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1、1ImprovingthestrengthofbrazedjointstoaluminabyaddingcarbonfibresMINGUANGZHUD.D.L.CHUNGCompositeMaterialsResearchLabatyStateUniversityofNewYkatBuffaloBuffaloNewYk142604400USATheadditionofshtbarecarbonfibrestoasilverbaseda
2、ctivebrazingalloy(63Ag—34Cu—2Ti—1Sn)resultedinupto30%improvementinthesheartensilejointstrengthofbrazedjointsbetweenstainlesssteelalumina.Theoptimumfibrevolumefractioninthebrazingmaterialwas12%.Thisimprovementisattributed
3、tothethinningmicrostructuralsimplificationofthealuminabrazereactionproduct(titaniumrich)layerthesofteningofthebrazingalloymatrixthestrengtheningofthebrazethereductionofthecoefficientofthermalexpansion.Thedepthoftitaniumd
4、iffusionintothealuminawasdecreasedbythefibreaddition.Thefirsttwoeffectsareduetotheabsptionoftitaniumbythefibres.Thisabsptionresultedinlesstitaniuminthebrazingalloymatrixabrazefibreparticulatereactionproduct(titaniumrich)
5、onthefibresthediffusionoftitaniumintothefibres.Incontrasttheuseofanactivebrazingalloywithalowertitaniumcontentbutwithoutcarbonfibresgavemuchweakerjoints.1.IntroductionThejoiningofaceramictoaceramicametaltoaceramicbybrazi
6、ng[12]isawidelyusedprocessinelectronicpackaginginthemanufacturingofenginesturbines.Thisprocessiscomplicatedbythepowettingbetweenthebrazingalloytheceramic.Onesolutiontothiswettingproblemisthepricoatingoftheceramicbyametal
7、butthissolutionaddsanextraexpensivesteptotheprocess.Anothersolutionistheuseofanactivebrazingalloy(ABA)whichisasilverbasedalloycontainingasmallproptionoftitanium[3—5].Thetitaniumreactswiththeceramicthusenhancingthewetting
8、ofthebrazingalloywiththeceramic.Itisnecessarytohavetitaniumreactwiththeceramicfromthewettingpointofview.Howeverthetitanium—ceramicreactionresultsinareactionlayerattheinterfacebetweenthebrazingalloytheceramic.Theceramicis
9、attackedbythereactionwhichresultsinmebrittlenessmicrostructuredefectsattheceramic—brazeinterface.Therefethetitanium—ceramicreactionthemicrostructurethicknessofthereactionlayershouldgreatlyaffectthequalityofthebrazedjoint
10、.Inparticularathinnerreactionlayerisexpectedtobebeneficialbecausethereactionproductismostprobablybrittlethereactionnecessarilydegradesthesurfaceoftheceramic[67].Howeverlittlewkhasbeendonetomodifythereactionlayerindertoim
11、provethequalityofthebrazedjoint.Inthiswkupto30%increaseinthesheartensilestrengthofstainlesssteelaluminajointswasachievedbyadding12vol%carbonfibrestotheactivebrazingalloy.Theadditionofcarbonfibresdecreasedthereactionlayer
12、thicknesssimplifiedthereactionlayermicrostructure.Alsotheadditionofcarbonfibressoftenedthesilver—copperbrazematrixwhichhelpedabsbmethermalstressgeneratedduringthecoolingdownstageofbrazing.Thecarbonfibresjustlikethecerami
13、creactedwiththetitaniuminthebrazingalloythusactedasatitaniumtraptherebycausingthicknessdecreasemicrostructuralmodificationinthereactionlayercausingthebrazingalloymatrixtodecreaseinhardness.Thehighthermalexpansioncoeffici
14、ent(CTE)ofthemetaltobejoinedtotheceramicofthebrazingalloycomparedwiththeceramiccausesthermalstresswhichdegradesthequalityofthebrazedjoint[8—11].ThisproblemmaybealleviatedbytheadditionofalowCTEmaterial(particlesfibres)int
15、hebrazingalloy[12].InthiswkcarbonfibresservedalsotodecreasetheCTEofthebrazingmaterial.Fromcompositetheycarbonfibresarealsoareinfcementthatincreasestheoverallstrengthofthebrazinglayer.HoweverthedecreaseofCTEthereinfcingef
16、fectarenottheonlyreasonsthatcarbonfibresincreasethebrazingjointstrengthasshowninthiswk.Theuseofcarbonfibresinasilverbasedbutnotactive(notcontainingtitanium)brazingalloy[13]inasilverbasedactivebrazingalloy[14]hadpreviousl
17、ybeenrepted.Howeverallpreviouswkhadbeenfocusedonmetalcoatedcarbonfibresasthemetalcoatingwassupposedtoenhancethewettingofthebrazingalloyonthecarbonfibres.Incontrastthiswkusedbarecarbonfibreswhicharelessexpensivethanmetalc
18、oatedcarbonfibresgavecomparableeffectsasthemetalcoatedfibresbecausethedissolutionofthecoppercoatingbythebrazingalloytookplaceanyway[14].Inadditionthiswkprovidesthefirstelucidationofthejointstrengthimprovementthroughexami
19、nationofthereactionlayerthickness3thicknessfmaximumshearjointstrengthfthatfibrevolumefraction(Fig.4).Increaseofthebrazethicknessbeyondtheoptimumvaluewoulddecreasethejointstrength.Inspiteofthegreaterbrazethicknessinthegra
20、dedjunctioncasethejointstrengthwashigher.Thejointsheartensilestrengthincreasedwithincreasingfibrecontentupto12vol%atwhichthestrengthwasmaximumcrespondingtoa30%increasefromthevalueat0vol%fibres.Above12vol%thesheartensiles
21、trengthdecreasedfromthemaximumvaluesuchthatthedecreasewasmuchlesssevereundertensionthanundershear.Thisdifferencebetweentensionsheartestingresultsat20vol%fibresisduetotheposityinthebrazingmaterial(tobeshownlaterinthispape
22、r)atthishighfibrecontentthegreaterdependenceofthejointstrengthonthestrengthofthebrazingmaterialundershearthanundertension.Thebrazethicknessisanimptantfactthataffectsthejointstrength.Anincreaseofthebrazethicknessincreases
23、thetotalamountoftitaniumwhichmayresultinexcessivereactionbetweentitaniumthebondingsurface.Howevertheadditionofcarbonfibresresultedinalargeincreaseinthetotalsurfaceareaavailabletoreactwithtitanium.Inderfurthertounderstthe
24、seeffectsweintroducetheconceptofthe‘‘titaniumfact’’whichisdefinedasthetotalvolumeofbrazingalloy(proptionaltotheamountoftitanium)dividedbythesumofthebondingsurfaceareacarbonfibresurfacearea.Assumingthattheamountoftitanium
25、perunitareaofthebondingsurfaceisthesameasthatperunitareaofthefibresurfacethetitaniumfactisproptionaltotheamountoftitaniumperunitareaofthebondingsurface.ThedashedcurvesinFig.5showthatthetitaniumfactincreaseswithincreasing
26、brazethicknessatallfibrecontents.ThenumberadjacenttoeachsolidlineistheshearjointstrengthinMPaattheparticularcombinationofbrazethicknesstitaniumfact.Thetitaniumfactsharplyincreasedwithincreasingbrazethicknesswhennocarbonf
27、ibrewaspresent.Howeverwiththeadditionofcarbonfibresthetitaniumfactincreasedmuchlessabruptlywithincreasingbrazethickness.Thehigherthecarbonfibrevolumefractionthelessabruptlydidthetitaniumfactincreasewithincreasingbrazethi
28、ckness.Thiseffectgivesanextraadvantagefthecarbonfibreaddition.Thatiswhenthebrazethicknesschangeswithinabrazedjoint(asmaybenecessitatedbythetopographyofthebondingsurfaces)thetitaniumfactdoesnotchangemuchwithinthebrazedjoi
29、ntifcarbonfibresarepresentsothatlocaloverbrazing(titaniumceramicoverreaction)underbrazing(titaniumbeinginsufficient)maybeavoided.Fig.5alsoindicatesthatifthetitaniumfactisbelow12(e.g.thecaseof20vol%fibres)thejointstrength
30、islow.Thismeansthattheuseofexcessivefibresdecreasesthetitaniumfacttoomuchthusresultsinadecreaseofthejointstrength.Indertoundersttheeffectofexcessivefibresthemicrostructureofanaluminaaluminajointwith30vol%fibresinthebrazi
31、ngmaterialwasexamined.Fig.6showsthecrosssectionalviewofthejointsuchthatonlytheptionofthecrosssectioncontainingthealuminabrazeinterface(lineneartoalmostparalleltotheleftedgeofFig.6)isshown.Largepeswereobservedattheinterfa
32、cewithinthebrazingmaterialindicatingthatthebrazingmaterialfailedtocoverthealuminasurfacecompletely.Thisisbecauseoftheinsufficientwettingofaluminabythebrazingalloyduetotheinsufficientamountoftitaniumatthealuminabrazeinter
33、face.Inotherwdsthereactionlayerwasnotcontinuousattheinterface.Theinsufficienttitaniumconcentrationatthisinterfaceisduetothetrappingoftitaniumbytheexcessivecarbonfibrespresent.Fig.7showsthemicrostructurewithinthebrazingla
34、yer.Largepeswereobservedatthefibrebrazeinterfaceduetoinsufficientwettingofthebrazingalloywiththefibres(associatedwiththeinsufficientconcentrationoftitanium).Fig.7showsseveralfibresindifferentientations.Fig.8showsanotherp
35、tionofthebrazingmaterial—aregioninwhichthefibres(e.g.thosenearthelowerleftcnerofFig.8)arewellsurroundedbythebrazingalloy.Inthisregionthefibreshinderedtheshrinkageofthebrazingalloyduringsolidificationthusresultinginsmallp
36、esinthebrazingalloybetweenthefibres.Fig.9showsscanningelectronmicrographsofthealuminabrazeinterfaceofaluminaaluminajointscontaining081020vol%carbonfibresinthebrazingmaterialthebrazethicknesswas40110140230lmrespectively.T
37、hesebrazethicknessesareaveragevaluesasthesurfaceofthealuminawasnotperfectlysmoothasshowninFig.9.ThereactionlayerattheinterfacewasfoundtodecreasewithincreasingfibrecontentasconfirmedbyXrayspectroscopyresults.Furthermethem
38、icrostructureofthereactionlayerwasmeunifmwhenfibreswerepresentthanwhenfibreswereabsent.Fig.10showstheelementaldistributionsacrosstheinterfacehizontallyaroundthedleofFig.9asobtainedbyenergydispersiveXrayspectroscopyusinga
39、nelectronbeamsizeofdiameter)0.8lm.Theseelementaldistributionsalloweddeterminationofthetitaniumrichreactionlayerthickness.Thereactionlayerthicknessthusdeterminedwereessentiallythesame(within$0.5lm)asthatdeterminedfromthem
40、icroscopeimage.ThevariationofthereactionlayerthicknesswithfibrevolumefractionisshowninTableII.Fig.10alsoshowsthattitaniumdiffusedintoalumina(whichmayresultinthedamageofthealuminamicrostructure).Fromthelengthofthetailinth
41、etitaniumconcentrationprofilethediffusiondepthintothealuminawasdeterminedasshowninTableIII.Thediffusiondepthdecreasedwithincreasingfibrevolumefraction.Thepresenceofatitaniumrichreactionlayerthediffusionoftitaniumintoalum
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