Thermal analysis in Ti-6Al-4V drilling

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To cite this version :

Ismaïl LAZOGLU, Gérard POULACHON, Christophe RAMIREZ, Mohammad AKMAL, Bertrand

MARCON, Frédéric ROSSI, José OUTEIRO, Michael KREBS - Thermal analysis in Ti-6Al-4V

drilling - CIRP Annals - Manufacturing Technology p.1-4 - 2017

Any correspondence concerning this service should be sent to the repository

Thermal

analysis

in

Ti-6Al-4V

drilling

Ismail

Lazoglu

(2)

a,

*

,

Ge´rard

Poulachon

(2)

b

,

Christophe

Ramirez

b

,

Mohammad

Akmal

a

,

Bertrand

Marcon

b

_,

_Fre´de´ric

_Rossi

b

_,

_{Jose´ Outeiro}

₍₂₎

b

_,

_Michae¨l

_Krebs

b

a

KocUniversity,ManufacturingandAutomationResearchCenter,Istanbul,Turkey b

ArtsetMetiersParisTech,LaBoMaP,Cluny,France

1. Introduction

Severalthermalmeasurementdevicesdedicatedformachining arealreadyavailableonthemarket[1].Outofthoseapparatus,the infraredCCDcameratogetherwiththethermocouple(TC)based techniquesarethemostwell-knownandefficientwaystomeasure thetemperatureinturning[2,3].Usually,knowingpreciselythe cutting temperature is mandatory to understand the thermal phenomena involved in the cutting zone and moreover for simulation’svalidationpurposes[4].Thefirst studiesin drilling wereinitiatedbyRumford[5]andSchmidtandRoubik[6]byusing acalorimetrictesttodeterminetheheatgeneration.Morerecently, AgapiouandStephenson[7]andRamirezetal.[8]haveusedtwo differentdevices tomeasure the temperatureduring a drilling operation.Respectively,thefirstoneconsistsofinsertingtwo K-typethermocouplewiresthroughthelubricationholesclosetothe cuttingedge.Thesecondmethodconvertsthetooltoanactual thermocouplebypositioningintheworkpieceaninsulatedwire. Whilethecuttingedgecutsthewire,itgeneratesa suddenhot junctionproducing a potentialdifference and thus, allowingto measurethetemperatureatthatspecificpositionandtime.

Since the early 2000, temperature measurements during drillingoperationsaremoreandmoreinvestigated.LeCozetal.

[9] and Kerrigan et al. [10] performed measurements with thermocouplesembeddedinsidethedrill.Thedataaretransferred totheacquisitionchainthankstoanon-boardwireless system located in the tool holder. Beno and Hulling [11] used a bichromaticpyrometerreceivingradiantlightthroughanoptical fiberlocatedintheworkpiece,inadirectionparalleltothedrillaxis tomeasurethetemperatureofthecuttingedge.Thesamekindof experimentalset-uphasbeenusedbyUedaetal.[12]for alloy steel,castironandaluminumalloysmachining.

Theabilitytosimulate,analyticallyornumerically,thephysics oftheprocessgivestremendouscontroltowardfurther improve-mentanddevelopmentofnewvista.Earlieryearsproducedanalytical models for cutting temperatures on simple tool geometries, nevertheless, latterprogressin computationalsciences, numerical techniquesweredevelopedforestimatingtemperaturesoncomplex tool geometries and processes. Komanduri and Hou [13] used differentheatsourcemodelstoestimatetemperaturedistributions. LazogluandAltintas[14]usedafinitedifferencenumericalmodelfor predictingtemperaturedistributioninchipandtoolforcontinuous andinterruptedmachiningoperations.LaterLazogluandIslam[15]

extended thesameapproach tooblique machiningoperationsfor temperatureprediction.Theunderstandingofthe thermo-mechani-cal phenomenawas furtherimproved with theintroduction ofa modelforthetransientheatpartitioncoefficientbetweenchipand toolrakefacebyIslametal.[16].

The present article will describe a newly designed device, namedRotaryToolTemperature(RTT),allowingtomeasurethe drilling temperature close to thecutting edge. This innovative devicehastheadvantagetobeeasilyassociatedwitharotating multi-component dynamometer for drilling and milling opera-tions.Comparisonsin-betweenexperimentsandsimulationsare presented.Asemianalyticalhybridapproachis adoptedforthe simulation of temperature fields on thedrilling tool. The heat generationsourceis estimatedusingthecuttingconditionsand materialmechanicalproperties,whereasthemechanicsofcutting provided the size of heat generation zone from the tool/chip contactlength.Finally,finiteelementanalysiswasemployedto achievethetemperaturedistributionprediction.

2. RotaryToolTemperaturedevice

Fig.1showsthenewmeasurementdevice,integratedtothe rotarydynamometerandtoolholderforsimultaneously measur-ing the temperature as well as forces and torques during the

* Correspondingauthor.

E-mailaddress:ilazoglu@ku.edu.tr(I.Lazoglu). ARTICLE INFO Keywords: Drilling Temperature Titanium ABSTRACT

Ti-6Al-4Viscommonlyusedespeciallyinaerospaceandbiomedicalindustries.Thisalloyisknownasa difficult-to-cut material. Dueto itspoor thermalproperties,the heatgeneratedduring machining processestrapsnearmaterialdeformationzones.Thiscausesdetrimentalhightemperaturesforthe cutting tools. This article combines the analytical and FEM modeling techniques to estimate the temperatureevolutionofcarbidetoolsinTi-6Al-4Vdrilling.Inthisarticle,anovelthermocouplebased temperaturemeasurementsetupisalsointroduced.Moreover,thesimulatedandmeasuredtemperatures undervariouscuttingconditionsforthedrillingofTi-6Al-4Varepresentedforthevalidation.

drillingoperations.ThisRTTdeviceconsistsof connectorswith coldjunctioncompensationstoacquiresignaluptosix thermo-couples,aninternalmemorytostorethesignalasafunctionof timeduringmachining,anon-boardlithiumbattery(3.6V,3.6Ah, anda maximum recommendedcontinuouscurrent of 130mA), andaninternalclocktosynchronizeallthesignals.TheRTTdevice can be mounted on a standard tool holder to measure the temperatureinbothdrillingandmillingoperations.Thesystem hastwoparts:theRTTitselfassembledandspinningtogetherwith the drill, and an external module in charge of the data communicationtothe computerthankstoanRJ45 connection. Theexternal moduleincludes an independent clock;this clock pulseisessential whenasimultaneousmeasurementofcutting forcesandtemperatureisnecessary.

Indeed,inthispeculiarcase,itisnecessarytosynchronizethe differentsignals;tothisend,itrequirestolaunchthetwoclocks withthesamepulserateandtriggeredsimultaneously.Whenthe clocksynchronizationiscomplete,thetwoacquisitionsystemsare unpluggedandthemachiningisperformed.Thecuttingforcesand theexternalclocksignalarerecordedindependently;afterwhich the temperature data are downloaded from the RTT on-board memoryandpost-synchronizedtotheothersignals.Inthepresent study, the RTT device is used to register signal from two thermocouples.Forthispurpose,thedrillisfirstlypreparedwith

two holes, obtained by electrical discharge machining (EDM), startingfromthebackofflutetoendeitherclosetothedrillcorner foronesideoratthemiddleofthecuttinglipfortheothersideof thedrill;thosetwotemperaturelocationsarelabeledasTCcorner andTCliprespectively.Thetwothermocouplewiresconnectedto theRTTdevicearethenslippedinside thelubricantcanalsand insidethepreviouslymentionedEDMholesasshowninFig.2.

Thetwothermocouplesaresecuredtothedrillbodyusingtwo different hightemperature ceramic glues: one is chargedwith silverparticles,asaresultimprovingitsthermalconductivityand used at the bottom of the EDM hole to connect the two thermocouplewireswiththedrillbody,theotheroneisusedto secure the wires to the drill permanently avoiding any signal disruptionwhenthetoolisturningduringthemachiningmotion. ThemaincharacteristicsoftheRTTdevicepresentedandusedin thisworkaresummarizedinTable1.

The thermocouples are used in a specific configuration to enableaninstantaneoustimeresponse.AsrepresentedinFig.2, thetwothermocouplewiresareindirectelectricalcontactwith thebottomofthe0.5mmdiameterholeinthetool.Therearetwo hotjunctions:K+/WCCoandWCCo/K.Asthetoolmaterialispart ofthehotjunction,theSeebeck’seffectresponseisinstantaneous. Theonlymeasurementtimedelaycomesfromthedataacquisition frequency.The exactlocationof themeasurement isat theK+/ WCCojunctionsince theelectromotive forcesgenerated bythe WCCo/K is close tozero. The RTT calibration was conducted undertheFDX07-029-2standard,andthethermaldriftchecked andresultedasnon-significantuntil708CinsidetheRTTforboth temperaturemeasurementandfortheclockgenerator.

3. Thermalmodeling

Thermal modeling requires the modeling of force field to estimatetheheatingloadforthedrillingsimulations.Thecutting forcesforthedrillingprocesswereestimatedusingthediscretized orthogonal to oblique transformation model as described by Altintas [17]. The geometry of the helical drill used for the developmentoftheobliquemodelisshowninFig.3.

dFtðzÞ¼KtcðzÞdAþ

D

bKte dFfðzÞ¼KfcðzÞdAþ

D

bKfe dFrðzÞ¼KrcðzÞdAþ

D

bKre 9 = ; (1) h¼c 2sin

k

t;

D

b¼

D

z cos

k

t

Fig.1.Experimentalsetupandschemeofthetemperatureacquisition.

Fig.2.Illustrationofthecornerandlipthermocouplelocationsinthedrill.

Table1

RTTdevicespecifications.

Memorycapacity 64MB

Acquisitionfrequency Upto2.25KHz

Accuracy 0.18K

Tool-holder Bodyof50mmdiameter

Eq. (1) determines the discrete force components along the tangential(dFt),feed(dFf)andradial(dFr)directionforaselected diskwiththedifferentialchipload(dA)of(

D

bh).Here,(Ktc,Kfc,Krc) and(Kte,Kfe,Kre)arethecuttingforceandedgeforcecoefficientsfor thetangential,feedandradialdirectionrespectively.Thecutting coefficientscalculatedandusedtopredictthecuttingforcesfor theseexperimentsareprovidedinTable2.

Themechanicalpropertiesoftheworkpiecematerialrequired by the transformation model for the cutting forces analytical modeling,andfora

D

zthickdiscretedisk(asshowninFig.3),were takenfromtheTi-6Al-4Valloymaterialdatabasereproducedin

[17].Theanalyticalcalculationofthetransitiontorquerequiredfor the selectedcutting conditions is in good agreement with the experimentallymeasuredtorqueasshowninFig.4.

Theexperimentdrillchiseledgewidthwandchiseledgeangle

c

was2mmand1318respectively.Thedrillgeometryiscomplex andchangesalongthecuttinglip.Thecriticalparameterssuchas chipflowangle

h

,effectiverakeangle

af

,obliqueshearangle

fi

, normal shearangle

fn

,localhelix angle

bz

vary andaffect the differentialcuttingforcesonthecuttinglip.Thevariationofthose anglesareillustratedinFig.5.

Theresultantcuttingforce(dFR)onadifferentialchiploadcan bedeterminedasthefollowing:

dFRðzÞ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi dF2 tðzÞþdFf2ðzÞþdFr2ðzÞ q (2)

The resultant cutting forces were then dissolved into their friction(dFu)andnormalcomponents(dFv)usingthefrictionangle (

ba

)inEqs.(3)and(4)asfollows:

dFuðzÞ¼dFcðzÞsinð

b

aÞ (3)

dFvðzÞ¼dFcðzÞcosð

b

aÞ (4)

Furthermore,thesameparameterswereusedtoestablishthe chipcontactlengthlcandchipvelocityVcasdescribedin[15]using Eqs.(5)and(6).

lc¼

hsec

h

sinð

f

nþ

u

nÞ

sin

f

nðcos

a

ncos

u

nsin

a

nsin

u

nÞ (5)

Vc¼

VsiniðzÞsin

f

nðzÞsec

h

ðzÞ

tan

f

iðzÞcos

a

nðzÞþsin

f

nðzÞtan

h

ðzÞ

(6) The power lost to friction for the discrete section along the cuttingedgeofthedrillingtoolistheproductofthedifferential friction force and the instantaneous chip velocity, given by Eq.(7).

dPuðzÞ¼dFuVcðzÞ (7)

Thesecondstepusedtheinformationproducedintheprevious steptodefinetheheatingloadandarea whichisactive onthe drillingtool.Theheatingloadinthiscaseforthetoolistheheat partition coefficient corrected sum of all products of the differentialfrictionforceandtheinstantaneouschipvelocityfor a discrete element along the cutting edge of the drilling tool, calculatedusingthefollowingequation:

Pu¼

X

dPuðzÞ; Pf¼

l

Pu (8)

Theheatpartitioncoefficientforthetool(

l

)wasselectedas 0.85and0.60correspondingtothetwocuttingspeedsasreported in[18].

Force,torqueandtemperaturemeasurementswereobtainedin drillingofTi-6Al-4Vtitaniumalloywitha12mmdiameterand308 nominal helix angle, uncoated, two fluted carbide drill in dry cuttingconditionsasgiveninTable3.Drillingdepthwaskeptat 20mminalltests.

Thefiniteelementsmodel(FEM)wasdevelopedforblindhole drilling.Thetetrahedraltypeelementswereselectedtomeshthe complexgeometryofthetool.Themeshdensitywasvariable,but tocapture thetemperaturedistributionaccurately itwasmade surethattheheatloadareashadveryfinemesh.Thetotalnumber oftetrahedralelementwas447,267withaminimumelementsize of2

m

m.Theboundaryconditionsonthetoolandworkpieceare selectedtoreflecttheexperimentalconditionsclosely. Thermo-dependentthermalconductivitywasusedforboththeworkpiece andthedrillmaterials.Thecuttingprocessissimulatedasatime dependentheattransfermodel,wheretheheatloadwillremain activeforthedurationofthecutasitwasintheexperiment.The heatload,ascalculatedearlier,isdefinedtobeactiveonlyatthe tool chipcontactzoneon thetool (Fig.6). ComsolMultiphysics software wasused for the FEManalysis on an Intel Xeon CPU W5590@3.33GHz,8coressystemformodelingthetemperature distributiononthedrillbit.Simulationtimeforeachconditionwas lessthan30min.

Table2

Cuttingcoefficientscalculatedandusedtopredictthecuttingforcesduringdrilling; KteandKfeare24and43Nmm1,respectivelyinallregions.

Cuttingforcecoefficients [Nmm2_]

Region1 Region2 Region3 Region4

Ktc 2445 2154 1953 1815

Kfc 1523 1042 774 606

Krc 892 649 481 372

Fig.4.Torquepredictionarecomparedwiththeexperimentaldataforthetwo feedrates.

Fig.5.Variationofanglesalongthedrillcuttinglip.

Table3

Cuttingspeedandfeedratevalues.

Test# Rotational speed[rpm] Cuttingspeed (Vc)[mmin1] Feedrate(f) [mmrev1_] 1 266 10 0.1 2 266 10 0.2 3 796 30 0.1 4 796 30 0.2

4. Experimentalvalidations

The drilling tests were performed on a DMC DMG 65 CNC machiningcenter.Testsateachconditionwereperformedthree timestoverifytherepeatability.Onlyonedrillhadbeenusedfor thewholeexperimentalcampaigntoavoiddispersion;aprevious campaignensurednotoolwearcanappearduringthetests.Axial forceFzandtorqueMzwerealsomeasuredbyusingarotarytype 9123CKistlerdynamometer(Fig.4).Forceandtemperaturesignals weresampledatthefrequenciesof2kHzand256Hz,respectively. During the drilling tests, the tool temperature at the two thermocouple locations specified previously were recorded. Simulatedandmeasured tooltemperaturesatthespecifiedtool cornerandtoollippointsarepresentedinFig.7.Asseeninthis figure, the simulation results and experimental measurements withthelipandcornerthermocouplesarematchingquitewellin trendsaswellasamplitudeswithlessthan10%differencesatthe maximum temperature points. For the cutting speed of 10mmin1_{andfeedrate of0.1mmrev}1_{,temperatureisabout} 3708Catthethermocouplelocations.Whenthecuttingspeedis increased to 30mmin1 _{and feedrate of 0.2mmrev}1_{, tool} temperatureat thethermocouplepointsare increasedtoabout 5008C.Simulationresultsshowthatthelocaltemperaturenearthe outercornerofthecuttinglip,thetooltemperaturecanbeeven higherasshowninFig.8.

5. Conclusion

Temperatureremainstobeoneofthemajorlimiting factors towardimprovingproductivityofadvancedengineeringmaterials liketitaniumalloys.Thermalanalysisofdrillingiscriticalforbetter understandingoftheprocess.Thisarticlecombinedtheanalytical and FEM modeling techniques to estimate the temperature evolution ofcarbide toolsin Ti-6Al-4Vdrilling.In thearticle,a new thermocouple based temperature measurement system, namedasRotaryToolTemperature(RTT)device,wasintroduced. Moreover, the simulated and measured temperatures under various cutting conditions for the drilling of Ti-6Al-4V were presentedasvalidationsforthethermalmodel.

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Fig.6.Chipcontactzonedividedintofourdiscreteregionsforthemechanicaland thermalsimulations.

Fig.7.Simulatedandmeasuredtooltemperatures:(a)forthethermocoupleatthe lippoint(atTClip)and(b)forthethermocoupleatthecornerpoint(atTCcorner)inthe drillingofTi-6Al-4VundertheconditionsgiveninTable3.

Fig. 8. Temperaturedistribution onthe drill cuttingedgeand toolbody for Vc=10mmin1andf=0.2mmrev1.