used for the materials of the electrode Itwas revealed that surfaces after modification have less cracksand higher corrosion resistance and wear resistance [2]Uhlmann and Roehner aiming to decrease the wear of toolelectrodes used boron-doped CVD (B-CVD) diamond andpolycrystalline diamond (PCD) as electrode materials formicro-EDM and experimental investigations on PCD andB-CVD diamond as tool electrode materials showed goodresults with respect to wear and process behavior underprocess conditions of micro-EDM [3]
Aiming at the problem that the rate of material removaldecreases rapidly in EDM with the increase of processingdepth Cao et al made some Cu-Cr composite electrodesand did EDM experiments with brass electrodes and Cu-Crcomposite electrodes -e removal rate and processingquality of different electrode materials were compared andanalyzed It was found that the removal rate of the compositeelectrode material was improved by about 2 times and themachining accuracy was improved significantly too [4] Tsaiet al proposed a new method of blending copper powderswith resin and chromium powders to make composite
HindawiAdvances in Materials Science and EngineeringVolume 2018 Article ID 2961879 8 pageshttpsdoiorg10115520182961879
electrodes It has shown that the composite electrodes ob-tained can achieve a higher material removal rate (MRR)than copper electrodes the recast layer is thinner and fewercracks are present on the machined surface [5] Khanra et aldeveloped a metal matrix composite (Cu-ZrB2) to get anoptimum combination of wear resistance and electrical andthermal conductivity e Cu-ZrB2 composite was de-veloped by adding dierent amounts of Cu and tested as anelectrode material at various process parameters of EDMduring machining of mild steel It was found that the ZrB2-40wt Cu composite shows more MRR with less toolremoval rate (TRR) than commonly used Cu tool [6] El-Taweel investigated the relationship between process pa-rameters in electrodischarge of CK45 steel and tool electrodematerial as Al-Cu-Si-TiC composite is produced usingpowder metallurgy (PM) technique e central compositesecond-order rotatable design had been utilized to planthe experiments and response surface methodology(RSM) was employed for developing experimental modelsAl-Cu-Si-TiC PM electrodes are found to be more sensitiveto peak current and pulse on-time than conventionalelectrodes [7] Senthilkumar and Reddy developed a newcopper-based metal matrix composite (Cu-B4C) for an EDMelectrode to get an optimum combination of wear resistanceand electrical and thermal conductivity e results showedthat copper composite with 40 boron carbide re-inforcement exhibited betterMRR and TRR compared to theconventional copper electrode [8]
Anil and Ccedilogun from the experimental study observedand compared the performance of solid copper and copper-coated stereo lithography (CCSLA) electrode It was foundthat the internal cooling channel formed by SLA technologyprolongs the life of the CCSLA electrode by dissipating theheat of coating [9] Wang et al through composite elec-troplating technology electrodeposited Cu-ZrB2 compositecoating on the side of composite electrode they made forexperiments It was concluded that the Cu-ZrB2 compositecoating can improve the electrical erosion resistance of theelectrode and can eectively guarantee the uniform wear ofthe end face of the electrode and reduce the tapering of themachined hole [10] Lv studied the machining process ofmicro-EDM with composite electrodes by means of com-puter simulation e Cu-based Ni-W alloy compositeelectrode was fabricated by the electroplating method forEDM experiments e electrical erosion resistance of thesurface material is enhanced and the sidewall wear of theelectrode can be eectively reduced and the shape accuracyof the machined hole can be greatly improved [11] Li et alstudied the wear mechanism of the Ni-TiNCu compositeelectrode in the case of high-frequency pulse current andfound out the inuence of the uctuation frequency ofdischarge current on electrode wear in micro-EDM It isshown that compared with the electrodes made from ho-mogeneous materials the high-frequency electromagneticproperties of the Ni-TiN composite layer can be used ef-fectively to inhibit the skin eect of high-frequency pulse onthe electrode and improve the distribution trend of currentdensity [12] Yuangang et al made the Cu-ZrB2 compositecoating electrodes by way of the electrode position process
on the basis of the dierence between the dischargingperformance of the electrodeposited coating and that of thematrix to ensure uniform wear of electrode bottom facesCompared with the conventional electrodes Cu-ZrB2composite coating electrodes display better wear resistanceon the same experimental conditions [13] Chiou et alpresented a comparative study of the performance of WCWC-coated Ag andWC-coated Cu electrodes for the micro-EDM milling and the experimental results showed that theWC-coated Ag electrodes yielded the lowest surfaceroughness and the WC-coated Cu electrodes achieved thehighest material removal rate which illustrated the eec-tiveness of the electrode coating method [14]
Based on the above research we can iexclnd that althougha great deal of achievements have been made in the researchabout EDM with multimaterial electrodes the related re-searches only stay at the level of technologic experimentse discharge mechanisms of EDM with multimaterialelectrode such as the inuence mechanism of multi-materials on the distribution of discharge breakdown andthe law of distribution absorption and conduction ofdischarge energy in multiphase materials and the mecha-nism of ejection process of multiphase molten metal ma-terials are still not very clear is paper carries out thetheoretical analysis and experimental veriiexclcation about thebreakdown probability of the multimaterial electrode inEDM and relevant research results can provide a referencefor further analysis of EDM mechanism with multimaterialelectrodes
2 Breakdown Process Analysis ofMultimaterial Electrode
EDM is a kind of nontraditional machining method that usesspark discharge to breakdown the dielectric between poles ofextremely close distance and removes the electrode materialby way of thermal eect of breakdown e machiningprocess of EDM with multimaterial tool is shown in Figure 1Substantially the discharge breakdown process of EDM isaected directly by the thermal iexcleld electron emission ofcathode electrons After the thermal iexcleld electron emissionunder the eect of high-strength electric iexcleld the electronavalanche ionizations are generated in the interelectrode
Workpiece
Pulse supply
Dielectric
Multimaterial electrode
Gap
Figure 1 Machining process of EDM with multimaterial tool
2 Advances in Materials Science and Engineering
dielectric when the electron avalanche ionizations reach theanode the dielectric is broken down and the dischargechannel is formed -erefore in the early stage of the dis-charge breakdown process the amount of thermal fieldelectron emission of cathode electrons at a certain locationcan directly affect the probability of discharge breakdown atthat location And the amount of thermal field electronemission is characterized by thermal field electron emissioncurrent density -at is to say the position with large thermalfield electron emission current density has the high proba-bility of discharge breakdown
21 Single Discharge Breakdown Process According to thethermal field electron emission theory when the tempera-ture on the tool surface is T (unit K) and a strong electricalfield with the electric field intensity E (Vcm) is exertedbetween electrodes the current density of cathode electronemission j is given as [15]
j(T) N(T) middot j(0) (1)
where j(T) is the current density of field electron emissionwhile temperature is T (Acm2) and N(T) is the coefficientof correction when temperature is T (K) relative to whentemperature is 0K which is given as
N(T) Q
sin(Q) (2)
where
Q 277 times 104T
ϕ
1113968
E (3)
where ϕ is the electron work function (J) and j(0) is thecurrent density of field electron emission while temperatureis 0 K (Acm2) which can be expressed as
j(0) 154 times 10minus6E2
ϕ
middot exp minus683 times 107ϕ32
Eθ 339 times 10minus4
E
radic
ϕ1113888 11138891113890 1113891
(4)
where θ(y) is the Nordheim functionAs to different materials copper and iron (Fe) for ex-
ample we set the same electron emission conditions T
300 K and E 4 times 107 Vcm when the Nordheim functionθ(y) is approximately equal to 1 then among all the pa-rameters in (1)ndash(4) only electron work function ϕ is differentbetween these two materials while the rest of them are all thesame Electron work function of commonmetals is shown inTable 1 [16]
Based on the thermal field electron emission theory thefield electron emission current density can be used tocharacterize the breakdown probability Where the thermalfield electron emission current density is large it is easier toform discharge breakdown and further the breakdownprobability increases A ratio coefficient of breakdown prob-ability Rp is used to characterize the numerical comparison ofthermal field electron emission current density of different
electrode materials -e ratio coefficient of breakdown prob-ability Rp(Fe-Cu) is
Rp(Fe-Cu) j(T)Fe
j(T)Cu (5)
By substituting the electron work function of copper andiron as 524 eV and 447 eV into (1)ndash(4) and comparing we canget that j(0) of copper and iron is approximately equal andN(T) of copper and iron is also approximately equal thereforethe thermal field electron emission current density of the two isapproximately equal which means the influence of differentmaterials on the breakdown process is basically the same
22 Effect of Continuous Discharges on BreakdownProcess For continuous spark discharges the change of dis-charge environment caused by the previous breakdown processcan lead to next breakdown process occurring near the pre-vious breakdown discharge channel [17]-e two factors whichhave the most influence on the breakdown process are thedielectricrsquos properties and the change of the temperaturearound the discharge channel However the change of di-electric property is mainly related to the dielectric deionizationability and the distribution of the debris interelectrodes whichhas little relationship with the electrode material so it will notbe discussed here -e change of the temperature around thedischarge channel is mainly due to the electric heating effect ofthe plasma caused by the previous breakdown (the temperatureof the central region of the discharge channel can reach above10000K) even during thematerial removal process part of theheat is taken away by the debris and the dielectric the tem-perature of the material in the discharge crater can still beseveral thousand degrees And during the pulse interval thetemperature at crater decreases due to thermal conduction-efollowing equation is the Fourier thermal conduction law
dQ minusλzT
zndA dt (6)
where Q is the amount of heat conducted (J) λ is the thermalconductivity of the material (WmK) zTzn is the temper-ature gradient A is the heat conduction area (m2) and t is theheat conduction time (s) And the relationship equation be-tween the amount of heat conduction Q and temperaturechange ΔT is given by
Q ΔTcρV (7)
where c is the specific heat capacity of material (JkgmiddotK) ρ is thedensity of material (kgm3) andV is the volume ofmaterial (m3)
Supposed that two different materials have the same cratersize and the same temperature gradient and the changes of
Table 1 Electron work function of common metals [16]
Materials Electron work function ϕ (eV)Copper 524Brass 334ndash524Zinc 334Iron 447Copper-tungsten alloy 454ndash524Tungsten 454
Advances in Materials Science and Engineering 3
thermal conductivity and specific heat capacity of the materialsare very small with the temperature the conduction time is pulseinterval time and then from (6) and (7) the ratio of temperaturedifference before and after the heat conduction near the dis-charge crater of copper and iron RΔT(Cu-Fe) is given as
RΔT(Cu-Fe) ΔTCu
ΔTFeλCuλFe
middotcFeρFecCuρCu
(8)
From Table 2 [18] the value of (8) can be calculated as483 which indicates that the decrease of temperature due toheat transfer in the discharge region of copper material isabout 5 times that of the discharge region of the iron ma-terial because the thermal conductivity of the copper ma-terial is higher than that of the iron material For thedischarge region with an initial temperature of 3500K if thetemperature of the iron material electrode is decreased byheat transfer of 200K then the cooling of the copper ma-terial electrode will reach about 1000K At the same timedue to the difference of the temperature in the dischargeregion the value of the temperature correction coefficientN(T) will be changed accordingly and then the difference ofthe thermal field emission current j(T) is significant -ethermal field emission current j(T) is given as [19]
j(T) 4πemkT
h3 1113946exp minusc +Ee minusEF
d1113874 1113875
middot ln 1 + exp minusEe minusEF
kT1113874 11138751113874 1113875 dEe
(9)
where e is the electron charge m is the electron mass k is theBoltzmann constant h is the Planck constant Ee is the energyof the electron and EF is the Fermi energy By substituting theaforementioned known parameters from logarithm table ofcurrent density as shown in Table 3 it can be seen that thethermal conductivity of iron is poor so that it is not easy toconduct heat away from discharge region and as a result thetemperature is high which leads to the thermal field emissioncurrent density of the iron electrode (j(T)Fe) 537ndash832 timesthat of the copper electrode (j(T)Cu) under the same con-ditions -at is to say the ratio coefficient of breakdownprobability Rp(Fe-Cu) is 537ndash832
-erefore under the same discharge conditions the ironelectrode is more prone to breakdown than copper one afterthe continuous pulse discharges Table 4 shows the calcu-lation results of the ratio of temperature differenceΔTCuΔTi and the ratio of thermal field electron emissioncurrent density j(T)ij(T)Cu of different materials under theconditions when the initial temperature is 3500K and thetemperature drop by heat transfer is 200K From Table 4 itcan be seen that under the same conditions the thermal fieldemission current density of brass is the highest that is
617ndash871 times that of copper as to the copper-tungstenalloy it is 105ndash162 times that of copper -is means theeffects of different materials on the breakdown process ofcontinuous discharges differ significantly A large amount ofthermal field emission current density leads to the highprobability of breakdown under the same conditions brass hasthe highest probability of breakdown iron takes the secondplace followed by the copper-tungsten alloy and copper hasthe least probability of breakdown
3 EDM Experiment of Multimaterial Electrode
31 Experiment Conditions -e equipment used in theexperiment is a self-built EDM lathe which mainly consistsof automatic feeding device power box and worktable Itsspindle speed is 01ms and it can be adjusted adaptivelyaccording to the discharge condition to meet the processingrequirements under different working conditions and toprevent arc discharge -e EDM sinking machining methodis adopted and deionized water is used as the working fluid-e processing parameters of multimaterial electrodes forEDM are shown in Table 5
-e multimaterial electrodes are formed by connectingthe cylindrical electrodes of two different materials in paralleland the ends of multimaterial electrodes are polished to bevery smooth -e fabricated multimaterial electrodes are shownin Figure 2 A series of continuous pulse discharge experimentsare performed on a range of combinations of multimaterialelectrodes twice more repeated experiments are conductedfor each electrode combination and then the electrode endfaces are observed with a high-power electron microscopeand the distribution of the discharge craters is recorded
32 Experimental Results andAnalysis Figures 3ndash5 show theexperimental results of different multimaterial electrodes ofiron and copper brass and copper and copper-tungsten and
copper respectively From the figures we can see that underthe same conditions of discharge the number of craters oniron electrodes is more than that on copper ones andsimilarly the number of craters on brass electrodes is morethan that on copper ones and the craters on copper-tungstenelectrodes are more than those on copper ones
As can be seen from Figure 3 for the same dischargebreakdown condition the discharge points are mostly dis-tributed on the surface of the iron electrode at the end ofdischarge and only a few discharge points exist on the surfaceof the copper electrode -is shows that the breakdownprobability of iron is obviously higher than that of copper Itcan also be found that the thermal conductivity of iron ismuchsmaller than that of copper which indicates that after con-tinuous discharge because of the low thermal conductivity ofiron less heat is transmitted from the iron to the outsideMeanwhile as the specific heat capacity of iron is higher thanthat of copper iron of the same temperature as coppercontains more heat and the temperature reduction of the ironis also smaller-e temperature of the iron electrode surface ishigher than that of the copper electrode which promotes theemission of the cathode thermal field electron emission onthe electrode surface therefore the breakdown probability ofthe iron electrode is elevated -e high-temperature burntraces on the surface of the iron electrode from Figure 3(a) canalso demonstrate the presence of high temperature on thesurface of the electrode after continuous discharges
As can be seen from Figure 4 a large number of dischargepoints can be found on the surfaces of both copper and brasselectrodes and the discharge points on the surface of the brasselectrode are much more than those of the copper surfacealthough the difference between the two is not as large as the
difference between copper and iron electrodes-is is becausethe conductivity of brass is slightly better than iron and itsspecific heat capacity is similar to that of copper undercontinuous discharge conditions the temperature differencebetween the left and the right of the multimaterial electrode isnot as large as the copper-iron electrode therefore the dif-ference of discharge points decreases
FromFigure 5 it is noted that although the discharge pointsof the copper-tungsten alloy electrode are more than those ofthe copper electrode the difference between them is obviouslyreduced compared with Figures 3 and 4 -is is because al-though the thermal conductivity of copper-tungsten alloy issmaller than that of copper its specific heat is smaller than thatof copper too these two aspects have the same effect on thetemperature difference of heat conduction -e reason why thebreakdown probability of copper-tungsten alloy is higher thanthat of copper is probably because the density of copper-tungsten alloy is greater which can increase the amount ofheat contained in the same volume of material andmake up forthe deficiency of smaller specific heat capacity And because thethermal conductivity of copper-tungsten alloy is lower than thatof copper the surface temperature of copper-tungsten alloymaterial is higher so its breakdown probability is increased
33 Experimental Data Processing We quantitatively de-scribe the number of discharge craters distributed on dif-ferent electrodes by using the discharge affecting area onelectrode surface which can be calculated by pixel analysisfunction of image processing software Matlab -e pixelanalysis method is a method of using the image processing
Table 4 Calculation results of RΔT(Cu-i) and Rp(i-Cu) of differentmaterials
RΔT(Cu-i)(ΔTCu)ΔTi Rp(i-Cu)(j(T)ij(T)Cu)
Iron 483 537ndash832Brass (Zn35) 300 617ndash871Copper-tungstenalloy (W70) 147 105ndash162
Table 5 Processing parameters of multimaterial electrodes forEDM
Items ParametersWorkpiece material Die steel
Multimaterial electrodes Iron and copper copper andcopper-tungsten brass and copper
Machining polarity Normal-ickness of workpiece(mm) 4
Pulse on time Ton (μs) 125Pulse interval time Toff (μs) 75Breakdown voltage Ub (V) 45Peak current Ip (A) 08Diameter of cylindricalelectrode (mm) 2
Machining time t (s) 30Dielectric Deionized water
(a)
(b)
Figure 2 Fabricated multimaterial electrodes of iron and copper(a) Side view (b) Bottom view
Advances in Materials Science and Engineering 5
software to accurately analyze the area of an irregular imageIt utilizes proportion relation between image pixels andimage area in the same photo by calculating the displaypixels of testing image to analyze the actual area of testingimage According to the proportion relation between theimage pixel and the actual area an equation is given by
testing area (At)reference area (Ar)
display pixels of testing area (Pt)
display pixels of reference area (Pr)
(10)
When analyzing the discharge affecting area the refer-ence area Ar is the known quantity as the diameter of thecylindrical electrode in the machining process is 2mm for
ease of calculation a square with the side length of 2mm istaken as a reference image then the reference area is 4mm2the display pixels of reference area Pr are the known quantityand it is the corresponding pixels of the reference area in theimage processing software -e display pixels of testing areaPt are the pixels corresponding to discharge affecting areaselected in the figure -e discharge affecting areas differ ondifferent electrode surfaces and the display pixels of testingarea are different -erefore by obtaining the display pixelsof testing area in the software substituting the knownquantities of reference area and the display pixels of ref-erence area the discharge affecting area on electrode sur-face can be calculated accurately from (10) -e discharge
(a) (b) (c)
Figure 3 Discharge point distribution on the surface of iron and copper electrodes
(a) (b) (c)
Figure 4 Discharge point distribution on the surface of brass and copper electrodes
(a) (b) (c)
Figure 5 Discharge point distribution on the surface of copper-tungsten and copper electrodes
6 Advances in Materials Science and Engineering
aecting areas on the surface of dierent multimaterialelectrodes are calculated as shown in Figure 6
As can be seen from Figure 6 the discharge point areasfrom large to small are followed by brass iron copper-tungsten alloy and copper e discharge point area ofcopper is the smallest in each set of data and the dischargepoint areas of brass iron and copper-tungsten alloy are ofseveral-fold relation with that of copper is is because thethermal conductivity of copper is greater than that of othermaterials and its speciiexclc heat capacity is the smallest so that ithas the minimum breakdown probability and this corre-sponds well with the previous analysis results of the ratiocoeyencient of breakdown probability On the end face of theiron and copper tool the average discharge area on ironsurface is 35 times as that on copper surface On the brass andcopper tool the average discharge area on brass surface is 26times as that on copper surface e reason for the reductionof area dierence is that the discharge area of the coppersurface is increasedWhile on the copper-tungsten and coppertool the average discharge area ratio is less than 2 e dif-ference in the breakdown probability of copper between iron
brass and copper-tungsten alloy decreases gradually is isbecause the thermal conductivity of iron is smaller than that ofother materials and the speciiexclc heat capacity is greater thanthat of other materials so that it has the maximum breakdownprobability with copper Besides the thermal conductivity ofcopper-tungsten alloy is the largest among those materialsand its speciiexclc heat capacity is the smallest therefore thedierence in the breakdown probability compared withcopper is the smallest among the three materials In additionit can also be seen from Figure 6 that the discharge area of thebrass and copper tool is larger than that of the other twogroups overall and this may be due to the low electronwork function of zinc in brass which is helpful to improvethe occurrence rate of discharge breakdown increasing thenumber of discharges on the brass surface At the same timedue to the inuence of the discharge breakdown the numberof discharges on the surface of copper is also increased
According to the experiment results breakdown prob-ability of dierent materials compared with copper Pb(i-Cu)can be obtained as
Pb(i-Cu) Testing area of material i Ai( )
Testing area of material i Ai( ) + testing area of copper ACu( ) (11)
Based on the experiment results of breakdown probabilityas well as the ratio of temperature dierence RΔT(Cu-i) and theratio coeyencient of breakdown probability Rp(i-Cu) formularyiexcltting is carried out by the adaptive iexclt method using Matlaband the breakdown probability iexcltting formula of dierentmaterials compared with copper is given asPb(i-Cu) 07658 + 007448 sin π middot RΔT(Cu-i) middot Rp(i-Cu)( )
minus 0269 exp minus 01093Rp(i-Cu)( )2
( )
(12)
From (12) the exact breakdown probability of certainmaterial compared to copper can be obtained and furtherthe breakdown probability between any two dierent ma-terials can also be obtained
e proposed model in this paper has the ability toanalyze and predict the electrode discharge breakdownprobability of dierent materials and the reasons for thedierent breakdown probabilities of dierent materials aregiven with a reasonable explanation by the model which isof some signiiexclcance for the understanding of dischargemechanism of EDM
0
0005
001
0015
002
Disc
harg
e are
a (cm
2 )
Experimental dataAverage value
Iron Copper Brass Cu-W CopperIron and copper tool Brass and copper tool Cu-W and copper tool
Copper
Figure 6 Discharge area on the surface of dierent multimaterial tools
Advances in Materials Science and Engineering 7
4 Conclusions
In this paper the influence law of surface breakdownprobability of multimaterial electrodes is analyzed accordingto the theory of field electron emission a mathematicalmodel of material property influencing discharge break-down in interelectrodes is established the model is verifiedby experimental method and the conclusions are drawn asfollows
(1) In single pulse discharge process the breakdownprobability of different material electrodes is ap-proximately the same and the discharge process isnot influenced basically
(2) Under continuous discharge conditions due to theheating effect of previous discharges the surfacetemperature of tool material with poor thermal con-ductivity and high specific heat capacity is higherwhich makes it easier to cause local discharge break-down and hence increases breakdown probabilityBreakdown probability is influenced by the propertiesof the materials such as thermal conductivity specificheat capacity and density
(3) -e repeatable experimental results of continuousdischarges have provided the relationship betweenbreakdown probability and different material propertiesand also proven the effectiveness of the dischargebreakdown probability model
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e financial support from the National Natural ScienceFoundation of China under Grant no 51405058 ScientificResearch Platform Foundation of Liaoning Province underGrant no JDL2016006 and Talent Special Foundation ofDalian City under Grant no 2016RQ054 is acknowledged
References
[1] R K Garg K K Singh A Sachdeva V S Sharma K Ojhaand S Singh ldquoReview of research work in sinking EDM andWEDM on metal matrix composite materialsrdquo InternationalJournal of Advanced Manufacturing Technology vol 50no 5ndash8 pp 611ndash624 2010
[2] N Mohri N Saito Y Tsunekawa and N Kinoshita ldquoMetalsurface modification by electrical discharge machining withcomposite electroderdquo CIRP Annals vol 42 no 1 pp 219ndash2221993
[3] E Uhlmann and M Roehner ldquoInvestigations on reduction oftool electrode wear in micro-EDM using novel electrodematerialsrdquo CIRP Journal of Manufacturing Science andTechnology vol 1 no 2 pp 92ndash96 2008
[4] M Cao Y Hao Y Cao and S Yang ldquoMechanism and ex-perimental research on small-hole EDM with Cu-Cr com-posite electroderdquo Sensors amp Transducers vol 174 no 7pp 268ndash272 2014
[5] H C Tsai B H Yan and F Y Huang ldquoEDM performance ofCrCu-based composite electrodesrdquo International Journal ofMachine Tools and Manufacture vol 43 no 3 pp 245ndash2522003
[6] A K Khanra B R Sarkar B Bhattacharya L C Pathak andM M Godkhindi ldquoPerformance of ZrB2-Cu composite as anEDM electroderdquo Journal of Materials Processing Technologyvol 183 no 1 pp 122ndash126 2007
[7] T A El-Taweel ldquoMulti-response optimization of EDM withAl-Cu-Si-TiC PM composite electroderdquo International Jour-nal of Advanced Manufacturing Technology vol 44 no 1-2pp 100ndash113 2009
[8] V Senthilkumar and M C Reddy ldquoPerformance analysis ofCu-B4C metal matrix composite as an EDM electroderdquo In-ternational Journal of Machining and Machinability of Mate-rials vol 11 no 1 pp 36ndash50 2012
[9] D Anil and C Ccedilogun ldquoPerformance of copper-coated stereolithographic electrodes with internal cooling channels inelectric discharge machining (EDM)rdquo Rapid PrototypingJournal vol 14 no 4 pp 202ndash212 2008
[10] J Wang Y Wang F Zhao et al ldquoFabricating electrode byelectrodepositing technology and its application in micro-EDM deep hole drillingrdquo China Mechanical Engineeringvol 20 no 15 pp 1848ndash1852 2009 in Chinese
[11] Q Lv Simulation and Experimental Study on the Fabricationof the Composite Electrode for MEDM Dalian University ofTechnology Dalian China 2009 in Chinese
[12] X P Li Y G Wang F L Zhao M H Wu and Y LiuldquoInfluence of high frequency pulse on electrode wear in micro-EDMrdquo Defence Technology vol 10 no 3 pp 316ndash320 2014
[13] W Yuangang Z Fuling and W Jin ldquoWear-resist electrodesfor micro-EDMrdquo Chinese Journal of Aeronautics vol 22no 3 pp 339ndash342 2009
[14] A H Chiou C C Tsao and C Y Hsu ldquoA study of themachining characteristics of micro EDM milling and itsimprovement by electrode coatingrdquo International Journal ofAdvanced Manufacturing Technology vol 78 no 9-12pp 1857ndash1864 2015
[15] J Jiang and J Weng Cathodic Electronics and Gas DischargePrinciple National Defense Industry Press Beijing China1980 in Chinese
[16] W Zhao Research on the Corrosion of Elecrodes and ItseoryFoundation in EDM Northwestern Polytechnical UniversityXirsquoan China 2003 in Chinese
[17] B Izquierdo J A Sanchez S Plaza I Pombo and N OrtegaldquoA numerical model of the EDMprocess considering the effectof multiple dischargesrdquo International Journal of Machine Toolsand Manufacture vol 49 no 3 pp 220ndash229 2009
[18] -e Engineering ToolBox Specific Heats for Metals httpswwwengineeringtoolboxcomspecific-heat-metals-d_152html
[19] M Li eoretical Basis of Electrical Discharge MachiningNational Defense Industry Press Beijing China 1989 inChinese
8 Advances in Materials Science and Engineering
CorrosionInternational Journal of
Hindawiwwwhindawicom Volume 2018
Advances in
Materials Science and EngineeringHindawiwwwhindawicom Volume 2018
electrodes It has shown that the composite electrodes ob-tained can achieve a higher material removal rate (MRR)than copper electrodes the recast layer is thinner and fewercracks are present on the machined surface [5] Khanra et aldeveloped a metal matrix composite (Cu-ZrB2) to get anoptimum combination of wear resistance and electrical andthermal conductivity e Cu-ZrB2 composite was de-veloped by adding dierent amounts of Cu and tested as anelectrode material at various process parameters of EDMduring machining of mild steel It was found that the ZrB2-40wt Cu composite shows more MRR with less toolremoval rate (TRR) than commonly used Cu tool [6] El-Taweel investigated the relationship between process pa-rameters in electrodischarge of CK45 steel and tool electrodematerial as Al-Cu-Si-TiC composite is produced usingpowder metallurgy (PM) technique e central compositesecond-order rotatable design had been utilized to planthe experiments and response surface methodology(RSM) was employed for developing experimental modelsAl-Cu-Si-TiC PM electrodes are found to be more sensitiveto peak current and pulse on-time than conventionalelectrodes [7] Senthilkumar and Reddy developed a newcopper-based metal matrix composite (Cu-B4C) for an EDMelectrode to get an optimum combination of wear resistanceand electrical and thermal conductivity e results showedthat copper composite with 40 boron carbide re-inforcement exhibited betterMRR and TRR compared to theconventional copper electrode [8]
Anil and Ccedilogun from the experimental study observedand compared the performance of solid copper and copper-coated stereo lithography (CCSLA) electrode It was foundthat the internal cooling channel formed by SLA technologyprolongs the life of the CCSLA electrode by dissipating theheat of coating [9] Wang et al through composite elec-troplating technology electrodeposited Cu-ZrB2 compositecoating on the side of composite electrode they made forexperiments It was concluded that the Cu-ZrB2 compositecoating can improve the electrical erosion resistance of theelectrode and can eectively guarantee the uniform wear ofthe end face of the electrode and reduce the tapering of themachined hole [10] Lv studied the machining process ofmicro-EDM with composite electrodes by means of com-puter simulation e Cu-based Ni-W alloy compositeelectrode was fabricated by the electroplating method forEDM experiments e electrical erosion resistance of thesurface material is enhanced and the sidewall wear of theelectrode can be eectively reduced and the shape accuracyof the machined hole can be greatly improved [11] Li et alstudied the wear mechanism of the Ni-TiNCu compositeelectrode in the case of high-frequency pulse current andfound out the inuence of the uctuation frequency ofdischarge current on electrode wear in micro-EDM It isshown that compared with the electrodes made from ho-mogeneous materials the high-frequency electromagneticproperties of the Ni-TiN composite layer can be used ef-fectively to inhibit the skin eect of high-frequency pulse onthe electrode and improve the distribution trend of currentdensity [12] Yuangang et al made the Cu-ZrB2 compositecoating electrodes by way of the electrode position process
on the basis of the dierence between the dischargingperformance of the electrodeposited coating and that of thematrix to ensure uniform wear of electrode bottom facesCompared with the conventional electrodes Cu-ZrB2composite coating electrodes display better wear resistanceon the same experimental conditions [13] Chiou et alpresented a comparative study of the performance of WCWC-coated Ag andWC-coated Cu electrodes for the micro-EDM milling and the experimental results showed that theWC-coated Ag electrodes yielded the lowest surfaceroughness and the WC-coated Cu electrodes achieved thehighest material removal rate which illustrated the eec-tiveness of the electrode coating method [14]
Based on the above research we can iexclnd that althougha great deal of achievements have been made in the researchabout EDM with multimaterial electrodes the related re-searches only stay at the level of technologic experimentse discharge mechanisms of EDM with multimaterialelectrode such as the inuence mechanism of multi-materials on the distribution of discharge breakdown andthe law of distribution absorption and conduction ofdischarge energy in multiphase materials and the mecha-nism of ejection process of multiphase molten metal ma-terials are still not very clear is paper carries out thetheoretical analysis and experimental veriiexclcation about thebreakdown probability of the multimaterial electrode inEDM and relevant research results can provide a referencefor further analysis of EDM mechanism with multimaterialelectrodes
2 Breakdown Process Analysis ofMultimaterial Electrode
EDM is a kind of nontraditional machining method that usesspark discharge to breakdown the dielectric between poles ofextremely close distance and removes the electrode materialby way of thermal eect of breakdown e machiningprocess of EDM with multimaterial tool is shown in Figure 1Substantially the discharge breakdown process of EDM isaected directly by the thermal iexcleld electron emission ofcathode electrons After the thermal iexcleld electron emissionunder the eect of high-strength electric iexcleld the electronavalanche ionizations are generated in the interelectrode
Workpiece
Pulse supply
Dielectric
Multimaterial electrode
Gap
Figure 1 Machining process of EDM with multimaterial tool
2 Advances in Materials Science and Engineering
dielectric when the electron avalanche ionizations reach theanode the dielectric is broken down and the dischargechannel is formed -erefore in the early stage of the dis-charge breakdown process the amount of thermal fieldelectron emission of cathode electrons at a certain locationcan directly affect the probability of discharge breakdown atthat location And the amount of thermal field electronemission is characterized by thermal field electron emissioncurrent density -at is to say the position with large thermalfield electron emission current density has the high proba-bility of discharge breakdown
21 Single Discharge Breakdown Process According to thethermal field electron emission theory when the tempera-ture on the tool surface is T (unit K) and a strong electricalfield with the electric field intensity E (Vcm) is exertedbetween electrodes the current density of cathode electronemission j is given as [15]
j(T) N(T) middot j(0) (1)
where j(T) is the current density of field electron emissionwhile temperature is T (Acm2) and N(T) is the coefficientof correction when temperature is T (K) relative to whentemperature is 0K which is given as
N(T) Q
sin(Q) (2)
where
Q 277 times 104T
ϕ
1113968
E (3)
where ϕ is the electron work function (J) and j(0) is thecurrent density of field electron emission while temperatureis 0 K (Acm2) which can be expressed as
j(0) 154 times 10minus6E2
ϕ
middot exp minus683 times 107ϕ32
Eθ 339 times 10minus4
E
radic
ϕ1113888 11138891113890 1113891
(4)
where θ(y) is the Nordheim functionAs to different materials copper and iron (Fe) for ex-
ample we set the same electron emission conditions T
300 K and E 4 times 107 Vcm when the Nordheim functionθ(y) is approximately equal to 1 then among all the pa-rameters in (1)ndash(4) only electron work function ϕ is differentbetween these two materials while the rest of them are all thesame Electron work function of commonmetals is shown inTable 1 [16]
Based on the thermal field electron emission theory thefield electron emission current density can be used tocharacterize the breakdown probability Where the thermalfield electron emission current density is large it is easier toform discharge breakdown and further the breakdownprobability increases A ratio coefficient of breakdown prob-ability Rp is used to characterize the numerical comparison ofthermal field electron emission current density of different
electrode materials -e ratio coefficient of breakdown prob-ability Rp(Fe-Cu) is
Rp(Fe-Cu) j(T)Fe
j(T)Cu (5)
By substituting the electron work function of copper andiron as 524 eV and 447 eV into (1)ndash(4) and comparing we canget that j(0) of copper and iron is approximately equal andN(T) of copper and iron is also approximately equal thereforethe thermal field electron emission current density of the two isapproximately equal which means the influence of differentmaterials on the breakdown process is basically the same
22 Effect of Continuous Discharges on BreakdownProcess For continuous spark discharges the change of dis-charge environment caused by the previous breakdown processcan lead to next breakdown process occurring near the pre-vious breakdown discharge channel [17]-e two factors whichhave the most influence on the breakdown process are thedielectricrsquos properties and the change of the temperaturearound the discharge channel However the change of di-electric property is mainly related to the dielectric deionizationability and the distribution of the debris interelectrodes whichhas little relationship with the electrode material so it will notbe discussed here -e change of the temperature around thedischarge channel is mainly due to the electric heating effect ofthe plasma caused by the previous breakdown (the temperatureof the central region of the discharge channel can reach above10000K) even during thematerial removal process part of theheat is taken away by the debris and the dielectric the tem-perature of the material in the discharge crater can still beseveral thousand degrees And during the pulse interval thetemperature at crater decreases due to thermal conduction-efollowing equation is the Fourier thermal conduction law
dQ minusλzT
zndA dt (6)
where Q is the amount of heat conducted (J) λ is the thermalconductivity of the material (WmK) zTzn is the temper-ature gradient A is the heat conduction area (m2) and t is theheat conduction time (s) And the relationship equation be-tween the amount of heat conduction Q and temperaturechange ΔT is given by
Q ΔTcρV (7)
where c is the specific heat capacity of material (JkgmiddotK) ρ is thedensity of material (kgm3) andV is the volume ofmaterial (m3)
Supposed that two different materials have the same cratersize and the same temperature gradient and the changes of
Table 1 Electron work function of common metals [16]
Materials Electron work function ϕ (eV)Copper 524Brass 334ndash524Zinc 334Iron 447Copper-tungsten alloy 454ndash524Tungsten 454
Advances in Materials Science and Engineering 3
thermal conductivity and specific heat capacity of the materialsare very small with the temperature the conduction time is pulseinterval time and then from (6) and (7) the ratio of temperaturedifference before and after the heat conduction near the dis-charge crater of copper and iron RΔT(Cu-Fe) is given as
RΔT(Cu-Fe) ΔTCu
ΔTFeλCuλFe
middotcFeρFecCuρCu
(8)
From Table 2 [18] the value of (8) can be calculated as483 which indicates that the decrease of temperature due toheat transfer in the discharge region of copper material isabout 5 times that of the discharge region of the iron ma-terial because the thermal conductivity of the copper ma-terial is higher than that of the iron material For thedischarge region with an initial temperature of 3500K if thetemperature of the iron material electrode is decreased byheat transfer of 200K then the cooling of the copper ma-terial electrode will reach about 1000K At the same timedue to the difference of the temperature in the dischargeregion the value of the temperature correction coefficientN(T) will be changed accordingly and then the difference ofthe thermal field emission current j(T) is significant -ethermal field emission current j(T) is given as [19]
j(T) 4πemkT
h3 1113946exp minusc +Ee minusEF
d1113874 1113875
middot ln 1 + exp minusEe minusEF
kT1113874 11138751113874 1113875 dEe
(9)
where e is the electron charge m is the electron mass k is theBoltzmann constant h is the Planck constant Ee is the energyof the electron and EF is the Fermi energy By substituting theaforementioned known parameters from logarithm table ofcurrent density as shown in Table 3 it can be seen that thethermal conductivity of iron is poor so that it is not easy toconduct heat away from discharge region and as a result thetemperature is high which leads to the thermal field emissioncurrent density of the iron electrode (j(T)Fe) 537ndash832 timesthat of the copper electrode (j(T)Cu) under the same con-ditions -at is to say the ratio coefficient of breakdownprobability Rp(Fe-Cu) is 537ndash832
-erefore under the same discharge conditions the ironelectrode is more prone to breakdown than copper one afterthe continuous pulse discharges Table 4 shows the calcu-lation results of the ratio of temperature differenceΔTCuΔTi and the ratio of thermal field electron emissioncurrent density j(T)ij(T)Cu of different materials under theconditions when the initial temperature is 3500K and thetemperature drop by heat transfer is 200K From Table 4 itcan be seen that under the same conditions the thermal fieldemission current density of brass is the highest that is
617ndash871 times that of copper as to the copper-tungstenalloy it is 105ndash162 times that of copper -is means theeffects of different materials on the breakdown process ofcontinuous discharges differ significantly A large amount ofthermal field emission current density leads to the highprobability of breakdown under the same conditions brass hasthe highest probability of breakdown iron takes the secondplace followed by the copper-tungsten alloy and copper hasthe least probability of breakdown
3 EDM Experiment of Multimaterial Electrode
31 Experiment Conditions -e equipment used in theexperiment is a self-built EDM lathe which mainly consistsof automatic feeding device power box and worktable Itsspindle speed is 01ms and it can be adjusted adaptivelyaccording to the discharge condition to meet the processingrequirements under different working conditions and toprevent arc discharge -e EDM sinking machining methodis adopted and deionized water is used as the working fluid-e processing parameters of multimaterial electrodes forEDM are shown in Table 5
-e multimaterial electrodes are formed by connectingthe cylindrical electrodes of two different materials in paralleland the ends of multimaterial electrodes are polished to bevery smooth -e fabricated multimaterial electrodes are shownin Figure 2 A series of continuous pulse discharge experimentsare performed on a range of combinations of multimaterialelectrodes twice more repeated experiments are conductedfor each electrode combination and then the electrode endfaces are observed with a high-power electron microscopeand the distribution of the discharge craters is recorded
32 Experimental Results andAnalysis Figures 3ndash5 show theexperimental results of different multimaterial electrodes ofiron and copper brass and copper and copper-tungsten and
copper respectively From the figures we can see that underthe same conditions of discharge the number of craters oniron electrodes is more than that on copper ones andsimilarly the number of craters on brass electrodes is morethan that on copper ones and the craters on copper-tungstenelectrodes are more than those on copper ones
As can be seen from Figure 3 for the same dischargebreakdown condition the discharge points are mostly dis-tributed on the surface of the iron electrode at the end ofdischarge and only a few discharge points exist on the surfaceof the copper electrode -is shows that the breakdownprobability of iron is obviously higher than that of copper Itcan also be found that the thermal conductivity of iron ismuchsmaller than that of copper which indicates that after con-tinuous discharge because of the low thermal conductivity ofiron less heat is transmitted from the iron to the outsideMeanwhile as the specific heat capacity of iron is higher thanthat of copper iron of the same temperature as coppercontains more heat and the temperature reduction of the ironis also smaller-e temperature of the iron electrode surface ishigher than that of the copper electrode which promotes theemission of the cathode thermal field electron emission onthe electrode surface therefore the breakdown probability ofthe iron electrode is elevated -e high-temperature burntraces on the surface of the iron electrode from Figure 3(a) canalso demonstrate the presence of high temperature on thesurface of the electrode after continuous discharges
As can be seen from Figure 4 a large number of dischargepoints can be found on the surfaces of both copper and brasselectrodes and the discharge points on the surface of the brasselectrode are much more than those of the copper surfacealthough the difference between the two is not as large as the
difference between copper and iron electrodes-is is becausethe conductivity of brass is slightly better than iron and itsspecific heat capacity is similar to that of copper undercontinuous discharge conditions the temperature differencebetween the left and the right of the multimaterial electrode isnot as large as the copper-iron electrode therefore the dif-ference of discharge points decreases
FromFigure 5 it is noted that although the discharge pointsof the copper-tungsten alloy electrode are more than those ofthe copper electrode the difference between them is obviouslyreduced compared with Figures 3 and 4 -is is because al-though the thermal conductivity of copper-tungsten alloy issmaller than that of copper its specific heat is smaller than thatof copper too these two aspects have the same effect on thetemperature difference of heat conduction -e reason why thebreakdown probability of copper-tungsten alloy is higher thanthat of copper is probably because the density of copper-tungsten alloy is greater which can increase the amount ofheat contained in the same volume of material andmake up forthe deficiency of smaller specific heat capacity And because thethermal conductivity of copper-tungsten alloy is lower than thatof copper the surface temperature of copper-tungsten alloymaterial is higher so its breakdown probability is increased
33 Experimental Data Processing We quantitatively de-scribe the number of discharge craters distributed on dif-ferent electrodes by using the discharge affecting area onelectrode surface which can be calculated by pixel analysisfunction of image processing software Matlab -e pixelanalysis method is a method of using the image processing
Table 4 Calculation results of RΔT(Cu-i) and Rp(i-Cu) of differentmaterials
RΔT(Cu-i)(ΔTCu)ΔTi Rp(i-Cu)(j(T)ij(T)Cu)
Iron 483 537ndash832Brass (Zn35) 300 617ndash871Copper-tungstenalloy (W70) 147 105ndash162
Table 5 Processing parameters of multimaterial electrodes forEDM
Items ParametersWorkpiece material Die steel
Multimaterial electrodes Iron and copper copper andcopper-tungsten brass and copper
Machining polarity Normal-ickness of workpiece(mm) 4
Pulse on time Ton (μs) 125Pulse interval time Toff (μs) 75Breakdown voltage Ub (V) 45Peak current Ip (A) 08Diameter of cylindricalelectrode (mm) 2
Machining time t (s) 30Dielectric Deionized water
(a)
(b)
Figure 2 Fabricated multimaterial electrodes of iron and copper(a) Side view (b) Bottom view
Advances in Materials Science and Engineering 5
software to accurately analyze the area of an irregular imageIt utilizes proportion relation between image pixels andimage area in the same photo by calculating the displaypixels of testing image to analyze the actual area of testingimage According to the proportion relation between theimage pixel and the actual area an equation is given by
testing area (At)reference area (Ar)
display pixels of testing area (Pt)
display pixels of reference area (Pr)
(10)
When analyzing the discharge affecting area the refer-ence area Ar is the known quantity as the diameter of thecylindrical electrode in the machining process is 2mm for
ease of calculation a square with the side length of 2mm istaken as a reference image then the reference area is 4mm2the display pixels of reference area Pr are the known quantityand it is the corresponding pixels of the reference area in theimage processing software -e display pixels of testing areaPt are the pixels corresponding to discharge affecting areaselected in the figure -e discharge affecting areas differ ondifferent electrode surfaces and the display pixels of testingarea are different -erefore by obtaining the display pixelsof testing area in the software substituting the knownquantities of reference area and the display pixels of ref-erence area the discharge affecting area on electrode sur-face can be calculated accurately from (10) -e discharge
(a) (b) (c)
Figure 3 Discharge point distribution on the surface of iron and copper electrodes
(a) (b) (c)
Figure 4 Discharge point distribution on the surface of brass and copper electrodes
(a) (b) (c)
Figure 5 Discharge point distribution on the surface of copper-tungsten and copper electrodes
6 Advances in Materials Science and Engineering
aecting areas on the surface of dierent multimaterialelectrodes are calculated as shown in Figure 6
As can be seen from Figure 6 the discharge point areasfrom large to small are followed by brass iron copper-tungsten alloy and copper e discharge point area ofcopper is the smallest in each set of data and the dischargepoint areas of brass iron and copper-tungsten alloy are ofseveral-fold relation with that of copper is is because thethermal conductivity of copper is greater than that of othermaterials and its speciiexclc heat capacity is the smallest so that ithas the minimum breakdown probability and this corre-sponds well with the previous analysis results of the ratiocoeyencient of breakdown probability On the end face of theiron and copper tool the average discharge area on ironsurface is 35 times as that on copper surface On the brass andcopper tool the average discharge area on brass surface is 26times as that on copper surface e reason for the reductionof area dierence is that the discharge area of the coppersurface is increasedWhile on the copper-tungsten and coppertool the average discharge area ratio is less than 2 e dif-ference in the breakdown probability of copper between iron
brass and copper-tungsten alloy decreases gradually is isbecause the thermal conductivity of iron is smaller than that ofother materials and the speciiexclc heat capacity is greater thanthat of other materials so that it has the maximum breakdownprobability with copper Besides the thermal conductivity ofcopper-tungsten alloy is the largest among those materialsand its speciiexclc heat capacity is the smallest therefore thedierence in the breakdown probability compared withcopper is the smallest among the three materials In additionit can also be seen from Figure 6 that the discharge area of thebrass and copper tool is larger than that of the other twogroups overall and this may be due to the low electronwork function of zinc in brass which is helpful to improvethe occurrence rate of discharge breakdown increasing thenumber of discharges on the brass surface At the same timedue to the inuence of the discharge breakdown the numberof discharges on the surface of copper is also increased
According to the experiment results breakdown prob-ability of dierent materials compared with copper Pb(i-Cu)can be obtained as
Pb(i-Cu) Testing area of material i Ai( )
Testing area of material i Ai( ) + testing area of copper ACu( ) (11)
Based on the experiment results of breakdown probabilityas well as the ratio of temperature dierence RΔT(Cu-i) and theratio coeyencient of breakdown probability Rp(i-Cu) formularyiexcltting is carried out by the adaptive iexclt method using Matlaband the breakdown probability iexcltting formula of dierentmaterials compared with copper is given asPb(i-Cu) 07658 + 007448 sin π middot RΔT(Cu-i) middot Rp(i-Cu)( )
minus 0269 exp minus 01093Rp(i-Cu)( )2
( )
(12)
From (12) the exact breakdown probability of certainmaterial compared to copper can be obtained and furtherthe breakdown probability between any two dierent ma-terials can also be obtained
e proposed model in this paper has the ability toanalyze and predict the electrode discharge breakdownprobability of dierent materials and the reasons for thedierent breakdown probabilities of dierent materials aregiven with a reasonable explanation by the model which isof some signiiexclcance for the understanding of dischargemechanism of EDM
0
0005
001
0015
002
Disc
harg
e are
a (cm
2 )
Experimental dataAverage value
Iron Copper Brass Cu-W CopperIron and copper tool Brass and copper tool Cu-W and copper tool
Copper
Figure 6 Discharge area on the surface of dierent multimaterial tools
Advances in Materials Science and Engineering 7
4 Conclusions
In this paper the influence law of surface breakdownprobability of multimaterial electrodes is analyzed accordingto the theory of field electron emission a mathematicalmodel of material property influencing discharge break-down in interelectrodes is established the model is verifiedby experimental method and the conclusions are drawn asfollows
(1) In single pulse discharge process the breakdownprobability of different material electrodes is ap-proximately the same and the discharge process isnot influenced basically
(2) Under continuous discharge conditions due to theheating effect of previous discharges the surfacetemperature of tool material with poor thermal con-ductivity and high specific heat capacity is higherwhich makes it easier to cause local discharge break-down and hence increases breakdown probabilityBreakdown probability is influenced by the propertiesof the materials such as thermal conductivity specificheat capacity and density
(3) -e repeatable experimental results of continuousdischarges have provided the relationship betweenbreakdown probability and different material propertiesand also proven the effectiveness of the dischargebreakdown probability model
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e financial support from the National Natural ScienceFoundation of China under Grant no 51405058 ScientificResearch Platform Foundation of Liaoning Province underGrant no JDL2016006 and Talent Special Foundation ofDalian City under Grant no 2016RQ054 is acknowledged
References
[1] R K Garg K K Singh A Sachdeva V S Sharma K Ojhaand S Singh ldquoReview of research work in sinking EDM andWEDM on metal matrix composite materialsrdquo InternationalJournal of Advanced Manufacturing Technology vol 50no 5ndash8 pp 611ndash624 2010
[2] N Mohri N Saito Y Tsunekawa and N Kinoshita ldquoMetalsurface modification by electrical discharge machining withcomposite electroderdquo CIRP Annals vol 42 no 1 pp 219ndash2221993
[3] E Uhlmann and M Roehner ldquoInvestigations on reduction oftool electrode wear in micro-EDM using novel electrodematerialsrdquo CIRP Journal of Manufacturing Science andTechnology vol 1 no 2 pp 92ndash96 2008
[4] M Cao Y Hao Y Cao and S Yang ldquoMechanism and ex-perimental research on small-hole EDM with Cu-Cr com-posite electroderdquo Sensors amp Transducers vol 174 no 7pp 268ndash272 2014
[5] H C Tsai B H Yan and F Y Huang ldquoEDM performance ofCrCu-based composite electrodesrdquo International Journal ofMachine Tools and Manufacture vol 43 no 3 pp 245ndash2522003
[6] A K Khanra B R Sarkar B Bhattacharya L C Pathak andM M Godkhindi ldquoPerformance of ZrB2-Cu composite as anEDM electroderdquo Journal of Materials Processing Technologyvol 183 no 1 pp 122ndash126 2007
[7] T A El-Taweel ldquoMulti-response optimization of EDM withAl-Cu-Si-TiC PM composite electroderdquo International Jour-nal of Advanced Manufacturing Technology vol 44 no 1-2pp 100ndash113 2009
[8] V Senthilkumar and M C Reddy ldquoPerformance analysis ofCu-B4C metal matrix composite as an EDM electroderdquo In-ternational Journal of Machining and Machinability of Mate-rials vol 11 no 1 pp 36ndash50 2012
[9] D Anil and C Ccedilogun ldquoPerformance of copper-coated stereolithographic electrodes with internal cooling channels inelectric discharge machining (EDM)rdquo Rapid PrototypingJournal vol 14 no 4 pp 202ndash212 2008
[10] J Wang Y Wang F Zhao et al ldquoFabricating electrode byelectrodepositing technology and its application in micro-EDM deep hole drillingrdquo China Mechanical Engineeringvol 20 no 15 pp 1848ndash1852 2009 in Chinese
[11] Q Lv Simulation and Experimental Study on the Fabricationof the Composite Electrode for MEDM Dalian University ofTechnology Dalian China 2009 in Chinese
[12] X P Li Y G Wang F L Zhao M H Wu and Y LiuldquoInfluence of high frequency pulse on electrode wear in micro-EDMrdquo Defence Technology vol 10 no 3 pp 316ndash320 2014
[13] W Yuangang Z Fuling and W Jin ldquoWear-resist electrodesfor micro-EDMrdquo Chinese Journal of Aeronautics vol 22no 3 pp 339ndash342 2009
[14] A H Chiou C C Tsao and C Y Hsu ldquoA study of themachining characteristics of micro EDM milling and itsimprovement by electrode coatingrdquo International Journal ofAdvanced Manufacturing Technology vol 78 no 9-12pp 1857ndash1864 2015
[15] J Jiang and J Weng Cathodic Electronics and Gas DischargePrinciple National Defense Industry Press Beijing China1980 in Chinese
[16] W Zhao Research on the Corrosion of Elecrodes and ItseoryFoundation in EDM Northwestern Polytechnical UniversityXirsquoan China 2003 in Chinese
[17] B Izquierdo J A Sanchez S Plaza I Pombo and N OrtegaldquoA numerical model of the EDMprocess considering the effectof multiple dischargesrdquo International Journal of Machine Toolsand Manufacture vol 49 no 3 pp 220ndash229 2009
[18] -e Engineering ToolBox Specific Heats for Metals httpswwwengineeringtoolboxcomspecific-heat-metals-d_152html
[19] M Li eoretical Basis of Electrical Discharge MachiningNational Defense Industry Press Beijing China 1989 inChinese
8 Advances in Materials Science and Engineering
CorrosionInternational Journal of
Hindawiwwwhindawicom Volume 2018
Advances in
Materials Science and EngineeringHindawiwwwhindawicom Volume 2018
dielectric when the electron avalanche ionizations reach theanode the dielectric is broken down and the dischargechannel is formed -erefore in the early stage of the dis-charge breakdown process the amount of thermal fieldelectron emission of cathode electrons at a certain locationcan directly affect the probability of discharge breakdown atthat location And the amount of thermal field electronemission is characterized by thermal field electron emissioncurrent density -at is to say the position with large thermalfield electron emission current density has the high proba-bility of discharge breakdown
21 Single Discharge Breakdown Process According to thethermal field electron emission theory when the tempera-ture on the tool surface is T (unit K) and a strong electricalfield with the electric field intensity E (Vcm) is exertedbetween electrodes the current density of cathode electronemission j is given as [15]
j(T) N(T) middot j(0) (1)
where j(T) is the current density of field electron emissionwhile temperature is T (Acm2) and N(T) is the coefficientof correction when temperature is T (K) relative to whentemperature is 0K which is given as
N(T) Q
sin(Q) (2)
where
Q 277 times 104T
ϕ
1113968
E (3)
where ϕ is the electron work function (J) and j(0) is thecurrent density of field electron emission while temperatureis 0 K (Acm2) which can be expressed as
j(0) 154 times 10minus6E2
ϕ
middot exp minus683 times 107ϕ32
Eθ 339 times 10minus4
E
radic
ϕ1113888 11138891113890 1113891
(4)
where θ(y) is the Nordheim functionAs to different materials copper and iron (Fe) for ex-
ample we set the same electron emission conditions T
300 K and E 4 times 107 Vcm when the Nordheim functionθ(y) is approximately equal to 1 then among all the pa-rameters in (1)ndash(4) only electron work function ϕ is differentbetween these two materials while the rest of them are all thesame Electron work function of commonmetals is shown inTable 1 [16]
Based on the thermal field electron emission theory thefield electron emission current density can be used tocharacterize the breakdown probability Where the thermalfield electron emission current density is large it is easier toform discharge breakdown and further the breakdownprobability increases A ratio coefficient of breakdown prob-ability Rp is used to characterize the numerical comparison ofthermal field electron emission current density of different
electrode materials -e ratio coefficient of breakdown prob-ability Rp(Fe-Cu) is
Rp(Fe-Cu) j(T)Fe
j(T)Cu (5)
By substituting the electron work function of copper andiron as 524 eV and 447 eV into (1)ndash(4) and comparing we canget that j(0) of copper and iron is approximately equal andN(T) of copper and iron is also approximately equal thereforethe thermal field electron emission current density of the two isapproximately equal which means the influence of differentmaterials on the breakdown process is basically the same
22 Effect of Continuous Discharges on BreakdownProcess For continuous spark discharges the change of dis-charge environment caused by the previous breakdown processcan lead to next breakdown process occurring near the pre-vious breakdown discharge channel [17]-e two factors whichhave the most influence on the breakdown process are thedielectricrsquos properties and the change of the temperaturearound the discharge channel However the change of di-electric property is mainly related to the dielectric deionizationability and the distribution of the debris interelectrodes whichhas little relationship with the electrode material so it will notbe discussed here -e change of the temperature around thedischarge channel is mainly due to the electric heating effect ofthe plasma caused by the previous breakdown (the temperatureof the central region of the discharge channel can reach above10000K) even during thematerial removal process part of theheat is taken away by the debris and the dielectric the tem-perature of the material in the discharge crater can still beseveral thousand degrees And during the pulse interval thetemperature at crater decreases due to thermal conduction-efollowing equation is the Fourier thermal conduction law
dQ minusλzT
zndA dt (6)
where Q is the amount of heat conducted (J) λ is the thermalconductivity of the material (WmK) zTzn is the temper-ature gradient A is the heat conduction area (m2) and t is theheat conduction time (s) And the relationship equation be-tween the amount of heat conduction Q and temperaturechange ΔT is given by
Q ΔTcρV (7)
where c is the specific heat capacity of material (JkgmiddotK) ρ is thedensity of material (kgm3) andV is the volume ofmaterial (m3)
Supposed that two different materials have the same cratersize and the same temperature gradient and the changes of
Table 1 Electron work function of common metals [16]
Materials Electron work function ϕ (eV)Copper 524Brass 334ndash524Zinc 334Iron 447Copper-tungsten alloy 454ndash524Tungsten 454
Advances in Materials Science and Engineering 3
thermal conductivity and specific heat capacity of the materialsare very small with the temperature the conduction time is pulseinterval time and then from (6) and (7) the ratio of temperaturedifference before and after the heat conduction near the dis-charge crater of copper and iron RΔT(Cu-Fe) is given as
RΔT(Cu-Fe) ΔTCu
ΔTFeλCuλFe
middotcFeρFecCuρCu
(8)
From Table 2 [18] the value of (8) can be calculated as483 which indicates that the decrease of temperature due toheat transfer in the discharge region of copper material isabout 5 times that of the discharge region of the iron ma-terial because the thermal conductivity of the copper ma-terial is higher than that of the iron material For thedischarge region with an initial temperature of 3500K if thetemperature of the iron material electrode is decreased byheat transfer of 200K then the cooling of the copper ma-terial electrode will reach about 1000K At the same timedue to the difference of the temperature in the dischargeregion the value of the temperature correction coefficientN(T) will be changed accordingly and then the difference ofthe thermal field emission current j(T) is significant -ethermal field emission current j(T) is given as [19]
j(T) 4πemkT
h3 1113946exp minusc +Ee minusEF
d1113874 1113875
middot ln 1 + exp minusEe minusEF
kT1113874 11138751113874 1113875 dEe
(9)
where e is the electron charge m is the electron mass k is theBoltzmann constant h is the Planck constant Ee is the energyof the electron and EF is the Fermi energy By substituting theaforementioned known parameters from logarithm table ofcurrent density as shown in Table 3 it can be seen that thethermal conductivity of iron is poor so that it is not easy toconduct heat away from discharge region and as a result thetemperature is high which leads to the thermal field emissioncurrent density of the iron electrode (j(T)Fe) 537ndash832 timesthat of the copper electrode (j(T)Cu) under the same con-ditions -at is to say the ratio coefficient of breakdownprobability Rp(Fe-Cu) is 537ndash832
-erefore under the same discharge conditions the ironelectrode is more prone to breakdown than copper one afterthe continuous pulse discharges Table 4 shows the calcu-lation results of the ratio of temperature differenceΔTCuΔTi and the ratio of thermal field electron emissioncurrent density j(T)ij(T)Cu of different materials under theconditions when the initial temperature is 3500K and thetemperature drop by heat transfer is 200K From Table 4 itcan be seen that under the same conditions the thermal fieldemission current density of brass is the highest that is
617ndash871 times that of copper as to the copper-tungstenalloy it is 105ndash162 times that of copper -is means theeffects of different materials on the breakdown process ofcontinuous discharges differ significantly A large amount ofthermal field emission current density leads to the highprobability of breakdown under the same conditions brass hasthe highest probability of breakdown iron takes the secondplace followed by the copper-tungsten alloy and copper hasthe least probability of breakdown
3 EDM Experiment of Multimaterial Electrode
31 Experiment Conditions -e equipment used in theexperiment is a self-built EDM lathe which mainly consistsof automatic feeding device power box and worktable Itsspindle speed is 01ms and it can be adjusted adaptivelyaccording to the discharge condition to meet the processingrequirements under different working conditions and toprevent arc discharge -e EDM sinking machining methodis adopted and deionized water is used as the working fluid-e processing parameters of multimaterial electrodes forEDM are shown in Table 5
-e multimaterial electrodes are formed by connectingthe cylindrical electrodes of two different materials in paralleland the ends of multimaterial electrodes are polished to bevery smooth -e fabricated multimaterial electrodes are shownin Figure 2 A series of continuous pulse discharge experimentsare performed on a range of combinations of multimaterialelectrodes twice more repeated experiments are conductedfor each electrode combination and then the electrode endfaces are observed with a high-power electron microscopeand the distribution of the discharge craters is recorded
32 Experimental Results andAnalysis Figures 3ndash5 show theexperimental results of different multimaterial electrodes ofiron and copper brass and copper and copper-tungsten and
copper respectively From the figures we can see that underthe same conditions of discharge the number of craters oniron electrodes is more than that on copper ones andsimilarly the number of craters on brass electrodes is morethan that on copper ones and the craters on copper-tungstenelectrodes are more than those on copper ones
As can be seen from Figure 3 for the same dischargebreakdown condition the discharge points are mostly dis-tributed on the surface of the iron electrode at the end ofdischarge and only a few discharge points exist on the surfaceof the copper electrode -is shows that the breakdownprobability of iron is obviously higher than that of copper Itcan also be found that the thermal conductivity of iron ismuchsmaller than that of copper which indicates that after con-tinuous discharge because of the low thermal conductivity ofiron less heat is transmitted from the iron to the outsideMeanwhile as the specific heat capacity of iron is higher thanthat of copper iron of the same temperature as coppercontains more heat and the temperature reduction of the ironis also smaller-e temperature of the iron electrode surface ishigher than that of the copper electrode which promotes theemission of the cathode thermal field electron emission onthe electrode surface therefore the breakdown probability ofthe iron electrode is elevated -e high-temperature burntraces on the surface of the iron electrode from Figure 3(a) canalso demonstrate the presence of high temperature on thesurface of the electrode after continuous discharges
As can be seen from Figure 4 a large number of dischargepoints can be found on the surfaces of both copper and brasselectrodes and the discharge points on the surface of the brasselectrode are much more than those of the copper surfacealthough the difference between the two is not as large as the
difference between copper and iron electrodes-is is becausethe conductivity of brass is slightly better than iron and itsspecific heat capacity is similar to that of copper undercontinuous discharge conditions the temperature differencebetween the left and the right of the multimaterial electrode isnot as large as the copper-iron electrode therefore the dif-ference of discharge points decreases
FromFigure 5 it is noted that although the discharge pointsof the copper-tungsten alloy electrode are more than those ofthe copper electrode the difference between them is obviouslyreduced compared with Figures 3 and 4 -is is because al-though the thermal conductivity of copper-tungsten alloy issmaller than that of copper its specific heat is smaller than thatof copper too these two aspects have the same effect on thetemperature difference of heat conduction -e reason why thebreakdown probability of copper-tungsten alloy is higher thanthat of copper is probably because the density of copper-tungsten alloy is greater which can increase the amount ofheat contained in the same volume of material andmake up forthe deficiency of smaller specific heat capacity And because thethermal conductivity of copper-tungsten alloy is lower than thatof copper the surface temperature of copper-tungsten alloymaterial is higher so its breakdown probability is increased
33 Experimental Data Processing We quantitatively de-scribe the number of discharge craters distributed on dif-ferent electrodes by using the discharge affecting area onelectrode surface which can be calculated by pixel analysisfunction of image processing software Matlab -e pixelanalysis method is a method of using the image processing
Table 4 Calculation results of RΔT(Cu-i) and Rp(i-Cu) of differentmaterials
RΔT(Cu-i)(ΔTCu)ΔTi Rp(i-Cu)(j(T)ij(T)Cu)
Iron 483 537ndash832Brass (Zn35) 300 617ndash871Copper-tungstenalloy (W70) 147 105ndash162
Table 5 Processing parameters of multimaterial electrodes forEDM
Items ParametersWorkpiece material Die steel
Multimaterial electrodes Iron and copper copper andcopper-tungsten brass and copper
Machining polarity Normal-ickness of workpiece(mm) 4
Pulse on time Ton (μs) 125Pulse interval time Toff (μs) 75Breakdown voltage Ub (V) 45Peak current Ip (A) 08Diameter of cylindricalelectrode (mm) 2
Machining time t (s) 30Dielectric Deionized water
(a)
(b)
Figure 2 Fabricated multimaterial electrodes of iron and copper(a) Side view (b) Bottom view
Advances in Materials Science and Engineering 5
software to accurately analyze the area of an irregular imageIt utilizes proportion relation between image pixels andimage area in the same photo by calculating the displaypixels of testing image to analyze the actual area of testingimage According to the proportion relation between theimage pixel and the actual area an equation is given by
testing area (At)reference area (Ar)
display pixels of testing area (Pt)
display pixels of reference area (Pr)
(10)
When analyzing the discharge affecting area the refer-ence area Ar is the known quantity as the diameter of thecylindrical electrode in the machining process is 2mm for
ease of calculation a square with the side length of 2mm istaken as a reference image then the reference area is 4mm2the display pixels of reference area Pr are the known quantityand it is the corresponding pixels of the reference area in theimage processing software -e display pixels of testing areaPt are the pixels corresponding to discharge affecting areaselected in the figure -e discharge affecting areas differ ondifferent electrode surfaces and the display pixels of testingarea are different -erefore by obtaining the display pixelsof testing area in the software substituting the knownquantities of reference area and the display pixels of ref-erence area the discharge affecting area on electrode sur-face can be calculated accurately from (10) -e discharge
(a) (b) (c)
Figure 3 Discharge point distribution on the surface of iron and copper electrodes
(a) (b) (c)
Figure 4 Discharge point distribution on the surface of brass and copper electrodes
(a) (b) (c)
Figure 5 Discharge point distribution on the surface of copper-tungsten and copper electrodes
6 Advances in Materials Science and Engineering
aecting areas on the surface of dierent multimaterialelectrodes are calculated as shown in Figure 6
As can be seen from Figure 6 the discharge point areasfrom large to small are followed by brass iron copper-tungsten alloy and copper e discharge point area ofcopper is the smallest in each set of data and the dischargepoint areas of brass iron and copper-tungsten alloy are ofseveral-fold relation with that of copper is is because thethermal conductivity of copper is greater than that of othermaterials and its speciiexclc heat capacity is the smallest so that ithas the minimum breakdown probability and this corre-sponds well with the previous analysis results of the ratiocoeyencient of breakdown probability On the end face of theiron and copper tool the average discharge area on ironsurface is 35 times as that on copper surface On the brass andcopper tool the average discharge area on brass surface is 26times as that on copper surface e reason for the reductionof area dierence is that the discharge area of the coppersurface is increasedWhile on the copper-tungsten and coppertool the average discharge area ratio is less than 2 e dif-ference in the breakdown probability of copper between iron
brass and copper-tungsten alloy decreases gradually is isbecause the thermal conductivity of iron is smaller than that ofother materials and the speciiexclc heat capacity is greater thanthat of other materials so that it has the maximum breakdownprobability with copper Besides the thermal conductivity ofcopper-tungsten alloy is the largest among those materialsand its speciiexclc heat capacity is the smallest therefore thedierence in the breakdown probability compared withcopper is the smallest among the three materials In additionit can also be seen from Figure 6 that the discharge area of thebrass and copper tool is larger than that of the other twogroups overall and this may be due to the low electronwork function of zinc in brass which is helpful to improvethe occurrence rate of discharge breakdown increasing thenumber of discharges on the brass surface At the same timedue to the inuence of the discharge breakdown the numberof discharges on the surface of copper is also increased
According to the experiment results breakdown prob-ability of dierent materials compared with copper Pb(i-Cu)can be obtained as
Pb(i-Cu) Testing area of material i Ai( )
Testing area of material i Ai( ) + testing area of copper ACu( ) (11)
Based on the experiment results of breakdown probabilityas well as the ratio of temperature dierence RΔT(Cu-i) and theratio coeyencient of breakdown probability Rp(i-Cu) formularyiexcltting is carried out by the adaptive iexclt method using Matlaband the breakdown probability iexcltting formula of dierentmaterials compared with copper is given asPb(i-Cu) 07658 + 007448 sin π middot RΔT(Cu-i) middot Rp(i-Cu)( )
minus 0269 exp minus 01093Rp(i-Cu)( )2
( )
(12)
From (12) the exact breakdown probability of certainmaterial compared to copper can be obtained and furtherthe breakdown probability between any two dierent ma-terials can also be obtained
e proposed model in this paper has the ability toanalyze and predict the electrode discharge breakdownprobability of dierent materials and the reasons for thedierent breakdown probabilities of dierent materials aregiven with a reasonable explanation by the model which isof some signiiexclcance for the understanding of dischargemechanism of EDM
0
0005
001
0015
002
Disc
harg
e are
a (cm
2 )
Experimental dataAverage value
Iron Copper Brass Cu-W CopperIron and copper tool Brass and copper tool Cu-W and copper tool
Copper
Figure 6 Discharge area on the surface of dierent multimaterial tools
Advances in Materials Science and Engineering 7
4 Conclusions
In this paper the influence law of surface breakdownprobability of multimaterial electrodes is analyzed accordingto the theory of field electron emission a mathematicalmodel of material property influencing discharge break-down in interelectrodes is established the model is verifiedby experimental method and the conclusions are drawn asfollows
(1) In single pulse discharge process the breakdownprobability of different material electrodes is ap-proximately the same and the discharge process isnot influenced basically
(2) Under continuous discharge conditions due to theheating effect of previous discharges the surfacetemperature of tool material with poor thermal con-ductivity and high specific heat capacity is higherwhich makes it easier to cause local discharge break-down and hence increases breakdown probabilityBreakdown probability is influenced by the propertiesof the materials such as thermal conductivity specificheat capacity and density
(3) -e repeatable experimental results of continuousdischarges have provided the relationship betweenbreakdown probability and different material propertiesand also proven the effectiveness of the dischargebreakdown probability model
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e financial support from the National Natural ScienceFoundation of China under Grant no 51405058 ScientificResearch Platform Foundation of Liaoning Province underGrant no JDL2016006 and Talent Special Foundation ofDalian City under Grant no 2016RQ054 is acknowledged
References
[1] R K Garg K K Singh A Sachdeva V S Sharma K Ojhaand S Singh ldquoReview of research work in sinking EDM andWEDM on metal matrix composite materialsrdquo InternationalJournal of Advanced Manufacturing Technology vol 50no 5ndash8 pp 611ndash624 2010
[2] N Mohri N Saito Y Tsunekawa and N Kinoshita ldquoMetalsurface modification by electrical discharge machining withcomposite electroderdquo CIRP Annals vol 42 no 1 pp 219ndash2221993
[3] E Uhlmann and M Roehner ldquoInvestigations on reduction oftool electrode wear in micro-EDM using novel electrodematerialsrdquo CIRP Journal of Manufacturing Science andTechnology vol 1 no 2 pp 92ndash96 2008
[4] M Cao Y Hao Y Cao and S Yang ldquoMechanism and ex-perimental research on small-hole EDM with Cu-Cr com-posite electroderdquo Sensors amp Transducers vol 174 no 7pp 268ndash272 2014
[5] H C Tsai B H Yan and F Y Huang ldquoEDM performance ofCrCu-based composite electrodesrdquo International Journal ofMachine Tools and Manufacture vol 43 no 3 pp 245ndash2522003
[6] A K Khanra B R Sarkar B Bhattacharya L C Pathak andM M Godkhindi ldquoPerformance of ZrB2-Cu composite as anEDM electroderdquo Journal of Materials Processing Technologyvol 183 no 1 pp 122ndash126 2007
[7] T A El-Taweel ldquoMulti-response optimization of EDM withAl-Cu-Si-TiC PM composite electroderdquo International Jour-nal of Advanced Manufacturing Technology vol 44 no 1-2pp 100ndash113 2009
[8] V Senthilkumar and M C Reddy ldquoPerformance analysis ofCu-B4C metal matrix composite as an EDM electroderdquo In-ternational Journal of Machining and Machinability of Mate-rials vol 11 no 1 pp 36ndash50 2012
[9] D Anil and C Ccedilogun ldquoPerformance of copper-coated stereolithographic electrodes with internal cooling channels inelectric discharge machining (EDM)rdquo Rapid PrototypingJournal vol 14 no 4 pp 202ndash212 2008
[10] J Wang Y Wang F Zhao et al ldquoFabricating electrode byelectrodepositing technology and its application in micro-EDM deep hole drillingrdquo China Mechanical Engineeringvol 20 no 15 pp 1848ndash1852 2009 in Chinese
[11] Q Lv Simulation and Experimental Study on the Fabricationof the Composite Electrode for MEDM Dalian University ofTechnology Dalian China 2009 in Chinese
[12] X P Li Y G Wang F L Zhao M H Wu and Y LiuldquoInfluence of high frequency pulse on electrode wear in micro-EDMrdquo Defence Technology vol 10 no 3 pp 316ndash320 2014
[13] W Yuangang Z Fuling and W Jin ldquoWear-resist electrodesfor micro-EDMrdquo Chinese Journal of Aeronautics vol 22no 3 pp 339ndash342 2009
[14] A H Chiou C C Tsao and C Y Hsu ldquoA study of themachining characteristics of micro EDM milling and itsimprovement by electrode coatingrdquo International Journal ofAdvanced Manufacturing Technology vol 78 no 9-12pp 1857ndash1864 2015
[15] J Jiang and J Weng Cathodic Electronics and Gas DischargePrinciple National Defense Industry Press Beijing China1980 in Chinese
[16] W Zhao Research on the Corrosion of Elecrodes and ItseoryFoundation in EDM Northwestern Polytechnical UniversityXirsquoan China 2003 in Chinese
[17] B Izquierdo J A Sanchez S Plaza I Pombo and N OrtegaldquoA numerical model of the EDMprocess considering the effectof multiple dischargesrdquo International Journal of Machine Toolsand Manufacture vol 49 no 3 pp 220ndash229 2009
[18] -e Engineering ToolBox Specific Heats for Metals httpswwwengineeringtoolboxcomspecific-heat-metals-d_152html
[19] M Li eoretical Basis of Electrical Discharge MachiningNational Defense Industry Press Beijing China 1989 inChinese
8 Advances in Materials Science and Engineering
CorrosionInternational Journal of
Hindawiwwwhindawicom Volume 2018
Advances in
Materials Science and EngineeringHindawiwwwhindawicom Volume 2018
thermal conductivity and specific heat capacity of the materialsare very small with the temperature the conduction time is pulseinterval time and then from (6) and (7) the ratio of temperaturedifference before and after the heat conduction near the dis-charge crater of copper and iron RΔT(Cu-Fe) is given as
RΔT(Cu-Fe) ΔTCu
ΔTFeλCuλFe
middotcFeρFecCuρCu
(8)
From Table 2 [18] the value of (8) can be calculated as483 which indicates that the decrease of temperature due toheat transfer in the discharge region of copper material isabout 5 times that of the discharge region of the iron ma-terial because the thermal conductivity of the copper ma-terial is higher than that of the iron material For thedischarge region with an initial temperature of 3500K if thetemperature of the iron material electrode is decreased byheat transfer of 200K then the cooling of the copper ma-terial electrode will reach about 1000K At the same timedue to the difference of the temperature in the dischargeregion the value of the temperature correction coefficientN(T) will be changed accordingly and then the difference ofthe thermal field emission current j(T) is significant -ethermal field emission current j(T) is given as [19]
j(T) 4πemkT
h3 1113946exp minusc +Ee minusEF
d1113874 1113875
middot ln 1 + exp minusEe minusEF
kT1113874 11138751113874 1113875 dEe
(9)
where e is the electron charge m is the electron mass k is theBoltzmann constant h is the Planck constant Ee is the energyof the electron and EF is the Fermi energy By substituting theaforementioned known parameters from logarithm table ofcurrent density as shown in Table 3 it can be seen that thethermal conductivity of iron is poor so that it is not easy toconduct heat away from discharge region and as a result thetemperature is high which leads to the thermal field emissioncurrent density of the iron electrode (j(T)Fe) 537ndash832 timesthat of the copper electrode (j(T)Cu) under the same con-ditions -at is to say the ratio coefficient of breakdownprobability Rp(Fe-Cu) is 537ndash832
-erefore under the same discharge conditions the ironelectrode is more prone to breakdown than copper one afterthe continuous pulse discharges Table 4 shows the calcu-lation results of the ratio of temperature differenceΔTCuΔTi and the ratio of thermal field electron emissioncurrent density j(T)ij(T)Cu of different materials under theconditions when the initial temperature is 3500K and thetemperature drop by heat transfer is 200K From Table 4 itcan be seen that under the same conditions the thermal fieldemission current density of brass is the highest that is
617ndash871 times that of copper as to the copper-tungstenalloy it is 105ndash162 times that of copper -is means theeffects of different materials on the breakdown process ofcontinuous discharges differ significantly A large amount ofthermal field emission current density leads to the highprobability of breakdown under the same conditions brass hasthe highest probability of breakdown iron takes the secondplace followed by the copper-tungsten alloy and copper hasthe least probability of breakdown
3 EDM Experiment of Multimaterial Electrode
31 Experiment Conditions -e equipment used in theexperiment is a self-built EDM lathe which mainly consistsof automatic feeding device power box and worktable Itsspindle speed is 01ms and it can be adjusted adaptivelyaccording to the discharge condition to meet the processingrequirements under different working conditions and toprevent arc discharge -e EDM sinking machining methodis adopted and deionized water is used as the working fluid-e processing parameters of multimaterial electrodes forEDM are shown in Table 5
-e multimaterial electrodes are formed by connectingthe cylindrical electrodes of two different materials in paralleland the ends of multimaterial electrodes are polished to bevery smooth -e fabricated multimaterial electrodes are shownin Figure 2 A series of continuous pulse discharge experimentsare performed on a range of combinations of multimaterialelectrodes twice more repeated experiments are conductedfor each electrode combination and then the electrode endfaces are observed with a high-power electron microscopeand the distribution of the discharge craters is recorded
32 Experimental Results andAnalysis Figures 3ndash5 show theexperimental results of different multimaterial electrodes ofiron and copper brass and copper and copper-tungsten and
copper respectively From the figures we can see that underthe same conditions of discharge the number of craters oniron electrodes is more than that on copper ones andsimilarly the number of craters on brass electrodes is morethan that on copper ones and the craters on copper-tungstenelectrodes are more than those on copper ones
As can be seen from Figure 3 for the same dischargebreakdown condition the discharge points are mostly dis-tributed on the surface of the iron electrode at the end ofdischarge and only a few discharge points exist on the surfaceof the copper electrode -is shows that the breakdownprobability of iron is obviously higher than that of copper Itcan also be found that the thermal conductivity of iron ismuchsmaller than that of copper which indicates that after con-tinuous discharge because of the low thermal conductivity ofiron less heat is transmitted from the iron to the outsideMeanwhile as the specific heat capacity of iron is higher thanthat of copper iron of the same temperature as coppercontains more heat and the temperature reduction of the ironis also smaller-e temperature of the iron electrode surface ishigher than that of the copper electrode which promotes theemission of the cathode thermal field electron emission onthe electrode surface therefore the breakdown probability ofthe iron electrode is elevated -e high-temperature burntraces on the surface of the iron electrode from Figure 3(a) canalso demonstrate the presence of high temperature on thesurface of the electrode after continuous discharges
As can be seen from Figure 4 a large number of dischargepoints can be found on the surfaces of both copper and brasselectrodes and the discharge points on the surface of the brasselectrode are much more than those of the copper surfacealthough the difference between the two is not as large as the
difference between copper and iron electrodes-is is becausethe conductivity of brass is slightly better than iron and itsspecific heat capacity is similar to that of copper undercontinuous discharge conditions the temperature differencebetween the left and the right of the multimaterial electrode isnot as large as the copper-iron electrode therefore the dif-ference of discharge points decreases
FromFigure 5 it is noted that although the discharge pointsof the copper-tungsten alloy electrode are more than those ofthe copper electrode the difference between them is obviouslyreduced compared with Figures 3 and 4 -is is because al-though the thermal conductivity of copper-tungsten alloy issmaller than that of copper its specific heat is smaller than thatof copper too these two aspects have the same effect on thetemperature difference of heat conduction -e reason why thebreakdown probability of copper-tungsten alloy is higher thanthat of copper is probably because the density of copper-tungsten alloy is greater which can increase the amount ofheat contained in the same volume of material andmake up forthe deficiency of smaller specific heat capacity And because thethermal conductivity of copper-tungsten alloy is lower than thatof copper the surface temperature of copper-tungsten alloymaterial is higher so its breakdown probability is increased
33 Experimental Data Processing We quantitatively de-scribe the number of discharge craters distributed on dif-ferent electrodes by using the discharge affecting area onelectrode surface which can be calculated by pixel analysisfunction of image processing software Matlab -e pixelanalysis method is a method of using the image processing
Table 4 Calculation results of RΔT(Cu-i) and Rp(i-Cu) of differentmaterials
RΔT(Cu-i)(ΔTCu)ΔTi Rp(i-Cu)(j(T)ij(T)Cu)
Iron 483 537ndash832Brass (Zn35) 300 617ndash871Copper-tungstenalloy (W70) 147 105ndash162
Table 5 Processing parameters of multimaterial electrodes forEDM
Items ParametersWorkpiece material Die steel
Multimaterial electrodes Iron and copper copper andcopper-tungsten brass and copper
Machining polarity Normal-ickness of workpiece(mm) 4
Pulse on time Ton (μs) 125Pulse interval time Toff (μs) 75Breakdown voltage Ub (V) 45Peak current Ip (A) 08Diameter of cylindricalelectrode (mm) 2
Machining time t (s) 30Dielectric Deionized water
(a)
(b)
Figure 2 Fabricated multimaterial electrodes of iron and copper(a) Side view (b) Bottom view
Advances in Materials Science and Engineering 5
software to accurately analyze the area of an irregular imageIt utilizes proportion relation between image pixels andimage area in the same photo by calculating the displaypixels of testing image to analyze the actual area of testingimage According to the proportion relation between theimage pixel and the actual area an equation is given by
testing area (At)reference area (Ar)
display pixels of testing area (Pt)
display pixels of reference area (Pr)
(10)
When analyzing the discharge affecting area the refer-ence area Ar is the known quantity as the diameter of thecylindrical electrode in the machining process is 2mm for
ease of calculation a square with the side length of 2mm istaken as a reference image then the reference area is 4mm2the display pixels of reference area Pr are the known quantityand it is the corresponding pixels of the reference area in theimage processing software -e display pixels of testing areaPt are the pixels corresponding to discharge affecting areaselected in the figure -e discharge affecting areas differ ondifferent electrode surfaces and the display pixels of testingarea are different -erefore by obtaining the display pixelsof testing area in the software substituting the knownquantities of reference area and the display pixels of ref-erence area the discharge affecting area on electrode sur-face can be calculated accurately from (10) -e discharge
(a) (b) (c)
Figure 3 Discharge point distribution on the surface of iron and copper electrodes
(a) (b) (c)
Figure 4 Discharge point distribution on the surface of brass and copper electrodes
(a) (b) (c)
Figure 5 Discharge point distribution on the surface of copper-tungsten and copper electrodes
6 Advances in Materials Science and Engineering
aecting areas on the surface of dierent multimaterialelectrodes are calculated as shown in Figure 6
As can be seen from Figure 6 the discharge point areasfrom large to small are followed by brass iron copper-tungsten alloy and copper e discharge point area ofcopper is the smallest in each set of data and the dischargepoint areas of brass iron and copper-tungsten alloy are ofseveral-fold relation with that of copper is is because thethermal conductivity of copper is greater than that of othermaterials and its speciiexclc heat capacity is the smallest so that ithas the minimum breakdown probability and this corre-sponds well with the previous analysis results of the ratiocoeyencient of breakdown probability On the end face of theiron and copper tool the average discharge area on ironsurface is 35 times as that on copper surface On the brass andcopper tool the average discharge area on brass surface is 26times as that on copper surface e reason for the reductionof area dierence is that the discharge area of the coppersurface is increasedWhile on the copper-tungsten and coppertool the average discharge area ratio is less than 2 e dif-ference in the breakdown probability of copper between iron
brass and copper-tungsten alloy decreases gradually is isbecause the thermal conductivity of iron is smaller than that ofother materials and the speciiexclc heat capacity is greater thanthat of other materials so that it has the maximum breakdownprobability with copper Besides the thermal conductivity ofcopper-tungsten alloy is the largest among those materialsand its speciiexclc heat capacity is the smallest therefore thedierence in the breakdown probability compared withcopper is the smallest among the three materials In additionit can also be seen from Figure 6 that the discharge area of thebrass and copper tool is larger than that of the other twogroups overall and this may be due to the low electronwork function of zinc in brass which is helpful to improvethe occurrence rate of discharge breakdown increasing thenumber of discharges on the brass surface At the same timedue to the inuence of the discharge breakdown the numberof discharges on the surface of copper is also increased
According to the experiment results breakdown prob-ability of dierent materials compared with copper Pb(i-Cu)can be obtained as
Pb(i-Cu) Testing area of material i Ai( )
Testing area of material i Ai( ) + testing area of copper ACu( ) (11)
Based on the experiment results of breakdown probabilityas well as the ratio of temperature dierence RΔT(Cu-i) and theratio coeyencient of breakdown probability Rp(i-Cu) formularyiexcltting is carried out by the adaptive iexclt method using Matlaband the breakdown probability iexcltting formula of dierentmaterials compared with copper is given asPb(i-Cu) 07658 + 007448 sin π middot RΔT(Cu-i) middot Rp(i-Cu)( )
minus 0269 exp minus 01093Rp(i-Cu)( )2
( )
(12)
From (12) the exact breakdown probability of certainmaterial compared to copper can be obtained and furtherthe breakdown probability between any two dierent ma-terials can also be obtained
e proposed model in this paper has the ability toanalyze and predict the electrode discharge breakdownprobability of dierent materials and the reasons for thedierent breakdown probabilities of dierent materials aregiven with a reasonable explanation by the model which isof some signiiexclcance for the understanding of dischargemechanism of EDM
0
0005
001
0015
002
Disc
harg
e are
a (cm
2 )
Experimental dataAverage value
Iron Copper Brass Cu-W CopperIron and copper tool Brass and copper tool Cu-W and copper tool
Copper
Figure 6 Discharge area on the surface of dierent multimaterial tools
Advances in Materials Science and Engineering 7
4 Conclusions
In this paper the influence law of surface breakdownprobability of multimaterial electrodes is analyzed accordingto the theory of field electron emission a mathematicalmodel of material property influencing discharge break-down in interelectrodes is established the model is verifiedby experimental method and the conclusions are drawn asfollows
(1) In single pulse discharge process the breakdownprobability of different material electrodes is ap-proximately the same and the discharge process isnot influenced basically
(2) Under continuous discharge conditions due to theheating effect of previous discharges the surfacetemperature of tool material with poor thermal con-ductivity and high specific heat capacity is higherwhich makes it easier to cause local discharge break-down and hence increases breakdown probabilityBreakdown probability is influenced by the propertiesof the materials such as thermal conductivity specificheat capacity and density
(3) -e repeatable experimental results of continuousdischarges have provided the relationship betweenbreakdown probability and different material propertiesand also proven the effectiveness of the dischargebreakdown probability model
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e financial support from the National Natural ScienceFoundation of China under Grant no 51405058 ScientificResearch Platform Foundation of Liaoning Province underGrant no JDL2016006 and Talent Special Foundation ofDalian City under Grant no 2016RQ054 is acknowledged
References
[1] R K Garg K K Singh A Sachdeva V S Sharma K Ojhaand S Singh ldquoReview of research work in sinking EDM andWEDM on metal matrix composite materialsrdquo InternationalJournal of Advanced Manufacturing Technology vol 50no 5ndash8 pp 611ndash624 2010
[2] N Mohri N Saito Y Tsunekawa and N Kinoshita ldquoMetalsurface modification by electrical discharge machining withcomposite electroderdquo CIRP Annals vol 42 no 1 pp 219ndash2221993
[3] E Uhlmann and M Roehner ldquoInvestigations on reduction oftool electrode wear in micro-EDM using novel electrodematerialsrdquo CIRP Journal of Manufacturing Science andTechnology vol 1 no 2 pp 92ndash96 2008
[4] M Cao Y Hao Y Cao and S Yang ldquoMechanism and ex-perimental research on small-hole EDM with Cu-Cr com-posite electroderdquo Sensors amp Transducers vol 174 no 7pp 268ndash272 2014
[5] H C Tsai B H Yan and F Y Huang ldquoEDM performance ofCrCu-based composite electrodesrdquo International Journal ofMachine Tools and Manufacture vol 43 no 3 pp 245ndash2522003
[6] A K Khanra B R Sarkar B Bhattacharya L C Pathak andM M Godkhindi ldquoPerformance of ZrB2-Cu composite as anEDM electroderdquo Journal of Materials Processing Technologyvol 183 no 1 pp 122ndash126 2007
[7] T A El-Taweel ldquoMulti-response optimization of EDM withAl-Cu-Si-TiC PM composite electroderdquo International Jour-nal of Advanced Manufacturing Technology vol 44 no 1-2pp 100ndash113 2009
[8] V Senthilkumar and M C Reddy ldquoPerformance analysis ofCu-B4C metal matrix composite as an EDM electroderdquo In-ternational Journal of Machining and Machinability of Mate-rials vol 11 no 1 pp 36ndash50 2012
[9] D Anil and C Ccedilogun ldquoPerformance of copper-coated stereolithographic electrodes with internal cooling channels inelectric discharge machining (EDM)rdquo Rapid PrototypingJournal vol 14 no 4 pp 202ndash212 2008
[10] J Wang Y Wang F Zhao et al ldquoFabricating electrode byelectrodepositing technology and its application in micro-EDM deep hole drillingrdquo China Mechanical Engineeringvol 20 no 15 pp 1848ndash1852 2009 in Chinese
[11] Q Lv Simulation and Experimental Study on the Fabricationof the Composite Electrode for MEDM Dalian University ofTechnology Dalian China 2009 in Chinese
[12] X P Li Y G Wang F L Zhao M H Wu and Y LiuldquoInfluence of high frequency pulse on electrode wear in micro-EDMrdquo Defence Technology vol 10 no 3 pp 316ndash320 2014
[13] W Yuangang Z Fuling and W Jin ldquoWear-resist electrodesfor micro-EDMrdquo Chinese Journal of Aeronautics vol 22no 3 pp 339ndash342 2009
[14] A H Chiou C C Tsao and C Y Hsu ldquoA study of themachining characteristics of micro EDM milling and itsimprovement by electrode coatingrdquo International Journal ofAdvanced Manufacturing Technology vol 78 no 9-12pp 1857ndash1864 2015
[15] J Jiang and J Weng Cathodic Electronics and Gas DischargePrinciple National Defense Industry Press Beijing China1980 in Chinese
[16] W Zhao Research on the Corrosion of Elecrodes and ItseoryFoundation in EDM Northwestern Polytechnical UniversityXirsquoan China 2003 in Chinese
[17] B Izquierdo J A Sanchez S Plaza I Pombo and N OrtegaldquoA numerical model of the EDMprocess considering the effectof multiple dischargesrdquo International Journal of Machine Toolsand Manufacture vol 49 no 3 pp 220ndash229 2009
[18] -e Engineering ToolBox Specific Heats for Metals httpswwwengineeringtoolboxcomspecific-heat-metals-d_152html
[19] M Li eoretical Basis of Electrical Discharge MachiningNational Defense Industry Press Beijing China 1989 inChinese
8 Advances in Materials Science and Engineering
CorrosionInternational Journal of
Hindawiwwwhindawicom Volume 2018
Advances in
Materials Science and EngineeringHindawiwwwhindawicom Volume 2018
copper respectively From the figures we can see that underthe same conditions of discharge the number of craters oniron electrodes is more than that on copper ones andsimilarly the number of craters on brass electrodes is morethan that on copper ones and the craters on copper-tungstenelectrodes are more than those on copper ones
As can be seen from Figure 3 for the same dischargebreakdown condition the discharge points are mostly dis-tributed on the surface of the iron electrode at the end ofdischarge and only a few discharge points exist on the surfaceof the copper electrode -is shows that the breakdownprobability of iron is obviously higher than that of copper Itcan also be found that the thermal conductivity of iron ismuchsmaller than that of copper which indicates that after con-tinuous discharge because of the low thermal conductivity ofiron less heat is transmitted from the iron to the outsideMeanwhile as the specific heat capacity of iron is higher thanthat of copper iron of the same temperature as coppercontains more heat and the temperature reduction of the ironis also smaller-e temperature of the iron electrode surface ishigher than that of the copper electrode which promotes theemission of the cathode thermal field electron emission onthe electrode surface therefore the breakdown probability ofthe iron electrode is elevated -e high-temperature burntraces on the surface of the iron electrode from Figure 3(a) canalso demonstrate the presence of high temperature on thesurface of the electrode after continuous discharges
As can be seen from Figure 4 a large number of dischargepoints can be found on the surfaces of both copper and brasselectrodes and the discharge points on the surface of the brasselectrode are much more than those of the copper surfacealthough the difference between the two is not as large as the
difference between copper and iron electrodes-is is becausethe conductivity of brass is slightly better than iron and itsspecific heat capacity is similar to that of copper undercontinuous discharge conditions the temperature differencebetween the left and the right of the multimaterial electrode isnot as large as the copper-iron electrode therefore the dif-ference of discharge points decreases
FromFigure 5 it is noted that although the discharge pointsof the copper-tungsten alloy electrode are more than those ofthe copper electrode the difference between them is obviouslyreduced compared with Figures 3 and 4 -is is because al-though the thermal conductivity of copper-tungsten alloy issmaller than that of copper its specific heat is smaller than thatof copper too these two aspects have the same effect on thetemperature difference of heat conduction -e reason why thebreakdown probability of copper-tungsten alloy is higher thanthat of copper is probably because the density of copper-tungsten alloy is greater which can increase the amount ofheat contained in the same volume of material andmake up forthe deficiency of smaller specific heat capacity And because thethermal conductivity of copper-tungsten alloy is lower than thatof copper the surface temperature of copper-tungsten alloymaterial is higher so its breakdown probability is increased
33 Experimental Data Processing We quantitatively de-scribe the number of discharge craters distributed on dif-ferent electrodes by using the discharge affecting area onelectrode surface which can be calculated by pixel analysisfunction of image processing software Matlab -e pixelanalysis method is a method of using the image processing
Table 4 Calculation results of RΔT(Cu-i) and Rp(i-Cu) of differentmaterials
RΔT(Cu-i)(ΔTCu)ΔTi Rp(i-Cu)(j(T)ij(T)Cu)
Iron 483 537ndash832Brass (Zn35) 300 617ndash871Copper-tungstenalloy (W70) 147 105ndash162
Table 5 Processing parameters of multimaterial electrodes forEDM
Items ParametersWorkpiece material Die steel
Multimaterial electrodes Iron and copper copper andcopper-tungsten brass and copper
Machining polarity Normal-ickness of workpiece(mm) 4
Pulse on time Ton (μs) 125Pulse interval time Toff (μs) 75Breakdown voltage Ub (V) 45Peak current Ip (A) 08Diameter of cylindricalelectrode (mm) 2
Machining time t (s) 30Dielectric Deionized water
(a)
(b)
Figure 2 Fabricated multimaterial electrodes of iron and copper(a) Side view (b) Bottom view
Advances in Materials Science and Engineering 5
software to accurately analyze the area of an irregular imageIt utilizes proportion relation between image pixels andimage area in the same photo by calculating the displaypixels of testing image to analyze the actual area of testingimage According to the proportion relation between theimage pixel and the actual area an equation is given by
testing area (At)reference area (Ar)
display pixels of testing area (Pt)
display pixels of reference area (Pr)
(10)
When analyzing the discharge affecting area the refer-ence area Ar is the known quantity as the diameter of thecylindrical electrode in the machining process is 2mm for
ease of calculation a square with the side length of 2mm istaken as a reference image then the reference area is 4mm2the display pixels of reference area Pr are the known quantityand it is the corresponding pixels of the reference area in theimage processing software -e display pixels of testing areaPt are the pixels corresponding to discharge affecting areaselected in the figure -e discharge affecting areas differ ondifferent electrode surfaces and the display pixels of testingarea are different -erefore by obtaining the display pixelsof testing area in the software substituting the knownquantities of reference area and the display pixels of ref-erence area the discharge affecting area on electrode sur-face can be calculated accurately from (10) -e discharge
(a) (b) (c)
Figure 3 Discharge point distribution on the surface of iron and copper electrodes
(a) (b) (c)
Figure 4 Discharge point distribution on the surface of brass and copper electrodes
(a) (b) (c)
Figure 5 Discharge point distribution on the surface of copper-tungsten and copper electrodes
6 Advances in Materials Science and Engineering
aecting areas on the surface of dierent multimaterialelectrodes are calculated as shown in Figure 6
As can be seen from Figure 6 the discharge point areasfrom large to small are followed by brass iron copper-tungsten alloy and copper e discharge point area ofcopper is the smallest in each set of data and the dischargepoint areas of brass iron and copper-tungsten alloy are ofseveral-fold relation with that of copper is is because thethermal conductivity of copper is greater than that of othermaterials and its speciiexclc heat capacity is the smallest so that ithas the minimum breakdown probability and this corre-sponds well with the previous analysis results of the ratiocoeyencient of breakdown probability On the end face of theiron and copper tool the average discharge area on ironsurface is 35 times as that on copper surface On the brass andcopper tool the average discharge area on brass surface is 26times as that on copper surface e reason for the reductionof area dierence is that the discharge area of the coppersurface is increasedWhile on the copper-tungsten and coppertool the average discharge area ratio is less than 2 e dif-ference in the breakdown probability of copper between iron
brass and copper-tungsten alloy decreases gradually is isbecause the thermal conductivity of iron is smaller than that ofother materials and the speciiexclc heat capacity is greater thanthat of other materials so that it has the maximum breakdownprobability with copper Besides the thermal conductivity ofcopper-tungsten alloy is the largest among those materialsand its speciiexclc heat capacity is the smallest therefore thedierence in the breakdown probability compared withcopper is the smallest among the three materials In additionit can also be seen from Figure 6 that the discharge area of thebrass and copper tool is larger than that of the other twogroups overall and this may be due to the low electronwork function of zinc in brass which is helpful to improvethe occurrence rate of discharge breakdown increasing thenumber of discharges on the brass surface At the same timedue to the inuence of the discharge breakdown the numberof discharges on the surface of copper is also increased
According to the experiment results breakdown prob-ability of dierent materials compared with copper Pb(i-Cu)can be obtained as
Pb(i-Cu) Testing area of material i Ai( )
Testing area of material i Ai( ) + testing area of copper ACu( ) (11)
Based on the experiment results of breakdown probabilityas well as the ratio of temperature dierence RΔT(Cu-i) and theratio coeyencient of breakdown probability Rp(i-Cu) formularyiexcltting is carried out by the adaptive iexclt method using Matlaband the breakdown probability iexcltting formula of dierentmaterials compared with copper is given asPb(i-Cu) 07658 + 007448 sin π middot RΔT(Cu-i) middot Rp(i-Cu)( )
minus 0269 exp minus 01093Rp(i-Cu)( )2
( )
(12)
From (12) the exact breakdown probability of certainmaterial compared to copper can be obtained and furtherthe breakdown probability between any two dierent ma-terials can also be obtained
e proposed model in this paper has the ability toanalyze and predict the electrode discharge breakdownprobability of dierent materials and the reasons for thedierent breakdown probabilities of dierent materials aregiven with a reasonable explanation by the model which isof some signiiexclcance for the understanding of dischargemechanism of EDM
0
0005
001
0015
002
Disc
harg
e are
a (cm
2 )
Experimental dataAverage value
Iron Copper Brass Cu-W CopperIron and copper tool Brass and copper tool Cu-W and copper tool
Copper
Figure 6 Discharge area on the surface of dierent multimaterial tools
Advances in Materials Science and Engineering 7
4 Conclusions
In this paper the influence law of surface breakdownprobability of multimaterial electrodes is analyzed accordingto the theory of field electron emission a mathematicalmodel of material property influencing discharge break-down in interelectrodes is established the model is verifiedby experimental method and the conclusions are drawn asfollows
(1) In single pulse discharge process the breakdownprobability of different material electrodes is ap-proximately the same and the discharge process isnot influenced basically
(2) Under continuous discharge conditions due to theheating effect of previous discharges the surfacetemperature of tool material with poor thermal con-ductivity and high specific heat capacity is higherwhich makes it easier to cause local discharge break-down and hence increases breakdown probabilityBreakdown probability is influenced by the propertiesof the materials such as thermal conductivity specificheat capacity and density
(3) -e repeatable experimental results of continuousdischarges have provided the relationship betweenbreakdown probability and different material propertiesand also proven the effectiveness of the dischargebreakdown probability model
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e financial support from the National Natural ScienceFoundation of China under Grant no 51405058 ScientificResearch Platform Foundation of Liaoning Province underGrant no JDL2016006 and Talent Special Foundation ofDalian City under Grant no 2016RQ054 is acknowledged
References
[1] R K Garg K K Singh A Sachdeva V S Sharma K Ojhaand S Singh ldquoReview of research work in sinking EDM andWEDM on metal matrix composite materialsrdquo InternationalJournal of Advanced Manufacturing Technology vol 50no 5ndash8 pp 611ndash624 2010
[2] N Mohri N Saito Y Tsunekawa and N Kinoshita ldquoMetalsurface modification by electrical discharge machining withcomposite electroderdquo CIRP Annals vol 42 no 1 pp 219ndash2221993
[3] E Uhlmann and M Roehner ldquoInvestigations on reduction oftool electrode wear in micro-EDM using novel electrodematerialsrdquo CIRP Journal of Manufacturing Science andTechnology vol 1 no 2 pp 92ndash96 2008
[4] M Cao Y Hao Y Cao and S Yang ldquoMechanism and ex-perimental research on small-hole EDM with Cu-Cr com-posite electroderdquo Sensors amp Transducers vol 174 no 7pp 268ndash272 2014
[5] H C Tsai B H Yan and F Y Huang ldquoEDM performance ofCrCu-based composite electrodesrdquo International Journal ofMachine Tools and Manufacture vol 43 no 3 pp 245ndash2522003
[6] A K Khanra B R Sarkar B Bhattacharya L C Pathak andM M Godkhindi ldquoPerformance of ZrB2-Cu composite as anEDM electroderdquo Journal of Materials Processing Technologyvol 183 no 1 pp 122ndash126 2007
[7] T A El-Taweel ldquoMulti-response optimization of EDM withAl-Cu-Si-TiC PM composite electroderdquo International Jour-nal of Advanced Manufacturing Technology vol 44 no 1-2pp 100ndash113 2009
[8] V Senthilkumar and M C Reddy ldquoPerformance analysis ofCu-B4C metal matrix composite as an EDM electroderdquo In-ternational Journal of Machining and Machinability of Mate-rials vol 11 no 1 pp 36ndash50 2012
[9] D Anil and C Ccedilogun ldquoPerformance of copper-coated stereolithographic electrodes with internal cooling channels inelectric discharge machining (EDM)rdquo Rapid PrototypingJournal vol 14 no 4 pp 202ndash212 2008
[10] J Wang Y Wang F Zhao et al ldquoFabricating electrode byelectrodepositing technology and its application in micro-EDM deep hole drillingrdquo China Mechanical Engineeringvol 20 no 15 pp 1848ndash1852 2009 in Chinese
[11] Q Lv Simulation and Experimental Study on the Fabricationof the Composite Electrode for MEDM Dalian University ofTechnology Dalian China 2009 in Chinese
[12] X P Li Y G Wang F L Zhao M H Wu and Y LiuldquoInfluence of high frequency pulse on electrode wear in micro-EDMrdquo Defence Technology vol 10 no 3 pp 316ndash320 2014
[13] W Yuangang Z Fuling and W Jin ldquoWear-resist electrodesfor micro-EDMrdquo Chinese Journal of Aeronautics vol 22no 3 pp 339ndash342 2009
[14] A H Chiou C C Tsao and C Y Hsu ldquoA study of themachining characteristics of micro EDM milling and itsimprovement by electrode coatingrdquo International Journal ofAdvanced Manufacturing Technology vol 78 no 9-12pp 1857ndash1864 2015
[15] J Jiang and J Weng Cathodic Electronics and Gas DischargePrinciple National Defense Industry Press Beijing China1980 in Chinese
[16] W Zhao Research on the Corrosion of Elecrodes and ItseoryFoundation in EDM Northwestern Polytechnical UniversityXirsquoan China 2003 in Chinese
[17] B Izquierdo J A Sanchez S Plaza I Pombo and N OrtegaldquoA numerical model of the EDMprocess considering the effectof multiple dischargesrdquo International Journal of Machine Toolsand Manufacture vol 49 no 3 pp 220ndash229 2009
[18] -e Engineering ToolBox Specific Heats for Metals httpswwwengineeringtoolboxcomspecific-heat-metals-d_152html
[19] M Li eoretical Basis of Electrical Discharge MachiningNational Defense Industry Press Beijing China 1989 inChinese
8 Advances in Materials Science and Engineering
CorrosionInternational Journal of
Hindawiwwwhindawicom Volume 2018
Advances in
Materials Science and EngineeringHindawiwwwhindawicom Volume 2018
software to accurately analyze the area of an irregular imageIt utilizes proportion relation between image pixels andimage area in the same photo by calculating the displaypixels of testing image to analyze the actual area of testingimage According to the proportion relation between theimage pixel and the actual area an equation is given by
testing area (At)reference area (Ar)
display pixels of testing area (Pt)
display pixels of reference area (Pr)
(10)
When analyzing the discharge affecting area the refer-ence area Ar is the known quantity as the diameter of thecylindrical electrode in the machining process is 2mm for
ease of calculation a square with the side length of 2mm istaken as a reference image then the reference area is 4mm2the display pixels of reference area Pr are the known quantityand it is the corresponding pixels of the reference area in theimage processing software -e display pixels of testing areaPt are the pixels corresponding to discharge affecting areaselected in the figure -e discharge affecting areas differ ondifferent electrode surfaces and the display pixels of testingarea are different -erefore by obtaining the display pixelsof testing area in the software substituting the knownquantities of reference area and the display pixels of ref-erence area the discharge affecting area on electrode sur-face can be calculated accurately from (10) -e discharge
(a) (b) (c)
Figure 3 Discharge point distribution on the surface of iron and copper electrodes
(a) (b) (c)
Figure 4 Discharge point distribution on the surface of brass and copper electrodes
(a) (b) (c)
Figure 5 Discharge point distribution on the surface of copper-tungsten and copper electrodes
6 Advances in Materials Science and Engineering
aecting areas on the surface of dierent multimaterialelectrodes are calculated as shown in Figure 6
As can be seen from Figure 6 the discharge point areasfrom large to small are followed by brass iron copper-tungsten alloy and copper e discharge point area ofcopper is the smallest in each set of data and the dischargepoint areas of brass iron and copper-tungsten alloy are ofseveral-fold relation with that of copper is is because thethermal conductivity of copper is greater than that of othermaterials and its speciiexclc heat capacity is the smallest so that ithas the minimum breakdown probability and this corre-sponds well with the previous analysis results of the ratiocoeyencient of breakdown probability On the end face of theiron and copper tool the average discharge area on ironsurface is 35 times as that on copper surface On the brass andcopper tool the average discharge area on brass surface is 26times as that on copper surface e reason for the reductionof area dierence is that the discharge area of the coppersurface is increasedWhile on the copper-tungsten and coppertool the average discharge area ratio is less than 2 e dif-ference in the breakdown probability of copper between iron
brass and copper-tungsten alloy decreases gradually is isbecause the thermal conductivity of iron is smaller than that ofother materials and the speciiexclc heat capacity is greater thanthat of other materials so that it has the maximum breakdownprobability with copper Besides the thermal conductivity ofcopper-tungsten alloy is the largest among those materialsand its speciiexclc heat capacity is the smallest therefore thedierence in the breakdown probability compared withcopper is the smallest among the three materials In additionit can also be seen from Figure 6 that the discharge area of thebrass and copper tool is larger than that of the other twogroups overall and this may be due to the low electronwork function of zinc in brass which is helpful to improvethe occurrence rate of discharge breakdown increasing thenumber of discharges on the brass surface At the same timedue to the inuence of the discharge breakdown the numberof discharges on the surface of copper is also increased
According to the experiment results breakdown prob-ability of dierent materials compared with copper Pb(i-Cu)can be obtained as
Pb(i-Cu) Testing area of material i Ai( )
Testing area of material i Ai( ) + testing area of copper ACu( ) (11)
Based on the experiment results of breakdown probabilityas well as the ratio of temperature dierence RΔT(Cu-i) and theratio coeyencient of breakdown probability Rp(i-Cu) formularyiexcltting is carried out by the adaptive iexclt method using Matlaband the breakdown probability iexcltting formula of dierentmaterials compared with copper is given asPb(i-Cu) 07658 + 007448 sin π middot RΔT(Cu-i) middot Rp(i-Cu)( )
minus 0269 exp minus 01093Rp(i-Cu)( )2
( )
(12)
From (12) the exact breakdown probability of certainmaterial compared to copper can be obtained and furtherthe breakdown probability between any two dierent ma-terials can also be obtained
e proposed model in this paper has the ability toanalyze and predict the electrode discharge breakdownprobability of dierent materials and the reasons for thedierent breakdown probabilities of dierent materials aregiven with a reasonable explanation by the model which isof some signiiexclcance for the understanding of dischargemechanism of EDM
0
0005
001
0015
002
Disc
harg
e are
a (cm
2 )
Experimental dataAverage value
Iron Copper Brass Cu-W CopperIron and copper tool Brass and copper tool Cu-W and copper tool
Copper
Figure 6 Discharge area on the surface of dierent multimaterial tools
Advances in Materials Science and Engineering 7
4 Conclusions
In this paper the influence law of surface breakdownprobability of multimaterial electrodes is analyzed accordingto the theory of field electron emission a mathematicalmodel of material property influencing discharge break-down in interelectrodes is established the model is verifiedby experimental method and the conclusions are drawn asfollows
(1) In single pulse discharge process the breakdownprobability of different material electrodes is ap-proximately the same and the discharge process isnot influenced basically
(2) Under continuous discharge conditions due to theheating effect of previous discharges the surfacetemperature of tool material with poor thermal con-ductivity and high specific heat capacity is higherwhich makes it easier to cause local discharge break-down and hence increases breakdown probabilityBreakdown probability is influenced by the propertiesof the materials such as thermal conductivity specificheat capacity and density
(3) -e repeatable experimental results of continuousdischarges have provided the relationship betweenbreakdown probability and different material propertiesand also proven the effectiveness of the dischargebreakdown probability model
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e financial support from the National Natural ScienceFoundation of China under Grant no 51405058 ScientificResearch Platform Foundation of Liaoning Province underGrant no JDL2016006 and Talent Special Foundation ofDalian City under Grant no 2016RQ054 is acknowledged
References
[1] R K Garg K K Singh A Sachdeva V S Sharma K Ojhaand S Singh ldquoReview of research work in sinking EDM andWEDM on metal matrix composite materialsrdquo InternationalJournal of Advanced Manufacturing Technology vol 50no 5ndash8 pp 611ndash624 2010
[2] N Mohri N Saito Y Tsunekawa and N Kinoshita ldquoMetalsurface modification by electrical discharge machining withcomposite electroderdquo CIRP Annals vol 42 no 1 pp 219ndash2221993
[3] E Uhlmann and M Roehner ldquoInvestigations on reduction oftool electrode wear in micro-EDM using novel electrodematerialsrdquo CIRP Journal of Manufacturing Science andTechnology vol 1 no 2 pp 92ndash96 2008
[4] M Cao Y Hao Y Cao and S Yang ldquoMechanism and ex-perimental research on small-hole EDM with Cu-Cr com-posite electroderdquo Sensors amp Transducers vol 174 no 7pp 268ndash272 2014
[5] H C Tsai B H Yan and F Y Huang ldquoEDM performance ofCrCu-based composite electrodesrdquo International Journal ofMachine Tools and Manufacture vol 43 no 3 pp 245ndash2522003
[6] A K Khanra B R Sarkar B Bhattacharya L C Pathak andM M Godkhindi ldquoPerformance of ZrB2-Cu composite as anEDM electroderdquo Journal of Materials Processing Technologyvol 183 no 1 pp 122ndash126 2007
[7] T A El-Taweel ldquoMulti-response optimization of EDM withAl-Cu-Si-TiC PM composite electroderdquo International Jour-nal of Advanced Manufacturing Technology vol 44 no 1-2pp 100ndash113 2009
[8] V Senthilkumar and M C Reddy ldquoPerformance analysis ofCu-B4C metal matrix composite as an EDM electroderdquo In-ternational Journal of Machining and Machinability of Mate-rials vol 11 no 1 pp 36ndash50 2012
[9] D Anil and C Ccedilogun ldquoPerformance of copper-coated stereolithographic electrodes with internal cooling channels inelectric discharge machining (EDM)rdquo Rapid PrototypingJournal vol 14 no 4 pp 202ndash212 2008
[10] J Wang Y Wang F Zhao et al ldquoFabricating electrode byelectrodepositing technology and its application in micro-EDM deep hole drillingrdquo China Mechanical Engineeringvol 20 no 15 pp 1848ndash1852 2009 in Chinese
[11] Q Lv Simulation and Experimental Study on the Fabricationof the Composite Electrode for MEDM Dalian University ofTechnology Dalian China 2009 in Chinese
[12] X P Li Y G Wang F L Zhao M H Wu and Y LiuldquoInfluence of high frequency pulse on electrode wear in micro-EDMrdquo Defence Technology vol 10 no 3 pp 316ndash320 2014
[13] W Yuangang Z Fuling and W Jin ldquoWear-resist electrodesfor micro-EDMrdquo Chinese Journal of Aeronautics vol 22no 3 pp 339ndash342 2009
[14] A H Chiou C C Tsao and C Y Hsu ldquoA study of themachining characteristics of micro EDM milling and itsimprovement by electrode coatingrdquo International Journal ofAdvanced Manufacturing Technology vol 78 no 9-12pp 1857ndash1864 2015
[15] J Jiang and J Weng Cathodic Electronics and Gas DischargePrinciple National Defense Industry Press Beijing China1980 in Chinese
[16] W Zhao Research on the Corrosion of Elecrodes and ItseoryFoundation in EDM Northwestern Polytechnical UniversityXirsquoan China 2003 in Chinese
[17] B Izquierdo J A Sanchez S Plaza I Pombo and N OrtegaldquoA numerical model of the EDMprocess considering the effectof multiple dischargesrdquo International Journal of Machine Toolsand Manufacture vol 49 no 3 pp 220ndash229 2009
[18] -e Engineering ToolBox Specific Heats for Metals httpswwwengineeringtoolboxcomspecific-heat-metals-d_152html
[19] M Li eoretical Basis of Electrical Discharge MachiningNational Defense Industry Press Beijing China 1989 inChinese
8 Advances in Materials Science and Engineering
CorrosionInternational Journal of
Hindawiwwwhindawicom Volume 2018
Advances in
Materials Science and EngineeringHindawiwwwhindawicom Volume 2018
aecting areas on the surface of dierent multimaterialelectrodes are calculated as shown in Figure 6
As can be seen from Figure 6 the discharge point areasfrom large to small are followed by brass iron copper-tungsten alloy and copper e discharge point area ofcopper is the smallest in each set of data and the dischargepoint areas of brass iron and copper-tungsten alloy are ofseveral-fold relation with that of copper is is because thethermal conductivity of copper is greater than that of othermaterials and its speciiexclc heat capacity is the smallest so that ithas the minimum breakdown probability and this corre-sponds well with the previous analysis results of the ratiocoeyencient of breakdown probability On the end face of theiron and copper tool the average discharge area on ironsurface is 35 times as that on copper surface On the brass andcopper tool the average discharge area on brass surface is 26times as that on copper surface e reason for the reductionof area dierence is that the discharge area of the coppersurface is increasedWhile on the copper-tungsten and coppertool the average discharge area ratio is less than 2 e dif-ference in the breakdown probability of copper between iron
brass and copper-tungsten alloy decreases gradually is isbecause the thermal conductivity of iron is smaller than that ofother materials and the speciiexclc heat capacity is greater thanthat of other materials so that it has the maximum breakdownprobability with copper Besides the thermal conductivity ofcopper-tungsten alloy is the largest among those materialsand its speciiexclc heat capacity is the smallest therefore thedierence in the breakdown probability compared withcopper is the smallest among the three materials In additionit can also be seen from Figure 6 that the discharge area of thebrass and copper tool is larger than that of the other twogroups overall and this may be due to the low electronwork function of zinc in brass which is helpful to improvethe occurrence rate of discharge breakdown increasing thenumber of discharges on the brass surface At the same timedue to the inuence of the discharge breakdown the numberof discharges on the surface of copper is also increased
According to the experiment results breakdown prob-ability of dierent materials compared with copper Pb(i-Cu)can be obtained as
Pb(i-Cu) Testing area of material i Ai( )
Testing area of material i Ai( ) + testing area of copper ACu( ) (11)
Based on the experiment results of breakdown probabilityas well as the ratio of temperature dierence RΔT(Cu-i) and theratio coeyencient of breakdown probability Rp(i-Cu) formularyiexcltting is carried out by the adaptive iexclt method using Matlaband the breakdown probability iexcltting formula of dierentmaterials compared with copper is given asPb(i-Cu) 07658 + 007448 sin π middot RΔT(Cu-i) middot Rp(i-Cu)( )
minus 0269 exp minus 01093Rp(i-Cu)( )2
( )
(12)
From (12) the exact breakdown probability of certainmaterial compared to copper can be obtained and furtherthe breakdown probability between any two dierent ma-terials can also be obtained
e proposed model in this paper has the ability toanalyze and predict the electrode discharge breakdownprobability of dierent materials and the reasons for thedierent breakdown probabilities of dierent materials aregiven with a reasonable explanation by the model which isof some signiiexclcance for the understanding of dischargemechanism of EDM
0
0005
001
0015
002
Disc
harg
e are
a (cm
2 )
Experimental dataAverage value
Iron Copper Brass Cu-W CopperIron and copper tool Brass and copper tool Cu-W and copper tool
Copper
Figure 6 Discharge area on the surface of dierent multimaterial tools
Advances in Materials Science and Engineering 7
4 Conclusions
In this paper the influence law of surface breakdownprobability of multimaterial electrodes is analyzed accordingto the theory of field electron emission a mathematicalmodel of material property influencing discharge break-down in interelectrodes is established the model is verifiedby experimental method and the conclusions are drawn asfollows
(1) In single pulse discharge process the breakdownprobability of different material electrodes is ap-proximately the same and the discharge process isnot influenced basically
(2) Under continuous discharge conditions due to theheating effect of previous discharges the surfacetemperature of tool material with poor thermal con-ductivity and high specific heat capacity is higherwhich makes it easier to cause local discharge break-down and hence increases breakdown probabilityBreakdown probability is influenced by the propertiesof the materials such as thermal conductivity specificheat capacity and density
(3) -e repeatable experimental results of continuousdischarges have provided the relationship betweenbreakdown probability and different material propertiesand also proven the effectiveness of the dischargebreakdown probability model
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e financial support from the National Natural ScienceFoundation of China under Grant no 51405058 ScientificResearch Platform Foundation of Liaoning Province underGrant no JDL2016006 and Talent Special Foundation ofDalian City under Grant no 2016RQ054 is acknowledged
References
[1] R K Garg K K Singh A Sachdeva V S Sharma K Ojhaand S Singh ldquoReview of research work in sinking EDM andWEDM on metal matrix composite materialsrdquo InternationalJournal of Advanced Manufacturing Technology vol 50no 5ndash8 pp 611ndash624 2010
[2] N Mohri N Saito Y Tsunekawa and N Kinoshita ldquoMetalsurface modification by electrical discharge machining withcomposite electroderdquo CIRP Annals vol 42 no 1 pp 219ndash2221993
[3] E Uhlmann and M Roehner ldquoInvestigations on reduction oftool electrode wear in micro-EDM using novel electrodematerialsrdquo CIRP Journal of Manufacturing Science andTechnology vol 1 no 2 pp 92ndash96 2008
[4] M Cao Y Hao Y Cao and S Yang ldquoMechanism and ex-perimental research on small-hole EDM with Cu-Cr com-posite electroderdquo Sensors amp Transducers vol 174 no 7pp 268ndash272 2014
[5] H C Tsai B H Yan and F Y Huang ldquoEDM performance ofCrCu-based composite electrodesrdquo International Journal ofMachine Tools and Manufacture vol 43 no 3 pp 245ndash2522003
[6] A K Khanra B R Sarkar B Bhattacharya L C Pathak andM M Godkhindi ldquoPerformance of ZrB2-Cu composite as anEDM electroderdquo Journal of Materials Processing Technologyvol 183 no 1 pp 122ndash126 2007
[7] T A El-Taweel ldquoMulti-response optimization of EDM withAl-Cu-Si-TiC PM composite electroderdquo International Jour-nal of Advanced Manufacturing Technology vol 44 no 1-2pp 100ndash113 2009
[8] V Senthilkumar and M C Reddy ldquoPerformance analysis ofCu-B4C metal matrix composite as an EDM electroderdquo In-ternational Journal of Machining and Machinability of Mate-rials vol 11 no 1 pp 36ndash50 2012
[9] D Anil and C Ccedilogun ldquoPerformance of copper-coated stereolithographic electrodes with internal cooling channels inelectric discharge machining (EDM)rdquo Rapid PrototypingJournal vol 14 no 4 pp 202ndash212 2008
[10] J Wang Y Wang F Zhao et al ldquoFabricating electrode byelectrodepositing technology and its application in micro-EDM deep hole drillingrdquo China Mechanical Engineeringvol 20 no 15 pp 1848ndash1852 2009 in Chinese
[11] Q Lv Simulation and Experimental Study on the Fabricationof the Composite Electrode for MEDM Dalian University ofTechnology Dalian China 2009 in Chinese
[12] X P Li Y G Wang F L Zhao M H Wu and Y LiuldquoInfluence of high frequency pulse on electrode wear in micro-EDMrdquo Defence Technology vol 10 no 3 pp 316ndash320 2014
[13] W Yuangang Z Fuling and W Jin ldquoWear-resist electrodesfor micro-EDMrdquo Chinese Journal of Aeronautics vol 22no 3 pp 339ndash342 2009
[14] A H Chiou C C Tsao and C Y Hsu ldquoA study of themachining characteristics of micro EDM milling and itsimprovement by electrode coatingrdquo International Journal ofAdvanced Manufacturing Technology vol 78 no 9-12pp 1857ndash1864 2015
[15] J Jiang and J Weng Cathodic Electronics and Gas DischargePrinciple National Defense Industry Press Beijing China1980 in Chinese
[16] W Zhao Research on the Corrosion of Elecrodes and ItseoryFoundation in EDM Northwestern Polytechnical UniversityXirsquoan China 2003 in Chinese
[17] B Izquierdo J A Sanchez S Plaza I Pombo and N OrtegaldquoA numerical model of the EDMprocess considering the effectof multiple dischargesrdquo International Journal of Machine Toolsand Manufacture vol 49 no 3 pp 220ndash229 2009
[18] -e Engineering ToolBox Specific Heats for Metals httpswwwengineeringtoolboxcomspecific-heat-metals-d_152html
[19] M Li eoretical Basis of Electrical Discharge MachiningNational Defense Industry Press Beijing China 1989 inChinese
8 Advances in Materials Science and Engineering
CorrosionInternational Journal of
Hindawiwwwhindawicom Volume 2018
Advances in
Materials Science and EngineeringHindawiwwwhindawicom Volume 2018
In this paper the influence law of surface breakdownprobability of multimaterial electrodes is analyzed accordingto the theory of field electron emission a mathematicalmodel of material property influencing discharge break-down in interelectrodes is established the model is verifiedby experimental method and the conclusions are drawn asfollows
(1) In single pulse discharge process the breakdownprobability of different material electrodes is ap-proximately the same and the discharge process isnot influenced basically
(2) Under continuous discharge conditions due to theheating effect of previous discharges the surfacetemperature of tool material with poor thermal con-ductivity and high specific heat capacity is higherwhich makes it easier to cause local discharge break-down and hence increases breakdown probabilityBreakdown probability is influenced by the propertiesof the materials such as thermal conductivity specificheat capacity and density
(3) -e repeatable experimental results of continuousdischarges have provided the relationship betweenbreakdown probability and different material propertiesand also proven the effectiveness of the dischargebreakdown probability model
Conflicts of Interest
-e authors declare that they have no conflicts of interest
Acknowledgments
-e financial support from the National Natural ScienceFoundation of China under Grant no 51405058 ScientificResearch Platform Foundation of Liaoning Province underGrant no JDL2016006 and Talent Special Foundation ofDalian City under Grant no 2016RQ054 is acknowledged
References
[1] R K Garg K K Singh A Sachdeva V S Sharma K Ojhaand S Singh ldquoReview of research work in sinking EDM andWEDM on metal matrix composite materialsrdquo InternationalJournal of Advanced Manufacturing Technology vol 50no 5ndash8 pp 611ndash624 2010
[2] N Mohri N Saito Y Tsunekawa and N Kinoshita ldquoMetalsurface modification by electrical discharge machining withcomposite electroderdquo CIRP Annals vol 42 no 1 pp 219ndash2221993
[3] E Uhlmann and M Roehner ldquoInvestigations on reduction oftool electrode wear in micro-EDM using novel electrodematerialsrdquo CIRP Journal of Manufacturing Science andTechnology vol 1 no 2 pp 92ndash96 2008
[4] M Cao Y Hao Y Cao and S Yang ldquoMechanism and ex-perimental research on small-hole EDM with Cu-Cr com-posite electroderdquo Sensors amp Transducers vol 174 no 7pp 268ndash272 2014
[5] H C Tsai B H Yan and F Y Huang ldquoEDM performance ofCrCu-based composite electrodesrdquo International Journal ofMachine Tools and Manufacture vol 43 no 3 pp 245ndash2522003
[6] A K Khanra B R Sarkar B Bhattacharya L C Pathak andM M Godkhindi ldquoPerformance of ZrB2-Cu composite as anEDM electroderdquo Journal of Materials Processing Technologyvol 183 no 1 pp 122ndash126 2007
[7] T A El-Taweel ldquoMulti-response optimization of EDM withAl-Cu-Si-TiC PM composite electroderdquo International Jour-nal of Advanced Manufacturing Technology vol 44 no 1-2pp 100ndash113 2009
[8] V Senthilkumar and M C Reddy ldquoPerformance analysis ofCu-B4C metal matrix composite as an EDM electroderdquo In-ternational Journal of Machining and Machinability of Mate-rials vol 11 no 1 pp 36ndash50 2012
[9] D Anil and C Ccedilogun ldquoPerformance of copper-coated stereolithographic electrodes with internal cooling channels inelectric discharge machining (EDM)rdquo Rapid PrototypingJournal vol 14 no 4 pp 202ndash212 2008
[10] J Wang Y Wang F Zhao et al ldquoFabricating electrode byelectrodepositing technology and its application in micro-EDM deep hole drillingrdquo China Mechanical Engineeringvol 20 no 15 pp 1848ndash1852 2009 in Chinese
[11] Q Lv Simulation and Experimental Study on the Fabricationof the Composite Electrode for MEDM Dalian University ofTechnology Dalian China 2009 in Chinese
[12] X P Li Y G Wang F L Zhao M H Wu and Y LiuldquoInfluence of high frequency pulse on electrode wear in micro-EDMrdquo Defence Technology vol 10 no 3 pp 316ndash320 2014
[13] W Yuangang Z Fuling and W Jin ldquoWear-resist electrodesfor micro-EDMrdquo Chinese Journal of Aeronautics vol 22no 3 pp 339ndash342 2009
[14] A H Chiou C C Tsao and C Y Hsu ldquoA study of themachining characteristics of micro EDM milling and itsimprovement by electrode coatingrdquo International Journal ofAdvanced Manufacturing Technology vol 78 no 9-12pp 1857ndash1864 2015
[15] J Jiang and J Weng Cathodic Electronics and Gas DischargePrinciple National Defense Industry Press Beijing China1980 in Chinese
[16] W Zhao Research on the Corrosion of Elecrodes and ItseoryFoundation in EDM Northwestern Polytechnical UniversityXirsquoan China 2003 in Chinese
[17] B Izquierdo J A Sanchez S Plaza I Pombo and N OrtegaldquoA numerical model of the EDMprocess considering the effectof multiple dischargesrdquo International Journal of Machine Toolsand Manufacture vol 49 no 3 pp 220ndash229 2009
[18] -e Engineering ToolBox Specific Heats for Metals httpswwwengineeringtoolboxcomspecific-heat-metals-d_152html
[19] M Li eoretical Basis of Electrical Discharge MachiningNational Defense Industry Press Beijing China 1989 inChinese
8 Advances in Materials Science and Engineering
CorrosionInternational Journal of
Hindawiwwwhindawicom Volume 2018
Advances in
Materials Science and EngineeringHindawiwwwhindawicom Volume 2018
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