Investigating the influence of built-up edge on forces and surface roughness in micro scale orthogonal machining of titanium alloy Ti6Al4V

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JournalofMaterialsProcessingTechnology235(2016)28–40

ContentslistsavailableatScienceDirect

Journal

of

Materials

Processing

Technology

j ou rn a l h o m epa ge :w w w . e l s e v i e r . c o m / l o c a t e / j m a t p r o t e c

Investigating

the

inﬂuence

of

built-up

edge

on

forces

and

surface

roughness

in

micro

scale

orthogonal

machining

of

titanium

alloy

Ti6Al4V

Samad

Nadimi

Bavil

Oliaei

a

_,

_Yi˘git

_Karpat

a,b,∗

a_Bilkent_University,_Department_of_Mechanical_Engineering,_Micro_System_Design_and_{Manufacturing}_Center,_Bilkent,_Ankara,_Turkey b_Bilkent_University,_Department_of_Industrial_Engineering,_Bilkent,_Ankara,_Turkey

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received3November2015

Receivedinrevisedform14March2016 Accepted7April2016

Availableonline11April2016 Keywords: Cutting Micromachining Built-upedge Titaniumalloy

a

b

s

t

r

a

c

t

The edge geometry of cutting tools directly inﬂuences the chip formation mechanismin

micro-mechanicalmachining, wheretheedge radiusand uncutchip thicknessarein thesame orderof

magnitude.Anuncutchipthicknessthatissmallerthanthecuttingedgeradiusresultsinalargenegative

rakeangleduringmachining,andbuilt-upedgeformationthenaffectsthemechanicsoftheprocess.In

thisstudy,micro-scaleorthogonalcuttingtestsontitaniumalloyTi6Al4Vwereconductedto

investi-gatetheinﬂuenceofbuilt-upedgeformationonthemachiningforcesandsurfaceroughness.Cutting

edgesinthesetestsareengineeredusingwireEDMtechniquetohaveanedgeradiusofaround2␮m

andclearanceanglesof7◦_and₁₄◦_._It_is_observed_that_machining_process_inputs_(uncut_chip_thickness,

cuttingspeed,andclearanceangle)affectthesizeofthebuilt-upedge,whichinturnaffecttheprocess

outputs.Itisobservedthatbuilt-upedgeformationprotectsthecuttingedgefromﬂankandcraterwear

undermicromachiningconditionsandtheinﬂuenceofbuilt-upedgeonthesurfaceroughnessvaries

dependingonthecuttingspeedanduncutchipthickness.Ourﬁndingsalsoindicateacloserelationship

betweentheminimumuncutchipthicknessandthemeanroughnessdepth(Rz)ofthemachined

sur-face.Theminimumuncutchipthicknessisfoundtobearound10%oftheedgeradiusinthepresenceof

built-upedge.

1. Introduction

Mechanicalmicromachiningisdeﬁnedasthemachiningof pre-cisionpartsmadeoutofawiderangeofengineeringmaterialswith complexsurfaces(Dornfeldetal.,2006).Asolidunderstandingof themechanicsofcuttingatthemicroscaleiscrucialinbuilding predictivemodelsandcontrollingthequalityofmicroparts.A com-monlyobservedphenomenon whichappearsduringcontinuous chipformationisbuilt-upedge(BUE)anditisknowntoaffect sur-faceroughnessandtoolwear.ABUEconsistsofmateriallayers whicharedepositedontothetoolsurface,changingthetool geom-etryand,hence,themechanicsoftheprocess.AstableandthinBUE isknowntoprotectthecuttingedge(KalpakjianandSchmid,2010).

∗ Correspondingauthorat:BilkentUniversity,DepartmentofIndustrial Engineer-ing,Bilkent,Ankara,Turkey.

E-mailaddress:ykarpat@bilkent.edu.tr(Y.Karpat).

Smalluncutchipthicknessesmachinedwithtoolshavinga com-parableedgeradiuscreatessuitableconditionsforBUEformation inmicroscalemachining.Gainingabetterunderstandingofthe inﬂuenceofBUEonprocessoutputshaveresultedinanincreased interestinmachiningresearch.

Due to the micro cutting tool fabrication process and tool materialgrainsizelimitations,theedgeradiuscannot beeasily decreased withoutsacriﬁcing thestrengthof thetool,which in turnaffectsthechipformationprocessandthesurfacequalityof themachinedworkpiece.Weuleetal.(2002)showedthe impor-tanceofedgeroundnesswhentungstencarbidemicroendmillsare usedtomachineferrousmaterials.Woonetal.(2008)studiedthe interactionbetweenuncutchipthicknessandedgeradiususing experimentaland ﬁnite element based techniques when micro machiningAISI1045.Theyshowedtheedgeradiusactinglikea negativerakeangleandfoundthataconstantstagnationpointon thetoolexistsforalargerangeofuncutchipthicknessvalues. How-ever,inanearlierstudy,Waldorfetal.(1999),studiedploughing http://dx.doi.org/10.1016/j.jmatprotec.2016.04.010

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S.N.B.Oliaei,Y.Karpat/JournalofMaterialsProcessingTechnology235(2016)28–40 29

Fig.1.(a)Schematicofwire-EDMprocessingtoobtainrequiredrakeandclearanceangles,(b)Edgeradiusmeasurementoftheinsertafteredgepreparation.

Table1 Experimentalconditions. InitialCutting EdgeRadius Clearance Angle UncutChip Thickness CuttingSpeed (m/min) 1.95␮m 7–14 0.2–0.4–0.6–0.8–1␮m 30–47–62–78

modelsduringorthogonalcuttingandfoundthatastablebuilt-up modeldescribedexperimentalobservationsbetterthana stagna-tionpointmodel.Fangand Dewhurst(2005)usedsliplinefield analysistopredictthesizeofthebuilt-upedge during machin-ing.KarpatandÖzel(2008)alsopresentedasliplinefieldbased approachformachiningwithroundedgedtools,includingbuilt-up edge.Karpat(2009)consideredtheinfluenceofcuttingedgeradius includingbuilt-upedgeduringmicroscalemachiningandstudied theeffectoffractureonmachiningoutputs.Childs(2013) devel-opeda finiteelementmodeltopredictbuilt-upedgeformation duringmachiningofsteelbyintegratingadamagemodel.Some preliminaryresultsonsimulatingbuilt-upedgeduringmicroscale machiningwerealsopresentedinChilds(2013).Adetailed inves-tigationofhowbuilt-upedgeinfluencesmicroscalemachiningis theaimofthisstudy.

Table2

EDXanalysisofsurfaceonthetool.

InsertSurface beforemachining

AnalysisofBUE InsertSurface

afterCleaning Co%5.19 Al%5.66 Co%4.65 W%94.81 Ti%82.82 W%93.95 V%3.46 Co%0.59 W%7.48

Inmicroscalemachining,thecuttingtooledgeradiusandthe amountofmaterialbeingcutareinthesameorderofmagnitude. Thereisavalueofuncutchipthicknessafterwhichcontinuouschip formationceases.Thiscriticalvalueisdeﬁnedasminimumchip thickness,whichisknowntobeafunctionofthetoolmaterial, cuttingedgeradius,andworkpiecematerial(Ikawaetal.,1992). Luccaetal.(1991,1993)indicatedtheimportanceoftheslidingand ploughingatthetoolworkpieceinterfaceduetoedgeradiusofthe toolduringultraprecisionmachiningconditions.Kimetal.(2004) analyzedtheperiodicityofforcesduringmicromillingand identi-ﬁedthetransitionbetweennon-cuttingandcuttingregimesnear

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30 S.N.B.Oliaei,Y.Karpat/JournalofMaterialsProcessingTechnology235(2016)28–40

Fig.2.InvestigationofthetoolsurfaceafterwireEDM(a)SEMimage,(b)Surfacetopographyoftherakeface.

theminimumchipthicknessvalue.Theyconcludedthatthe peri-odicityofcuttingforcesisaffectedbytheminimumchipthickness, feedpertooth,andcuttingpositionangle.Theyobservedalocal maximuminthethrustforcesthattakesplacearoundminimum chipthicknessvalue.Junetal.(2006)investigatedthedynamicsof microendmillingandobservedpeaksinthethrustforcearound minimumchipthicknessduringmicromachiningof ferriteand pearlite.Theyconcludedthattheminimumchipthicknesseffect causesinstabilityatlowfeedrates.Thesamephenomenonwas observedbyLiuetal.(2004)andnamedasfeedrateinstability.Son etal.(2005)studiedminimumuncutchipthicknessandits rela-tiontotheedgeradiusduringdiamondturningofvariousmaterials. Theydevelopedarelationshipbetweenminimumuncutchip thick-ness,frictionangle,andedgeradius.Malekianetal.(2012)used aminimumenergybasedmethodtomodelminimumuncutchip thicknesswhilemachiningaluminumalloy.Theyfoundthe stagna-tionpointtobeapproximately23%oftheedgeradius,anditisnot aconstantpointbutaregiononthetool.CubaRamosetal.(2012) emphasizedtheimportanceofbuilt-upedgeformationduring tran-sitionfromploughingtocuttingduringmicroscalemachiningof AISI1045anddevelopedanewmodelfortheestimationof

mini-mumchipthickness.Inthisstudy,minimumuncutchipthickness phenomenoninthepresenceofBUEisinvestigatedbyfocusingon theinteractionbetweenBUEandsurfaceroughness.

Duetoitshighreactivitywithcuttingtool materialsandits lowthermalconductivity,titaniumalloysareconsidered difﬁcult-to-cut,sorapid toolwearisan importantissuethataffectsthe qualityofthemachinedproducts.BUEformationduring machin-ingoftitaniumundermacroscalemachiningconditionshasbeen wellstudiedinliterature.TheissuesrelatedtoBUEandtoolwear havebeensummarized inEzugwuand Wang(1997),Armendia etal.(2010)andPramanikandLittlefair(2015).Thepsonthiand Özel(2015)studiedmicromillingoftitaniumalloyTi6AL4Vand observedBUEwhencuttingedgeisworn.Thewearmechanisms oftitaniumalloyswereshown tobequitedifferentfromsteels and nickel alloys inHartung and Kramer(1982). It wasshown thattoolwearwasgreatlyreducedwhenadhesionoccursbetween toolandchip.Theadhesionlayerpreventsslidingattheinterface andimprovestoollifeundercertainconditions.Inarecentstudy, Kümmeletal.(2014)showedthattoolwearperformancemaybe improvedwhencuttingtoolswithintentionalbuilt-upedgeare employedinmachining.Kümmeletal.(2015)usedlasersurface

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Fig.3.(a)Theexperimentalsetupformicroorthogonalexperiments,(b)Microstructureofthetitaniumworkpieceusedinthisstudy,(c)Atypicalchipproducedduring micromachiningexperiments.

texturingtocreatedifferentdimplestructuresandchannelsonthe rakefaceofthetool.ThedimpletexturesareshowntoincreaseBUE adhesiononthetool.Inthisstudy,electricaldischargemachining (EDM)techniqueisusedtopreparethecuttingedges.UsingEDM processcreatesmicroscalecratersonthesurfaceofthetool,which mayalsopromoteBUEadhesionduringmachining.

Inthisstudy,theinfluenceofbuilt-upedgeonprocessforces, surfacequalityandminimumchipthicknessduringmicro machin-ingoftitaniumalloyhasbeeninvestigatedindetail.Cuttingedges withsmalledgeradiiwerespecificallyfabricatedanduncutchip thicknessvaluesweresettobelessthantheedgeradiusto cre-atemicroscalemachiningconditions.Cuttingedgesandmachined surfaceswereinvestigatedaftertheteststorevealtheinfluenceof built-upedgeonprocessoutputs.

2. Experimentalprocedure

2.1. Fabricationofcuttingedgemicrogeometry

Thecuttingedgemicrogeometryisobtainedthroughmicrowire electricdischargemachining(wireEDM)technique.TheEDM tech-niqueisadvantageousintermsofcontrollingthemicrogeometry oftheedge.ThefeasibilityofedgepreparationusingEDM

tech-niquewasshownbyYussefianetal.(2010).Inthisstudy,theedge geometriesoftheuncoatedcarbidetools(DCMW11T304H13A) with0◦rakeanglearemodifiedbyusingthewireEDMdevice (Sod-ickAP250L).TheUandVaxesofthewireEDMmachineareutilized toobtainnecessaryclearance(relief)anglesonthecuttingedges asshowninFig.1a.AnEDMwirediameterof100␮mwasused. ThewireEDMmachineusesoilasdielectricfluidinordertoobtain superiorsurfacequality.Cuttingedgeswithtwodifferentclearance angles(7◦ and14◦)werepreparedandedgeradiiofeachinsert weremeasured.Fig.1bshowstheprofileofacuttingedgebefore andafteredgepreparationwithEDMmachining.

Alaserscanningmicroscope(KeyenceVK-X100)wasusedto measuretheedgeradiusandclearanceangleofthetools.Theedge radiusafterEDMmachiningis measuredas1.95␮mwhere the initialedgeradiusoftheinsertwas25␮m.Adetaileddiscussion ontheinﬂuenceofedgeradiusonmachiningandthe uncertain-tiesrelatedtoitsmeasurementsweredescribedinDenkenaand Biermann(2014).Scanningelectronmicroscopeimageofthetool surfaceafterwireEDMisshowninFig.2a.Thesurfaceroughness (Ra)oftheinsertwasmeasuredas0.11␮musingalaserscanning

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Fig.4.SEMimagesofthecuttingedgesaftermachiningtests:a)0.2␮m,b)0.4␮m,c)0.6␮m.

2.2. Experimentalsetupfororthogonalmicrocuttingtests

Athree-axishybridmicromachiningcenter(MikrotoolsDT110), capable of performing micro turning processes, was used to conductmicro orthogonal cuttingexperiments. The machine is equipped with a spindle with maximum rotational speed of 3000rpm.Titaniumalloyshaftswith10mmouterdiameterand 0.25–0.35mmwallthicknesswereprepared.Onlyoneendofthe shaftwasmachinedtobehollow.Thedepthofhollowsidewaskept at3–4mm.InsertswithengineerededgeswereplacedonaSDACR 1212K11Stoolholder,whichwasthenattachedtotheKistlermini dynamometer(9256,max250N)asshowninFig.3.Micro machin-ingforcesweremeasuredandtransferredtoaPCthroughadata acquisitioncard(NationalInstrumentsPXI series).Thetitanium alloyTi6Al4V(␣+␤)workmaterialwithlamellarmicrostructure

(grainsof average25␮m lengthand 5␮m thickness), which is showntobesuitableformicromachiningapplicationsinAttanasio etal.(2013),wasusedinexperiments(Fig.3b).Ithadahardnessof 328HV.Theworkpiecewasfedtowardsthecuttingtoolwith nec-essaryfeedratetoobtaintherequireduncutchipthicknessduring orthogonaltests.Themachiningtimeanddistancewerechecked usingahighspeedmicroscopetomakesurethatthedesiredfeed ratewasachievedduringtests.Fig.3cshowsatypicalcontinuous chipproducedduringmicromachiningexperiments.

Inthis study,micro scaleorthogonalmachiningexperiments ontitaniumalloyTi6Al4Vwereperformedbyusingcuttingedges speciﬁcallyproducedforthatpurpose.Adifferentcuttingedgewas usedineachtest.Cuttingtoolshaveclearanceanglesof7and14◦. Thereasonforusingalowclearanceangleistochangemachining conditionsinthecuttingzoneandobserveitsinﬂuenceonthe

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Fig.5. (a)ScanningelectronmicroscopeimageoftheBUEregionandcorrespondingEDXmap,(b)EDXspectrumanalysisofthebuiltupedgezoneontherakeface,(c)EDX spectrumanalysisofthetoolsurface,(d)ColormapoftheTiandCelementsontheBUE.

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Fig.7. 3DEdgeproﬁlemeasurements.Leftcolumn7◦_,_right_column₁₄◦_clearance_angle._Uncut_chip_thickness_from_top_to_bottom_0.4,_0.6,_0.8,_and₁_␮m._Cutting_speed_is

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Fig.8.IncreasingBUEtiplengthwithincreasinguncutchipthicknessa)0.4,b)0.6,c)0.8,and,d)1␮m.

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Fig.10. (a)2Dedgeproﬁlecorrespondingto14◦_clearance_angle_at_0.4_␮m_uncut_chip_thickness_after₂_min_of_continuous_machining,_(b)_Tip_length_of_BUE_after₉_min_of

continuousmachining(62m/mincuttingspeed).

cessoutputssuchasbuiltupedgeformationandsurfaceroughness. Arangeofcuttingspeedvalueswasconsidered,setat30,47,62, and78m/minbasedonpracticalmachiningconditionsoftitanium alloyTi6Al4Vwithinthelimitsofthespindle.Uncutchip thick-nessvalueswereconsideredas0.2,0.4,0.6,0.8and1␮m,which areallsmallerthantheedgeradius.Experimentalconditionsare summarizedinTable1.Theforcesduringmicromachiningwere alsomeasured.Theexperimentswereconductedover12sforeach cuttingconditionduringforcemeasurementstudies.

3. InvestigationofBUEaftermicromachiningexperiments Experimentswereperformedateachconditionandthecutting edgesoftheinsertsareinvestigatedaftereachexperiment.Fig.4 showsthescanningelectronmicroscopeimageoftherakefaceof thecuttingedgewherethebuilt-upedgeformationcanbeclearly seen.

ArelativelylargeBUElength(around20–100␮m),compared totheuncutchipthicknessisobserved.Basedontheseimages, thethicknessoftheBUEislargeratthetipofthetooland grad-uallydecreasesonthetoolrakeface.HartungandKramer(1992) observedbuiltuplayer(BUL)andbuiltupedge(BUE)formation whilemacroscalemachiningtitaniumalloyTi6Al4Vwithdifferent cuttingtoolmaterials.Theyobservedthattoolwearisconsiderably reducedatcertainconditionsasalayerpreventsrelativeslidingat thetoolchipinterfaceandlimitsthediffusionrateofthetool con-stituents.Athighspeeds,thisprotectivelayerisremovedandtool wearincreasesquickly.Thethicknessofthislayerisdetermined bythebalancebetweentherateofdiffusionofthetoolmaterial throughit,andtherateofdissolutionofthereactionlayerinthe workmaterial.Theyobservedtheabsenceofcobaltandalayerof TiCwasformedonthetoolsurfacewhichwasreplenishedbythe carbonatomsremovedfromtheWCgrainsbelow.Similar observa-tionswerealsomadebyIkutaetal.(2002),Armendiaetal.(2010), Gerezetal.(2009)andArrazolaetal.(2009)underconventional

machiningcaseswhereelevatedtemperatures(700–800◦C)exist atthetoolchipinterface.Atmicroscalemachiningthetemperature riseatthetool-chipinterfaceisexpectedtobelowercomparedto macroscalemachiningconditions.

Inordertoinvestigatethechemicalcompositionofthetooland BUE,anEDXanalysiswasperformedandtheresultsareshownin Fig.5.AscanningelectronmicroscopeimageoftheBUEregioncan beseeninFig.5a.Titanium,aluminum,andvanadiumelements inthebuilt-upedgeregioncanbeclearlyidentiﬁedasshownin Fig.5banditsdetailedchemicalcompositionisgivenTable2.The EDXanalysisofthetoolsurfacebeforemachiningisalsoshownin Fig.5canditschemicalcompositionisalsogiveninTable2.Acolor mapofTiandCelementsontheBUEandtoolsurfacesareshownin Fig.5d.TheEDXanalysiswasperformedforalluncutchipthickness valuesandsimilarresultsareobserved.

InordertoinvestigatetheinﬂuenceofBUE onthetool-chip interface,thesectionof thetool under theBUE isalso investi-gatedaftercleaningit usinganultrasonicprocess.EDXanalysis wasrepeatedontheregionwhereBUEexisted.Fig.6showsthe EDXspectrumofthetoolsurfacewhereCo(%4.65)andW(%93.95) wasdetected.TheamountofCoisslightlylowercomparedtothe originaltoolsurface(seeTable2).Thisisinaccordancewiththe observationsofHartungandKramer(1982),althoughthe differ-enceisquitelow.

Fig.7showstheedgeproﬁlesobtainedthroughalaserscanning microscopewhereshapesofbuilt-upedgeforeachcuttingcasecan beseen.Fig.8showstheopticalmicroscopeimagestakenfromthe ﬂankfaceofthecuttingedges.

Fig.9showsedgeproﬁlesofcuttingedgeswheretheshapeof thebuiltupedgearoundthetooltipcanbeseen.Basedonthese images,asuncutchipthicknessincreases,theaveragetiplengthof BUEincreases(fromaround10_␮mto19_␮minFig.8).Thelength ofBUEontherakefaceislongeratloweruncutchipthickness. NosigniﬁcantdifferenceintermsofBUEsizewasobserveddueto differentclearanceangles.

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Fig.11.Forcevs,timemeasurements.Leftcolumn7◦_,_right_column₁₄◦_clearance_angle._Uncut_chip_thickness_from_top_to_bottom_0.4,_0.6,_0.8,_and₁_␮m.

Fig.10ashowstheedgeprofileofthetoolafter12sof contin-uousmachiningaftercleaningoftheBUE.Nosignofcraterand flankwearwasobserved,confirmingtheprotectiveeffectofBUE. However,theedgeprofilemeasurementrevealedthatedgeradius increasedto3–4␮mattheendofthemachiningtest.Edge round-ingisaknownformoftoolwearduringmicroscalemachining.The machiningprocesswasfurtherlengthenedto2and9minof contin-uousmachining.Theedgeradiuswasalsomeasuredtobearound 3–4␮mandagainnosignofflank/craterwearwasobservedonthe tool.Fig.10bshowsthetiplengthofBUEafter9minofmachining fromflanksideofthetool.

Theincreaseinedgeradiusmayhavehappenedduring burn-inperiodofthetool,anditfurtherpromotestheaccumulationof materialinfrontofthetool.TheinﬂuenceofBUEonmachining forcesandsurfacequalityareinvestigatedinthenextsection. 4. Forcemeasurementsduringmicroscalemachiningof titaniumalloyTi6AL4V

Measuringforcesduringmicromachiningtestsallowsthe inves-tigationoftheprocessforces inthepresence ofBUEformation. Fig.11 shows theforcemeasurementsobtainedfor 7◦ and 14◦ clearanceangletoolsfordifferentuncutchipthicknessvaluesat 62m/mincorrespondingto12sofcutting.Thesemeasurements correspondtothecuttingedgeimagesshowninFig.7.Each

orthog-onalcuttingexperimentwasrepeatedat leastthree timeswith upsharptools,andconsistentresultswereobtained.Themeasured forceswereﬁlteredwithalowpassﬁlterandaveragemachining forcesandforcevariationsaroundaverageforceswerecalculated. Machiningforceswereobservedtoincreaseattheinitialstages ofmachining,indicatingchangingcontactconditionsatthetooltip andBUEformationontherakeface.Increasingaverageforcemay berelatedtoedgeroundingandincreasingforcevariationsmaybe relatedtoBUEformation.Thethrustforce(Ft)seemstobeaffected

bytheBUEmorethanthecuttingforce(Fc).Forcemeasurements

followasteadytrendindicatingtheformationofastableBUE. Fig.12showstheinfluenceofcuttingspeedonthrustand cut-tingforcesfordifferentuncutchipthicknessvalues.Thrustforces increasewithincreasingcuttingspeedfor0.4-0.6_␮muncutchip thickness.Thereisasignificantjumpinthrustforcesat62m/min for uncut chip thicknesses of 0.8–1␮m. These resultsindicate theinfluenceofincreasingbuilt-upedgetiplengthwith increas-inguncut chipthickness andcuttingspeed. Ascuttingspeed is increasedto78m/min,thrustforcesplateau,whichmaybedue tosharptipofBUE.Cuttingforcesmostlypeakedaround62m/min andtendtodecreaseat78m/min.

Fig.13showstheaveragecuttingandthrustforcesasafunction ofuncutchipthicknessforcuttingedgeswith7◦and14◦clearance angles.AlthoughnocleardifferenceinBUEshapeswasobserved between7and14clearanceangles,forcemeasurementsincreaseas

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Fig.12.Meanvaluesof(a)thrustand(b)cuttingforcesasafunctionofcuttingspeed.

clearanceangledecreasesasexpected.Cuttingforcemeasurements increasewithincreasinguncutchipthickness.Thrustforce mea-surementsfor7◦clearanceangleexhibitapeakat0.6␮mandthen plateaubetween0.8and1␮m.Thrustforcemeasurementfor14◦ clearanceangletoolexhibitedasimilartrend,howeveritplateaus between0.8and1␮muncutchipthickness.Thepresenceofa built-upedgeisknowntobeimportantsincealargecontactareawiththe workpieceandthetoolexistsasaresultofBUE.Therefore,larger thrustforcesareobtainedinthemeasurements.

5. EffectofBUEonsurfaceroughness

Inmicro scalemachining,theinfluence ofmachining condi-tionsonthesurface roughness is quiteimportant as shown in CubaRamosetal.(2012).Thissectioninvestigatestheinfluence ofBUEonsurfacequality. Asmentioned earlier,theuncutchip thicknesswherethethrustforcereachesitspeakvalueis identi-fiedasminimumchipthicknesswhichdefinesthetransitionfroma shearing-dominatedtoaploughing-dominatedmachiningregion. Inpreviousstudies,theinfluenceofthesurfaceroughnessbeing cutisusuallyneglected.Inthisstudy,aftereachtest,themachined surfacesare investigated by using a laser scanning microscope (KeyenceVK-X100) for contactlesssurface roughness measure-mentswhere arithmeticmeanheight(Ra)and averagedistance betweenthehighestpeakandlowestvalley(Rz)valuesare consid-eredtostudytheeffectofbuilt-upedgeonsurfaceroughness.In measurements,imagestitchingcapabilityofthelasermicroscope isusedtoinvestigatealargeareafordetailedsurface characteriza-tion.Noisefilteringandplanartiltcorrectionisappliedtoaregion ofinterestof(750␮m×200␮m)beforeextractingRaandRz

val-ues.Acut-off(␭c)valueof80␮mwasusedtoremovetheinﬂuence ofwavinessfromsurfaceroughnessmeasurements.Fig.14shows surfacetopologiesaftermachiningat62m/minat0.4and1␮m uncutchipthicknesses.Theridgesonthemachinedsurfacedueto nonuniformBUEtipformationcanbeclearlyseen.

Fig.15showsthesurfaceroughnessmeasurementsasafunction ofspeedforvariousuncutchipthicknessvalues.Itisinterestingto seethatsurfaceroughness(Rz)valuesimprovearound62m/min cuttingspeed,correspondingwithhighcuttingand thrustforce values(Fig.12).After62m/mincuttingspeed, thesurface qual-ity deteriorates signiﬁcantly mainly due to scratching effect of BUE.Theresultsindicatethatthereexistsacomplexrelationship betweenuncutchipthickness,cuttingspeed,BUEformation,and surfaceroughness.Relativelybettersurfacequality valueswere obtainedatloweruncutchipthicknessvalues.

InFig.15,at0.2␮muncutchipthickness,averagesurface rough-ness(Rz)valueof0.13␮mwasmeasured.Asfor0.4␮muncutchip

thickness,averagesurfaceroughness(Rz)valueof 0.16␮m was

measured.ItmustbenotedthattheRzanduncutchipthickness valuesarequiteclosetoeachother.Inordertoobservethechip formationduringmicroscalemachining,ahighspeedmicroscope (KeyenceVW9000series)isused.Duetolimitationsofthehigh speedmicroscope,therecordingwasonlymadefromthebackside ofthechip.Basedonthevideorecordings,under0.4␮muncutchip thicknessvalue,thechipformationtransitionedfromcontinuous todiscontinuous.Themediaﬁlesattachedtotheonlineversion ofthepaperclearlyshowcontinuous(Video1for0.8␮muncut chipthickness)andintermittent(Video2for0.2␮muncutchip thickness)machiningcycles.Itmustbenotedthatinthe experi-mentalsetupusedinthisstudy,theworkpieceiscontinuouslyfed

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towardsthecuttingedge.Therefore,intermittentchipformation isexpectedwhenuncutchipthicknessislessthanminimumchip thickness.Basedontheseﬁndings,thetransitionfromcontinuous tointermittentchipformationhappensaround0.4␮m.Assuming thatedgeradiusis4␮m,theminimumuncutchipthicknessin thepresenceofBUEisaround10%oftheedgeradius.Thisratio issmallerthanthosereportedforstagnationpointassumptionin literature.

6. Conclusions

Thisstudyinvestigatestheinﬂuenceofbuilt-upedgeontool wear,forces,andsurfaceroughnessundermicroscalemachining conditions.Theuncutchipthickness,cuttingspeed,andclearance angleareconsideredasprocessvariables.Basedonourﬁndings: • AstableBUEformationexistsundermicroscalemachining

con-ditionsofthetitaniumalloyTi6Al4Vconsideredinthisstudy. • Duringbreak-inperiod,thecuttingedgeradiusincreaseswhich

promotesBUEformation,protectingthetoolsfromcraterand ﬂankwear.EDXanalysisoftheinsertsurfaceindicatedasmall changeinthechemical analysisofthesurface.Butitisnotas signiﬁcantasthoseinmacroscalemachiningprocesses. • Machininginputconditionsareshowntoaffectthesizeandshape

ofBUE,whichaffectmachiningforcesandsurfaceroughness val-ues.Acuttingspeed of62m/minresultedinlargerforcesand bettersurfaceroughnessvalues.

Fig.13.Meanvaluesofcuttingandthrustforcesat62m/mincuttingspeedfor cuttingedges.

• Ourfindingsconfirmtheinfluenceofsurfaceroughnesson min-imumuncutthickness.InthepresenceofBUE,theratioofuncut chipthicknesstoedgeradiusiscalculatedtobearound10%.

Acknowledgements

The authorswould like tothank The Scientiﬁc and Techno-logicalResearchCouncilofTurkey(TÜB˙ITAK-110M660,National

Fig.14.Lasertopographyimagesofthesurfacesmachinedat62m/mincuttingspeedata)0.4␮muncutchipthickness,b)1␮muncutchipthickness,c)0.8␮muncutchip thicknessat78m/min(with300×heightmagniﬁcation).

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Fig.15.Surfaceroughnessanalysisasafunctionofcuttingspeed(a)Ra,(b)Rz.

YoungResearcherCareerDevelopmentProgram)andState Plan-ningOrganization of Turkey (HAMIT-Micro System Design and ManufacturingResearchCenter).

AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound,in theonlineversion,athttp://dx.doi.org/10.1016/j.jmatprotec.2016. 04.010.

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