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Published paper
Mumtaz, K.A., Todd, I., Hopkinson, N. and Vora, P. (2015)
AlSi12 In-Situ Alloy
Formation and Residual Stress Reduction using Anchorless Selective Laser Melting
.
Additive Manufacturing, 7. pp. 12-19.
AlSi12
in-situ
alloy
formation
and
residual
stress
reduction
using
anchorless
selective
laser
melting
Pratik
Vora,
Kamran
Mumtaz
∗,
Iain
Todd,
Neil
Hopkinson
CentreforAdvancedAdditiveManufacturing,DepartmentofMechanicalEngineering,UniversityofSheffield,UK
a
r
t
i
c
l
e
i
n
f
o
Articlehistory:
Received10February2015
Receivedinrevisedform19March2015 Accepted9June2015
Availableonline12June2015
Keywords:
AdditiveManufacturing Selectivelasermelting Residualstressreduction Insitualloying Aluminiumalloys
a
b
s
t
r
a
c
t
RapidmeltpoolformationandsolidificationduringthemetalpowderbedprocessSelectiveLaserMelting (SLM)generateslargethermalgradientsthatcaninturnleadtoincreasedresidualstressformation withinacomponent.Metalanchorsorsupportsarerequiredtobebuiltin-situandforciblyholdSLM structuresinplaceandminimisegeometricdistortion/warpageasaresultofthisthermalresidualstress. Anchorsareoftencostly,difficultandtimeconsumingtoremoveandlimitthegeometricfreedomof thisAdditiveManufacturing(AM)process.AnovelmethodknownasAnchorlessSelectiveLaserMelting (ASLM)maintainsprocessedmaterialwithinastressrelievedstatethroughoutthedurationofabuild.As aresultmetalcomponentsformedusingASLMdonotrequiresupportstructuresoranchors.ASLMlocally meltstwoormorepowderedmaterialsthatalloyundertheactionofthelaserandcanformintovarious combinationsofeutectic/hypo/hypereutecticalloyswithanewlowersolidificationtemperature.This newalloyismaintainedinasemi-solidorstressreducedstatethroughoutthebuildwiththeassistance ofelevatedpowderbedpre-heating.InthispapertheASLMmethodologyisdetailedandinvestigations intoprocessingofalowtemperatureeutecticAl-Sibinarycastingalloyisexplored.TwotypesofAl powderswerecompared;pre-alloyedAlSi12andelementalmixAl+12wt%Si.Thestudyestablishedan understandingofthelaserin-situalloyingprocessandconfirmedsuccessfulalloyformationwithinthe process.Differentialthermalanalysis,microscopyandX-Raydiffractionwereusedtofurtherunderstand thenatureofalloyingwithintheprocess.ResidualstressreductionwasobservedwithinASLMprocessed elementalAl+Si12andgeometriesproducedwithouttherequirementforanchors.
©2015TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).
1. Introduction
Technologiessuchas SelectiveLaserMelting(SLM),Electron BeamMelting(EBM),andDirectMetalDeposition(DMD)etc.utilise metalpowdersasfeedstockmaterialtoproduceparts.SLMmetal powderssuchasstainlesssteel,aluminium,titanium,nickelbased alloysetc.arematerialsofinterestwithinaerospace,medicaland automotiveindustriesetc.[1–4].Inrecentyearstherehasbeena notableindustryuptakeofSLMandEBMtechnologiesto manufac-tureenduseparts.Thisismainlyaresultoftheprocesses’geometric freedomthatisaffordedtodesignerswhenmanufacturingfully densecomponentsfromavarietyofalloys.Howevertheclaimthat SLMorEBMoffers“unlimited”designfreedomisuntruedueto therequirementforsupports/anchorsthatpreventcertain geome-triesfromgeometricallydistortingasaresultofthermalresidual stress.Furthertothisanchorsareoftencostly,difficultandtime
∗Correspondingauthor.Tel.:+4401142227789.
E-mailaddress:k.mumtaz@sheffield.ac.uk(K.Mumtaz).
consumingtoremove.Becauseofthelimitationsanchorsexertover theprocess,todayeffortstolimitthenumberofsupports/anchors andminimiseresidualstressremainsamajorresearchpriority.
2. Stressdevelopmentandrequirementfor anchors/supportsduringSLM
DuringSLMprocessing,highheatintensitiesaregeneratedby thelasersource,thisis requiredtoensurecompletemelting of metalpowderparticlesandminimisepartporosity[5,6].However therapidheating/meltingofmaterialisfollowedbyarapid solid-ificationthatinducesthermal variationsthatcauseareasofthe scanned/processedlayertoexpand/contractatdifferentrates, sub-sequentlygeneratingresidualstresswhichcancauseacomponent togeometricallydistort/warp[5,7,8].Anchorsaremetallurgically fusedtothesubstrateplateandvariouslocationsacrossthelaser meltedcomponent,forciblyholdinggeometriesinplace.Anchors are made from the same material as the SLM component and are alsoformed through the layerby layer melting of powder fromthepowderbed.Anexampleofwarpageand therequired
http://dx.doi.org/10.1016/j.addma.2015.06.003
Fig.1. OverhanginggeometriesmostpronetowarpageduringSLM.
anchoringstructure can beseen inFig. 1.Typically large over-hanging/unsupportedgeometriesbuiltparalleltothepowderbed requirethemostsupport/anchoring[9].
Anexampleofanun-anchoredzincSLMcomponentcanbeseen inFig.2(a),thezincpartcurlsupduetoitbeingun-anchoredto thesubstrate,thistypeofwarpagecanbecontrolledwiththeuse ofanchors.Fig.2(b)showsasteel(316L)SLMcomponentmade withandwithoutanchors.Thenumberandlocationofanchorsare dependentonthepartgeometryandbuilddirection/orientation
[10].Howevertheinclusionofanchorsdoesnotguaranteea com-ponentwillbewarpfree,largestresspronegeometriescanrip anchorsfromthesubstrateduringabuildcausingittofail.Once abuildiscompletethetaskofremovinganchorsaddstimeand costtothebuild.Thisisparticularlythecasewithcobaltchrome dentalcopings,crownsandbridgesfabricatedusingSLM.These componentsrequiremultiplesupportstructuresthatlaterresultin time-consumingsurfacefinishingoperationsinordertopreserve thedentalsurfaceintegrity.Inmanycasestheanchorscannotbe accessedorremoved,asaresultthegeometricfreedomofthe pro-cessiscompromisedasaccessisrequiredtoeventuallyremove theseanchors.Evenafteranchorremovalpartscanstillwarpdue totheremainingstress,thiscanbepreventedbyrelievingthestress throughfurnaceheatingcyclespriortoanchorremoval[11].
The polymer AM process Laser Sintering (LS) uses lasersto process polymers from a powder bed. LS uses the same layer bylayerfabricationmethodasSLM toproducepartsbutisable to do so without therequirement for anchors. LS uses special ‘supercooling’polymersandcarefulprocessingtemperature con-troltoenableanchorlessLSpartstobemade[12].LSmaterials nylon11and12(supercoolingpolymers)havere-freezing temper-atures(138–143◦C)thatarelowerthantheirmeltingtemperature (185–189◦C)[13].DuringLSpowderbedpre-heattemperatures aremaintainedabovethematerial’ssolidificationtemperaturebut belowitsmeltingtemperature[14],thelaserscansregionsofthe powderbedcausingthepolymertomelt,morepreheatedpowder isdepositedandprocesseduntilthepartiscomplete.Theprocessed
materialisnotgivenanopportunitytorapidlysolidifybutisinstead heldinasemi-solid/stressreducedstatepreventingpartwarpage. Materialsthatarenotlaserprocessedremainsolid,whereasthe bedtemperatureensuresthatthelasermeltedpolymerremains partiallyliquidthroughoutthebuildanddoesnottransitionback intoasolid.Afterbuilding,thepartisallowedtocooloverseveral hoursandcompletelysolidify.Eliminatingrapidmaterial solidifi-cationduringabuildreducesacomponent’stendencytowarpand eliminatestherequirementforanchorsleadingtogreater geomet-ricfreedomandreducedpost-processingoperations.Furthermore, LSpartsdonotneedtobephysicallyattachedtoasubstrateand thereforepartscanbestackedontopofeachother,increasingbuild volumewithintheprocess.
2.1. Eutecticalloysolidificationcharacteristics
The super coolingbehaviour of nylon 11 and 12 during LS processing allows parts to remain in a semi-solid state,stress reducedandremovestherequirementsforanchors.However,there arenoknown‘supercoolingmetals’inexistencetodaythathavea melttemperaturethatissignificantlyhigherthanthere-freezing temperature.Howevertherearecombinationsofmetalsthatwhen combinedinspecificproportions,formanalloythathasalower solidification/freezingtemperaturethanoneormoreofthe individ-ualmaterialspriortoalloying.Thesealloysareknownas‘eutectic alloysystems’withthelowestpossiblemeltingpointformingthe eutecticpoint[15].
Duetothelow solidificationtemperatureofeutectic compo-sitionstheyareextensivelyusedassolderingandcastingalloys. Useofeutecticcompositionsminimiseenergyusageandalloy seg-regationduring thecastingprocess[16]. Eutecticsystems form whenalloyingadditionsformaloweringoftheliquiduslinesfrom bothmeltingpointsofthepureelements.Ataspecific composi-tionthereisaminimummeltingpoint.Asimplebinaryeutectic systemistypifiedbythemetallicalloyofbismuthandzinc.Pure elementalbismuthexhibitsanequilibriumfreezingpointof270◦C
Fig.3.BinaryphasediagramcontainingmaterialAandB.
andpureelementalzincexhibitsanequilibriumfreezingpointof 420◦C.Whenbismuthandzincarefullyalloyedinproportionsof 97wt%and 3wt%respectivelya meltpointof 254◦C isformed, thisis16◦Clowerthanthemeltingpointofbismuth[17].A sim-plebinaryeutecticphasediagramisshowninFig.3.Alpha,beta, solidandliquidphasesareshownwithrespecttovaryingmaterial compositionsand temperature,TE representstheeutectic
melt-ingpoint.Eutecticmaterialproportionscanvaryfromtheexact eutecticpointcreatinghypoorhypereutectics(alloyscontaining aeutecticsystem) withvariablesolidificationtemperaturesand materialproperties.Therangeofcompositionsthathavethe poten-tialtoformeutecticsisbroad,rangingfromaluminiumalloysto highertemperaturenickels.
2.2. ProcessingusingAnchorlessSelectiveLaserMelting
Removingoralleviatingstressbuildupandtherequirement foranchorswithinSLMcanbeachievedbypreventingpartsfrom completelysolidifyingduringprocessingormaintaininginastress reduced state. ASLMhas beendeveloped toprevent processed metalfromcompletelysolidifyingduringanSLMbuild[18].Thisis achievedbyformingaeutecticalloyoreutecticsystem(hyper/hypo eutectic)fromtwoormoreun-alloyedmaterialsandmaintaining
Itstillmayalsobepossibletopre-heatthepowderbedto tem-peraturesbelowtheeutecticmelt pointsothatstressesarenot developedoraresufficientlyrelieved.Stressescanbesufficiently relaxedifthebedtemperatureallowsdiffusionalrelaxationofthe material[19].Dependantonthematerial,theserelaxationkinetics areinitiated between40 and60%of thesolidification tempera-tureofthematerialandabove,thisisalsotimedependant.When thelaserscansregions ofthepowderbed theindividualA and Bpowderswillmelt and forma eutectic alloyin-situ thatwill nowonlysolidifyattemperaturesbelowtheeutecticsolidification point.Becausethebedtemperatureissetneartheeutecticpoint themelted/alloyedregionswillnotrapidlysolidifyorifwithinthe diffusionaltemperaturerangewillgeneratelessstressthanthose formedduringconventionalSLM.Othereutecticcompositionssuch asAl66Mgofferlargeprocessingwindowsof212◦C(temperature differencebetweeneutecticmeltpointandlowestmeltingpoint ofindividualun-alloyedmaterial).Alargeprocessingwindowmay beadvantageousasthebedtemperaturecontrolwouldnotneed toberegulatedaspreciselycomparedtothatofasmallprocessing window.Furthertothisa largeprocessingwindowmayreduce unwantedsolidstatesinteringofunprocessedpowdersdueto pre-heattemperaturebeingfarlowerthanthemelttemperatureof theun-alloyedmaterial.Thissolidstatesinteringor“caking”of materialcausesmaterialdepositionissuesduetoagglomeration.
2.3. Aluminiumandinsitualloying
Thereisawideinterestinprocessingaluminiumalloysusing SLMtoproducecomplexpartswithimproveddesign.Aluminium
isofparticularinterestbecauseithasexcellentstrengthtoweight ratios,heattransferpropertiesandcorrosionresistantwith sev-eralotherbenefits[20,21].Amongawide varietyofaluminium alloys,thereisaninterestinprocessingafewspecificaluminium alloysandthereforestudieshavefocusedonimprovingthe abil-itytoprocesscommercialaluminiumalloy(inpre-alloyedform). AluminiumalloyssuchasAlSi12, AlSi10Mg,Al6061and others havebeenprocessedusingSLM[20,22].Recentfocushasbeenon processingAl–Sicastingalloysduetothelowerdifferencebetween meltingandsolidificationandfavourablesubstratewetting char-acteristicscomparedtowroughtaluminiumalloyssuchasAl6061 atroomtemperature[21,23].
Toimproveprocessingofaluminiumanditsalloysseveral meth-odshavebeenappliedoftenbywayofhardwaremodificationor upgrade.Brandletal.[22]usedAlSi10Mgpowderwithaheated bedtobuildparts.Inthestudydifferenttypesofbuildplatforms wereusedsuchaswithoutandwithoutpreheatingupto300◦C toanalyse residualstresses and fatigueresistanceusing exper-imentaland computeraidedsimulation techniques.Theheated platformaidedinreducingresidualstressesinpartsbuilt result-inginhighfatigueresistance.Inadditionthepeakhardeningheat treatmentinfluencedthefatigueresistanceofthesample consider-ably.Aluminiumhasbeenwellknownforitshighreflectivityand highthermalconductivitypropertiesandthesehavebeenexplored invarious applications.Therefore thesepropertiesresultin rel-ativelylowerabsorptionoflaserenergyduringtheprocessthus makingitoneofthedifficultmaterialstoprocesswithSLM.This leadstoimperfectionsinpartssuchaslowdensityduetolackof completemelting,tracesofun-melted powdersinparts,balling duetolackofsufficientenergy.Thereforetoovercomesomeof theseissueslaserscanningspeedcanbereducedforlowpower devicesorhighpowerlaserscanbeutilised.Inaddition,aluminium developsanoxidecoatingwhenincontactwithoxygeninthe atmo-sphere.Toimprovemeltingandbreakdownofoxides,oftenhigh powerlasersareusedorlaserspeedsarereducedforlowpower lasersthusaffectingbuildrates(approx.4mm3/sfora100Wlaser).
Buchbinderetal.[1]useda highpowerlasersourceof1kWto producepartswithcloseto100%partdensityandwithimproved buildratewithAlSi10Mg.Densitiesofapprox.99.5%wereachieved athigherbuildrateof21mm3/swith1kWlaserthusachieving
faster builds.Similar densitywas alsoachievedby 500W laser sourceatscanspeedof1200mm/sresultinginlowerrateofbuild speed of9mm3/s comparedtolater. InSLM thecoolingrateis
adifficultprocessing parametertocontrolduetothenatureof theprocessingarrangementhoweverthecoolingrateplaysakey roleinthemicrostructureformation.Thecoolingrateofapprox. 7×106K/sisoftenconsideredtobeachievedinSLMprocessing
[24].
TraditionallySLMfeedstockmaterialswerepre-alloyed pow-ders such as AlSi12, AlSi10Mg, Ti6Al4V etc. Often the powder compositionofanalloywasadaptedfromwell-established con-ventional manufacturing processes such as casting, welding, forging,powdermetallurgyetc.Thesematerialsperformedwellin partsmanufacturedbyconventionalprocesses.OccasionallySLM processingofsuchmaterialscouldbedifficult.Themicrostructures obtainedbySLMarecharacterisedbyrapidsolidificationandlarge thermalgradientsleadingtoformationofintermetallicphaseswith combinationof largeandfinegrainssizes; thisis generallynot thecase withconventional processing. Therefore differentpost processing techniques suchas Hot Isostatic Pressing(HIP) and heattreatmentsarenormallyappliedtoimprovedensity, micro-structuresand/orprecipitates.Thereforetherehasbeenaneedfor customfeedstockmaterialthatwouldhavesuitablecharacteristics forSLM. Priortothis investigation,in situmethodsof develop-ingalloyshavebeenstudiedusingSLM.Astudyundertakenby Bartkowiaketal.[20]utilisedcustomAl–CuandAl–Znpowders.
Theauthors’blendedelementalpowdersandproduceddifferent compositionsforin-situprocessingusingSLM.Theresultsfound thattherewerefinemicrostructureswithhomogenouslydissolved intermetallicphasesina metalmatrix. Similarin-situworkhas beenundertakentodevelopMetalMatrixComposites(MMC)using theSLMprocesses[25–27].Thisworkreportedformationofin-situ MMCwithimprovedmechanicalproperties.HoweverGuetal.[27]
alsoreportedexcessiveenergy,leadingtothedisappearanceofTiC phaseswithinthematerial.Processingblendedpowdersfor selec-tiveinsitualloyingcanallowuserstodeveloptheirowncustom powdersfromreadilyavailableelementalcompositions.
3. Materialsandexperimentalsetup
AlSi12isaeutecticalloythatsolidifiesat577◦C,itsindividual constituentsAlandSimeltat660◦Cand1414◦Crespectively,a modifiedRenishawSLM125wasusedtoconductstandardSLM andASLMprocessingofmaterials.Aseriesofcubeswereproduced andanalysedtotestin-situalloying,followedbythermal, chem-icalandopticalanalysis.Anumberofun-supportedoverhanging flat‘T’shapedgeometrieswerecreatedandobservedforgeometric distortion(warpage).
3.1. Materialproperties
Twoseparate batches of metallic powders/mixes wereused within this investigation. First, aluminium–silicon pre-alloyed eutecticpowder(AlSi12)andsecond,anelementalblendof com-merciallypurealuminium(Al)andsilicon(Si)powderswereused. The AlSi12andAl powdersweregas atomisedwithirregularly shapedmorphologyasseeninFig.5(a)and(b).Theparticlesize analysisofpowderswasundertakenusingaCoulter160powder particlesizeanalyser;particlesizedistributionwasshowntobe 20–90m.TheSipowderwassourcedfromFischerScientificand hadawideparticlesizerangewithirregularmorphologydueto millingduringmanufacture.ThepowderhadD10,D50,D90 val-uesof1m,10mand64mrespectivelyandabulkdensityof 2.3g/cm3.Aspecificpowderparticlesizedistributionwasobtained
bysieving. PostsievingtheSiparticlesizedistributionwassub 100m.ThepureAlpowderhadD10,D50,D90valuesof20m, 38mand66mrespectivelyandabulkdensityof2.7g/cm3.The
AlSi12alloypowderhadD10,D50,D90valuesof18m,33mand 59mrespectively.AlandSielementalpowdermixeswerecreated byfirstmeasuringtheweightofindividualmaterialsintheircorrect eutecticproportions.Theywerethencombinedandplacedwithin amixingdrumwithceramicballstobreakupagglomeration.This mixturewasthenrevolvedwithinaplanetarymixerat1000rpm for6minwithregularintervalsat2mintostirthepowder manu-ally.Thisassistedinbreakingupofpowderlumpsthatmayhave formedduringmixing.Afterthefinal2minofthemixingcyclethe powderwasthendirectlytransportedandplacedwithintheSLM materialhopperforprocessing.
3.2. SLM/ASLMsetup
Fig.5.SEMimagesforpowdermaterial;(a)AlSi12and(b)Al+Si12.
thelaserspotwas35mandpowderlayerthicknesswas50m. TheSLMchamberwasmodifiedtoallowsubstratepre-heatingup to380◦C.
3.3. Thermalanalysis
ADifferentialThermalAnalysis(DTA)wasperformedon unpro-cessed powders and samples produced using pre-alloyed and elementalmixesofpowdertoconfirmalloyingandphasetransition temperatures.APerkinElmerDTA7equipmentwasusedforthis analysis.Asmallweighedsamplewasplacedinanaluminacrucible andacontrolledheatcyclewasapplied.Thematerialtemperature wasrampedupto800◦Cat25◦C/min.Thesamplewasheldfor 15minatelevatedtemperatureandramped downat25◦C/min untilitreachedroomtemperature.Thecompleteheatcyclewas performedundercontrolledargonatmosphereforallsamples.This analysisshowedmeltingandsolidificationofsamplesintheform endothermicandexothermicpeaksplottedinatemperaturevs. dT/Tplot.
3.3.1. Microstructuralandphasecompositionanalysis
Sampleswerecross-sectionedand mountedinthex–zplane i.e.perpendiculartobuilddirectioninordertoanalysetheformed meltpools.ASTMstandard E407-07wasadaptedtoattain opti-mumresults.ANikonlightopticalmicroscopewasusedtoanalyse themeltpoolmicrostructureandmelttrackcharacteristics. Micro-scopic analysis was also used to observe traces of un-melted material.PhasecompositionanalysiswasperformedusingSiemens D-5000X-RayDiffraction(XRD)equipmentwithCuK␣radiation (=1.5418 ˚A)at40kVand30mA,usingacontinuousscanmodeat 1◦/min.Theobtainedpeakswereanalysedusingadatabasefrom theInternationalCentreforDiffractionData(PDF-4+2013).
3.3.2. Warpmeasurementandresidualstressreduction
Anoptical–analyticalmethodwasused tomeasure geomet-ricdistortion/warpage.Severalimagesweretakenofunanchored overhangingsamplesusinganopticalmicroscope.Theseimages werethenanalysed using imageprocessingsoftware knownas ImageJ.Thedataobtainedwasthenplottedandawarpprofilewas studied.Fig.6showshowwarpheight/distancewasmeasuredon thelengthofanunsupportedoverhanginggeometry.
4. Resultsanddiscussion
4.1. Thermalanalysis
Figs.7–10showthethermalanalysisplotsforpowderandlaser meltedsamplesproducedfrompre-alloyedandanelementalmix
Fig.6.Warpmeasurementofun-anchoredoverhanginggeometry.
Fig.7. AlSi12powderdirectlytestedusingDTA.
Fig.9. Al+Si12powderdirectlytestedusingDTA(solidlineindicatesfirstheatcycle, dashedindicatessecondheatcycle).
oftheAl–Sialloysystem.AsseeninFigs.7and8,forpre-alloyed materialasinglemeltingandsolidificationpeakwereobtainedfor powderandSLMprocessedsamples.Theplotindicatedtheonset temperatureformeltingandcoolingpeakswereapproximatelyat 570◦C(±3◦C) witha heattransferrateof25◦C/min.Thelower valueoftheeutecticonsettemperatureforsolidificationcouldbe onaccountofminorundercoolingorDTAsystemlag.Theoverlapof onsettemperatureofpeaksisacharacteristicbehaviourofmetals; astheinternalenergyreducestheliquidphasebeginstotransform tosolidphaseatthesametemperaturei.e.withoutsignificantsuper cooling.
Fig.9showsthermalcyclesappliedtoanAl+Si12powdermixed sample.Twothermalcycleswereappliedtothesample;firstto alloyAlandSi powderandsecondtoverifysuccessofalloying. Duringthefirstthermalcycle(solidlineinFig.9)thesolidto liq-uidphasetransformationbeganat640◦C.Asingleexothermicpeak wasobservedthatsuggestedsilicondissolvedintoliquidphase alu-miniuminthecrucible.Oncooling,theonsettemperatureofthe solidifiedmaterialwasobservedat570◦C.Theonsetsolidification temperatureindicatedinsitualloyingofaluminiumandsiliconat neareutectic proportions.On applicationofthesecondthermal cycle(dashedlineinFig.9),theonsettemperaturesofexothermic andendothermicpeakswerefoundtooverlapat570◦C;thus con-firmingalloyingofAlandSimaterialasaresultofthefirstthermal cycle.Howeversecondaryexothermicandendothermicpeakswere observedsuggestingincompletealloyingforaportionofthesample withinthecrucible.Thiscouldbeduetolackofstirring mecha-nismsinamoltenstatewithinthecruciblethusresultinginasmall volumeofsiliconrichalloyandthereforeformingahyper-eutectic
Fig.10. ASLMprocessedAl+Si12sampletestedinDTA.
Fig.11.AlSi12pre-alloyedmicrostructure–SLMprocessed.
alloythathadanonsetmeltingtemperatureof650◦Cand solidifica-tiontemperatureof600◦C.AsshowninFig.10theASLMprocessed Al+Sisamplesshowasimilarplotasthepre-alloyedsamples;the endothermicandexothermicpeaksoverlappedat560◦C,thus con-firmingasuccessfulinsitualloyingofelementsduringtheASLM process.
4.2. Alloymicrostructure
Fig.11showsthemicrostructureobtainedbyprocessingAlSi12 pre-alloyedpowder.Amicrostructureintheformofa fine alu-miniumsolidsolutionwitheutecticstructurewasobserved.This concurredwithatypicalrapidlycooledAl–Sisystem[1].The inter-actiondurationbetweenthelaserexposureandpowderparticles forascanspeedof261mm/s,was120s.Thustheresultingfine microstructurewasobtained.
Fig.12showsthemicrostructureobservedinaninsitualloyed Al+Si12powder processedusingASLM.Themelt track consoli-dationshowedcompletemeltingofpowders,thussuggestingthe alloyingof Al–Si powders.The white structures(dendrites)are aluminiumsolidsolutionandthedarkerregionsareAl–Si eutec-tics.Marangoniconvectioninthemeltpoolswouldhaveenhanced materialmixing andthusalloyingwithaneartohomogeneous spreadofeutecticstructuresinthesolidifiedmeltpool.Asaresult ofthisasingleendothermicpeakoftheSLMprocessedsamplewas observedinFig.10.Inaddition,thedirectionalgrowthofdendrites suggestedathermalgradientbetweenthetopandbottomofthe meltpool.Also,thedendritesobservedinafewareaswerecoarse,
Fig.13.XRDpatternsofaluminium–siliconsystem(Al–Si).(a)AlSi12PApowder,(b)Al+Si12EMpowder,(c)SLM-AlSi12PAand(d)ASLM-Al+Si12EM.
suggestingslowercoolingwithinthemeltpool.Howeverthe eutec-ticstructureseemstobereasonablyuniform,suggestingsuccessful insitualloyingofAl–Sipowders.
4.3. Phasecompositionanalysis
For further determination of phases present, XRD was employed.Fig.13showsanXRDplotforPre-Alloyedpowder(PA), ElementalMixedpowder(EM)and SLMprocessedsamples.The
␣-Alandtracesof␣-siliconcanbeobservedclearlywithpowder samples.PriortoSLMprocessing,AlandSipeakscanbeclearly observed.Thedecreaseinintensityofaluminiumandsiliconpeaks suggesteddissolvingofprimarysiliconintothealuminiummelt poolandformingaeutecticphase.Thereforetheanalysisfurther confirmedin-situalloyingofprimaryaluminiumandprimary sil-iconwhenprocessedunderalaser.ItshouldbenotedthatAlSi12 doesnotformanintermetallicandhencenonewidentifiablephase. Itisacceptedthatelementalpowdermixeswillhaveless homo-geneityofdissolvedSiascomparedtopre-alloyedpowders,asa resultthoroughand consistentpowderpreparation (i.e.mixing) hadbeenemployedpriortoSLMprocessing.
4.4. Anchorlesscomponents–stressreductionusinganchorless selectivelasermelting
SamplesproducedusingSLM(pre-alloyedpowder)andASLM (elementalpowders)atroomtemperatureand100◦Cpowderbed pre-heatingdisplayedsimilarun-anchoredoverhangcapabilities. Boththeseprocesseswereonlyabletoproduceanun-warped over-hangifthelengthoftheoverhangremainedunder2mm,however asthelengthoftheoverhangincreasedabove2mmtheoverhangs wouldwarptosuchalevel thattheSLM buildswouldfail due tothematerialdepositionmechanismcollidingwiththewarped portionofthepart(asshowninFig.14).Aspowderbedpre-heat temperaturewasincreasedto380◦CtheSLMprocesswasunable tosuccessfullydeposit pre-alloyedAlSi12forprocessing dueto materialagglomeration.TheASLMprocesshoweverwasableto depositelementalAl+Si12mixesatelevatedtemperaturewithout agglomerationduetothehighermeltingtemperaturesofeachof theseindividualelements(comparedtopre-alloyedAlSi12).Fig.15
displaysthemeasurementsofwarpheightforASLMsamples pro-ducedat380◦Cpre-heating.Thesamplesdisplayedminorsignsof warpagebutnottotheextentobservedinSLM,nordiditcause
Fig.14.ASLMoverhangcomponentbuiltat100◦C.
theASLMbuildtofailduetocollisionswiththewiperarm.The maximumwarpheightof1mmwasmeasuredwhenanoverhang lengthof5mmwasused.Itisenvisagedthatthiswarpheightcanbe reducedwithfurthertuningofpowderbedpre-heattemperatures. InanattempttofurthertestthecapabilitiesofASLMprocessing, a 10mm unsupported overhang wasproduced using an Al+Si blend.AsseeninFig.16,thissampledisplayedminimalsignsof warping(1mmwarpage).Itsundersideshowedsignsofsatellite formationduetobeinganunsupportedlayer,furtherparameter
Fig.16. UnsupportedoverhangcomponentsproducedusingtheASLMprocessfrom elementalAl+Si12powder.
optimisationmayfurtherreduceitsprevalence.Fig.16alsoshows anunsupportedspringlikegeometryfabricatedusingASLM,this geometrypossessedanunsupportedhorizontaloverhangof20mm (a×10greateroverhangcapabilitythanSLM’s2mmoverhang). Aswouldbeexpectedwiththis springlikegeometry,itcanbe compressedandwillafterreturntoitsoriginalgeometry.These componentsdemonstratethatASLMiscapableofproducing com-ponents withminimal warpage as a result of reduced residual stressfromin-situalloyedmaterialsandelevatedpre-heating.The processedmaterialmayhaveremainedinasemi-solidstate(above 577◦C)for a prolongedperiod duringthebuild(as opposedto rapidsolidificationwithinconventionalSLM)asthelaserenergy wouldhaveinputtedheatintothesurroundingpowderbedwith thesubstratepre-heatingactingasaheatlossreducer.The ele-vatedpre-heatingofthepowderbedwouldhavereducedthermal gradientandthusreducedresidualstressformation.Alternatively, evenwiththeheatinputfromthelasertheprocessedmaterialmay havecompletelysolidifiedduringthebuildduetothepre-heatbed temperaturebeing197◦Cbelowtheeutecticsolidification temper-atureofthenewlyformedalloy.Ifthisisthecase,itsuggeststhatthe pre-heatingtemperaturewasinthediffusionrangeofthematerial facilitatingarelaxingofstresses.
5. Conclusion
This study presented preliminary work investigating in-situ alloyingofanAl–SieutecticalloysystemusingSLMandASLM.The resultsobtaineddemonstratedsuccessfulalloyingofaluminium andsilicon.Oncomparisonbetweenthemicrostructureofin-situ alloyedmaterialandprealloyedmaterial;theprimaryaluminium in form of dendrites and silicon in form of eutectic structure wereobserved.TheDTAanalysisandphasecompositionanalysis complementedthesefindingsandincreasedconfidenceinthe in-situalloyingapproach.Utilisationofin-situalloyingmethodfor developingnewmaterialscouldbepotentiallycosteffectiveand encouragefasterdevelopmentofmaterialsforSLMthatwouldyield thesamepropertiesofconventionalmaterials.
Evenwhen elevated pre-heating wasapplied topre-alloyed AlSi12, SLM wasunabletobuildoverhangs greaterthan 2mm. Thiswasduetosolidstatesinteringofloosepowderand subse-quentagglomerationduringdeposition.HoweverduringASLMthe heatinputfromthelasercombinedwiththesubstratepre-heating mayhaveallowedtheprocessedmaterialtobemaintainedina semi-solidstateforanextendedperiodoftimewithinthebuild thusreducingresidualstressesfromdeveloping.Alternatively,the processedmaterialmaynothavebeenheldinasemi-solidstate foraprolongedperiodwithinthebuild,thiscouldbearesultof thepre-heatingtemperaturebeingheld197◦Cbelowtheeutectic solidificationpoint.Ifthiswasthecase,itwouldindicatethata prolongedsemi-solidstatemaynotberequiredduringabuildto reducestressandthatrelaxationasaresultofthematerialbeing heldwithinitsdiffusionaltemperaturerangemaybesufficient.The overhangingtestsamplesproduceddemonstratedthattheASLM processiscapableofreducing stresswithincomponentsduring
abuild. ComparedtoconventionalSLM,ASLMhasthepotential toreducethenumberofsupportsrequiredtomanufacturemetal components,builtfrommaterialsthatformeutecticsystems.
Acknowledgements
TheauthorswouldliketothankEPSRCfortheirsupportduring thisresearch(grantnumberEP/I028331/1).
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