Tool wear analysis and improvement of cutting conditions using the high-pressure water-jet assistance when machining the Ti17 titanium alloy

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To cite this version :

Yessine AYED, Guénaël GERMAIN, Amine AMMAR, Benoit FURET - Tool wear analysis and

improvement of cutting conditions using the high-pressure water-jet assistance when machining

the Ti17 titanium alloy - Precision Engineering - Vol. 42, p.294-301 - 2015

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ContentslistsavailableatScienceDirect

Precision

Engineering

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

Tool

wear

analysis

and

improvement

of

cutting

conditions

using

the

high-pressure

water-jet

assistance

when

machining

the

Ti17

titanium

alloy

Y. Ayed

a,∗

_,

_G.

_Germain

a

_,

_A.

_Ammar

a

_,

_B.

_Furet

b a_Arts_et_Métiers_ParisTech,_LAMPA,₂_bd_du_Ronceray,₄₉₀₃₅_Angers_Cedex,_France b_IUT_Nantes,_IRCCyN,₂_av._du_Professeur_Jean_Rouxel,₄₄₄₇₅_Carqefou,_France

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received17January2015

Receivedinrevisedform7April2015 Accepted9June2015

Availableonline20June2015 Keywords: Toolwear Toollife Highpressure Cuttingforces Titaniumalloy

a

b

s

t

r

a

c

t

ThispaperpresentsexperimentalresultsconcerningthemachinabilityofthetitaniumalloyTi17with andwithouthigh-pressurewaterjetassistance(HPWJA)usinguncoatedWC/Cotools.Forthispurpose, theinﬂuenceofthecuttingspeedandthewaterjetpressureontheevolutionoftoolwearandcutting forceshavebeeninvestigated.Thecuttingspeedhasbeenvariedbetween50m/minand100m/min andthewaterjetpressurehasbeenvariedfrom50barto250bar.Theoptimumwaterjetpressurehas beendetermined,leadingtoanincreaseintoollifeofapproximately9times.Comparedtoconventional lubrication,anincreaseofabout30%inproductivitycanbeobtained.

1. Introduction

Titaniumalloyshavebeenwidelystudiedduetotheir remark-ableproperties,notablytheirstrength/densityratiowhichpresents interestingadvantagesintheaviationandaerospaceﬁelds. How-ever, the machining of these alloys is particularly challenging, whichhasadirectimpactonproductivity,surfaceintegrity and toollife.

Thehighchemicalreactivityandthelowthermalconductivityof thesealloyspresentfavourableconditionsfortoolwear.Inaddition, thehightemperatureinthecuttingzoneandthehighcuttingforces accentuatetheevolutionofthewear.Accordingto[16],thereisa criticaltemperatureatwhichthedegradationofthecuttingtool isaccelerated.Thisisaround740◦C,760◦Cand900◦CforWC/Co, PCDandCBNrespectively. Thislimitsthechoiceofcuttingtool materialsandcoatings.Regardlessofthetypeofmachinedmaterial, thetoolmustfulﬁlanumberofcharacteristicstohavebetterwear resistance:

• Maintain good mechanical properties at high temperatures (hardness,rigidityandtenacity).

∗ Correspondingauthor.

E-mailaddress:yessine.ayed@ensam.eu(Y.Ayed).

• Haveahighchemicalinertiatoresistthehighchemicalafﬁnity ofsomealloys.

• Haveanadaptedcuttinggeometry.

Accordingto[11,19–21]theuncoatedtungstencarbideWC/Co seemstobeagoodchoiceforthemachiningoftitaniumalloys.

High-pressurewaterjetassistancepresentsanefﬁcientsolution todeceleratetoolwearandtoenhancetoollife.Usingthis tech-nique,theincreaseintoollifefortitaniumalloysvariesbetween 185%[3]and460%anditcanreach780%whenmachiningInconel

718[8,15].Inaddition,HPWJAallows,amongotherthings,to

effec-tivelylubricatethecuttingzoneandthustoreducethetemperature inthetoolbymorethan40%[13].Furthermore,thefragmentation ofchipsisensured.Thestudyoforthogonalcuttingwithhigh pres-surecoolantassistance[12]showedthatchipbreakingiscontrolled bythenozzlediameterusedandthepressureofthewaterjet.

Theeffectofthelubricanttypehasbeentested by[17].The study is based on the comparison of the performance of the water-solubleoilandtheneatoilduringthehigh-pressure cool-ingassistedmachiningofTi–6Al–4V.Therespectivetoollifefor theconventional lubrication, neatoiland water-solubleoilare 4min,5minand14min(Vc=100m/min,f=0.2mm/rev,P=100bar,

dnozzle=0.8mm).Theinﬂuenceofthedirectionofthelubricantjet

andﬂowratehasbeentheobjectofthestudycarriedoutby[7]

fortheturningofAISI1045steel.Whentheﬂuidwasappliedon http://dx.doi.org/10.1016/j.precisioneng.2015.06.004

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Table1

ChemicalcompositionandmechanicalpropertiesofTi17.

Chemicalcomposition(%) Mechanicalproperties

Ti Al Mo Cr Zr Sn UTS(MPa) p0.2%(MPa) A%

82.5 5.16 4.08 4.06 2.01 2.06 1110 1020 5

therakeandtheﬂankfacesofthetool,thebestresultshavebeen obtained.Ithasbeenmentionedthatthelubricantjetaccelerates craterwearbypulling-outtheadherentlayers.

The turning of Ti–6Al–4Vat high cutting speeds using PCD inserts has been tested by [5,10]. The study showed that an improvementofthetoollifeofupto9to21timescouldbeobtained. Different gradesof CBNtools have beenused whenmachining Ti–6Al–4V[9].Theyseemtobesensitivetocoolantpressure,the improvementoftoollifecanreach150%.

Theexplanationoftoolwearmechanismswhenmachining tita-niumalloys[1]andnickel-basedalloyshasbeenthesubjectofother studies[2,6,22].

Thepresentstudyprovidesacontributiontotheoptimization ofthisprocesswhenmachiningTi17titaniumalloy.Itisbasedona widerangeofexperimentaltestsandawell-informedstudyofthe toolwearmechanisms.

2. Materialandexperimentalsetup

2.1. Materialpresentation

The studied material is the Ti17 titanium alloy

(Ti–5Al–2Sn–4Mo–2Zr–4Cr); its chemical composition and its mechanicalpropertiesaresummarizedinTable1.

Atroomtemperature,theTi17alloyiscomposedofan˛phase andaˇphase.Thetransustemperatureislocatedat880◦C;above thistemperature,onlythe_ˇphase exists.The_˛phaseexistsin differentmorphologies:˛grainboundary(˛GB),˛Widmanstätten

grainboundary(˛WGB)and˛WImorphology[18].The

microstruc-tureofthematerialispresentedinFig.1. 2.2. Experimentalsetup

InordertodeterminetheVCmin(minimumcuttingspeed),

pre-liminarytestsbasedonthe“PrE66-520-4”standardhavebeen done.Thecuttingspeedhasbeenvariedfrom5m/minto90m/min. At the end of these tests, the cutting speed of 50m/min was selected.Afterthisphase,toolwearexperimentswereundertaken atdifferentcuttingspeedsandwithdifferentlubricationpressures. ThetoolusedinalltestsistheSeccoJetStreamPCLNR2525M12.The purposeofthesetestswastoevaluatetheeffectivenessofHPWJA underthefollowingconditions:

Fig.1. MicrostructureofTi17.

Table2

Waterjetpressureandﬂowrate.

Pressure(bar) 50 100 150 200 250

Flowrate(l/min) 9 15 20 23 26

Table3

Geometricalparametersofthecuttingtoolandthecuttingparameters. Toolgeometry

Noseradiusr(mm) 1.2

Edgeradiusrˇ(m) 30

Rakeangle(◦₎ ₇

Flankangle(◦₎ ₆

Cuttingedgeangle(◦₎ ₉₅

Cuttingparameters

CuttingspeedVC(m/min) 50;61;75;81;88;100

Depthofcutap(mm) 1.5

Feedratef(mm/rev) 0.1

-CuttingspeedVC(m/min):50;75;61;81;88and100.

-Waterjetpressureandﬂowrate(Table2).

Allmachiningoperationshavebeencarriedoutwitha “LEAD-WELLLTC25iL”CNClathe(Fig.2).Ahigh-pressurepumphasbeen usedtosupplythehigh-pressurejet(400bar,38l/min).Thepump tankisﬁlledwith200Loflubricantconsistingof95%waterand5% solubleoil.TheexperimentalsetupisillustratedbyFig.3

The depth of cut and the feed rate were maintained con-stant(1.5mmand0.1mm/revrespectively).Foreachtest,anew uncoatedtungstencarbide(H13A)toolinsertwasused.The mea-surementofthethreecuttingforcecomponentswasdoneusing aKistler9257Bdynamometer.Thegeometricalparametersofthe toolandthecuttingparametersaresummarizedinTable3.

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Fig.3. Experimentalsetup.

Toolwearhasbeenfollowedandcontrolledbyusingastereo binocularmicroscopeandascanningelectronmicroscope,by mon-itoringthecuttingforces.Over270measurementsrelatedtothe20 testshavebeencarriedout.Atthebeginningoftheexperimental work,aseriesofrepeatabilitytestswereperformed.Thesametypes ofwearwasobservedandthemeasurementerroroftheﬂankwear isestimatedtobe0.02mm.Fig.4showsthemainweartypes.

3. ToolweartestsatVC=50m/min

Fortheinitialcuttingspeedof50m/min,threelubricationtypes wereselected:conventionallubrication(Conv-Lub),andtwo high-pressurelubricationconditions(P=100barandP=250bar).Fig.5

showstheevolutionofflankwear(VB)asafunctionofthe machin-ingtime.Forthetotaldurationofthetest(30min),flankweardoes notexceed0.26mm.ItcanalsobenotedthatwithHPWJA(250bar), theevolutionofflankwearisstableinthebeginningofthetest. However,towardstheendofthetest,itsevolutionbecomesvery rapid.Moreover,withapressureof100bar,theflankwearisstill lessthan0.2mm[1].Hence,itwouldbeinterestingtotestother lubricationpressurestoinvestigatetheireffectontoolwear.

Fig.5.Flankwearevolution,VC=50m/min.

Fig.6showstheSEMimagesofthetoolrakefaceattheendof thethreetrials.Fortheconventionallubricationcondition,layers ofdepositsandalargenotchhavebeennoticedonthecuttingedge (Fig.6(a)).

Bahattetal.[2]explainsthatthecombinationofthehigh con-tactpressurebetweenthetoolandthechip,coupledwiththehigh cuttingtemperature,causesweldingtooccurbetweenthesurface ofthechipandtherakeface.Devillezetal.[6]proposesthesame explanation.

Childs[4] showsthat thestickingbetweenthechipandthe carbidegrainsisduetodiffusionbetweenthechipandthetool. Thus,anincreaseinthecuttingtemperaturepromotesthe diffu-sionbetweenthetoolandthechip.Itistruethattheadherentlayers couldprotectthetool.However,whentheformationoftheselayers reachesasaturationstate,theyarepulled-out.Whenthishappens, theyaredetachedalongwithfragmentsofthetool,thuscausing adhesivewear[6,22].

However,withthehigh-pressurelubrication,acraterisvisible onthetoolrakeface(Fig.6(b)and(c)).Furthermore,the build-upoftheadherentlayershasbeenreducedduetotheactionof thelubricantjet.Infact,Thelubricantjetpullsouttheadherent layersleadingtotheaccelerationofthecraterwear[1,7].However, undertheseconditionstheevolutionoftheﬂankwearremainslow comparedtoconventionallubrication.Overtime,thiswillweaken

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Fig.6.Craterwear,(a)conventionallubrication,(b)100bar,(c)250bar.

thetoolinsert.Thecuttingedgewilleventuallycollapseunderthe actionofthecuttingforces.

Increasingthepressureofthewaterjetcausesfastertearing awayofthedeposedlayersleadingtotheaccelerationofthecrater wear.Hence,theoptimalpressureshouldbedeterminedwhich, firstly,coolsthetoolandallowsaccesstothecuttingarea(a pres-sureof50barseemstobeinsufficientinthis case).Secondly,it reducestheflankwear(apressureof100bargivesthebestresults). Still,thisdoesnotaffecttheglobaltoollife.Infact,toolwearis moreseverewithconventionallubricationduetothehigher cut-tingtemperatureandthehigherfrictioncoefficientcomparedto HPWJA.Under theseconditions,thewearrateis higherthanin HPWJAduetotheactivationofmanywearmechanisms.Infactwith conventionallubricationthecombinationofhighcuttingforcesand hightemperaturecausestherapidcollapse ofthecuttingedge, especiallyforhighcuttingspeeds.

Fortheseteststhecuttingspeedwasincreasedby50%relativeto thereferencecuttingspeedof50m/min.Fig.7showstheevolution ofﬂankwearfortheﬁvelubricationconditions.Forthe conven-tionallubricationcondition,toolwearincreasesveryquicklyand thetoollifedoesnotexceed3min.However,withhigh-pressure lubrication,toollifehasbeengreatlyimproved.Indeed,thebest performancehasbeenrecordedat100bar.

Table4 summarizes theresults of thewear tests.It canbe

noticedthattoollifehasbeenmultipliedby8fortthepressure

Table4

ToollifeatVC=75m/min.

Conv-Lub 50bar 100bar 150bar 250bar

Toollife(min) 3 18 26 23 16

of100bar.However,increasingthepressurebeyond100bardoes notimprovetheresults.

Theformationandtheevolutionofnotchwearhavealsobeen noted.Whenthenotchwearexceeds0.21mm,burrsareformedon theworkpiece.Fig.8showstheevolutionofnotchwear(VBn).In fact,itgivesthemeanvaluesandprovidesaglobalestimationover themachiningtime.

Thevariationofthecuttingandtheaxialforces,fordifferent lubricationconditions,areplottedinFig.9.ItshowsthatFaand

FCincreaseasafunctionofmachiningtimeandconsequentlythe

toolwear.Furthermore,foranincreaseof0.3mmofﬂankwear,the variationoftheaxialforceFareaches350Nwhilethevariation

ofthecuttingforce_FCexceeds100N.Itwasalsonotedthatthe

evolutionoftheaxialandthecuttingforcesfollowstheevolution offlankwear,thesametrendsarenoticed.Furthermore,whenthe axialforceexceeds600N,thecorrespondingflankwearisabout 0.3mmforalltests.So,theaxialforceismoresensitivetoflankwear thanthecuttingforce.Fig.10showstheevolutionoftheaxialforce asafunctionoftheflankwearfordifferentlubricationconditions. Globally, the use of high-pressure assistance significantly increasesthetool life.However,whenthejet pressureexceeds 100bar,thetoollifebeginstodecrease(comparedtoitsvalueat 100bar),andthesurfaceoftheworkpieceisaffectedbyscratches. Thesescratchesaremostlikelycausedbytheimpactoffragmented chips.Moreover,whenthepressureincreases,thechipsare bro-kenintosmallerpiecesandprojectedathigherspeeds.Thismay intensifythescratchesonthesurfaceoftheworkpiece.

Itisnecessarytonotethatthisphenomenonisobservedinthe contextofthisstudyontheTi17alloy.However,thismightnotbe thecaseforotheralloysandmaterials.

Forthisseriesoftests,thecuttingspeedisincreasedby75% rel-ativetothereferencecuttingspeed.Fig.11showstheevolutionof

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Fig.9.Evolutionoftheaxialandthecuttingforces.

Table5

ToollifeatVC=88m/min.

Conv-Lub 50bar 100bar 150bar 250bar

Toollife(min) 1 6 9 8.5 9

flankwearforthedifferentconditions.Withconventional lubrica-tion,theTi17alloyisnotmachinableatthiscuttingspeedbecause thetoollifeisabout1min.Infact,theflankwearevolveslinearly reaching0.45mminonly90s.However,withhigh-pressure assis-tancethetoollifehasbeenincreasedsignificantlywithvaluesup to9minandwithalinearevolutionoftheflankwear.Indeed,the curvesdonotexhibitthecatastrophicevolutionofwear.

Fig.12showstherakeandﬂankfacesofthetoolattheendof theweartests.WithConventionallubrication,thecuttingedgehas completelycollapsed.Indeed,toolwearseemstobeverysevere.In contrast,withHPWJA,toolwearismoreregular.

Table5showstheeffectofthehigh-pressurewaterjetassistance

onincreasingthetoollife.Withapressureof50-bar,toollifeis multipliedby6andwiththepressureof100baritismultiplied by9.Accordingtotheﬂankwearcurves,increasingthepressure seemstohavenoeffectbeyond100bar.However,asforthecutting speedof75m/min,scratcheshavebeenobservedontheworkpiece. Theyaremoreintenseandaccompaniedbytheweldingofchip fragmentsontheworkpiece.

Asithasbeenpreviouslynoted,undertheactionofthewater jet,chipsarefragmentedintoverysmallfragmentswhichdonot exceed3mm,someofwhicharerecycledbypassingbetweenthe cuttingedgeand theworkpiece.Thesecouldbeweldedonthe workpiece,asshowninFig.13.

Fig.10.Evolutionoftheaxialforceasafunctionofﬂankwear(VC=75m/min).

Theanalysisofthecuttingforceevolutionduringthe machin-ingtimeshowstheexistenceofpeaks.Thesepeaksmaycorrespond tothephenomenonofchipsrecycling.Thechangeincuttingforce Fcanreach50N.Fig.14(a)and(b)respectivelyshowsthe evo-lutionofthecuttingforceafter5minand9minofhigh-pressure waterjetassistedmachining.Furthermore,fromthesecurves,it canbenotedthatthefrequencyofforcepeakshasbeenincreased. Hence,itcanbeconcludedthatthechiprecyclingfrequencyhas alsobeenincreased.Theapproximatenumberofthesepeakshas beenmultipliedbytwo.

Flankwearcouldbeacceleratedwiththephenomenonofchip recyclingwhichcouldalsoexplainthecurvesofFigs.5and7.

Forthisseriesofexperiments,thecuttingspeedhasbeen dou-bledcomparedtothereferencecuttingspeed.AccordingtoFig.15, toollifedoesnotexceed4.5minevenwithhigh-pressurewater jetassistance.Thus,itcanbededucedthattheeffectivenessofthe assistancehasbeengreatlyreduced.Infact,thelubricantjet can-notdissipatetheheatgeneratedinthecuttingzoneandthewear continuestoevolve.

However, compared withconventional lubrication, Table 16

shows that the tool life can be multiplied by 9 with a pres-sureof100bar.Withconventionallubrication,thetoollastsonly a few seconds before collapsing. As hasalready been noticed, forpressuresexceeding100bar,thesurfaceoftheworkpieceis affectedbyscratchesandweldedfragmentsofchips.

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Fig.12.Flankandcraterwearattheendoftoollife,VC=88m/min.

Fig.13. Scratchesandweldedchipfragmentsonthesurfaceoftheworkpiece(150barand250bar).

7. Discussion

Fig.17summarizestheresultsobtainedforthecuttingspeeds of75m/min,88m/minand100m/min.Theﬁrstﬁndingisthattool lifedropsrapidlywhencuttingspeed increases.Thisisbecause theincreaseofthecuttingspeedcausesanincreaseofthecutting powerandsothedissipationofheatintheworkpieceaswellas thetool.Theadditionalgenerationofheatcausesatemperature riseinthecuttingzone.Thereafter,thewearmechanismswhich areactivatedathightemperaturesdominate,thusacceleratingtool wear.

Inthecaseofconventionallubrication,weartakesplacevery quicklyandcausesthecollapseofthecuttingedgeafterafew min-utes(VC=75m/min),orafterafewseconds(VC=100m/min).The

high-pressurewaterjetallowsthecontinuouscoolingofthecutting area.However,theeffectivenessoftheassistancedecreasesasthe cuttingspeedbecomesmoreimportant.Certainly,high-pressure waterjetassistancecanincreasethetoollifegreatlycomparedto theconventionallubrication,butitreachesitslimitsforhighcutting speeds.

In the range of cutting speeds in which assistance has cer-tainadvantagesandisstilleffective,anoptimumpressurecanbe

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Fig.16.ToollifeatVC=100m/min.

Fig.17.Variationofthetoollifeinfunctionofthecuttingspeedandthelubricant jetpressure.

determined.Indeed,increasingthelubricantjetpressureallowsthe convectioncoefﬁcienttoincreaseandsodecreasesthetemperature inthecuttingzone.Thiscouldpresentabarriertosomewear mech-anisms,thusincreasingtoollife.InthecaseofTi17,thebestresults havebeenobtainedat100bar.Moreover,theanalysisofthecutting forceandtheworkpiecesurfacehasallowedthedetectionofforce peaks,scratchesandweldedchipsforthehighestpressuresused

Fig.18.IdentiﬁcationoftheTaylorlaw.

inthisstudy.Alltheseelementshavemadeitpossibletoidentify thepressureof100barastheoptimumpressure.

8. Determinationofthemaximumcuttingspeedfor T=15min

Inthispartofthestudy,themaximumtoollifehasbeenfixedat 15min(i.e.thevaluecommonlyusedinindustry).Themain objec-tiveistodeterminethemaximumcuttingspeedthatcanensurethe 15minofmachiningusingconventionalandhigh-pressure lubri-cation.Forthispurpose,theTaylorlawcoefficients(Eq.(1))have beenidentified.Fig.18showstheobtainedresults.

T=CvVcn (1)

FortheconventionalandHP(100bar)conditions,theanalytical calculationofthecuttingspeedsgives respectively61.06m/min and81.40m/min.

Followingthis calculation,twotests atVC=61m/minand at

VC=81m/minhavebeencarriedout.Fig.19(a)showsthetoolwear

evolutionforthesetests.Itcanbeseenthattheresultsofanalytical calculationsareconﬁrmed.

Forconventionallubricationconditionafterrun-inand stabi-lizationphases,thewearincreasessigniﬁcantly;indeed,itgoes from0.16to0.3between12minand16min(_VB=0.14mm).

How-ever,inthecaseofwaterjetassistance(100bar),wearincreases linearlyfromthesixthminute.Itgoesfrom0.23to0.31between 11minand15min(VB=0.08mm).Moreover,itcanbenoticed

thattheevolution ofwearinthecase ofHP assistanceismore controlledandpresentsalinearevolution.Bycontrast,forthe con-ventionalmachining,wearisrapidlyacceleratedfrom10minof

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machining,whichresultsinacatastrophictoolwear.So,evenifthe twoconﬁgurationshavethesametoollife,HPassistanceprovides betterwearcontrol.

Fig.19(b)showstheevolutionofthecuttingandtheaxialforces. Forthecuttingforce,itsevolutionisthesameforbothlubrication conditions.Theevolutionoftheaxialforceseemstofollowthe evo-lutionofwear.Theseobservationsconﬁrmthattheaxialforceof 600Nisasignofﬂankwearof0.3mm.

For a tool life of 15min, the transition from VC=61m/min

(conventionallubrication)toVC=81m/min(100bar)signiﬁesan

increaseinproductivityofabout32%.Toolliferemainsbetween 9minand15minforthecuttingspeedsofupto88m/min.Inthis range,Ti17isnotmachinablewithconventionallubrication.

9. Conclusion

Thisstudyisfocusedontheinﬂuenceofpressureandcutting speedontheeffectivenessofthehigh-pressurewaterjetassistance. Thelubricantpressurehasbeenvariedfrom50barto250barand thecuttingspeedfrom50m/minto100m/min.

TheeffectofHPassistanceisclearlydiscernible.Infact, exper-imentaltestshaveshownthattoollifecanbeincreasedupto9 timeswithapressureof100bar.Moreover,beyondacertain cut-tingspeedthematerialisnolongermachinablewithconventional lubrication.However,usingHPassistance,itispossibletocontinue machiningandeventoincreasethecuttingspeed.Forthesame toollifeof15min,thecuttingspeedcanbeincreasedby30%using HPassistance.Theefﬁciencyofwaterjetassistancedecreaseswith increasingcuttingspeed.

Theinﬂuenceofwearontheevolutionofthecuttingforceshas beenstudied.Ithasbeennotedthatacertainvalueoftheaxialforce presentsasignofﬂankwear(0.3mm).

It has been remarked that, for pressures beyond 100bar, scratches on the surface of the workpiece have been noticed. Moreover,weldedfragmentsofchipshavebeenobservedonthe machinedsurface,whichcanbetheresultofchipsrecycling phe-nomenon.

Acknowledgement

TheauthorswouldliketothanktheFrenchregion“Paysdela loire”forfundingtheproject.

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