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Temperaturedependentdynamicstrainageinginselectivelasermelted316L
BenediktDiepold*,SteffenNeumeier,AnnalenaMeermeier,HeinzWernerHöppel,TorstenSebald,
MathiasGöken
BenediktDiepold,,AnnalenaMeermeier,öppel,TorstenSebald,
öken
Friedrich-Alexander-UniversitätErlangen-Nürnberg,DepartmentofMaterialsScience&Engineering,
InstituteI:GeneralMaterialsProperties,91052Erlangen,Germany
E-Mail:benedikt.******@
TorstenSebald:ArianeGroup,82024Taufkirchen,Germany
Keywords:Serrations,Portevin-LeChateliereffect(PLC),dynamicstrainageing(DSA),strength,
microstructure,strainratesensitivity(SRS),laserbeammelting(LBM)
Abstract
AdditivelymanufacturedausteniticstainlesssteelAISI316L(,X2CrNiMo17-12-2)is
,thehightemperaturepropertiesof
,selectivelasermelted(SLM)316Lwas
testedinthesolutionannealedconditionbycompressionandtensiletestsattemperaturesbetween
25°Cand877°
conventionallyproducedreferencematerialduetohardeningbyahighdislocationdensityandafine
,tensiletestsrevealedalossinductilityoftheSLMmaterialattemperatures
between300°Cand627°Cwheretheelongationtofracturewasreducedfrom65%to39%.
increasednormalizedworkhardeningrateandanegativestrainratesensitivity,dynamicstrainageing
Thisarticlehasbeenacceptedforpublicationandundergonefullpeerreviewbuthasnot
beenthroughthecopyediting,typesetting,paginationandproofreadingprocess,whichmay
doi:.
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Additivemanufacturing(AM)receivedincreasingattentionfortheproductionofcomplexpartsfor
mechanicalpropertiesoftheirselectivelasermelted(SLM)316Lalloyatroomtemperature,proven
bynumerouspublications[1]–[5],farlowerattentionhasbeendevotedtothehightemperature
,AM316Lisadditionally
strengthenedbyfinedislocationcellswhichdissolvebetween600°Cto1000°C[6].Duetothat,
superiortensilepropertiesofAM316Lwerealsoobtainedattemperaturesof250°C[7]and800°C
[8].Forlongtimeexposuresunderhightemperatures,however,unstabledislocationcellscan
deterioratethecreepproperties[9][10].Saeidietal.[8]alsoobservedthepropagationofLudersbands,
whicharecloselyrelatedtodynamicstrainageing,at800°,nosystematicstudyonthe
influenceoftheSLMmicrostructureonthemechanicalpropertiesovertheentiretemperatureregime
fromroomtemperatureto900°,sinceAISI316Lisa
widelyusedausteniticstainlesssteelforapplicationsatelevatedtemperaturesanditisknownfrom
theconventionallyproducedmaterial,thatdynamicstrainageingoccursatelevatedtemperatures.
Theimprovementoftheprocessparametersinrecentyearswithrespecttothelaserpower,hatch
distance,scanstrategy,
ahighstrength[11].ThehighthermalgradientsandhighsolidificationratesintheSLMprocesslead
AcceptedArticle
introducessignificantstressesinaccordancewiththetemperaturegradientmechanism(TGM)[12].
RecentresultsofBertschetal.[13]showedthatthesestressesandstrainscontributestronglytothe
,
:.
areknowntobebeneficialforthemechanicalproperties[1]–[3][5].
Themainstrengtheningmechanismsoftheausteniticsteel316LaresolidsolutionandHall-Petch
,additionalstrengtheningbythedendritic
dislocationscontributingtothestrengthofthematerial[1][14][15].Onemodeltounderstandthe
fundamentalsofstrengtheningbycellulardislocationstructureswasestablishedbyMughrabi[12-14].
Thesocalledcompositemodeldescribesthestressdistributioninthesoftcellandhardwallregions
andenablesthecalculationofthetotalstrengtheningeffectbyamodifiedTaylorequation,usinga
geometricconstantfortheheterogeneouscompositeαhet,dependingonthewallthicknessandcell
diameter[18].ItwasalsofoundbyBlumandReppich[19][20]forcellulardislocationnetworks
developedunderstaticcreeploads,thatthestrengtheningcontributionisinverselyproportionaltothe
celldiameter(σ=kGb/dcell).Gallmeyeretal.[15]foundthatthecalculationofthedislocationdensity
onthebasisofthecelldiameterof620nmisnearlyequaltothedislocationdensityobtainedby
,bothresultina
strengtheningcontributionof297MPaforas-
theas-builtconditionayieldstrength(YS)between427MPaand640MPaduetothedescribed
variousstrengtheningmechanisms[21]–[23].Subsequentsolutionheattreatmentsleadtodislocation
,aloweryieldstrength
between364MPaand436MPa[8][22],heattreatmentsareoftencarriedoutin
ordertoreduceresidualstressesanddistortionsaftertheremovalofthebuildingplatform[24].
AcceptedArticle
Althoughthe316Lalloyexhibitsahighcorrosionresistanceandgoodmechanicalpropertiesathigh
temperatures,itisknownthattheconventionallyproduced316Lisaffectedbydynamicstrainageing
(DSA),compare[25]–[31]:Atintermediatetemperaturesthediffusionvelocityofsoluteatomsbecomes
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lockingandavalanche-likeunlockingofdislocationsleadingtoserrationsinthestressstraincurveat
temperaturesbetween250°Cand600°C[25].ThiseffectiscalledafterPortevinandLeChatelier
(PLC-effect),whofirstdescribedthisphenomenon[32].Additionally,alsothemechanicalproperties
,316Lshowsahugeductilitydecreaseinthis
temperaturerange[25][27][33].Themagnitude,however,varieswiththeappliedprocessingtechnologies
,itisimportanttoinvestigatethepropertiesofSLMprocessed,andsolutionannealed
316Lfromroomtemperaturetoelevatedtemperaturesinordertoanalyzepossibledifferences
betweenadditivelymanufacturedandconventionallyproduced316Landtoevaluateitspotentialfor
,themicrostructureand
itsinfluenceonthecompressionandtensilestrengthisanalyzedanddiscussedhere.
AcceptedArticle
:.
Additivemanufacturingof316LsampleswascarriedoutonaM290(EOSGmbH,Krailling,
Germany)SLMmachineatArianeGroupGmbH(Taufkirchen,Germany).Thecompositionofthe
-upprocesswascarriedoutwithalayerthicknessof
40µmandtheusageofthestandard316LprocessparametersofEOSincludingarotatingstripe
,inaccordancewithSongetal.[34],
2,7J/mm².The316Lpowderhadaparticlesizesbetween20and65µmandwassuppliedbyOC
OerlikonCorporationAG(Pfäffikon,Switzerland).Samplesformicrostructuralinvestigationswere
thefollowing:x-directionpointstotherightsideandy-directiontothebacksideoftheSLM-machine
-
conventionallyproducedrodofthesamesteelwithadiameterof10cmwasinvestigated.
Table1:Chemicalcompositionofthe316Lpowder.
wt.%FeCrNiMoMnCuPSSiCN
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Alladditivelymanufacturedsamplesweresubjectedtoaheattreatmentinavacuumfurnacepriorto
°Candheldfor
-treatedcondition.
AcceptedArticle
Samplesformicrostructuralanalysisweregrounddowntoagritsizeof2500,mechanicallyand
electrochemicallypolishedusinga3µmdiamondsuspensionandanelectrolyteA2(StruersGmbH,
Willich,Germany).Themicrostructurewasanalyzedwithascanningelectronmicroscope(SEM)
:.
ZeissCrossbeam540(Oberkochen,Germany)usingbackscatteredelectron(BSE)contrastandthe
electronbackscattereddiffraction(EBSD)-detector(OxfordNordlysnano).TheEBSD
measurementswith1000x750pixelswerecarriedoutat20kV,withabinningof4x4andastepsize
of1µ,thesoftwareChannel5(HKL)
linesininversepolefigure(IPF)mappingsindicatelowangleboundarieswithmisorientation
between2°and15°andthickerlineshighanglegrainboundarieswithanglesabove15°.The
elementalcompositionofinterdendriticregionswasdeterminedbyenergy-dispersiveX-ray
spectroscopy(OxfordX-Mat150).
Tensileandcompressiontestswerecarriedoutattemperaturesof25°C,300°C,627°C,777°Cand
877°°C,400°C,500°Cand
677°.
ThecompressiontestswereperformedonanInstron4505(InstronGmbH,Darmstadt,Germany)
testingmachine,equippedwithaHegewald&Peschke(Meß-undPrüftechnikGmbH,Nossen,Germany)
,verticallybuiltSLMrodswithadiameterof12mmwereturnedto4mmand
dividedintoseveralsamples,eachwithalengthof7mm(seeFigure1a).Theconventionalreference
testswereconductedinathree-,obtained
fromalinearvariabledifferentialtransformer(LVDT).Theinitialstrainratewas10-3s-
strainratejumpsto10AcceptedArticle-4s-1and10-5s-
strainratesensitivity(SRS)m=dln(σ)/dln(dε/dt)wasdeterminedatthefirstandsecondtransition
,thestresswasfittedwithastraight
,the
stressdifferencewithintheplasticstrainintervalfrom5%%strainandthenormalizedwork
:.
hardeningratewascalculatedbytheequationθ=[(σ–σ)/]/E[25].The
εpl==
temperaturedependentYoung’smodulusprovidedbythetensiletestswasusedforthecalculations.
Thevalueswererelatedtothetruestressσandtrueplasticstrainεpl.
ThetensiletestswerecarriedoutonaZwick1456(ZwickRoellGmbH&,Ulm,Germany)
diameterof6mm(seeFigure1b).Eachsamplewasheatedupandheldatthedesiredtesting
ratewassetto10-3s-
’smoduliwerecalculatedbytheslopeofthefirstloadingofthesample.
Figure1:a)Examplesoftensileandcompressiontestsamplesb)schematicdescriptionofthedeterminationand
calculationoftheworkhardeningrateandstr