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AlSi12 In-Situ Alloy Formation and Residual Stress Reduction using Anchorless Selective Laser Melting

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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.

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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

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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

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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

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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 20–90␮m.TheSipowderwassourcedfromFischerScientificand hadawideparticlesizerangewithirregularmorphologydueto millingduringmanufacture.ThepowderhadD10,D50,D90 val-uesof1␮m,10␮mand64␮mrespectivelyandabulkdensityof 2.3g/cm3.Aspecificpowderparticlesizedistributionwasobtained

bysieving. PostsievingtheSiparticlesizedistributionwassub 100␮m.ThepureAlpowderhadD10,D50,D90valuesof20␮m, 38␮mand66␮mrespectivelyandabulkdensityof2.7g/cm3.The

AlSi12alloypowderhadD10,D50,D90valuesof18␮m,33␮mand 59␮mrespectively.AlandSielementalpowdermixeswerecreated byfirstmeasuringtheweightofindividualmaterialsintheircorrect eutecticproportions.Theywerethencombinedandplacedwithin amixingdrumwithceramicballstobreakupagglomeration.This mixturewasthenrevolvedwithinaplanetarymixerat1000rpm for6minwithregularintervalsat2mintostirthepowder manu-ally.Thisassistedinbreakingupofpowderlumpsthatmayhave formedduringmixing.Afterthefinal2minofthemixingcyclethe powderwasthendirectlytransportedandplacedwithintheSLM materialhopperforprocessing.

3.2. SLM/ASLMsetup

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Fig.5.SEMimagesforpowdermaterial;(a)AlSi12and(b)Al+Si12.

thelaserspotwas35␮mandpowderlayerthicknesswas50␮m. 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.

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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,was120␮s.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,

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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

(9)

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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