ADDITIVEMANUFACTURINGDESIGNCONSIDERATIONSFORPRODUCTION

INAEROSPACECONSIDERATIONSFORTHEFULLVALUESTREAM

CAITLINOSWALDAdditiveManufacturingSpecialist

LAIInternational

ADDITIVEMANUFACTURINGDESIGN:CONSIDERATIONSFORPRODUCTION

INTRO

Asadditivemanufacturingcontinuestomatureandbeginstotransferfromafadtoarobustproductionprocess,

complementarymethodsofdesigningforadditivemanufacturingneedtodevelopinparallel.Todaythemajorityof

designengineersweretrainedunderconventionaleducationmethodsofsubtractivemanufacturing.Switchingthat

paradigmwilltakemorethanadjustingafewrules,butanevolutionineducation,practiceandmindset.When

designersareurgedtobecreativeandinnovative,thedesignspaceboxwhichmeetsfit,form,andfunction

requirementsexpandsexponentiallywhenusingadditive.Thisnewideaofstartingfromnothinginsteadofstarting

fromablockofmaterial,whileoverwhelmingforsome,becomeslimitlessforstudentsandyoungdesignengineers

withoutdecadesofstrictrulesandguidelineshanginginthemidst.However,thefastpaceofadditivemanufacturing

adoptionrequiresthosewhopreviouslytrainedforconventionalmanufacturingtorapidlyretrainandrethinktheway

theydesigntomeetthevastnewoptionsaffordedbyadditive.It’simportanttorealizethatwhile“complexityisfree”

therearestillguidelinesandinstructionthatcanmakeyouradditivedesignasefficientaspossiblewhenconsidering

theparametersofcost,time,andquality.ThefollowingpaperfollowsPartAofatwo-partseriesfocusedonAdditive

ManufacturingDesign.Theserieswillcondensesomeoftheselimitlesspossibilities,andcapturetheadvancing

strategiesofadditivemanufacturingdesign.

THEOPTIMALADDITIVEDESIGNENGINEER

Fordecades,thethreeidealsofdesign,manufacturing,andcosthavelivedinsilos.Whileit’scleareveninthe

conventionaldesigncyclehowimportantitisforthesethreeskillsetstocollaborateandunderstandeachother,itis

DesignEngineer

ManufacturingEngineerCostAnalyst

magnifiedinanadditivemanufacturingenvironment.Whereconventionaldesignstakeweeksmaybemonthstobe

realizedandmanufactured,theadditivecycleisquick,sometimesovernight,anddesignfaultscanbeidentified

immediately.Withthesedesign,manufacturing,andcostiterationshappeningsoquickly,it’sreasonabletoseehow

thesethreeskillsetscanstarttolearnfromeachotherandmeldintooneoptimizedapproach.Byintegratingthe

impactonpostprocessingoperationsandcostconsiderationsupfront,combinedwiththedesignopportunitiesthat

additivemanufacturingprovideswithregardstocomplexity,theidealdesigncanbefoundrapidly,significantly

reducingthedevelopmenttimelineforproducts.

THEADDITIVEDESIGNPROCESS

Today,thedesignprocessforadditivemanufacturingoftenstartswithanexistingdesign,especiallywhenthegoalisto

removecostorweightfromacurrentproduct.Thepartselectionprocess,materialdecision,andadditivetechnology

(forexampleEBMvslaser)selectionscouldrepresentanentirediscussionandarticleonitsown,butwewillfast

forwardtowhathappensafterthosedecisionshavebeenmadeandit’stimetoopenaCADpackageandcreatethe

additivedesign.

Thedesigncycleisnotlinear.Thedecisionsmadeduringeachpieceofthedesignwillaffectandcontributetothe

others.Therearemanypossibleoutcomestothecycledependingonwhatparameterneedstobeoptimized.For

example,aone-offdevelopmentpiece,whereleadtimeisthemostimportantfactor,mayleadtoaflatandlow

orientationpartwhichreducesbuildheightanddoesnotconsiderthequantityofpartswhichcouldnesttogether.This

wouldbeinefficientinaproductionenvironmentbecausethenumberofbuildsneededtomeetproductionquantities

wouldequallyincreasewithlaborcostsforset-upandteardown,build-to-buildconsumablecosts,andmachinedown-

timebetweenbuilds.Thedesignthatyouusefortheone-offdevelopmentpiecewillbecompletelydifferentthanone

thatyoudesigntooptimizeforaproductionvolumewhichmaximizespartsperbuild,nestsmultiplelayersofparts,and

reducespostprocessingstepswithuniquefeatures.Investigatingeachofthesepuzzlepiecesseparatelyisdifficult

sincetheyarerelatedbycauseandeffect,howevertherearesomebasicguidelinesandconsiderationswhichshouldbe

drawnuponwheninthedesigncycle.

MATERIALPROPERTIES

Itisessentialtounderstandthatthematerialpropertiesofadditivemanufacturedmetalarenotequivalenttowrought

orcast.Additivelymanufacturedmetalshavetheirowndesignsystem,andshouldbetreatedassuch.Duetothe

temperaturegradientsduringmetaladditivemanufacturingprocesses,materialpropertiesandmicrostructurecanbe

anisotropicinnaturebeforeperformingpostthermaltreatment.Thermalprocessingcanalterthemicrostructureto

becomemoreisotropic,buttherestillmaybesomeorientationandgeometriceffectswhichremain.Theeffectsmay

beseenonmonotonicordynamicproperties.It’simportanttohaveanunderstandingofhowsignificanttheeffectsare

withinthematerial/processcombinationchosen.Iftherearedesignfeaturesthatwouldbelimitingifnotbuiltina

certaindirection,it’simportantforthedesignengineertointegratethatdesignruleintothebeginningstageoftheir

orientationdecisions.

POSTPROCESSING

Intheadditivemanufacturingprocess,therearetwotypesofCADmodelswhichexist.Oneisthefinalgeometrywhich

iswhatthefinalproducedpartwillbeinspectedto.Thedatumsareidentified,machinedtoleranceswillbecalledout,

surfacefinishrequirementswillbezoned,assemblynotes,etc.

ThesecondCADmodelwhichisequallyimportantistheas-builtmodel,orthemodelwhichisusedastheinputintothe

additivemachine,thiswilllookdifferentthanthefinishedmodels.Holesmaybefilledin,supportstructurewillbe

added,machiningstockwillbeadded,andmore.Tounderstandwhatconsiderationsshouldbeputintothe“as-built”

model,agenericprocessmapisprovided.

Therequiredpostprocessingstepswilllookslightlydifferentdependingonthematerialandtechnologyutilized.For

example,ElectronBeamMelting(EBM)doesnotrequirestressrelieformechanicalremovalofthepartsfromthebuild

plate,butdoesrequirepowderremovalofthepartiallysinteredpowderthatsurroundstheparts.LaserPowderBed

Fusion(LPBF)requiresstressreliefandeitherwireEDMorabandsawproceduretoremovethepartsfromthebuild

Build Buildremoval

ThermalProcessing

SurfaceFinishMethod

Machining Inspection&Testing

plate.ThermalprocessingforallmetaladditivemanufacturingoftenincludesaHotIsostaticPressing(HIP)processto

reducevoidsandporositywithinthematerialwithparametersbasedonmaterialchoice,andadditionallysome

materialsmayrequireaheattreattoaltertothedesiredmicrostructure.

BUILDANDPOWDERREMOVAL:

Asanexpansiontotheexampleabove,differentadditivetechnologiesofferadvantagesanddisadvantageswhenit

comestobuildandpowderremovalconsiderationswhendesigningapart.

ELECTRONBEAMMELTING(EBM)

TheconsiderationsforEBMwithregardstobuildplateremovalsurroundtwoaspects.First,itisidentifiedasabest

practicetonotbuilddirectlyontheplate,especiallyifthepartshavestrictchemistryrequirements.Therecouldbe

someinfluenceofthestainlesssteelplateforthefirstfewlayersofthebuild,soitisadvisedtostartpartsatleast3-

5mmfromthetopofthebuildplate.Those3-5mmcanbefilledbyeithersupportstructure,orsolidmaterialtobelater

cut-off,whicheverismoreefficientforthepart.OneadvantagefortheEBMprocessisthatduetolowresidualstress

fromtheentirebuildareaatelevatedtemperatures,lesssupportsarerequiredandthepartsusually“pop”offofthe

plateeasilywithslightpressureorwiththeuseofarubbermallet.

ThebiggestconsiderationduringpartremovalofanEBMbuildispowderremoval.Ononehandthepre-heatstepin

theEBMprocessprovidesbetterpartintegrity,butforpowderremovalitprovidesachallengethatmustalsobe

consideredwhendesigningyourpart.Duringthepre-heatstepintheEBMprocess,thepowdersurroundingthesolid

materialispartiallysinteredmakingitdifficultsometimestoremovefromlongcavities.Thispartiallysinteredpowder

doesnotflowfreely,however,itiscrucialthepowderisremovedbeforesendingthepartsthroughanythermal

processinglikehotisostaticpressing(HIP)orheattreatment.Designingsweepingradiiwhilemaximizinglineofsite

cavitiesintoapartcanmakepowderremovalmoresuccessful.

Figure1(a) Figure1(b) Figure1(c)

Figure1showslineofsightpowderremovalexamples:A)simpleradiusB)elongatedradiusC)straightcenterwithradii

onexitswheretheblacklinedepictshowfarthecompressedairinthepowderrecoverysystemwouldbeabletoreach

intothecavitywithlineofsight.Itisnotalwaysintuitiveastowhichgeometrywouldbebest.

Figure2showstheadvantageoflargeradiiinsteadofcornerpoints.Itismucheasiertoremovepowderfromasmooth

surfacethanonewithmanydiscontinuities.

LASERPOWDERBEDFUSION

Considerationsinlaserpowderbedwithregardstobuildremovalareslightlydifferent,butnolesscomplex.Inthelaser

systems,thereisabenefittothe“cool”buildprocessasthepowderdoesnotbecomesinteredandthusflowseasily.

Whenbuildingintricateinteriorcavitiesisrequired,theremovalprocessinlaserprovidesanadvantage.However,even

thoughthepowderflowsmoreeasilyoutofcavities,itcanstillbecomplexduetotherequirementofremovingthe

powderbeforestressreliefandbeforethepartscanberemovedfromtheplate.Therefore,iftheinternalcavitiesare

buriedwithinthesupportstructureorcannotbereachedbythevacuum,powderremovaloptionsmustbedesigned

intotheparts.Ifanorientationcannotbeconstructedtoallowforfullpowderremoval,itiscommontoaddholestothe

as-builtmodeltoeasethepowderremovalprocessofcavities,asseeninFigure3below.Theseholescanthenbe

pluggedlateronbyaweld.

Figure2(a) Figure2(b)

Inlasersystems,thepartsandtheplateareessentiallyweldedtogethermakingitabitmorecomplextoseparate,

addingtimetotheoverallprocess.PartsmustberemovedbymechanicalmethodssuchasWireEDMorabandsaw.

WireEDMisamoreaccurateprocess,sotheadditionalstockneededunderneaththepartsmayonlybe(3mm),

whereaswiththebandsaw,itisrecommendedtoaddatleast(5mm)ofsolidmaterialorsupportstructureunderneath.

THERMALPROCESSING

Thermalprocessinginmostcasesdoesnotdefineorchangemanydesigns,unlessthereissignificantdistortionduring

thethermalprocessduetodrasticallyvaryingthicknessescomparedtothepartlengthorheight.Thisisanotherarea

wherechoosingtherightadditivetechnologymayberequired.Largepartscanbedifficulttoproduceonlasersystems

duetothe“cool”processthatmaystrugglewithresidualstress.Stressreliefisrequiredonmostlaserbuilds,buteven

moresoonpartsthatspanthebuildarea.EBMmaybeabetterchoiceingeneralforlargepartsinboththehorizontal

andverticalorientationssincethestressreliefstepisnotrequired.Ifgeometricdistortionisstillanissueforapart,

thereareafewoptionswhichcanhelpthepartsholdtheirshapethroughthethermalprocessing.Forinstance,adding

asacrificialgussetorframearoundthinwalledcomponentstolaterbemachinedoffcanprovidestabilityduringthe

thermalprocessingsteps.

SURFACEFINISHMETHOD

Thesurfacefinishofmetalpowderbedfusiontechnologiesisrougherthanacasting,andalsodependsonthe

orientationofthesurface.Topsurfacescanbefairlysmooth.Verticalsurfacesnormallyhavearoughbutconsistent

surfacefinish,andsurfaceswhichareorientatedatangleslessthan90degreesfromthebuildplatecanbetheroughest.

Ingeneral,thelaserpowderbedsystemshavepowderandlaserparametersthatareoptimizedforasmoothersurface

finish.EBMhasfocusedonoptimizingforcostviabuildspeed,andthereforepartsarerougherwithafasterbuildtime

trade-off.Itisoptimalforcosttokeepthesesurfacefinishesas-built,butmanytimesstructuralandflowrequirements

orpartaestheticscandrivetheneedforvarioussurfacefinishmethodstobringdownthesurfacefinishmeasurements.

Thesurfacefinishmethodchoseninfluencestheimpactondesignchanges,buttheultimategoalistounderstandthe

materialremovalrateandcompensatefortheremovalintheas-builtmodel.Forexample,ifatumblingprocess

Figure3

removes0.005”fromthesurface,itwouldberecommendedtoadda0.005”enveloptotheas-builtCADfilesoafter

postprocessingthefeatureswillcomeintoconformingthicknesses.

Whenattemptingtocompensateforsurfacefinishmethods,designersmustadditionallyconsiderthephysicsofthe

processitself.Forexample,cornerswillhaveamoreaggressivematerialremovalrateinavibratorybowlthanaflat

surface,soprofiletolerancesmayhavetoopeninthoselocations,asseenbelowinFigure4.Foranycriticalfeaturesor

tolerances,differentmaskingtechniquesduringthesurfacefinishprocessmayaidinmaintainingtheas-builtsurfaceso

thematerialdoesnotdegrade,orcanbelatermachined.

Inadditiontoexternalsurfacefinishmethods,internalcavitiesmanytimesrequiresurfacefinishmethodstosmooth

outchannelsforflowrequirements.Similarly,thedesignersmustconsiderthematerialremovalratesofthosemethods

andcompensatefortheminthemodel.

MACHINING

Asdiscussedpreviously,mostpartsneedtobemachinedatsomelevelbeforebecomingafinalproduct.Critical

featuresandtighttoleranceswhichcan’tbeachievedbytheprintingprocessusuallymustbebroughtintoconformance

byconventionalCNCprocesses.ThereareafewdesignconsiderationsupfrontwhichmakestheCNCprocessmore

effective.

First,thedesignengineerandthemanufacturingengineershouldcollaboratetounderstandhowthepartwillbeheld

duringtheCNCprocessset-ups.UtilizingtheadvantagesoftheAdditiveManufacturingprocess,datumfeaturescanbe

addedtotheas-builtmodeltoreducecustomtooling,oratminimum,provideconsistenttoolingacrossapartfamilyof

similargeometries.Tabs,pins,holes,slots,orevenatemporaryhandlecanallbeaddedtotheprintedparttohelpalign

anyfixturesandtooling,andcanbelatercut-off.Havingthoseconversationsupfrontwillreducedevelopment

iterationsandspeedupthesetupandactualdevelopmenttime.

As-BuiltSurfaceTumbledSurface

DimensionalProfileAllowance

Figure4

Anotherdesignconsiderationwithregardstomachiningiswrapstock.Justaswithsurfacefinishmethods,therewillbe

somematerialremovalinordertobringcriticaltolerancesintoconformance.Addingwrapstocktothosefeatureswill

ensurethereisenoughmaterialtherefortheCNCprocesstoremove.Theamountofwrapstockaddedshouldbe

enoughtoreduceriskoftherenotbeingenoughmaterial,butlimitedsoastonotaddtoomuchtimeandcosttothe

CNCprocess.Thiswillvarydependingonthematerial,thegeometryoftheparts,aswellasthetechnologybeingused.

Figure5belowshowsthedifferencebetweenthe(a)finalpartgeometryand(b)theas-builtgeometrywiththeholes

filledin,andbottomstockaddedforpostmachining.

Insummary,theoptimaladditivemanufacturingdesignermustwearmultiplehats.Theyneedtohavean

understandingofthefullmanufacturingvaluestream,costanalysis,andevenstructuralandmaterialsengineering.

Onestepbetteristohaveexpertsavailabletoprovideinsightintothedesignprocessasit’sproceedingina

collaborativeatmosphere.Thedesigncyclewasdiscussed,wherethepiecesareallinterconnectedandincludecause

andeffectcorrelationsbetweenthem.Focusedfirstonmaterialpropertiesandpostprocessingtherearemanydesign

considerationswhicharedependentonmaterial,technology,andrequiredoperations.Inthenextseries,some

producibilityrulesandguidelineswillbereviewed,alongwithtopologyoptimization,andotherdesignforfunction

opportunitiesaswell.

FormoreinformationonDesignforAdditiveManufacturingandotherAdditiveManufacturingtrainingopportunities,

contactCaitlinOswaldatLAIInternational.coswald@laico.com

Figure5(a) Figure5(b)