See discussions, stats, and author profiles for this publication at: http://www.researchgate.net/publication/270474649Simulation of cracking phenomena during laserbrazing of ceramics and cemented carbideARTICLEinSCIENCE AND TECHNOLOGY OF WELDING & JOINING NOVEMBER 2014Impact Factor: 1.38 DOI: 10.1179/1362171814Y.0000000241DOWNLOADS23VIEWS374 AUTHORS, INCLUDING:Kimiaki NagatsukaOsaka University14 PUBLICATIONS 31 CITATIONS SEE PROFILEYoshihisa SechiKagoshima Prefectural Institute of Industri10 PUBLICATIONS 23 CITATIONS SEE PROFILENinshu MaOsaka University62 PUBLICATIONS 69 CITATIONS SEE PROFILEAvailable from: Ninshu MaRetrieved on: 14 August 2015SimulationofcrackingphenomenaduringlaserbrazingofceramicsandcementedcarbideK.Nagatsuka*1,Y.Sechi2,N.Ma3andK.Nakata1To investigate the mechanism of the occurrence of the microcracks produced in laser brazing ofsiliconcarbideandcementedcarbideusingasilvercoppertitaniumalloyasthefillermetal, anumerical simulation for thermal cycle and thermal stress was performed using the finite elementmethod. Thelaser-irradiatedareaof thecementedcarbidewasheatedselectively, andalmostuniform temperaturedistributioninthefillermetalwasobtained.Thetensilethermalstressinthesiliconcarbideappearedneartheinterfacebetweenthesiliconcarbideandthefillermetal,andthemaximumprinciplestressat about 400Kduringthecoolingprocessreachedthefracturestrength of the silicon carbide. This resulted in the formation of microcracks in the silicon carbidewhichwasobservedintheexperiment.Keywords:Laserbrazing,Microcrack,Siliconcarbide,Activefillermetal,Finiteelementmethod,Thermal stressIntroductionSiliconcarbide (SiC) has several functional character-istics, suchas a lowcoefcient of thermal expansion(CTE); highthermal conductivity; goodhightempera-ture mechanical properties; superior wear, corrosion,heat, and oxidation resistances; and semiconductingproperties. The dissimilar brazing of silicon carbide/metals usinganactiveller metal is atechniqueusedcommonly to produce cutting tools, ceramic heatexchangers, optical devices, andpower devices.1,2Thecemented carbides (WCCo alloys) as classied intometal materials have low CTE and high rigidity, makingthem suitable for fabricating structural materials.3Thus,dissimilar joints of SiC/WCCo alloys are idealfor forming cutting tools. However, certainproblemsremain with conventional brazing, such as materialdeterioration, the formation of a thick brittle compoundlayer, long treatment times and the occurrence ofthermal stressstrain in the joint.1,4These problemsarise because conventional furnace brazing requires longtreatment times in order to ensure the heating andcoolingof the whole component. Onthe other hand,laser brazing, in which a laser is used as the heat source,has attractedattentionas anewmethodfor formingdissimilarmetalmetal jointsaswell asceramicsmetaljoints.5,6Incontrast toconventional furnace brazing,laser brazing can potentially minimisethe growth of thebrittleinterfacial compoundlayerandpreventmaterialdeterioration, becausetheheatingandcoolingtimesinthis process are short, only the selected part of thecomponentneedstobeheated,andtheamountofheatinputissmall.Inaddition,thisprocessuseslessenergy.Laser brazing has been performed at atmosphericpressuretoformdissimilar metalmetal joints usingashielding inert gas. For instance, aluminium alloysteel,6magnesium alloyNi-plated steel,7aluminium alloytitanium alloy8joints have been formed using thistechnique.Inaddition,thistechniquehasbeenbroughtinto practical use in the automotive industry. As aninstance of ceramicsmetal joint, Huang et al.9haveemployedthedissimilar laser brazingtojoindiamondgritssteelsubstrateusingaCuTiSnmetalpowderasthe ller inorder tofabricate brazeddiamondtools.Sudmeyer et al.10have reportedthe effect of surfacepatterningonthejoint strengthof laserbrazedsiliconcarbidesteel joints formed using AgCuTi andSnAgTi as the ller metals. They foundthat the jointstrength increased after surface patterning. In a previousstudy,wehaduseddissimilarlaserbrazingonceramicssuch as sialon,11silicon carbide,12hexagonal boronnitride,13diamond,14and graphite,15while using a WCCoalloyasthemetalmaterial.Theeffectoftheactive-elementcontentofthellermetal onthejointstrengthand the interfacial reaction between the ceramics and theller metal was investigated. However, little data isavailableonthestressandtemperaturedistributionsofjointsformedbyshortdissimilarbrazingprocess, suchas laser brazing, has been reported.16,17The thermalstress during the cooling process caused by the mismatchinthe CTEbetweenceramics anda ller metal, andbetweenceramics andametal substrate ofteninducestheoccurrenceofcracksinthebrazedceramics.18,19Inaddition,thetemperaturedistributionandheatingtimealsoaffectthediffusionoftheactiveelement, whichis1Joining and Welding Research Institute, Osaka University, Ibaraki,Osaka,Japan2Kagoshima Prefectural Institute of Industrial Technology, Kagoshima,Japan3JSOLCorporation,Tokyo,Japan*Correspondingauthor,email nagatuka@jwri.osaka-u.ac.jp 2014Instituteof Materials, MineralsandMiningPublishedbyManeyonbehalf of theInstituteReceived12June2014; accepted4August2014DOI 10.1179/1362171814Y.0000000241 ScienceandTechnologyof WeldingandJoining 2014 VOL 19 NO 8 682necessary for joining the ceramics. Therefore, in thisstudy, we performed a numerical simulation of thethermalcycleandthermalstressduringlaserbrazingofsiliconcarbide andaWCCoalloytoinvestigate themechanism of the occurrence of the cracks in the siliconcarbidepartofthelaserbrazedjoint.LaserbrazingprocessThe experiments were performedusingsiliconcarbideblocks,platesoftheWCCoalloy,andAg28Cu1?7Ti(mass-%) ller metal sheets. The silicon carbide block wasof recrystallised silicon carbide (.99mass-% SiC, poros-ityof17%)andhaddimensionsof56563?5mm. TheWCCo alloy plate (ISO K10 grade) was used asthe substrate; this haddimensions of 1061062mm.The thickness of the ller metal sheet was 0?1mm, and itssizewaskeptat363mmtoensurethatitdidnotowout of the joint interface. The dimensions and geometriesofthematerialsusedweredesignedbasedonaturningtool. For the temperature measurements, a WCCo alloyplate with a hole was prepared by electric dischargemachining. Thehole was locatedat themiddleinthesquare onthe lower surface of the plate andhadthediameter of 2mmandthedepthof 1?8mm. Figure1shows a schematic illustration of the laser brazingprocess. Thetestspecimenwasshapedlikeatophat.Allermetal sheetwassandwichedbetweenthesiliconcarbideblock(placedabovethesheet)andtheWCCoalloy plate (placed belowthe sheet) and placed in avacuumchamber.Thetopofthespecimenwascoveredwitha transparent quartz glass plate, whichnot onlyacted asawindowfor the vacuum chamberduring laserbeamirradiation, but alsoxedthespecimeninplace.The vacuumchamber was evacuated to less than 1021Pa,andArgasof99?999%puritywaspumpedinuntil thepressure was equal to atmospheric pressure. Thisevacuation and substitution cycle was performed vetimes before brazing. During brazing, Ar gas was made toow through the chamber at a rate of 5Lmin21.Radiationfromthe yttriumaluminiumgarnet (YAG)andlaser diode (LD) lasers was simultaneouslytrans-ferredcoaxiallyviaoptical brestothelaserheadunit,and was used to irradiate the sample through thetransparent quartz glass plate onthe topside of theWCCo alloy plate at an irradiation angle of 85u. The LDlaserwascomplemented theYAGlaserinmeltingallofthe ller metal. Table1 lists the laser brazing conditions,which were set according to the standard JIS Z3261 BA-8and the conditions used in previous studies.12The portionof the WCCoalloy plate aroundthe siliconcarbideblockwassubjectedtolaserirradiationfor36s.Duringlaser heating, the temperature ofthe WCCoalloyplatewas monitored with an R-type thermocouple, which wasinsertedintothe holeofthe WCCoalloyplatebeneaththe joint. The thermocouple was xed at the middle in thesquare,andthedistancebetweenthethermocoupleandtheinterfaceofthellermetalsheet/WCCoalloyplatewas 0?2mm, as shown in Fig.1b.NumericalsimulationanalysisFiniteelementmodelA thermo-elasticplastic analysis was performed using thenite element methodtosimulatethe temperatureandstress distributions induced by the laser brazing of siliconcarbideandtheWCCoalloyusingtheAgCuTialloyas the ller metal. The explicit nite element method solverinLS-DYNA(version971) wasused.20Figure2showstheniteelementmodel oflaserbrazingthatwasused.Thismodel consistedof threeparts(thesiliconcarbideblock, the ller metal sheet, and the WCCo alloy plate).The dimensions of these material components corre-sponded to those of the experimental specimens. The meshsizearoundthellermetal was0?160?160?1mm. Theyellow line (beam element) indicates the heat input to theWCCoalloy, andit movesaroundthesiliconcarbideblock along the red line. The number of nodes andelements is 148830 and133797, respectively. Table2lists the material properties,2125andFig.3 shows theaimageof laserheating; bcross-sectionof laserbrazingapparatus1 Schematicillustrationof laserbrazingTable1 Laser-brazingconditionsPulsedYAGaverageoutput/WPulsedYAGwavelength/nmCWLDoutput/WCWLDwavelength/nmPulsefrequency/HzScanningtime/s Atmosphere134 1064 20 808 100 36 Arflow(5Lmin21)Nagatsukaet al. Simulationof laserbrazingofceramics/cementedcarbideScienceandTechnologyof WeldingandJoining 2014 VOL 19 NO 8 683temperature dependence of these properties for siliconcarbide, the AgCuTi ller metal, and the WCCoalloy.2125Becausethesiliconcarbideisverybrittle, itsyield strength used in the simulation is almost equal to thefracture strengthwhichis about ahalf of the bendingstrengthbasedonexperimental approximation.26Asili-concarbide/ller metal/WCCoalloyjoint was formedafter the laser heating in the experimental phenomena. Inthesimulation, thejoinedshapeof siliconcarbide/llermetal/WCCoalloywasdividedbyniteelementmesh,and then was heated by laser. Because the thermal stress insiliconcarbide duringheatingwouldbe verylow, dueto the lowyield strength of the ller metal at hightemperatures. Inotherword, thethermal stressinducedinthesiliconcarbideduringheatingwasreleasedduetoyield strength of the ller metal being low at hightemperatures.BoundaryconditionsFixingofWCCoalloyTo prevent the spinning and parallel displacement of themodel,theapexesatthebottomsurfaceoftheWCCoalloywerexed.HeatinputThe heat input was calculated as the mathematicalproduct of the energy output of the YAG and LD lasersandtheirlaserabsorptivities.ThevaluesofthelaserabsorptivitygwerecalculatedusingBramsonsformula27,28g~0:365Rl 1=2{0:0667Rl z0:006Rl 3=2(1)whereRandlaretheelectricalresistivityinVm(WC:0?53mVm)andthewavelengthofthelaserinm(YAGlaser: 1064nm, LDlaser: 808nm), respectively. Thegvalues of theYAGandLDlaser calculatedbyequa-tion(1) were 0?23 and 0?26, respectively. Bramsonsformulais onlyapplicable for surface-oxide-layer-freemetalsspecimenheatedinvacuum. Thepresenceofanoxide layer will increase the absorptivity.28However,this formulawouldbeapplicableforthelaser brazingprocess,becausetheoxidationofWCwaspreventedbyAr gas owing continuously in the chamber during laserheating.HeatdensitydistributionAGaussiandistributionof the heat densityinahalfspherewasusedinthethermalconductionsimulation20q~6|31=2Qp:p1=2a3e{3x2a2 {3y2a2 {3z2a2 (2)whereqistheheatdensityat(x,y,z),Qisthenetheatinput from laser power, and a is the radius of the heatingzone(a50?2mm).MovementofheatsourceTheyellowline(beamelement)iswheretheheatinputto the WCCo alloy. The heat was then transferredalong the red square, which had sides of 7mm, as showninFig.2. Themovingspeedwas0?778mms21. Thus,theheatingtimewas36s.HeattransferandradiationThe heated joint was cooled by the heat convectiontothe owing Ar gas (5Lmin21) andthroughheataspecicheat; bthermal conductivity; cyieldstrength; dYoungsmodulus2 Finiteelement model of laserbrazingTable2 Material propertiesof siliconcarbide, AgCuTi llermetal andWCCoalloy2125Density/6103kgm23Meltingtemperature/KSpecificheat*/JK21kg21Thermalconductivity*/Js21m21K21Coefficientofthermalexpansion/61026K21Youngsmodulus*/GPaYieldstrength*/MPaSiliconcarbide 2?7 3003 700 50 4?5 80 50{Fillermetal 9?0 1063 114 219 19?6 83 272WCCoalloy 14?9 224 143 5?0 640 5230*Temperaturedependence.{Fracturestrength.Nagatsukaet al. Simulationof laserbrazingofceramics/cementedcarbideScienceandTechnologyof WeldingandJoining 2014 VOL 19 NO 8 684radiation. Theheatlostbytheconvectionandthatbytheradiationwerecalculatedtogetherbythefollowingsimpliedequation20qc~h(Tc{Ts) (3)where qc, h, Tc, andTsare the coolingheat ux, thecoefcient of heat transfer between the environmentwithinthe chamber andthe surface of the joint, thetemperatureof theenvironment (293K), andthesur-face temperature of the joint, respectively. Figure4showsthetemperaturedependenceoftheheat-transfercoefcient.ResultsanddiscussionAppearanceandmicrostructureofbrazedjointFigure5aandbshows the appearanceandthecross-sectionalmicrostructureofthelaser-brazedjointrepeti-tively. The laser irradiatedarea was observedontheWCCo alloy around the silicon carbide block.Although no macrocrack was found on the brazedsilicon carbide, a microcrack was observed in the siliconcarbideblockneartheedgeof thebrazedllermetal.The mechanism of the microcrack occurrence wasdiscussedbasedonthesimulationresultsasbellows.TemperaturechangeduringlaserbrazingFigure6shows the measuredandcomputedtempera-turechangesintheWCCoalloyduringlaserbrazing.The temperature measurement was carriedout intheholewhichwaslocatedatthemiddleinthesquareonthe lower surface of the WCCoalloyplate andhadthe diameter of 2mmandthe depthof 1?8mm. Thedistance between the measuring point and the llermetal/WCCo alloy interface was 0?2mm, and theposition at which the temperature was computedcorrespondingtothemeasuredposition. Thetempera-ture increasedrapidly after the laser irradiation. Themaximummeasuredtemperature was 1099K, incon-trast tothehighest computedtemperature, whichwas1082K.Amarginoferrorofapproximately 10Kinthemaximummeasuredtemperaturewasobservedoverthree measurements. After laser irradiation, the tem-perature of the joint decreased. There was a goodagreement between the measured and computed heatingand cooling curves. The simulation model and theaccuracywereveried.aappearance; bcross-section3 Temperature-dependent material propertiesof siliconcarbide, AgCuTi llermetal andWCCoalloy4 Temperature dependence of heat-transfer coefcientbetweenenvironment inside of chamber andsurface ofjointNagatsukaet al. Simulationof laserbrazingofceramics/cementedcarbideScienceandTechnologyof WeldingandJoining 2014 VOL 19 NO 8 685Figure7shows the computedtemperature distribu-tionbefore(0s), during(9, 18, 27and36s), andafter(300s) the laser irradiation of the ller metal at the WCCoalloysurface.Thebrokenlinesrepresentthesiliconcarbideblock.Whenlaserirradiationwascompletedat36s, the temperature reached the highest one. Thedifference in the temperature on the ller metal atvarious locations was smaller than 50K during the laserirradiation and cooling process. In other words,althoughthelaserirradiatedareaof theWCCoalloywas selectively heated during laser irradiation, thetemperature distribution of the ller metal becameapproximatelyuniformduringheatingandcoolinginspiteoftheselectivelaserheating.Theseresultsprovedthat a uniform brazed joint can be successfully obtainedby proposed selective laser heating. The entire llermetal component was heatedtoatemperature higherthanthemeltingpointofthellermetal(1063K).Thebrazed joint was cooled to approximately the roomtemperature(293K)after300s.Stress distribution and microstructure of brazedjointFigure8 shows the distribution of the maximumprincipal stress (residual stress) onthe brazedside of5 aappearanceandbcross-sectional microstructureof laserbrazedjoint6 Measured and computed temperature changes in WCCoalloyduringlaserbrazing7 Computed temperature distribution before (0 s), during (9, 18, 27, and 36 s), and after (300s) laser irradiation of llermetal andWCCoalloysurfaceNagatsukaet al. Simulationof laserbrazingofceramics/cementedcarbideScienceandTechnologyof WeldingandJoining 2014 VOL 19 NO 8 686thesiliconcarbideblockaftercooling.Itisknownthatknowingthemaximumprincipal stress is essential forstudyingcrackformationandfractureinbrittlemateri-als, suchasceramics.29,30Thelargesttensilemaximumprincipal stress exceeding the fracture strength of siliconcarbide(50MPa) aroseoutsidethebrazedller metaledge. Figure9 shows the X, Y and Z axis stress (residualstresses) distributions of anXZcross-sectionaroundthebrazedller metal after cooling. Thetensilestresswas concentrated in the silicon carbide block outside thebrazed ller metal edge (at position (1) in Fig.9). The Xaxisstresswasthelargestamongall thestresseswithinthis XZ cross-section. Figure10 shows the relationshipbetween the temperature and the X axis stress atposition(1),whichisshowninFig.9.TheXaxisstressincreasedwithadecreaseinthetemperatureduringthecooling process, andreachedthe fracture strengthofsilicon carbide at approximately 400K. This largetensile thermal stress was presumably caused by themismatchbetweentheCTEofthellermetal andthatof silicon carbide. The CTEof silicon carbide usedis 4?561026K21, that of the ller metal used is19?661026K21, andthatoftheWCCoalloyusedis5?061026K21. Thus, the CTE of the ller metal is fourtimes those of silicon carbide and the WCCo alloy. Theller metal expanded during heating and contractedduringcooling. However, the contractionof the llermetalwasrestricted bythesiliconcarbideandtheWCCo alloy because these materials formed a joint.Therefore, thetensilestress intheller metal andthecompressive stress in the silicon carbide and the WCCoalloywithintheareawherethesiliconcarbideandtheWCCoalloywereincontactwiththellermetalaroseas the thermal stress. As aresult, the tensile thermalstress in the silicon carbide and the WCCo alloyaround the ller metal outside the contact area alsoaroseduetotheequilibriumforceof thecompressivestress. The position where the largest tensile thermalstressoccurredcorrespondedtothepositionwherethecrack was formed, as can be seen from Fig.5. Thus, themicrocrackwascausedbythethermalstressduetothemismatchintheCTEs of siliconcarbideandtheAg28Cu1?7Ti (mass-%) ller metal. Onthebasisof thiscomputation model, in our future studies, the possibilityandeffectiveness of thermal stress reductionmethodscan be investigated by changing the composition and thethickness of the ller metal. Furthermore, the laserpowerandthelaserscanningpatternsofthisdissimilarjoiningprocess canbeoptimisedfor thehighefcientproduction.ConclusionsAnumerical simulationof the temperature andstressdistributionsofabrazedjointofsiliconcarbide/silver28mass-% copper1?7mass-% titanium ller metal/cementedcarbide was performed, and these results werecompared with those of brazing experiments. Thefollowingconclusionswereobtained.1. There was a good agreement between the measuredand computed heating and cooling curves. Based on the8 Maximumprincipal stress (residual stress) distributiononbrazedsideof siliconcarbideaftercooling9 X, YandZaxisstress(residual stress) distributionsonXZcross-sectionaroundbrazedllermetal aftercoolingNagatsukaet al. Simulationof laserbrazingofceramics/cementedcarbideScienceandTechnologyof WeldingandJoining 2014 VOL 19 NO 8 687resultsofthethermalconductionanalysis,althoughthelaser-irradiated area of the cemented carbide wasselectivelyheatedduringlaserirradiation, thetempera-ture distribution of the filler metal became approxi-matelyuniformduringheatingandcoolinginspiteoftheselectivelaserheating.2. Alargetensilethermal stressinducedbythelaserbrazing, reached the fracture strength of the siliconcarbide at about 400K during cooling. The largest stressarose in the silicon carbide outside the brazed filler metalarea. This tensile thermalstress caused the formation ofamicrocrackinthesiliconcarbideblockonthebrazedjoint.References1. Y. Liu, Z. R. HuangandX. J. Liu: Joiningof sinteredsiliconcarbideusingternaryAg-Cu-Ti activebrazingalloy, Ceram. Int.,2009,35,34793484.2. H. Chin, K. Cheong and A. 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