MicrostructuralCharacteristicsofLaser WeldedJointsofGH909 Alloy

Objective Owingtotheexcellentperformanceandthelowcoefficientofthermalexpansionathightemperatures Fe-Co-Ni-basedsuperalloyshavebeen widelyusedintheprecisionstructuralcomponentsofaerospace Asthese alloyslackaCrelement thereisnovolumeexpansioneffectcausedduetosolid-phasetransformation Meanwhile thepresenceofNb Ti andAlmakesthesealloyspronetoelementsegregationandcrackingduringsolidification Certainproductswillbediscontinued andflightsafetymaybeseriouslyaffected Basedonthis laserweldingwas performedonGH909alloytoinvestigatethemicrostructureevolutionandtheorientationcharacteristicsofthejoints Manystudieshavebeenpublishedontherelationshipbetweentheweldability themicrostructure andthejoint propertiesof900seriessuperalloys However therehavebeenfewstudiesonthejointevolutionofmicrostructures andcrystalorientationproperties Asaresult weinvestigatedthemacroscopicforming microstructure andcrystal orientationpropertiesoflaser-weldedjointsmadeoftheGH909alloytorichrelatedtheroryofGH909superalloy weldingmetallurgyandexpandapplicationfieldoflaserwelding Methods Thisstudyconductedexperimentalinvestigationson2 mmthicknessGH909alloyplates Thewelded articlewastransverselysectioned withalinecutmachineintothreesamplesfor microstructuralexamination Sampleswerefabricatedviaaseriesofroughgrinding finegrinding andmechanicalpolishing Theelectrolyteand aquaregiadiluentwereusedtoremovethestressonthesurfaceanddisplaythepartitionandthemicrostructural characteristicsofthejointsusingthescanningelectron microscopyinbackscatteredelectron modeandelectron backscattereddiffractiontechnique ResultsandDiscussions TheweldofGH909alloypresentedatypical????hourglassshape andtheweld-zonegrain sizewaslargerthanthatofthebasemetalandheat-affectedzone Fig 3 Typicalcolumnargrainsprecipitatedin themiddleandthelowerpartsoftheweld whileasmallnumberofequiaxedgrainsformedonthetopoftheweld Fig 4 Duetothesimilarshapeofthefusionline thegrowthofgrainsatthetopandthebottomoftheweld demonstratedhorizontalsymmetry Planaraciculargrainsformedpreferentiallyintheuppernear-weldzoneand weretransformedintoelongatedcolumnardendriteswithstablegrowthviacompetitionselection Moreover a changeoccurredinthemiddlenear-weldzoneinvolvingcells cellulardendritic andcolumnardendritic Migrated grain boundaries were observedin the heat-affected zone andtwo growth modes including epitaxialand nonepitaxialgrowths wereobservednearthefusionlineofthe weld Fig 5 Thegrainsinthe middle weld exhibitedtypicaldiscontinuousgrowthpatterns Thegrainchangedfromcoarsetofine anditwasquicklyreplaced bynewgrains Thegrainboundaryexpansionandliquefactionoccurredwhilethegrainscoarsenedintheheataffectedzone resultinginformingthemigratedgrainboundary Fig 7 Twogrowthmodes includingepitaxialand nonepitaxialgrowths existedinthepositionnearthefusionlineofthe weld Theanalysisofgrainboundary orientationdifferenceshowedthattheheat-affectedzone averagevalue 43 45 degrees andweld averagevalue 38 16 degrees nearthefusionline wereprimarilycomposedoflarge-anglegrainboundaries >15 degrees--50 degrees with moregrain boundariesoforientationdifferencebetween>50 degrees and63 degrees formedinthenonepitaxialgrowthmodeoftheweld The graingrowthdirection wasconsistentintheupper weld indicatinganagminatedfeature gamma columnargrains continuouslygrewandeventuallyformedalargeperforatedgrain Fig 8 Thegrainboundaryorientationdecreased graduallyfromtheheat-affectedzonetothecenteroftheweld Conclusions Thejoint????sweldcross-sectionhadatypical????hourglassshape Typicalcolumnarcrystalsappearedin themiddleandlowerwelds withafewequiaxedcrystalzonesatthetopofweld Thegrainintheweldzone coarsenednoticeablywhencomparedtothatinthebasemetalandheat-affectedzone Neartheweldzoneofthe upperfusionline needle-likecrystalsinaplaneformexisted whichformedelongatedcolumnardendriticcrystals throughcompetitionandeliminationmechanism Neartheweldzoneofthemiddlefusionline thetransitionfrom cellularcrystalsandcellular-likedendritestocolumnardendriteshappened Thereweremigratinggrainboundaries intheheat-affectedzone Moreover twomodes includingepitaxialandnonepitaxialgrowths emergednearthe fusionlineoftheweldseam Thegrainsinthemiddleofweldgrewinatypicaldiscontinuouspattern withthegrain increasingfromfinetocoarse However thesegrainswerequicklyreplacedbynewgrains Thegraingrowth directionwasconsistentatthetopoftheweld indicatingaggregativecharacteristics Thecontinuousgrowthofthe columnarcrystalsresultedinalargeperforatedgrain