In-situ infrared thermography measurements to master transmission laser welding process parameters of PEKK

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To cite this version :

Villar, Miguel

and Garnier, Christian

and Chabert, France

and

Nassiet, Valérie

and Samélor, Diane

and Diez, Juan Carlos and

Sotelo, Andres and Madre, Maria A. In-situ infrared thermography

measurements to master transmission laser welding process parameters of

PEKK. (2018) Optics and Lasers in Engineering, vol. 106. pp. 94-104.

ISSN 0143-8166

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In-situ

infrared

thermography

measurements

to

master

transmission

laser

welding

process

parameters

of

PEKK

M. Villar

a

_,

_C.

_Garnier

a ,∗

_,

_F.

_Chabert

a

_,

_V.

_Nassiet

a

_,

_D.

_Samélor

b

_,

_J.C.

_Diez

c

_,

_A.

_Sotelo

c

_,

M.A.

Madre

c

a_{LGP-ENIT-INPT,}_{Université de}_Toulouse,₄₇_Avenue_d’Azereix,_Tarbes_cedex_{BP1629-65016,}_France

b_CIRIMAT,_{Université de}_Toulouse,_CNRS,_INPT,_UPS,_ENSIACET-4_allée_Emile_Monso,_Toulouse_cedex₄_{BP44362-31030,}_France c_Institute_of_materials_science_of_Aragon,_{CSIC-University}_of_Zaragoza,_Calle_de_Pedro_Cerbuna,_12,_Zaragoza_50009,_Spain

a

r

t

i

c

l

e

i

n

f

o

Keywords:

Assemblingprocess Transmissionlaserwelding Polymericmaterial PEKK

Infraredthermography Variabilitytest

a

b

s

t

r

a

c

t

Thetemperaturefieldalongthethicknessofthespecimenshasbeenmeasuredduringtransmissionlaserwelding. Polyetherketoneketone(PEKK)isaveryhighperformancethermoplasticwithtunableproperties.Wehaveshown thatthisgradeofPEKKcanbeturnedtoquasi-amorphousorsemi-crystallinematerial,duetoitsslowkinetics ofcrystallization.Itsglasstransitiontemperatureis150°C.Theeffectofitscrystallineratedirectlyimpacts itsopticalproperties:thetransmittanceofquasi-amorphousPEKKisabout60%intheNIRregion(wavelength rangefrom0.4to1.2µm)whereasitislessthan3%forthesemi-crystallinematerial.Theweldingtestshavebeen carriedoutwithan808nmlaserdiodeapparatus.Theheatfieldisrecordedduringtheweldingexperimentby infraredthermographywiththecamerasensorperpendiculartothelasersheetandtothesample’slengthtofocus ontheweldedinterface.Thestudyisdividedintwosteps:firstly,asinglespecimenisirradiatedwithanenergy densityof22J.mm−2_:_the_whole_sample_thickness_is_heated_up,_the_maximum_temperature_reaches₂₂₂_±₇_°C.

ThistemperaturecorrespondstoaboutTg+ 70°C,butthepolymerdoesnotreachitsmeltingtemperature.After

that,weldingtestswereperformed:atransparent(quasi-amorphous)sampleastheupperpartandanopaque (semi-crystalline)oneasthelowerpartwereassembledinstaticconditions.Themaximumtemperaturereached attheweldedinterfaceisabout295°Cwhentheupperspecimenisirradiatedfor16swithanenergydensity of28J.mm−2_._The_temperature_at_the_welded_interface_stays_above_T

gduring55sandreachedthemelting

temperatureduring5sbeforerapidcooling.Theseparametersaresuitabletoassemblebothpolymericpartsina strongweld.Thisworkshowsthatinfraredthermographyisanappropriatetechniquetoimprovethereliability oflaserweldingprocessofhighperformancethermoplastics.

1. Introduction

Highperformancethermoplasticsgraduallyreplacemetallicalloys insomeindustrialﬁelds,suchasaerospaceandpowerelectronics. In-deed,theyattesthighmechanicalstrengthcombinedtoheatresistance higherthan200°Caswellasresistancetocorrosion,withthe advan-tageoflightness.Thelatterresultsinprogresstowardssustainability. Amongthem,thepolyaryletherketone(PAEK)familyhasbeen demon-stratedtobethemostresistanttothermo-oxidativedegradation[1] .In particular,polyetherketoneketone(PEKK)withanEther/Ketoneratioof 1

2,keepsaconservativemodulushigherthan1GPauptoitsglass tran-sition(Tg)atabout150°C.TheadvantageofPEKKisitsslower

crystal-lization,allowingtoeasilyturnPEKKintoamorphousorsemi-crystalline polymer,comparedtowell-knownpolyetheretherketone(PEEK).More

∗ _{Corresponding}_author.

E-mailaddress:[email protected](C.Garnier).

knowledgeandknow-howaboutassemblingprocessesofPAEKwould seeminglyleadtowidespreadindustrialuses.

Among assembling processes for thermoplastics, laserwelding is anenvironmentally-friendly,fast,reliableandnon-contactprocess[2] . Weldingbytransmissionrequirestheupperparttobetransparentto thelaserwavelengthwhereastheinferiorpartabsorbsthesame wave-length.Thus,theenergyofthebeamisstoppedattheinterface, allow-ingbothpolymericpartstoheatup.Earlytheoriesonweldingstrength werebasedon diﬀusion[2] .Upon temperature,theinterdiﬀusionof polymericchainstakesplaceattheinterface,creatingmacromolecular entanglementsbetweenbothparts,resultinginastrongassembly.Laser weldingisinprincipleapplicabletoanythermoplastic,providedthat theupperpartistransparenttothelaserwavelength.Thisprocessis alreadyappliedintheindustrywithcommonpolymerssuchas polycar-bonateandPMMA[3] .

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Sinceamorphouspolymersarecommonlytransparenttolaserbeam, lasertransmissionweldingisfeasible.Thus,thebeamgoesthroughthe materialwithoutinducingmajorchangesinitschemicalstructure.From aphysical pointof view,a theory todescribetheadhesion of simi-larpolymers,alsocalledautohesion,basedonthediﬀusionof macro-moleculesaboveTgwasproposedin1960sbyVoyutskii[4] andVanenin

[5] .Then,thetheoryofreptationexposedbyDeGennes[6,7] described themotionof polymericchainsandwas successfulin predictingthe molecularweightdependencyoftheself-diﬀusion.Fromthesetheories, anumberofattemptshavebeenmadeinordertolinktheprocess pa-rameterstotheinterfacialstrengthinweldingprocesses.Amongthem, Wool’smodel[8] ,predictingthemechanicalenergyrequiredtoseparate twoweldedpolymericpartsasafunctionoftime,pressure,temperature andmolecularweight,appearstobestﬁttheexperimentaldata.

Weldingasemi-crystallinepolymerisachallenge:mostofthe theo-riespredictingtheinterfacialstrengthdonottakeintoaccounttherole ofcrystallinityonself-diffusionandtheroleofthekineticsof crystal-lization.Moreover,lasertransmissionweldingrequirestheupperpart tobetransparentandtostaytransparenttothelaserbeamallalongthe process.Semi-crystallinepolymerssuchasPAEKareopaqueto visible-nearinfraredlight.Nevertheless,somestudiesgoinsidemore knowl-edgeabouttransmissionlaserweldingofPEEK:Amanatetal.[9] give theparametersrangestogetstrongweldsof250µmthickfilmswith aninfraredabsorbingmediumattheinterface,withapulsedfiberlaser withawavelengthof1060nm.Asaresult,somebubblesaretrappedat theinterface:theexplanationgivenbytheauthorsisthepresenceof wa-terinthespecimens.Potenteetal.[10] weldedundriedandpre-dried PEEK,theynoticedbubblesintheundriedPEEKweldsonly.Sometimes, heatdamagesarereported[9] onamorphousPEEKatapowerof20W withafocalplanespeedof4mm.s−1_._Despite_the_fact_that_authors_agree

abouttheeffectofwater,theprocessparameters,suchaspowerand samplespeed,arenotwelldefinedyet.Additionally,theexperimental studiesoflaserweldingprocesslackconclusivelinksbetweenprocess parametersandtheireffect onassemblies.Forinstance,theeffectof laserpoweronthetemperatureofspecimensisindefiniteuntilnow.

Measuringthetemperaturesinvolvedduringthelaserwelding pro-cessismeaningfulformanyreasons:(a)checkingthatthetemperature reachedishighenoughtoinsureself-diffusionaccordingtoautohesion theoriestogetinterfacialstrength(b)checkingthatthetemperature in-sidethepolymericpartsstayslowenoughtoavoidthermaldegradation ofthematerial,and(c)locatingtheHeat-AffectedZone(HAZ),thelatter haspossiblyundergonemorphologicalchangesandthus,modification ofpropertiesanddimensions.In-situtemperaturemeasurementswould allowgettinginsightthetransmissionlaserweldingprocesstomakeit morereliable.

Measuringthetemperatureattheinterfaceisademandingtask be-causebothpolymericpartsareinclosedcontactallalongtheprocess. So,contact-freetemperaturemeasurementtechniquessuchasinfrared (IR)thermographyhavebeenimplementedtothelaserweldingprocess insomestudies[11–14] .Inparalleltotheexperimentalwork,these au-thorshavedevelopedanumericalsimulationtopredictthetemperature attheinterface.Tovalidatetheirmodel,thechosenmaterialsareeither PMMAassemi-transparentelementandABS/PCasabsorbentpartor, inanotherstudy,polycarbonatebasedFRP(ﬁberreinforcedpolymer). Bothareamorphouspolymers,withwell-knowntransmission,reﬂection andabsorptionfactorsatnear-andmedium-IRspectrum.Todate,the laserweldingofPEKKhasnotbeenreported,duetothenoveltyofthis high-performancethermoplastic.Also,theoriginalityofthisworkisto weldtwosamplesofthesamepolymertunedintoamorphous(A-PEKK, transparent)andsemi-crystalline(C-PEKK,opaque)state.

AlthoughIRthermographyseemstobesuitedtothetransmission laserweldingprocess,nostudyusesitasatechniqueforcontrolling tem-peraturesdistributionalongthesample’sthickness.Intheexisting stud-ies,measurementswereperformedwiththecamerasensorparallelto theweldedinterface.Withsuchanexperimentalsetup,thetemperature ismeasuredattheupperorlowersurfaceofthepolymericparts.These

Fig.1. ChemicalstructureofPEKK.

calculated/experimentalresultswereimplementedtonumerical simu-lation,usingahybridcodethatcombinesMietheory[15,16] andthe MonteCarlomethod[17] toquantifythescatteringphenomena.The ex-perimentalandnumericalresultsmatchforthetemperaturesmeasured attheupperandlowersurfaces,nevertheless,thetemperatureatthe interfaceisforecastfromthesemeasurementsbutnotreallymeasured. Withthecamerasensorperpendiculartothelasersheetand perpendic-ulartothesample’slength,thetemperatureattheinterfaceofwelded assemblyisreportedfortheﬁrsttime.

Inthiswork,thefeasibilityofdeterminingthetemperature distri-butionattheinterfacealongthesample’sthicknessduringthewelding processofPEKKisoutlined.Toachievethis,theheatfieldisrecorded byinfraredthermographywiththecamerasensorperpendiculartothe lasersheetandperpendiculartheweldedinterface.Thisstudyisdivided intwosteps:Firstly,atestoflaserbeam/specimeninteractionwas per-formedinordertobringabetterunderstandingoffastheatingeffects duetohighconcentratedpowerlaserspotonasemi-crystallinePEKK. Then,fortheweldingtest,anassemblymadeofamorphousPEKKasthe upperpartandsemi-crystallinePEKKasthelowerpartwasused.The effectsofprocessparametersonthepropertiesofPEKKareexplained. 2. Materialsandmethods

AhighperformancePAEK,thepolyetherketoneketone(PEKK) Kep-stan6002byArkema waschosen.Itschemicalstructureis shownin Fig. 1 ,itismadeofetherandketonecopolymersin1

2 ratio.

Quasi-amorphousandsemi-crystallinePEKKspecimenswereused fortheweldingtest.Inthefollowing,quasi-amorphousPEKKwillbe re-ferredasA-PEKKandsemi-crystallinePEKKasC-PEKK.Bothspecimens wereprocessedbycompressionmoldingfromdriedpellets.An800kN PinetteEmidecauLAB800Photplatespresswasused;withcontrolled heatingandcoolingtemperaturesfrom2to10°C.min−1_._For_controlling

thecrystallinerateandplatedimensions,thesamplesweremadeintwo steps.Intheﬁrststep,thepelletsweremeltedat340°C,slightly com-pressedandcooleddowntoroomtemperature.Thecoolingratewas 2°C.min−1_for_the_crystallized_sample;_and_liquid_nitrogen_was_used_for

afastcoolingratetogetthequasi-amorphoussample.Inthesecondstep, thesampleswereﬂattenedupto2mmthickwhileheatingat2°C.min−1

untilglasstransitioninordertoremoveanyresidualstress.The com-pressionmoldedspecimensweredriedduring24hat140°Ctoremove waterbeforewelding.

Transmissionfactorsandreﬂectionfactorsinthewavelengthrange of400–1100nmhavebeenmeasuredwithaPerkinElmerLambda19 spectrophotometerequippedwithanintegratingsphere.Theincidence beamanglewas8° withrespecttotheperpendiculardirectionofthe samplesurface.Beforetheexperiments,thedevicewascalibratedwith aBaSbackground.Testswereperformedat20°C.

Transmissionfactorsinthewavelengthrangeof3–5µmhavebeen measuredwithaPerkinElmerSpectrumOnespectrophotometerinthe transmissionmode.Theincidencebeamangleis0° withrespecttothe perpendiculardirectionofthesamplesurface.Testswereperformedat 20°C.

DiﬀerentialScanningCalorimetry(DSC)scanswerecarriedoutwith aDSCQ200fromTAInstrumentscalibratedwithindium,startingat roomtemperatureto400°Cwitharampof5°C.min−1_._Samples_were

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Fig.2. Schemeoftheprojectedlaserbeamonthespecimen,a)singlespecimen, b)assembly.

studiedasprocessed,bykeepingtheirthermomechanicalhistoryof com-pressionmoldingprocess,tobeascloseaspossibletoindustrial condi-tions.Glasstransitiontemperature(T_g),coldcrystallizationtemperature (Tcc)andmeltingtemperature(Tm)wereobtained.Coolingrampswere

performedat2°C.min−1_and₁₀_°C.min−1_from₄₀₀_to₂₀_°C.

Crystalliza-tionenthalpyandmeltingenthalpywerecalculatedbyintegrationof theareaoftheexothermicandendothermicpeaksrespectively.

Experimental near infrared laser device performs a continuous 808nmbeam,projectingasingle100×1mm2lasersheet,asrepresented

inFig. 2 .Lasersourceisfedby90diodesbranchedon6stacksof15 diodes.Laserpower issetbydiode feedingintensity,starting power emissionat13Aandreaching230Wfor40A.Nonetheless,poweris spreadoutallalongthelasersheet,soonlyactualirradiatedzonemust beconsideredininteractionforenergycalculations.Eventhoughlaser weldingisusuallyperformedindynamicsetup,statictestswerecarried outforamoreaccurateinterfacialmeasurement,andtosuppress para-siteaccumulativethermaleffectsappearingfordynamiclaserwelding. Infraredtemperaturefieldsmeasurementswereperformedwithan IRcameraCEDIP©_Jade_III_MWIR_retrofitted_FLIR©_titanium_SC₇₂₀₀

withathermalresolutionof20mKat30°C.Theuseddetectorisa mid-waveIndiumAntimonide(InSb)oneandthesensoris aclose-upx1 coveringaspectralrangefrom3.70to5.15µmwithafocalplanearray (FPA)windowingof7.6×9.9mm2_._Based_on_literature,_we_assume_the reﬂectivityofthematerialsismuchlessthan1[11] andso,thereﬂection factorisassumedtobe lessthan15%foramorphousspecimens.The detectorresolution is320×256 pixelsandthepitchof thesensoris 30µm.Soon,thepixelsizeonbothdirectionsis0.03mm.Calibration anddatarecordingweremanagedbyAltairsoftware.

Specimenswerelaidout300mmfarfromthecloseupx1sensorto havethebestsharpness.Inordertomeasuretheinterfacialtemperature andthedistributionofthermalﬁeldalongbothdirections(thicknessand lengthofthesample),side-byrecordingswereperformedscopingthe regionofinterest(Fig. 3 ).Thewholeexperimentsweredoneina25°C inathermoregulateddarkroomtoreduce theeﬀectofenvironment onIRmeasurement.Withregardtomeasuretherapidtemperaturerise

Fig.3. Infraredareaofinterestofthesamples,a)singlespecimen,b)assembly.

Fig.4. Experimentallaserdevice.

duringlaserirradiation,integrationtimewassetupat16µs,obtaining atemperaturerangefrom140to300°C.IRMovieswererecordedat 125Hzwith5imagesaveragingconducivetominimizingtherandom noise.Soon,theeﬀectivefrequencywas25Hz.Thewholeexperimental setupisshowninFigs. 4 and5 .

3. Resultsanddiscussion

UnlessthetechniqueofDSCiscontrovertedfortheanalysisofthe crystallinityof PAEKbecauseofthenearnessofmelting and crystal-lization[18] ,wechooseittocomparethethermaltransitionsofthe quasi-amorphousandcrystallized materialsbecauseof thesimplicity of use.ThethermaltransitionsattheﬁrstheatingofA-PEKKand

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C-Fig.5. Layoutofinfraredrecordingscene. 50 100 150 200 250 300 350 −1.5 −1 −0.5 Temperature (°C) Heat Flow (mW)

C−PEKK−Annealed at 250°C − Heating 2°C/min A−PEKK−Heating 2°C/min

Fig.6. DSCthermogramofA-PEKKandC-PEKK:ﬁrstheatingat2°C.min−1_of

moldedsamples.

PEKK are seen in Fig. 6 . For both materials, the thermograms dis-playaglasstransitionat150°C,whenthemacromolecularchainshave gained enough mobility tomove slightly. Then, for A-PEKK, a cold crystallizationpeakis observed withamaximum at220°C followed byadoublemeltingpeakcenteredat300°C.Theenthalpyof crystal-lization(ΔHc=18.6±0.5J.g−1)islowerthantheenthalpyofmelting

(ΔH_mA=20.0±0.5J.g−1₎ _indicating _that_the_specimens _were_slightly

crystallinebeforetesting,duetothecompressionmoldingprocess.The crystallinerateisusuallygivenby:ΔHm/ΔH100%,assumingΔH100%is

122J.g−1 _like_for _PEEK _[19]_,_it_gives _an_{insigniﬁcant} _rate_of _0.01%

foramorphousA-PEKK.ForC-PEKK,nocold-crystallizationoccurs,the coolingrateappliedduringthecompressionprocessisslowenoughto al-lowthemacromoleculestoself-organizedintocrystallinephaseatmost astheycan.TheenthalpyofmeltingisΔHmC=25.6±0.5J.g−1,

corre-spondingto20.9%of crystallinitystillassumingΔH100%=122J.g−1.

SincetheC-PEKKisprocessedwithalongannealingtime,thismelting enthalpyapproachesthemaximumcrystallinityreachedbythisgrade ofPEKK.

Weldingasemi-crystallinepolymerisachallenge,sincetheupper partastostaytransparenttothelaserbeamallalongtheprocess.As soonasthecrystallinerateincreases,anysemi-crystallinepolymerturns transparenttosemi-transparenttoopaqueforhighestcrystallinerates. Itiswellknownthatitispossibletocontrolthecrystallinityby apply-ingacoolingratefasterthanthekineticsofcrystallization.Thisway, PEKKcanbeobtainedastransparentoropaquematerial.Thekineticsof crystallizationisslowforthisgradeofPEKKcomparedtootherPAEK

50 100 150 200 250 300 350 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Temperature (°C) Heat Flow (mW)

PEKK−Annealed at 250°C − Cooling 10°C/min PEKK annealed at 250°C − Cooling 2°C/min

Fig.7. DSCthermogramofPEKK:coolingrampsfrommeltingstateat380°C.

400 500 600 700 800 900 1000 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Wavelength (nm) Transmission

C−PEKK

A−PEKK

Fig.8. TransmissionfactorforA-PEKKandC-PEKKmeasuredby spectropho-tometry.

[16] ,allowingthoroughlycontrollingthecrystallinityalong the pro-cessingsteps.Togofurtherthekineticsofcrystallizationofthisgrade ofPEKK,coolingrampsatvariousrateshavebeenperformed,asseenin Fig. 7 .Fromthemeltingstate,nocrystallizationoccurswhenthecooling rateis10°C.min−1_or_faster._The_glass_transition_is_broad_at₁₅₀_°C._At

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400 500 600 700 800 900 1000 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Wavelength (nm)

Specular + Diffuse Reflection

C−PEKK

A−PEKK

Fig.9. ReﬂectionfactorforA-PEKKandC-PEKKmeasuredby spectrophotom-etrywithanintegratingsphere.

400 500 600 700 800 900 1000 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Wavelength (nm) Calculated Absorption

C−PEKK

A−PEKK

Fig.10. Absorbancefactor(𝛼)forA-PEKKandC-PEKK,calculatedwithEq.(1).

3.4 3.6 3.8 4 4.2 4.4 4.6 4.8 5 5.2 5.4 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Wavelength (µm) Transmission

C−PEKK

A−PEKK

Fig.11. TransmissionfactorforA-PEKKandC-PEKKmeasuredby spectropho-tometry. 0 5 10 15 20 25 140 150 160 170 180 190 200 210 220 230 240 Time (s) Temperature (°C)

0,98

0,96

0,91

0,85

0,80

Fig.12. Impactoftheemissivityratevariabilityonmaximumtemperature.

Table1

Impactoftheemissivityvariabilityontemperaturemeasurements.

Emissivity𝜺 0.80 0.85 0.91 0.96 0.98

Maxtemperature(°C) 232.25 227.55 222.35 218.34 216.81

𝚫T(°C) + 9.9 + 5.2 – − 4.01 − 5.54

2°C.min−1_,_an_exothermic_peak_indicates_the_{crystallization:}_it_could_be

interpretedastheorganizationoftheshortestchains,thelatterhaving moremobilitythanthelongestones.Theglasstransition,stillat150°C islessobviouswhenthecoolingrateislower.Asacomparison,the crys-tallinityofPEEKismaximumevenwithacoolingrateof25°C.min−1

[16] .

Forthetransmissionlaserweldingprocess,thepolymericmaterial usedastopelementhastobetransparentenoughforthelaserbeamto gothrough.Inthesameidea,thelowerelementoftheassemblyhastobe absorbenttothewavelength.TheopticalpropertiesofbothA-PEKKand C-PEKKhavebeendeterminedinthevisible-nearinfraredrange,from 400to1100nm.Anythermoplasticistransparentwhenamorphousand itturnstoopaquewhenitissemi-crystalline.

TheabsorbanceA𝜆,transmittanceT𝜆andreﬂectanceR𝜆arelinkedto

theintensityofthelightﬂux𝜑𝛾

𝜆byEq. (1 ),where??istfortransmitted

flux,iforincidentfluxandrforreflectedflux:

𝐴_𝜆+𝑇_𝜆+𝑅_𝜆=1 𝑇_𝜆= 𝜑𝑡 𝜆 𝜑𝑖 𝜆 𝑅_𝜆= 𝜑𝑟 𝜆 𝜑𝑖 𝜆 (1)

Thetransmissivitycoefficientandthereflectivity coefficienthave beenmeasuredusingaspectrophotometerwithanintegratingsphere. Forthesametemperature,thetransmissionfactorisinfluencedbythe surfaceroughness,thelightincidenceangleandthesamplethickness. The results of transmission andreflection are shown in Figs. 8 and 9 respectively.ThetransmissionoftheamorphousPEKKisnegligible between 200 and400nm,then, thetransmission factorincreases to reach65±5%atmaximumfor1100nm.Atthewavelengthof inter-est(808nm),thetransmissionfactoris60±5%at20°Cforthe2mm thicksamples.Forthesemi-crystallinesample,thetransmissionisabout 3%alloverthestudiedwavelengthrange.

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0 50 100 150 200 250 300 0 50 100 150 200 250 Time (s) Maximum temperature (°C) Maximum temperature

a)

b)

c)

a)

b)

c)

Fig.13. IRthermogramduringandafterirradiation:temperature(°C)versustime(s).Theverticalboldblacklinesindicatethetimewhentheimageswererecorded.

0 2 4 6 8 10 80 100 120 140 160 180 200 220 Distance (mm) Temperature (°C) a) 0 0.5 1 1.5 2 2.5 3 3.5 4 80 100 120 140 160 180 200 220 Distance (mm) Temperature (°C) b)

Fig.14. IRproﬁlesoftheHAZ– a)AlongYaxis(lengthwise)– b)AlongZaxis(thickness).

Fig.15. Opticalphotographyofthesampleafterlaserbeam/specimen interac-tionexperiments:ontheleft:topview,ontheright:sideview.

Globalreflectionistheadditionofthespecularreflectionanddiffuse reflection.Fortheamorphoussample(A-PEKK),theglobalreflection factorisatmost12%at1000nm.

From the transmission factor and the reﬂection factor, the ab-sorbancefactor(𝛼)hasbeencalculated.TheresultsareshowninFig. 10 .

Theabsorbanceiscloseto100%forthelowestwavelengthanddecrease withwavelength.At808nm,theabsorbanceisabout25%.

Tosum up,at 808nm, the transmissionfactor is about 60%for thequasi-amorphoussample, A-PEKKreferred astransparentsample, whereasthetransmissionfactorofthesemi-crystallineC-PEKKisless than3%.Thelatterisconsideredasopaquematerialinthefollowing. Theseresultsconﬁrmwhatissupposedinotherstudies[21] ,the reﬂec-tionfactorisverylow(lessthan12%)comparedtotransmissionand absorptionfactor.

Theblackbodyisoftenusedasareferencefordeterminingthe emis-sivitycoefficientofmaterials.Theblackbodyabsorbslightinthe visi-blewavelengthrangeandtransmitsintheinfraredrange.Despite poly-meremissivitymeasurementshavebeenshowntobehighlyimprecise [20,21] ,polymersareusuallyconsideredtohavehighemissivity coef-ficients[12,22] ,theirbehaviorwithinvisibleandIRrangearecloseto thoseoftheblackbody.DuetothenoveltyofthepolymersofthePAEK family,afewopticalpropertiesareavailableintheliterature. Never-theless,theiropticalpropertiesareexpectedtobethesameasanyhigh performancepolymerinthiswavelengthrange.Toconfirmthis assump-tion,thetransmissionfactorhasbeenmeasured.Thesensorusedfor IRthermographymeasurementscoversaspectralrangefrom3.70to

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0 20 40 60 80 100 120 140 160 180 200 0 50 100 150 200 250 300 Time (s) Maximum temperature (°C) Maximum temperature

a)

b)

c)

a)

b)

c)

b)

140 160 180

Fig.16. IRthermogramforweldingtest:duringandafterirradiationoftheassembly:temperature(°C)versustime(s).Theverticalboldblacklinesindicatethe timewhentheimageswererecorded.

Fig.17. Photographyoftheweldedsamples:Ontheleft:topviewofthe as-sembly,onthemiddle:sideview,ontheright:bottomviewoftheassembly– dimensionsgiveninmm.

5.15µm.Inthisrange,thetransmissionislessthan5%forA-PEKKand C-PEKK,asseeninFig. 11 .

Assuminglikeotherauthors[13] thatthereﬂectionfactorislow,we concludethatthePEKKabsorbsalmostthewholeincidentradiationin the3.70to5.15µmrange.

Thetemperaturecalculatedfromtheinfrared thermography mea-surementsdependsontheemissivityofthematerials.Toestimatethe impact of temperature variation due to theemissivity, a variability testwasundertaken. Theemissivitywassetup as0.91forthe semi-transparentPEKKand0.96fortheopaqueone;thesevaluesarein agree-mentwithpreviousstudies[12,23] .Forourvariabilitytest,the emissiv-ityfactorvariesfrom0.8to0.98.Theeﬀectontheresultingmaximum temperatureisplottedinFig. 12 .TheresultsarereportedinTable 1 , where𝜀=0.91ischosenasareference:variationsofthemaximum

tem-peraturearewithintherangeof−5.5°C–+9.9°C.Standarddeviation𝜎

andaverageµ ofthisdatasetarerespectively6.44°Cand223.46°C. ThecoeﬃcientofvariationC_v,canbecalculatedasfollow:

𝐶_𝑣 = (1+ 1 4𝑛

)

×σ

μ (2)

wherenisthepopulation,𝜎isthestandarddeviationandµ isthe

av-erage.Withexperimentaldata,C_v is0.03.Duetotheverylowvalue ofCv,thefollowingemissivitycoeﬃcientsareﬁnallyused:0.91forthe transparentmaterialand0.96foropaqueone.

3.1. Laserbeam/specimeninteraction

Inordertosetupalltheinfraredmeasurements,experimentswere performedonasinglesample.Laserpower,measuredinWatt,isset dur-ingagiventime(s)uponthespottedarea(mm2₎_brings_energy_density_in

J.mm−2_._Energy_density_was₂₂_J.mm−2_for_the_laser_{beam/specimen} in-teractiontests.AnIRmoviewasrecordedusingtheparametersdefined intheexperimentalsection.Alltheimageswereprocessedseekingto matchmeasuredareatothespecimensize.Aplotofthemaximum tem-peratureandthreeimagesatdifferentstepsareprovidedinFig. 13 to showaqualitativedistributionofthethermalfieldsinthesample thick-ness.

Duetotheintegrationtimeapplied,onlytemperatureabove140°C isconsidered.Thisthermogramconﬁrmsthatrealirradiationtimelasts about20s.Theheatingrateisveryfast;itiscalculatedat600°C.min−1

inaverage.

Attheendoftheirradiationat22J.mm−2_,_the_maximum tempera-turereachedis222±7°C.Thistemperatureisabout70°Chigherthan theglasstransitionofthepolymer.Thewholethickness(2mm)ofthe sampleisaﬀecteduniformlybythetemperaturerise.Theheat-aﬀected zone(HAZ)iswiderinthedirectionparalleltothelaserbeamatthe endoftheirradiation.Assoonastheirradiationisstopped,theHAZ be-comeswiderandwiderduetoheatconduction.Afterirradiation, ther-malwavespropagateperpendicularlytoleadtoahomogeneouscircular

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shapeHAZintheobservedplane,attheendoftheexperiment.The cool-ingrateofthesampleis270°C.min−1_in_the_ﬁrst₁₀_s_and_decreases_to

150°C.min−1 _in_the_following₂₀_s._According_to_the_results_obtained

byDSC,thetemperaturealongthesample’sthicknesswashigherthan theglasstransition(150°C)during34sforanirradiationof21s.The polymerdoesnotreachitsmeltingpointwiththistimeandpowerof exposure.

ThedimensionsoftheHAZisdeterminedusingprofileonboth di-rectionsfortemperatureabove150°C.InFig. 14 , bothprofilesbring HAZsize:4mmlengthwiseand4mmallalongthesample’sthickness. Thewholethicknessisthermallyaffected,thatmatcheswithvisual ob-servationinFig. 15: theHAZappearsonbothsurfacesofthesample, wheretheaffectedzoneslookbrighterandsmootherthantherestofthe sample.Thesameisobservedontheuppersurfaceofthesample. 3.2. Weldingtests

Afterthetestoflaserbeam/specimeninteraction,IRmeasurements wereperformedduringweldingofacoupleof2mmthickspecimens. A-PEKKwasusedastopelement,C-PEKKforthebottom.Theenergy densitywasdeterminedpreviouslyandfixedat28J.mm−2_for_this_test. Theevolutionof themaximum temperatureof theassemblywas recordedduringwelding,stillwiththecamerasensorperpendicularto thelasersheetandperpendiculartothesample’slength.Theresultsare plottedinFig. 16 ,aswellasimagesatdifferentstepsoftheprocessto haveaqualitativedistributionofthethermalfields.

Aspreviously,onlytemperatureabove140°Cisconsidereddueto theintegrationtimeapplied.Realirradiationtimeisabout16s.As ex-pectedintransmissionlaserwelding,theenergyofthebeamisstopped attheinterfacesincethelowerpartisabsorbentat808nm.The max-imumtemperatureduringtheprocessis295±7°Cforanirradiation

at28J.mm−2_._This_temperature_is_about₁₄₀_°C_higher_than_the_glass transition,andabout20°Chigherthanthebeginningofthemeltingof PEKKoccurringat275°C.Theheatingrateduringirradiationismore than600°C.min−1_._Unlike_for_the_laser_{beam/specimen}_interaction

pre-sentedinFig. 13 ,theheat-aﬀectedzoneinthelowersampleis symmet-ricalinbothdirections,alongsideandperpendicularlytothelasersheet, whereastheHAZisstretchedoutintheupperpart.

Whentheirradiationisstopped,thetemperatureinthelowerpart graduallydecreases.Thecoolingrateisabout300°C.min−1_the_ﬁrst₁₀_s

justaftertheirradiationisstopped.ThetimeneededtoreachT_gon cool-ingfromthemaximumtemperature(295°C)is42s.Theaveragecooling ratebetweentheirradiationisstoppedandtheTgisabout200°C.min−1.

FromDSCresults,weexpectnocrystallizationofA-PEKKuponcooling, thecoolingrateistoofast.

Uponcooling,theHAZintheupperpartincreases,andmore,the HAZinthelowerparttremendouslystretchesout.Inall,thetemperature attheinterfaceof bothsampleswashigherthantheglass transition during55sandhigherthanthemeltingtemperatureduring5sforan irradiationtimeof16s.

ThephotographyoftheresultingassemblyispresentedinFig. 17 . ThedimensionsoftheHAZmeasuredfromthesepicturesarein agree-mentwiththedimensionsobtainedbyinfraredimages.

4.Conclusion

Thetemperaturedistributiononthesurfacealongthesample’s thick-nesshasbeenmeasuredduringtransmissionlaserwelding.The origi-nalityofthisworkistoplacethecamerasensorperpendiculartothe lasersheetandperpendiculartotheweldedinterface.Withsuchan ex-perimentalsetup,thetemperaturesattheweldedinterfaceandalong thethicknessofthespecimenshavebeenmeasured.Weareawarethat thetemperaturesmeasuredarenotrepresentativeoftheonesinsidethe sample(xaxis),duetothediﬀerenceofpropagationofheatwaves in-sidethematerialandinair.Nevertheless,thetemperaturemeasuredon

thesurfaceofassembliesishelpfultolinktheprocessparameters(laser powerandlaserspeed)tothemechanicalstrengthofweldedinterfaces. Forthisstudy,ahigh-performancethermoplastic,PEKKwaschosen, itsglasstransitionwasmeasuredat150°Canditsmeltingrangebetween 275and320°C.Thematerialwasprocessedbycompressionmoldingto getquasi-amorphoussamples,referredasA-PEKKandfullycrystallized, referredasC-PEKK.Theiropticalpropertiesaswellaskineticsof crys-tallizationhavebeenoutlined.Itistheﬁrsttimethelaserweldingof PEKKisreported.

Thelaser beam/specimeninteractiontestwas performedwith C-PEKKirradiatedat22J.mm−2_._The_maximum_temperature_reached_by

thesampleisabout220°Candthetemperaturealongthesample’s thick-nessstaysaboveTgduring34swhentheirradiationsteplasts21s.When

theirradiationisstopped,theheat-aﬀectedzone(HAZ)hasinitiallya longitudinalshapealongthewholethicknessofthesample.Theshape oftheHAZbecomessymmetricalintheobservedplanefor20safterthe irradiationisstopped.

Forthe welding test, anassembly made of a transparent (quasi-amorphous)sampleastheupperpartandopaque (fullycrystallized) sampleasthelowerpartwasused.Themaximumtemperaturereached attheinterfaceisabout295°Cwhentheupperspecimenisirradiated for16swithanenergydensityof28J.mm−2_._The_temperature_along thesample’sthicknessstaysaboveTgduring55sandabovethemelting

temperatureduring5s.Thistimeisobviouslyenoughtoallow macro-moleculestoself-diﬀuseandtocreateentanglements.Indeed,the ob-tainedassemblylooksstrong,althoughnomechanicaltesthasbeen per-formeduntilnow.

ThisgradeofPEKK,withaslowkineticsofcrystallizationissuitable forlasertransmissionwelding.Withtheenergybeamandirradiation timestudied,themaximumtemperatureinsidethesampleiskeptfar fromthedegradationofPEKK.Finally,thelocationandsizeofthe heat-aﬀectedzone(HAZ)havebeendetermined.Thisworkisasteptowards morereliabilityofthelaserweldingprocess.Infutureworks,the in-terfacialstrengthofweldedassemblieswillbestudiedbymechanical tests.

Acknowledgment

TheauthorswouldliketothanktheFrenchMinistryofHigher Edu-cationandResearchforFinancialsupportthroughaPhDfunding. References

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M.Villarisamechanicalengineerwithaspecializationinmaterialsscience,thermalandenergytechnologies.Afterbeinginvolvedinsomeindustrialprojects, hewentontodevelophisscientiﬁcproﬁlewithaPhDthesisinPolymerPhysics.HisPhDthesistopicsapplythetransmissionlaserweldingtoassemblea newkindofpolymerofthePAEKfamily.Forthetimebeing,theworkshehasaccomplishedcoversthethermal,opticalandmechanicalcharacterizationof materialsandprocesses.

C.GarnierhasbeengraduatedbytheNationalEngineeringSchoolofTarbes.In2011,hedefendedhisPhDthesisuntitled“DynamicBehaviorofLRI’s compositestructures:applicationtoimpactandfatigue”.In2013,hejoinedtheLaboratoryofManufacturingEngineeringinTarbes,partoftheUniversityof Toulouse,France,asassociateprofessor.Hisresearchinterestsincludeallaspectsofcompositematerials(manufacturing,behavior,FEM,inspection,tests). Hisresearchspeciallyfocusesoncompositematerialbehaviorinstatic,fatigueandmultiaxialfatigue,onexperimentalmechanicsandonthermalﬁelds measurements.

F.ChabertobtainedaPhDdegreeinMaterialsandProcessEngineeringin2004attheInstitutNationalPolytechniqueofGrenoble– Franceintheﬁeldof materialsforfuelcells.SheworkedaspostdoctoralfellowsuccessivelyattheChemicalEngineeringDepartmentattheUniversityofMelbourne,Australia andESPCIParistech,France.Since2010,sheisateacher-researcherattheInterfaceandFunctionalMaterialsgroupoftheLaboratoryofManufacturing EngineeringinTarbes,partoftheUniversityofToulouse.Herresearchfocusesonpolymerprocessing,assemblingprocessesforthermoplasticsaswellas nanoandmicro-compositeswithcontrolledproperties.Shehasco-authored4patentsand15scientiﬁcarticles.

V.NassietisProfessorofmaterialsengineeringattheNationalEngineeringSchoolofTarbes(ENIT)since2012.ShegraduatedwithaPh.D.inPhysics fromtheUPPAUniversity,France,in1995.AssociateProfessorfrom1997to2012attheLaboratoryofManufacturingEngineeringinTarbes,partofthe UniversityofToulouse,sheholdsahabilitationthesis(HDR)attheInstitutNationalPolytechniqueofToulouse,France.Herresearchinterestsfocusonthe physical,chemicalandmechanicalpropertiesofadhesiveinterfacesinmulti-materialsandoninterfacedurability.Themainaimistheestablishmentof reliablerelationshipsbetweenthepolymeric(adhesiveorresinmatrix)structureanditspropertiessuchasmechanical,rheological,thermalandadhesives intotheinterface.

D.SamélorhasbeengraduatedinMaterialsEngineeringbyEvry-Val-d’EssonneUniversityin2001.ShecompletedherMasterswithspecializationonmethods ofpreparationandcharacterizationofthinﬁlmsinthesameuniversityin2002.In2003,shejoinedtheCIRIMATlaboratoryinToulouseasanengineer.Her activitiesarefocusedonpreparationofceramics,metalsandcompositesbyMetalOrganicChemicalVaporDeposition(MOCVD)andcharacterizationofthe coatingsbyconventionalormorespeciﬁctechniques.

J.C.DiezteachesattheUniversityofZaragoza’sSchoolofEngineeringandArchitecture(DepartmentofMaterialsScience).HeholdsaPhysicsDegree andPh.D.fromtheUniversityofZaragoza.Since1989heteachesinthisUniversity,insubjectsmainlyrelatedwithGeneralPhysics,MaterialsScience, TechnologyofCeramicsandMaterialsLaserProcesses.Heismainlyinterestedontheuseoflasertechnologytoimprovethepropertiesoffunctionalceramics (HTcsuperconductorsandthermoelectricoxides).Hehasalsoexperienceintheuseoflaserradiationonmetals,polymers,andcompositematerials,especially onthemoldsectorandinaeronauticapplications.

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A.SotelohadhisDoctoraldegreeinChemistryattheUniversidaddeZaragozain1994.Hehasgotatwo‐yearspostdoctoralpositionatMax‐PlanckInstitute forMetalsResearchinStuttgart(Germany)in1995.In1997hegottwoyearsMarieCuriegrantinCRISMATLaboratoire,Caen(France).From2000heis ProfessorattheUniversidaddeZaragozaintheDepartmentofMaterialsScience.Hehaspublishedmorethan135scientiﬁcarticlesandbookchapters,most ofthemrelatedtothermoelectricandsuperconductingceramicswithemphasisonlaserprocessingtechniques.

M.A.MadrehasbeengraduatedinChemistrybytheUniversityofZaragoza.ShehasgotPh.D.inEngineeringbytheUniversidaddeZaragozawith qualification:CumLaudewithExtraordinaryDoctorateAward.Since1991sheisUniversityProfessorattheDpt.MaterialsScienceintheSchoolofEngineering andArchitecture.Herresearchinterestsincludeaspectsof:Thermoelectricandsuperconductingceramics.Texturingofceramicsbylasertechniques,laser appliancesinotherfields,solutionchemicalsynthesistechniques,andmechanicalproperties.Shehaspublishedmorethan110scientificarticlesandbook chapters.