343 

AdvancesinProductionEngineering&Management ISSN1854‐6250

Volume14|Number3|September2019|pp343–354 Journalhome:apem‐journal.org

https://doi.org/10.14743/apem2019.3.332 Originalscientificpaper

  

Evaluation of the sustainability of the micro‐electrical discharge milling process 

Pellegrini, G.a, Ravasio, C.a,* aUniversity of Bergamo, Department of Management, Information and Production Engineering, Dalmine (BG), Italy  

A B S T R A C T A R T I C L E   I N F O

Thesustainabilityevaluationofanindustrialprocessisanactualissue:apro‐cessshouldnotonlygrantpartqualityandhighproductionratesatthelowestcost,butitshouldminimizeitsimpactontheenvironmentaswell.Micro‐EDM(Electrical Discharge Machining) is widely used in micro machining for itssmall force andhighprecision and environmental aspects related this tech‐nology are taken into account. In this paper, an evaluation of themicro‐EDmilling process concerning the sustainabilitymanufacturingwasmade. Forthis purpose, amethod to assess the sustainability process was developed,takingintoaccounttheenergeticconsumption,theenvironmentalimpact,thedielectricconsumption, thewearof theelectrodeand themachiningperfor‐mance.Thismethodwasappliedfortheexecutionofmicro‐pocketsusingtwoworkpiecematerials,twotypesofelectrodeandfivetypesofdielectric,bothliquid and gaseous. This analysis permits the identification of the criticalaspectsofthemicro‐EDmillingprocessformthepointofviewofthesustain‐ability.Thecomparisonbetweenthedifferentsolutionsintermsofelectrodematerialanddielectricunderlinesinterestingconsiderationsabouttheusageofnon‐traditionaldielectrics.Asregardselectrodematerial,theenvironmen‐tal impact processwhen brass electrode is adopted is lower than tungstencarbide electrode. As concerns dielectric,water reveals to be themost sus‐tainabledielectric;vegetableoilandoxygen,provedtobevalidsubstitutestotraditionaldielectricsunderseveralviewpoints,includingsustainability.

©2019CPE,UniversityofMaribor.Allrightsreserved.

Keywords:Electricaldischargemachining(EDM);Micro‐electricaldischargemachin‐ing(micro‐EDM);Micro‐electricaldischargemilling(micro‐EDmilling);Sustainability;Sustainabilityindex;Dielectricfluid

*Correspondingauthor:chiara.ravasio@unibg.it(Ravasio,C.)

Articlehistory:Received29May2019Revised13September2019Accepted16September2019

  

1. Introduction  

Whendealingwithprocessselection,sustainabilityissuesareachievingincreasingimportance.Aprocessshouldnotonlygrantpartqualityandhighproductionratesatthelowestcost,butitshouldminimizeitsimpactontheenvironmentaswell.Whereasquality,productivityandcostcanbeevaluatedusingacceptedtechniques,sustainabilityevaluationisstillmatterofresearch.Thesimplestwayofassessingsustainabilityisthroughindicators[1],althoughamorecomplexmethod of life cycle assessment has been proposed by international standards [2, 3]. In bothcases, data collected from either production practice or experimental tests are supplied to amodelwhichmakesthemdimensionallyhomogeneousandthenaggregatesthem,generallybymeansofweightingfactors.Ascalarvalue(index)isthenevaluatedandusedtorankdifferentprocessconditions.Todefineaprocessindex,knowledgeaboutthesubjectisnecessarytoiden‐tify the factors [4]: in general, energy requirements (for both themain process and auxiliaryactions),materialusage(forbothworkandconsumables),processfluidsmanagement(suchascoolants,lubricantsanddielectrics),wasteproduction,healthandsafetyissuesshouldbetakenintoaccount.Amongmanyotherprocesses,sustainableproductionthroughelectricaldischarge

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machining(EDM)hasbeenstudied[5‐7].AlbeitarelativelysmallnumberofpartsarecurrentlyproducedviaEDM,whencomparedtometalcuttingoperations,EDMisoneofthemostenergyintensiveprocesses [8].Similarconsiderationscanbeextended tomicro‐EDM,bywhichsmallbutincreasingvolumesareprocessedandthehighspecificpowerconsumptionishigher[9,10].

InEDM,material removal is effectedbya sequenceof electricaldischargesbetweenwork‐pieceandaconductivetool(electrode),removingsmallportionsofmaterialfrombothsides,i.e.frombothworkandtool.Toimprovetheprocess,suchdischargestakeplaceinadielectricme‐dium,whosepurposeisreducingthesparksize,localizingtheenergysupply,contributingtoendthesparkandtoremovethevaporizedmaterial[11].TwomainEDMtechniquesbecamepopu‐lar,namelydie‐sinkEDM(oftensimplyquotedasEDM)andWireEDM(WEDM). EDMprocesses showmany interesting properties, since they are able to produce complexshapes and they are not affected by themechanical strength of the workpiece. On the otherhand, somenegative factors limit the fieldof applicationof such technique.First, thematerialremovalrate(MRR)iscriticallylow:forthisreason,agooddealofresearchhasbeenfocusedonimproving theMRR [12,13].Moreover, toolwear rate (TWR) is alwaysnonnegligible if com‐paredwithMRR, affectingmaterial consumption and geometrical accuracy. Then, the processoutputdependsonseveralparameters;processoptimizationisrequiredtoachievegoodresultsbutitisoftennontrivial.Assaid,whenassessingprocessperformanceforEDMoperations,MRRandtoolweararemainlytakenintoaccount.Otherimportantfactsaregeometricalaccuracyandsurfaceintegrity.Recently,energyconsumptionandhealthimpactareconsideredaswell[14]. EDMcanbeusedtomachinesmallfeatures(havingcharacteristicsizeofabout1mmorless),inthiscasethetermmicro‐EDMisoftenused.Micro‐EDMisbasedonthesamephysicprincipleas allEDMoperations, yet eachdischarge conveys less energy and thedischarge frequency ishigher [15]. In this way, improved accuracy (small gaps are mandatory for small parts) isreachedandtheenergyeffectivenessisgenerallysmaller,sincethefractionofenergyusedforvaporizingthedielectricisrelativelyhigher[16,17]. Severaltypesofoperationsareusedinmicro‐EDM:micro‐diesinkandwireEDMaresimilarto their macro‐scale counterpart, whereas some specific processes includemicro‐ED drilling,micro‐EDmilling,micro‐ED grinding andmicro‐wireED grinding (micro‐WEDG). Inmicro‐EDmillingarotatingtool(ofsimplegeometry)ismovedwithrespecttotheworkasintraditionalendmilling,suchaspocketmillingorcontouringapplications.Comparedwithothermicro‐EDMprocesses, it achieves better flushing and tool stability [18]. In micro‐ED milling, machiningtimesareoftenquite long,butmachiningperformancemaybe improvedbyeitheroptimizingprocessparameters[19‐21]orexploitingothertechniques,asithasbeenreportedin[22]. ForbothEDMandmicro‐EDM,processoptimizationmaybecarriedoutinseveralways,byconsideringmachiningparametersorbyselectingsuitablematerialsforeitherelectrodeordie‐lectric fluid.Electricalparameteroptimization involves theselectionof current,voltage, sparktimeandsparkinterval.Forthispurpose,regressiontechniquesareoftenused[23].Ingeneral,commercialEDMdevicesareprovidedwithmachiningprogrammes, includinganoptimalcom‐bination of parameters as a function ofmaterials (for bothwork and tool) and of the surfacerequirements(eitherroughingorfinishing);thus,finetuningofelectricalparametersisseldomfeasiblebytheenduser,whocanonlyselectamongasetofmachiningprogrammes.Incaseofmicro‐EDM,aRCgeneratorisveryoftenemployed:duetothephysicsofthepowercircuit,dis‐chargecurrentanddurationcannotbeindependentlychosen,sofurtherreducingthedegreesoffreedomforoptimization[8]. Furtherchancesforoptimizationareprovidedbytheselectionofprocessmaterials,forboththedielectricand theelectrode.Dielectricselection isdealtwith inmanystudies [24].Atpre‐sent,keroseneanddeionizedwater(especially forWEDM)aremainlyused[14,25].Desirablepropertiesforadielectricfluidarelowspecificgravity,highflashpointandoxygencontent,lowviscosityandtoxicity,highbreakdownvoltageandbiodegradability[26]. Other solutions includeorganicoils, aerosolsandgases [27,28].Theuseof avegetableoilprovedtoachievesignificant improvementswithrespecttokeroseneintermsofMRR,surfacefinishandintegrity,besidesbeingmoredesirableforsustainabilityissues[6].GasassistedEDM(sometimes referred to asdry EDM)have been studiedby severalAuthors. Both air [29] and

Evaluation of the sustainability of the micro‐electrical discharge milling process

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oxygen[30]werestudied,interestingfindingshavebeenreportedinthelasttwodecades.MRR,TWR,surfacequalityandintegritymaybeimprovedbyusingdryEDM[11]. Whenevaluatingdielectricperformances,healthproblemshavebeenoftenconsidered.Die‐lectric fluidsmayreleasevapoursand fumes; theymayrequirea treatment(filtering,deioniz‐ing)and theybecomeexhaustedover time. Ithasbeenpointedout thataquantitativeassess‐mentofhealthimpactrequiresmoredataaboutpollutantconcentrationinfumesthatisheavilydependentondielectrictype[31,32]. Dielectric circulation and filtering generally use a significant fraction of the overall powerconsumption[10].Inthisway,bothsustainabilityandprocesscostareaffected.Itisworthnot‐ingthatenergyconsumptioninEDMisreportedtobethemainsourceofbothcostandsustain‐abilityissues.Arelevantpartofenergyconsumptionisindependentfrommachiningparameters[33],sothemainimpactonbothcostandsustainabilityisduetomachiningtime;thus,MRRcon‐siderationareofprimaryrelevance. The sustainability evaluation of themicro‐EDM process is a complex task because severalaspects have to be taken into account. Sustainability assessment through indexesmay be thesimplestway,yetitrequiresknowledgeaboutprocessdata.Inthepresentpaperasustainabilityindexispresentedformicro‐EDmillingandusedtocomparedifferentchoicesofdielectricflu‐ids,bothliquidandgas.Anexperimentalcampaignwascarriedouttosupplydatatothemodel.Fivedielectrics, twoworkmaterials(titaniumandstainlesssteel)andtwoelectrodematerials(brassandtungstencarbide)weretakenintoaccountforthestudy.

2. Materials and method 

2.1. Development of the Sustainability Index 

Theideaofsustainablemanufacturingisnotyetfullydefinedduetothepresenceofseveralin‐terpretationsofthesustainabilityconcept[34].Infact,severaldomainscanbeconsideredso,asafunctionofthese,theexpressionsustainablemanufacturingmayassumedifferentmeanings.Inthis paper, the concept of sustainability is strictly connected to themanufacturing operationsandtheaspectsrelatedtostakeholders,technologies,services,supplychainarenotincluded.Inthisview, the issues thatare taken intoaccount toevaluate theoverall environmental impactare: the energy consumption, thematerials requirement, thewastemanagement, the processsafetyandthestaffhealth(Fig.1).

ASustainabilityIndex(SI),expressedineuro,wasdevelopedrepresentingtheenvironmentalimpactintermsofquantityoftheconsumedresourcesandpollutioneffectscreatedbytheEDMmachining.Inthisway,whentheindexassumeshighvalues,theprocessispoorlysustainable,whileforlowvaluestheprocesshasalowerimpactandthereforeismoresustainable.

Fig.1Sustainablemanufacturingissues

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TheelementstakenintoaccounttoformulateSIare:

Energeticconsumption: It represents theelectrical energyconsumptionof themicro‐EDMma‐chine.Theenergeticsustainability( )iscalculatedasfollows:

∙ (1)

where representstheabsorbedenergyofthemachineexpressedin[kWh]and thecostperunitfortheelectricityin[€/kWh].

Electrodewear:Thesustainabilityrelatedtotheelectrodewear( )canbeevaluatedas:

∙ (2)

where isthevolumeoftheconsumedelectrodein[mm3]and istheunitcostpervolumeofthetoolexpressedin[€/mm3].

Dielectric:Toestimatetheimpactofthedielectric, thepurchasecostofboththedielectricandthefiltersandthecostsforitsdisposalweretakenintoaccount.Thepurchasecostofthedielec‐tric ( )expressed in [€], charged toanEDMoperationhavinga timedurationof (erosiontime),wasevaluatedas:

∙ ∙ (3)

where isthepriceperlitreofthedielectricin[€/l], isthevolumeofthedielectrictank[l]and isthelifetimeofthedielectricin[h].Itmustbenotedthattheerosiontime,expressedin[h],istimeintervalfromthestarttotheendofmachiningcycleandthereforeincludestimepe‐riodinwhichthemachinedoesnoterode(forexamplethetimeusedtocontrolthelevelofwearofthetool).Strictlyspeaking,onlytheactivetimeshouldbetakenintoaccountbutconsideringthat thedurationof themillingoperation ismuch longer thanthepassive time,assumingthatthis latter isnegligible leadstoanacceptableapproximation.This information is inagreementwith[10]foundfordie‐sinkingEDMofmacrocomponents.

Thepurchasecostofthefilter( )in[€]forthedielectricunitcanbeestimatedas:

∙ (4)

where isthecostofthefilterin[€] andLfisitslifetimeexpressedin[h].Itisworthnotingthatfiltermanagementinvolvestheuseofsignificantamountofenergy;inthismodel,however,such value has already been included in overall energy consumption and therefore is not ac‐countedhere.

Thedismantlingcostofthedielectricin[€]isdefinedas:

∙ ∙ (5)

where istheunitarydielectricdismantlingcostforliterin[€/l].Thedielectricsustainabilitytakesintoaccountthethreeovermentionedelements:

∙∙ (6)

Notedthatthetotaleffectofdielectriconsustainability isproportional tomachiningtimeandthereforeitisdirectlydependentonMRR.

Processperformance:Thisfactorisrelatedtothesustainabilityimpactofscrapproductionthatcan be evaluated by multiplying the scrap rate (i.e. the probability of producing a non‐conformingpart)bytheaveragecostofeitherdisposingorrepairingthepart.Ifapartcannotberepaired,itsdisposalcostisevaluatedbytakingintoaccountthecostoftheelectrodewear,ofthemachinetimeandoftherawworkpiece.Whenapartcanberepaired,theassociatedcostislower than thedisposal cost.On average, the cost forproducing a nonconformingpart (tobeeitherscrappedorrepaired)isafractionγofthetotaldisposalcost.Scraprateshouldbeesti‐

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mated throughastatisticalanalysis; for thereportedexperiments,however,asimplified tech‐nique,basedondeviationsofthemachinedslotdepthfromitsnominalvalue,waspreferred.Asuitablesmoothingfunctionwasusedtolinkdeviationstoscrapprobability.

Onthisbasis,theperformancesustainabilitycanbeestimatedasfollows:

∙ ∙ ∙ ∙ (7)

where isthehourlycostofthemicro‐EDMmachine,expressedin[€/h],andkdescribesthevalueoftheworkpiece,forsakeofsimplicity,itsvaluewassettozero.

Environmentalimpact:Itrepresentsanevaluationoftheenvironmentalimpactoftheuseddie‐lectric.Severalaspectsweretakenintoaccountsuchasthefirehazard,thegenerationoffumesandvapours,thepossibleskinirritationoftheoperators,thegenerationoftoxicfumes,thedustformation,thepossiblere‐useofthedielectricand,finally,thedielectricandfiltersdismantling.Foreachofthem,aqualitativeevaluationfrom0to3wasmadebasedonliteraturedata[7,35](increasingthevalue,theenvironmentalimpactismoresevere).Thepenaltycoefficient wasdefinedastheratiobetweenthesumoftheseevaluationsandthemaximumachievablepoints.

Theenvironmentalsustainability( )wasevaluatedasfollows:

1 ∙ ∙ (8)

whereβisanunitarycoefficientin[€/h].Infrontoftheabovedescription,theSustainabilityIndexin[€]iscalculatedas:

(9)

ThedevelopedSustainabilityIndexisaffectedbytheprocessperformance,intermsofmachin‐ingtimeandelectrodewear,experimentallyevaluating.

2.2. Experimental cases 

ASarixSX‐200machinewasusedtorealizemicro‐millingtests.Micro‐pocketsontwotypesofworkpiecematerialsandusingtwodifferentelectrodesweremachined.Fig.2showsthedimen‐sionsofthepockethavingdepth0.1mm.Themillingstrategywaslayer‐by‐layerandthedepthof each layerwas0.003mm,adoptinga roughingenergy.Whendealingwithmilling, thema‐chinebuilderallowstoselectamongsomebuiltinsetsofprocessparameters(machiningstrate‐gy, i.e. roughing, finishingetc.).For thepresentcase, themachiningstrategy labelledroughingwasselected.Itcanbenotedthatwithineachstrategy,theelectricalparametersdependontheworkpiecematerial, theelectrodecharacteristicsandthetypeofdielectric(onlykeroseneandwater are included). Themachining parameters affect strongly themachiningperformance intermofmachiningtime,electrodewearandgeometricalcharacteristics.Initsturn,thesustaina‐bilityindexdependsstronglyonmachiningtimeandonelectrodewear(seetheequationsofthesub‐indexes).Theworkpiecematerialswerestainlesssteel (AISI316L)andtitanium(Ti6Al4V);asregards

the tool, tubularelectrodesmadeof twodifferentmaterials, tungstencarbide(WC)andbrass,havingexternaldiameterof0.3mmandinternaldiameterof0.12mm,wereused. Asregardsthedielectric,fivetypesofdielectricwereused,bothliquidandgaseous:Kerosene(HEDMA111),demineralizedwater,vegetableoil (soyabean),airat10barandoxygenat9.5bar.TheirpropertiesarereportedinTable1.Thegaseousdielectricswereinjectedinthema‐chiningzonethoroughthetubularelectrode(Fig.3).Whentheunconventionaldielectricswereused,sincetheyarenotincludedinthesoftware,kerosenedatawereselected.

Fig.2Geometryofthemicro‐pocket

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Fig.3Implementationofdry‐EDM

For each pocket the energy consumption was measured using a watt‐metro Christ EL‐EKTRONIK(CLM1000ProfessionalPlus)placedinwaytoincludethewholepowerusage.Whengaseousdielectricswereused, thedielectricunitwasdisabledtomeasuretheactualelectricalenergyadsorbedbythemachining.Inthecaseofcompressedairasdielectric,theenergycon‐sumedbythecompressorwasmeasuredandincluded.Attheendofeachpocket,theelectrodewascutusingthewireEDMunit torestore thesame initialelectrodeconditions foreachtest.Foreachmilling, theEDMmachine records themachining time, theelectrodewear, themeanerosionspeed,theerodedvolumeandtheactualdepthofthepocket.Table2reportsthevaluesofthecoefficientsusedintheequationstocalculatethesustainabilityindex.

Table3reports, foreachdielectricusedintheexperimental investigation,theevaluationofthealltheaspectstakenintoaccountforthedeterminationofthepenaltycoefficient( ).Thiscoefficientwasusedintheformulaoftheenvironmentalsustainability( ).Foreachaspect,arankingindexfrom0to3wasevaluated.Coefficient istheratiobetweenthesumofallindex‐esandtheworstpossiblescore(allindexesequalto3).

Table1PropertiesofthedielectricsTypeofdielectric Dynamicviscosity

[g/(m·s)]Density[g/dm3]

Dielectricrigidity[kV/mm]

Thermalconductivity[W/(m·K)]

Specificheat[J/(g·K)]

Dielectricconstant

Kerosene 1.64 781 14‐22 0.14‐0.149 2.1‐2.16 1.8Water 0.92‐1 1000 65‐70 0.606‐0.62 4.19 80.4Vegetableoil 48.4 915‐925 62‐65 0.14‐0.16 1.67 2.86Air 0.019 1.205 3 0.016‐0.026 1.005 1.000536Oxygen 0.021 1.43 0.92‐2.6 0.026 0.92 1.00049

Table2Valuesofthecoefficients

Kerosene H2O Vegetableoil Air Oxygen[€/kWh] 0.156[€/mm] Brass:0.024362;WC:0.1054[€/l] 9.63 0.25 1.4 0 1.05[l] 25 25 25 40[h] 1000 1000 1000 33.33[€] 117[h] 1000[€/l] 0.215 [€/h] 40

Table3Determinationofthepenaltycoefficient Kerosene Water Vegetableoil Compressedair OxygenFirehazard 3 0 0 0 2Fumesproduction 3 3 3 0 0Skinirritation 3 0 0 0 0Toxicfumes 3 0 1 0 0Dustproduction 0 0 0 3 3Dielectricre‐use 1 2 1 0 3Dielectricdismantling 3 1 3 0 0Filtersdismantling 3 3 3 0 0Total 19 9 11 3 8 0.79 0.37 0.46 0.12 0.33

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3. Results and discussion 

Figs.4and5showtheerosiontimeandthevolumeoftheelectrodewearobtainedmillingAI‐SI304andTi6Al4Vusingbrass andWCelectrodes varying the typeof dielectric. Thebars forbrass electrodeusinggaseousdielectrics areomitted since these conditionsdidnotpermit torealizethetestaspreviouslyunderlined.Ingeneral,waterasdielectricoffersanoptimalsolutionforallthetestedconditionswhentheobjectiveistominimizethemachiningtime.Whenbrasselectrodeisused,vegetableoiliscomparabletokeroseneespeciallyforAISI304whileusingWCelectrodethereisaremarkabledifferenceontheperformancebetweentheoil‐baseddielectrics:themachiningoccursinafasterwayusingvegetableoilthankerosene.Asfarasgaseousdielec‐tricsareinvolved,someinterestingresultsareobtained:whilecompressedairdoesnotrepre‐sentavalidalternative,theoxygenisoneofthebestsolutionstominimizethemillingtime. Asregardstheelectrodewear,thegasdielectrics,especiallytheoxygen,minimizethewear.Ingeneral,thewaterasliquiddielectricobtainsgoodresults.Itisherebyconfirmedthatvegetableoiliscompetitivewithkerosene.Carbideelectrodesshowalowerelectrodewearthanbrass[36].

Usingthesedata,thedevelopedSustainabilityIndexwascalculated(Fig.6).Severalconsider‐ationscanbemade.First,theenvironmentalimpactprocesswhenbrasselectrodeisadoptedislowerthanWCelectrode.Infact,inthreeoutoffivesub‐indexesofSI,machiningtimeplaysanimportantroleandtherefore theprocessesconsuming lowertimearemoresustainable.Fixedtheelectrode,thedielectricthatrevealstobethemostsustainableisthewater.Ingeneral,kero‐seneislesssustainablethantheothersliquiddielectrics.Vegetableoilisanappreciabledielec‐tric inall thetestedconditions.Asregardsgaseousdielectrics,compressiveairgivesworstre‐sultswhileoxygen isvery interesting.On titaniumsheets,oxygen is thebestsolution tomini‐mizethesustainabilityindexwhileonstainlesssteelgivesgoodresults.

Anyway, this analysis on the globalSI does not allow identifying the problematic issues ofeachexperimentedconditions.For thisreason,Figs.7and8showthecontributionof the fivesustainabilitysub‐indexestotheglobalSI.

Fig.4MachiningtimeforAISI304andTi6Al4VusingbrassandWCelectrodevaryingthedielectric

Fig.5WearofbrassandWCelectrodewhenAISI304andTi6Al4Vismachinedvaryingthedielectric

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Usingbrasselectrode,wearandenvironmentalsub‐indexesareespeciallyrelevant.Theen‐ergeticcomponentandthatonerelatedtotheperformancearealmostconstantforallthecondi‐tions.Regardingtheotherscomponents,thedielectricsdisplaythelargestpercentagevariation,yet itscontributiontoSI isrelativelysmall.Thedielectricandenvironmentalsustainability in‐dexesarehighforkerosene,mediumforvegetableoilandlowforwaterforbothworkpiecema‐terials.SimilarremarksarevalidforelectrodewearwhenAISI304ismachined,whileelectrodewearisalmostthesameforTi6Al4V.

There is a different situationwhenWCelectrode is used.The energetic component resultsalwayssmallexceptforairasdielectric.Forliquiddielectrics,eachsub‐indexcanberankedasfollows:highforkerosene,mediumforvegetableoiland lowforwater.Themaincomponentsare wear and environmental sub‐indexes. Overall performance of air dielectric is poor whileoxygen proves to be competitivewith liquid dielectrics especially because it allows low elec‐trodewear.

Anyway, the critical aspects on the formation of the globalSI varyingworkpiece and elec‐trodematerialanddielectrictypecanbeunderlinedthoughFigs.6and7.Inviewofthisanaly‐sis,itispossibletotakeactionsaimingtoreduceingeneralthesustainabilityindex(andthere‐fore to improve the sustainability level) focusing on the aspects causing more sustainabilityproblemsfollowingaParetologic.

 Fig.6SustainabilityindexforAISI304andTi6Al4VusingbrassandWCelectrodevaryingthedielectric

 Fig.7CompositionofthesustainabilityindexforAISI304andTi6Al4Vusingbrasselectrodevaryingthedielectric

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 Fig.8CompositionofthesustainabilityindexforAISI304andTi6Al4VusingWCelectrodevaryingthedielectric

A furthercomparisonbetweenworkingconditions(combinationsofelectrodematerialanddielectric)canbemadebyevaluatingeachsinglesub‐index,normalizing thesumofallvaluescorresponding to all working condition. The percentage distribution of each sub‐indexes arereportedinFigs.9and10,showingarankingoftheexperimentalconditionsforbothworkma‐terials.ConsideringAISI304,ingeneraltheuseofcompressedairisnotappreciableexceptfortheindicatorrelatedtothedielectricsustainability.Thecombinationkeroseneasdielectricandtungstencarbideaselectrodeexpendsalotofresourcesrespecttheotherssolutions.OxygenasdielectricandWCelectrode is thebest solution for thesub‐indexes regarding theenergycon‐sumption, the electrode wear and the environmental impact but need improvements for theperformanceanddielectriccomponents.Waterandvegetableoilalwaysgivegoodresults.

 Fig.9 Percentagedistributionof sub‐indexesondifferentexperimental conditions in termsof electrodematerialanddielectricforAISI304

 Fig.10Percentagedistributionofsub‐indexesondifferentexperimentalconditionsintermsofelectrodematerialanddielectricforTi6Al4V

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AlsoforTi6Al4V,waterandvegetableoilshowgoodperformancefromthepointofviewofthesustainability.Itisconfirmedthatcompressedairisnotacompetitivedielectric.Asregardsthecombinationoxygenasdielectricandtungstencarbideaselectrode,thecriticalaspectisonlythedielectricsustainabilitywhiletheotherssub‐indexesareveryinteresting.

Theadvantageoftheproposedindexmeetstherequirementstobeeasilyimplementedinin‐dustrialapplications.Anyway,thepresentedresultsintermsoftheeffectofthetypeofelectrodeanddielectriconthelevelofsustainabilityofthemachiningareinfluencedbytheadoptedpa‐rametersandbythechoiceofthevaluesofthecoefficients.Theindexcouldbeimprovedtakingintoaccountothersustainabilityissues.Forexample,thepollutioneffectsduetothecontamina‐tionfromdustsofbothelectrodeandworkpiececouldbeconsideredintothesub‐indexrelatedtheenvironmental.Moreover,otheraspectsrelatedthequalityofthemachiningcouldbetakenintoaccountsuchasroughnesssurface.

Theproposedindexcanbeimplementedindifferenttechnologicalsituationssuchasmicro‐EDMdrillingorWEDM.Infact,thesedifferentapplicationsofthesametechnologyhaveincom‐monthesamephysicalprincipleofmaterial removalbasedon theerosion thoroughelectricaldischargesbetweentheworkpieceandtheelectrodetoolthatoccurinadielectricfluid.Theas‐pectstakenintoaccountfortheelaborationofthesustainabilityindexareingeneralcommontootherEDMprocessesandthereforethemodelcansupportsimilarworksonothersapplications.Infact, themainfactorsrelatedtosustainabilityofEDMprocessesaretheenergeticconsump‐tion,theelectrodewear,theusageofthedielectric,theeffectofthedielectricontheenvironmen‐talandtheprocessperformanceintermsoftheprobabilityofproducinganon‐conformingpart.

4. Conclusion 

The micro‐ED milling process was evaluated concerning the sustainability manufacturing. Aglobal index, named Sustainability Index, taking into account the energetic consumption, theenvironmentalimpact,thedielectricconsumption,thewearoftheelectrodeandthemachiningperformance (i.e. the scrapping/repairing rate)was developed. The index estimates the envi‐ronmentalimpactintermsofbothquantityofconsumedresourcesandpollutioneffectscreatedby the process. Itwas applied for an experimental case, in particular the execution ofmicro‐pocketsonstainlesssteelandtitaniumsheetsusingtwotypesofelectrodeandfivetypesofdie‐lectric,both liquidandgaseous.Foreachworkpiecematerial, theeffectsofboth theelectrodematerialandthetypeofdielectriconthesustainabilityprocessperformancewereanalysed.Inthisway,foreachconditionintermofworkpiecematerial/electrodematerial/dielectricthecrit‐icalaspectsrelatedtothesustainabilitycanbeidentified.Focusingontheseaspects,actionsoffindingsolutionsminimizingtheenvironmentalimpactoftheprocesscanbeundertaken.Unu‐sualdielectrics, suchasvegetableoil andoxygen,proved tobevalid substitutes to traditionalonesunderseveralviewpoints,includingsustainability.

Theproposed index tomeasure the sustainability ofmicro‐EDMmilling processmeets therequirementstobeeasilyimplementedinindustrialapplications.Theindexprovidesatoolthatcanassistthedecision‐makingstageoftheselectionoftheproductandprocessconditionsaim‐ing the minimization of the environmental impact. The obtained results can improve theknowledgeofalternativedielectrics,notyetusedinindustrialapplications.Finally,theaspectstakenintoaccountfortheelaborationofthesustainabilityindexareingeneralcommontootherEDMprocessesandthereforeitcansupportsimilarworksonothersapplicationssuchasmicro‐EDMdrillingorWEDM.

Acknowledgements 

TheauthorswishtothankDr.FabioCacciaandDr.AlessioPalombafromtheUniversityofBergamofortheircontri‐butionintheexperimentalresearch.

 

Evaluation of the sustainability of the micro-electrical discharge milling process

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