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HybridAdditiveandSubtractiveMachineTools-

ResearchandIndustrialDevelopments

JosephM.Flynn1,AlborzShokrani1,StephenT.Newman1andVimalDhokia1*

*Email:v.dhokia@bath.ac.uk1DepartmentofMechanicalEngineering,UniversityofBath,BathBA27AY,UK

Abstract

Bysynergisticallycombiningadditiveandsubtractiveprocesseswithinasingleworkstation,therelativemeritsofeachprocessmaybeharnessed.This facilitatesthemanufactureof internal,overhangingandhighaspectratiofeatureswithdesirablegeometricaccuracyandsurfacecharacteristics.Theabilitytowork,measureandthen rework material enables the reincarnation and repair of damaged, high-value components. Thesetechniques present significant opportunities to improve material utilisation, part complexity and qualitymanagementinfunctionalparts.

The number of single platform workstations for hybrid additive and subtractive processes (WHASPs) isincreasing.Manyoftheseintegrateadditivedirectedenergydeposition(DED)withsubtractiveCNCmachiningwithinahighlymobilemulti-axismachinetool.Advancednumericalcontrol(NC),andcomputeraideddesign(CAD),manufacture(CAM)andinspection(CAI)softwarecapabilitieshelptogoverntheprocess.

This research reviews and critically discusses salient published literature relating to the development ofWorkstations for Hybrid Additive and Subtractive Processing (WHASPs), and identifies future avenues forresearchanddevelopment. Itreportsonstate-of-the-artWHASPsystems, identifyingkeytraitsandresearchgaps.Finally,a futurevision forWHASPsandotherhybridmachinetools ispresentedbaseduponemergingtrendsandfutureopportunitieswithinthisresearcharea

Keywords: Hybridmanufacturingprocesses;Machinetooldesign;Additivemanufacturing;Subtractivemanufacturing

1. IntroductionTheuseofadditivelymanufacturedmetalcomponentsintight-toleranceandcriticalapplicationsislimitedbytheattainableaccuracy,uniformityofmaterialsproperties,andsurfacequality.Prevailingqualityissuesinadditivemanufacturerelatetopartresolutionduetothesmallestbuilt-element,partdensity,partiallybondedparticulateandresidualstresses.Untilsuchatimeasastep-changeinbuild-materialorenergydeliverymethodsismade,itwillnotbepossibletoimproveparttoleranceswithoutasignificantincreaseincost-to-build-rateratio.Thismeansthatobtainingtheresolutionrequiredtoachieveconformingpart intighttoleranceapplications iscurrentlynotfeasible.Assuch,additivelymanufactured metal parts almost always require post-processing to improve part qualitycharacteristicsandrelieveresidualstresses.

Onepossiblesolutiontoovercometheselimitationsisto`hybridise’two,ormore,processestocreatea heightened capability. At the present time, workstations for hybrid additive and subtractiveprocessing (termed ‘WHASPS’ by the authors) are emerging on the machine tool market. Thesemachinescombineanadditivemanufacturingprocess,withaconventionalsubtractiveprocess,suchasCNCmachining.WHASPsarecreatingsignificantopportunitiesinthedesignandmanufactureoffinishedparts,andalso inthereincarnationandremanufactureofhigh-valuecomponents[1].Theabilitytobothaddandsubtractmaterialhelpstoaddressgeometricalchallenges,suchasinternaland

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overhanging features,andpartswithahigh ‘buy-to-fly’ ratio [2].Theseadvantageshelptoreducematerialwastage,andexcessiveconsumptionoftooling.

Therearealreadyreviewpapersinthefieldofhybridadditiveandsubtractivemanufacturing.Wanget al. [3] discuss the repair of parts via laser-based additive manufacturing processes. This dealspredominantly with welding-based processes and gives a general discussion on the necessarycomponents for an integrated system. Similarly, reviews have been undertaken relating to hybridmanufacturingprocesses[4],[5];however,thesedonotgointodetailaboutspecificconfigurations,themes and challenges in HASPs. Lorenz et al. [6] have recently published a review of hybridmanufacturing processes and machine tools that incorporate directed energy deposition (DED)processes.Thisreviewishighlyfocusedanddoesoffercoverageofalternativeadditivemanufacturingprocesses.Intermsofprocessplanningandmanufacturingstrategies,SimhambhatlaandKarunakaran[7]introducestrategiestomanufactureundercutandinternalgeometriesusingHASPs,andKulkarnietal. [8]have reviewedprocessplanning in layeredmanufacturing. In recenthistory thisareahasdrawnsignificantattentioninacademiaandindustry, includingseveralcommercialisedsystems.Assuch, this review aims to update and extend previous works, covering manufacturing processexploitation,machineconfigurationanddesignprinciples.Finally,futurechallengesandopportunitiesinWHASPsareidentified,concludingwithafuturevisionofthisarea.

2. Additivemanufacturingofmetalcomponentsanditslimitations

Thecurrentadditivemanufacturingprocesslandscapecompriseseightprocessfamilies,asdefinedbythe “Standard Terminology for AdditiveManufacturing Technologies,” which is part of the ASTMF2792-12A standard series [9] (see Figure 1). In addition to those detailed in this standard, `coldspraying’hasbeenadded,whichreferstoanadditiveprocessthatpropelspowderedmaterialatasubstrateatasufficientlyhighvelocitytocauseadhesionandmaterialbuild-up[10].Inmetaladditivemanufacturing (MAM), material extrusion, sheet lamination, powder-bed fusion, directed energydepositionandcoldsprayingareused[10];however,industryhaspredominantlyfocusedonpowderbed fusion and directed energy deposition [11]. In both of these processes, high-localisedtemperaturesareusedtoeitherfusepowderwithinabed,orcreateameltpoolintowhichpowderedmetalisdepositedonthebuildsurface.Bytheirverynature,thesehigh-localisedtemperaturescausemanyissuesinMAMparts.

2.1. Limitationsofadditivelymanufacturedmetalparts

Consideringdirectedenergydeposition(DED)andpowderbedfusion(PBF)processes,limitingfactorsinclude:partresolutionoraccuracyduetosmallestbuiltelement,unsatisfactorysurfacequality,pooruniformity in material properties, and mechanical properties e.g. residual stresses. These issuesnecessitatepost-processingtoachievethedesiredpartproperties.

Partresolutionislargelydefinedbythesmallestbuilt-element.InbothPBF[13]–[16]andDED[17]–[19]processes, theresolution isdeterminedbythemelt-poolgeometry,which isaffectedby laserpower,scanningvelocity,hatchspacingandlayerthickness.InDED,feedstockdeliveryalsodefinestheprocessresolution,asmaterialfeedrateandthespatialdistributionofthedepositedparticulatechangetheshapethedepositedtrack[20],[21]e.g.width,heightanddilution.InbothPBFandDEDprocesses,thethermalhistoryofthebuildcanaffectthemeltpoolgeometry,aspreviouslyheatedmaterialcanbere-meltedbyadjacentscans[22]–[26].Also,errorsthatcompoundoverthecourseof

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Figure1:ASTMF2792-12A[9]standardterminologyforadditivemanufacturingprocesses,withdescriptionquotedfrom

theWohlersReport2014[12].Coldsprayinghasbeenaddedusingthedescriptionof[10]

thebuildleadtoachangeinstandoffdistancebetweenfeedstockoutletandsubstrate[18].This,too,canleadtoachangeinmelt-poolgeometry.

TemperaturegradientsandthegeometryofthemeltpoolcaneachhavedetrimentaleffectsonMAMprocesses. Temperature gradients and associated surface tension can cause rapid hydrodynamicmotionsknownasMarangoniflow,resultinginthe`dishing’or`humping’ofthesolidifiedelement[27].Also,longthinmeltpoolscanresultthe‘balling’ofmaterial,whichdegradessurfaceroughnessandpartdensity [28]–[30].Otherprocessphenomenondegradethesurfacequality (roughness)ofMAMcomponents,whicharediscussed in [31]–[34].Oneof themost fundamentalof these is the‘staircaseeffect,’whichisaresultofthelayer-wiseapproximation(zerothorder)ofpartgeometries,affectingbothPBFandDED.Furthertothis,thepartialbondingofparticulateisacommoncauseofsurfacequalitydegradation.InPBF,thisoccursasaresultofconductiontosurroundingpowder.InDEDprocesses,propelledparticulatemaypassthroughtheheatsource,adheringtoany(hot)surfaces[21].

ThematerialpropertiesofMAMcomponentsarerelatedtothedensityofthebuiltmaterialandtheformationofanappropriatemicrostructureduringandafterDED[19],[35]–[38]andPBF[16],[39]–[46]processes.ForsomePBFprocesses(SelectiveLaserMelting-SLM), ithasbeenfoundthatthemicrostructureisdependentonlaserpower,scanningvelocity,hatchspacingandlayerthickness[16].Thisislargelyduetotheireffectsontherapidsolidificationofthemoltenmaterial.Scanningspeedaffectsgrain coarseness, grainalignmentandmaterialdensity,whereashatch spacingaffectspartdensity and grain orientation [16]. Another layer of complexity exists due the dependence ofmicrostructureonthematerialusedandpartgeometry[23].

ResidualstressinMAMpartsisoneofthegreatestconcernsinbothPBF[47]andDEDprocesses[22],[48].Localisedheatingandphasetransformationsinmaterialsinducestresseswithinthepart,whichcanexceedtheyieldstrengthofthematerialandcausepartdistortionorevenfracture.Researchhas

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begun relating residual stress to thermal gradients, subsequently reducing induced stresses viachangesinprocessingparameters,suchaslaserpower,scanningvelocityandpreheatingofthepart[24],[25].Itissomewhatunanimouslyagreedwithintheliteraturethatpost-processingtoalleviateresidualstressesisanessentialpartofanyMAMprocess.

2.2. Mechanicalfinishingofadditivelymanufacturedmetalparts

The finishing of additively manufactured metal components may be categorised into threemechanisms, namely: (i) machining and mechanical conversion e.g. machining, shot-peening andgrinding; (ii) thermal processes including laser and electron beam melting; (iii) chemical andelectrochemical processes, such as etching and electropolishing.Machining and othermechanicalsubtractiveprocesseshavebeenwidelyusedinnear-netshapingprocesses,suchasmoulding,castinganddie-casting.Thishasnowbeenextendedtoadditivemanufacturing,allowingfeaturegeometriestoberealisedwithgreateraccuracyandsurfacequalityviasubtractiveprocessing.Togivecontexttosurfacequalityexpectations,aerospaceapplicationshavereportedlyspecifiedsurfaceroughness0.8μm<Ra<1.6μm.

Spieringsetal.[49]usedCNCturningtofinishAMpartsbuiltinAISI316and15-5HPsteels,resultinginasurfaceroughnessRaof0.4μm.ItwasalsonotedthatfinishingofAMpartshadlimitedaffectonthefatiguestressat106cycles,butsignificanteffectat107cycles.Tamingeretal.[50]utilisedhigh-speedmilling (HSM) to finish aluminium AM parts. HSMwas found to produce highly favourablesurface roughness (8-56 μin RMS) and waviness (400 μin RMS); however, compared with othersubtractiveprocesses,HSSintroducedlargeresidualstressesinthefinishedsurface.

GrindinghasalsobeenusedtofinishMAMparts.WithAISI316Lsteel,Löberetal.[51]wereabletoreducetheas-builtsurfaceroughness(15μm)to0.34μm.Rossietal.[52]reportedthatonhorizontalsurfaces,thesurfaceroughness(Ra)wasreducedfrom12μmto4μm,andonverticalsurfacesfrom15μmto13μminNickel-Iron-Copperparts.Thisclearlyillustratestheimportanceoforientationandbuild-direction.Complexandintricategeometriesposeachallengeforconventionalgrinding.Inanattempttoalleviatethis,Beauchampetal.[53]usedshape-adaptivegrindingtofinishTi6Al4VMAMparts.Withthisprocess,asurfaceroughness(Ra)of~10nmwasachievedbyusingthreedifferentdiamondabrasivepellets.

3. AnintroductiontoWHASPs

ThisresearchgivesaninsightintothetechnologicalandprocessdevelopmentsthathavefurtheredfieldofWHASPs.Most,ifnotall,WHASPsexhibitkeymodules,arrangedintosuitableconfigurations.ThegeneralarchitectureofaWHASPisdescribedinFigure2.Aswillbeseenthroughoutthisreview,thedefinitionofanewWHASPalmostalwaysbeginswithatargetmotionplatforme.g.anexistingmachine tool. This platform is typically optimised in its layout for either additive or subtractiveprocessing.Thesecondaryprocessisthenintroducedviasomeformofintegration,whichmightbethephysicalmountingof an additivedepositionhead,or the introductionof a separate industrialrobottodeliverthesecondaryprocess.Tobeabletointerchangebetweenprocesses,someformofcontroller logic or physical reconfiguration of themachinemust be present. The controller of themachineisresponsibleformotionandtheauxiliarycommandsthatfacilitateadditive,subtractiveand,inmanycases,metrologyprocessesduringmanufacture.Thiscontrollerreceivesinstructionsfromthesoftware layer, which encapsulates the representation of the part geometry, process sequences

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(processplan)andanyinspectionrequirements.Thepresenceof in-processsensingandmetrologypermitsabidirectionalexchangebetweenthesoftwareandcontrollerlayers,resultinginanadaptiveorreactiveprocessplan.Eachlayerofthisarchitectureisgivenadedicatedsectioninthisreview.The`Hardware’layerisdiscussedforacademicresearchandindustrialdevelopmentsinSections4.1and5.1, respectively.Similarly, the `Controller’ layer isdiscussed inSection4.2and5.2and `Software’layersarediscussedinSections4.3and5.2.

TheremainingsectionsofthispapercorrespondtothelayersofthearchitectureproposedinFigure2.Theselayersareexploredforbothresearchandindustrialdevelopments,whicharelatercomparedandcontrastedtoidentifyemergingtrendsinWHASPdevelopment.

3.1. ThegeneralisedthehybridadditiveandsubtractiveprocessIn general, Figure 3 describes the process interactions in a hybrid additive and subtractivemanufacturingprocess.AnygivenprocessmayexhibitsomeoralloftheseinteractionsasaWHASPcreatesanewpart,oroperatesonanexistingpart.Themanufactureofnewpartsnecessarilystartswith theadditionofnewmaterial viaadditiveprocessing.Conversely,part repairor reincarnationtypicallystartswithameasurementorcharacterisationstagetoidentifythepositionandorientationofthepart,orthenatureofadefectinrelationtothemachine’scoordinateframeofreference.

Hybridadditiveandsubtractiveprocessingmaybeundertakeninanopenorclosed-loopfashion.Tocontinuetoprocessadditivelyorsubtractively,withoutconductingsomeformofverificationontherecentprocessoutcomes,istoconductanopen-loopprocess.Conversely,tocharacterisetherecent

Figure2:AgeneralarchitectureforWHASPs,coveringaspectsfromhardware,controllerandsoftwarecapabilities

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Figure3:Processinteractionswithinahybridadditiveandsubtractiveprocess,showingopenandclosed-loop

operations

processoutcomesusingmetrologyandsensingcapabilities,beforecommittingtofurtheradditiveorsubtractiveprocessing,istoundertakeaclosed-loopoperation.Likewise,processcompletionisopenorclosed-loopdependingonwhichofthethreetypesofoperationisfinal.

Once processing begins, there are a total of 12 interactions within and between processes: (1)consecutive, open-loop additionofmaterial, (2) consecutive acquisitionofmeasurement data, (3)consecutive, open-loop subtraction of material, (4) interchange from additive to subtractiveprocessing,withoutverificationofadditiveoutcomes, (5) interchange fromsubtractive toadditiveprocessing, without verification of subtractive outcomes, (6) verification of additive processingoutcomes,(7)verificationofsubtractiveprocessingoutcomes,(8)additiveprocessingwithadditionalinsight from prior measurement and characterisation, (9) subtractive processing with additionalinsight prior measurement and characterisation, (10) verified process completion, based onmeasurementorcharacterisationofthefinalpart, (11)unverified(open-loop)processcompletion,endingwith additive processing, and (12) unverified (open-loop) process completion, endingwithsubtractiveprocessing.Whererequired,theremainingsectionsofthisreviewshallreferbacktothisinteraction diagram to help describe the process interactions and strategies employed in eachimplementation.

3.2. Motivationsforhybridisationofadditive&subtractivetechnologies

MAMpartsrequirefurtherpost-processingtorefinegeometricalaccuracy, improvesurfacequalityandrelieveresidualstresses.ConventionalmechanicalmechanismsforfinishingofmetalpartsmaybeadvantageousinfinishingMAMcomponents,duetoeaseofhardwareintegration,andtheabilityto selectively process material, producing the required surface characteristics imposed by somecritical applications (e.g. aerospace and medical). As many of the existing mechanical finishingtechniquesrequire`line-of-sight’toaccessoverhangingorinternalfeatures,itisadvantageoustobe

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abletoselectthe intervalatwhichfinishingoccurs i.e.buildmaterial, finishthismaterialandthensubsequentlyaddmorematerial.

Theabilitytofluentlyaddandsubtractmaterialfromaworkpiececreatessignificantopportunitiesinthemanufactureofnewpartsandtheremanufactureofwornordamagedparts[3].Remanufactureisregardedasacostandenergyefficientwaytoextendtheusefullifeofpartsandproducts,receivingattentionformcivilianandmilitaryarenas[54].WHASPsprovidetheopportunityforrawmaterialstobetransformed intofinishedpartsusingonlyonevisit toasinglemachinetool, increasingprocesscapability[55].

The ability to work, inspect and then rework material until the part conforms to tolerances andspecificationsmayprovideastep-changeinqualitymanagement.Therealisationoftheseconceptsleadstoreductionsincostsincurredowingtofloorspacerequirements,generationofscrapandswarf,andpotentiallyimprovedprocessingtimes.Moreover,highlycomplexpartswithexternalandinternalfeaturesorhighgeometricalprecisionandsurfacequalitycanbeproduced[7],[56].Assuch,WHASPsmay overcome existingmanufacturing challenges, thereby satisfying the objective of “1+1=3” forhybrid manufacturing as defined by Lauwers et al. [5]. For these reasons, research into thedevelopment of workstations for hybrid additive and subtractive processing will be of significantimportancetohighvaluemanufacturingofthefuture.

4. Hybridadditiveandsubtractivemanufacturingprocesses–research

BasedonthearchitecturedetailedinFigure2,academicresearchrelatingtoWHASPsmaybebrokendown into the constituent layers, namely: the hardware, controller and software layers. Theproceedingsectionsreportontheliteraturefromtheperspectiveofeachoftheselayers.

4.1. ThehardwarelayerFigure 4 gives a cross-section of how academic research has addressed the `Hardware’ layer ofWHAPSs.Thediagramshouldbereadfromtoptobottom,firstselectingasubtractiveprocess,thenanadditiveprocess,thirdlyamotionplatformconfigurationischosenandfinallyamethodofprocessinterchangeorreconfiguration.Thenumberofreferencesineachboxofisindicativeoftheabundanceofresearchconcerningagivenconfiguration.

Figure4showsthatsubtractiveprocessingisalmostexclusivelylimitedtoCNCmachining,withasingleexampleofSelectiveLaserErosion(SLE).Similarly,DirectedEnergyDeposition(DED)dominatestheadditive processing tier, with a small number of cases considering powder bed fusion (PBF) andmaterial extrusion. As a final introductory observation, WHASPs are largely built upon existingcommercialmachinetools,withadaptationsintheformofadditiveprocessintegration.Typically,theonlyvariantsonthisthemeincludetheadditionofextraroboticmanipulators,orthedevelopmentoflow-costin-housemachinetools,whichcloselyresemblecommercialsystems.Eachconfigurationwillnowbeexaminedindetail,withacriticalassessmentofitscapabilityandsuitabilityinanindustrialsetting.

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Figure4:Abreakdownofthehardwareconfigurationsdevelopedinacademicresearch,groupedaccordingtoprocess

inclusion,thetypeofmotionplatformusedandthemethodofmachinereconfigurationandinterchangebetween

additiveandsubtractiveprocesses.

4.1.1. CNCmachiningwitharc-baseddirectedenergydeposition

ThehybridisationofCNCmachininganddirected-energydepositionisthemostabundantcombinationinacademicresearch.Oneformofthisisarc-baseddirectedenergydeposition,whereanelectricalpowersupplyisusedtoestablishanarcbetweenananodeandcathode.Theheatfromthisarccreatesamelt-poolintowhichwire-fedorpowderedmaterialisdeposited.

The use of arc-based directed energy deposition has been realised through the mounting of anadditivehead(weldingtorch)withinamachinetool,oronanindustrialroboticmanipulator.Merzetal. [57] developed ‘Shape Deposition Manufacturing’ (SDM), which hybridised a newly definedadditiveprocess, ‘Microcasting,’ andCNCmachining. InMicrocasting an arc is initiatedwithin theweldingheadbetweentheelectrodeandthefeedstockwire.Thewireismeltedinthearc,depositing

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astreamofrelativelylargedropletsontothesubstrate.MechanicaltestingshowedthatMicrocastingsurpassedthespecifiedtensilestrengthof308weldments.

In this researcha test-bed facilitywasdeveloped,which included fourdistinctprocessingstations,namely:5-axisCNCmachining,aroboticdepositionstation,partcleaningandshotpeeningstations.A further robotperformed transfersof thepart between stations. Thedeposition stationhad theabilitytodepositprimarymaterial(stainlesssteel)orsupportmaterial(copper)ontothesubstratetoassistwithoverhangingfeatures.Thepresenceofcleaningandshot-peeningstationsfacilitatestheremovalofcuttingfluidresidueandreliefofresidualstressesfromtheadditiveprocess,respectively.

Later in1997-1998,Amonetal. [58], [59]extendedtheworkof [57]bymodellinga ‘Microcasting’droplet impacting on an ambient substrate. As a result of this modelling, torch power, dropletdeposition rate, droplet size and free-fall distance were optimised to reduce the likelihood ofinterlayerde-bondingandexcessivethermalstressbuild-upinsingleanddual-materialparts.Amonet al. [19] also theorised about the integration ofmultiplemanufacturing processes into a singlemachinetool.Morespecifically,Amonetal.[58],[59]describethemountingofanadditiveheadtotheZ-axisofaCNCmillingmachine;aconfigurationthatwouldlaterbecomepopularinresearch-ledWHASPimplementations.SeeFigure7andFigure12forexamplesofthisconfiguration.

In2005,Songetal.[60]alsosoughttointegrateanadditiveprocesswithinsingle,commercial3-axismachinetool.InthisresearchGasMetalArcWelding(GMAW)wasutilisedinasimilarmannertothatproposedbyMerzetal.[57]andAmonetal.[58],[59].ThisresearchintegratedtwoGMAWweldingheadsbymountingthemadjacenttothespindleofacommercial3-axismachinetool.Thisfacilitatedthedepositionofdifferentmaterialsordepositionwidthsi.e.coarseandfine.Theprincipleaimofthisinvestigationwastoanalysetheeffectsofdifferentweldingparametersonthebuiltmaterial,suchaswelding voltage, current and speed. Itwas found thatbydepositinga layerofmetal, followedbyplanarmilling and then depositing the next layer resulted in high density parts (>90%),with finalsurfaceroughness2μm(Ra)aftermillingandtensilestrengththatiscomparabletowiremildsteel.Thisresearchwassupplementedin2006bySongandPark[61]whodemonstratedthemanufactureofmulti-material components using the same set-up. In a cubic specimen, amild steel core wasenshroudedwithinastainlesssteellayerusingtwoadditiveheads.Twodistinctmaterialswereclearlyevident on themicrograph; however, the authors expressed concerns over the induced stress inmaterialswithdissimilarthermalexpansioncoefficients.

Figure5:Complexfeatures,includingatriangularhelicalduct(left)andahollowtorus(right),manufacturesusing3-axis

hybridadditiveandsubtractivemanufacturingprocesses[7]

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Akulaetal. [62], [63]developedan in-housemachine tool toaccommodatebothCNCmillingandGMAW (MIG / MAG) welding. By developing a machine tool and associated PLC-based controlinternally,theauthorsretainedtheabilitytoredesignbothhardwareandsoftwaremodules.Thefinalhybridprocessadoptedthenotionofdepositinglayer,planarmillingthislayer,andthendepositingthenextlayer.Onceanear-netshapewasachieved,profilemillingwasundertakentocompletethepart.TheresearchundertakenbyAkulaetal.[62],[63]focusedonoptimisingtheprocessparametersforadditive/subtractiveprocesses.Theauthorsclaimedthatbyusingthismethod,thecycletimeformanufacturingmouldsanddiescouldbesignificantlyreduced.Furthermore,integratingtheirsystemintoanexistingcommercialCNCcentrecouldreducecapitalinvestment.Investigationsindicatedthatthe desiredmaterial properties formoulds and dies could not be entirely achieved by arc-baseddirected energy depositionmethods. Akula et al. [62] concluded that partsmanufactured by thismethodaremechanicallyinferiortotheircounterpartsmanufacturedconventionally;however,afterCNC milling, similar geometrical accuracy is achieved. The overall part accuracy is process andworkpiecedependent;however figures stated in [64]describepartaccuraciesof±0.030mmforacombinedDEDandCNCmachiningprocesses.Inanotherstudy,Akulaetal.[63]analysedtheeffectof deposition parameters in additive/subtractive manufacturing and highlighted variations withinmaterialmicrostructureandinpart/buildplatedistortionduetounevenheatingandcoolingduringtheweldingprocess.

Karunakaranetal.[65]–[67]reportedontheintegrationoftheGMAW(MIG/MAG)additiveweldingprocess(asdescribedbyAkulaetal.[62],[63])intocommercial3-axismachinetools.Theyemphasisedthat the integration should not interfere with the existing capabilities of a CNC machine tool.Therefore,apneumaticactuatorconfigurationwasusedtoraiseandlowertheadditiveheadbetweenmanufacturingprocessestoavoidcollisions.Theweldingpowersourcewasalsohousedwithinthemachinetool’sprotectivepanels[67].Inthisresearch,theauthorscommentedonthepossibleuseofautomatictoolchangingtoreconfigureany3or5-axismachinetool;however,theydisregardedthisnotionduetotheneedtoestablishelectrical,gasandwirefeedstockconnections[65],[66].Inacasestudy[65],theauthorsfoundthatthisWHASPcansignificantlyreducethecostsandthetimerequiredformanufacturinganymetal toolordieascomparedtoother individual techniques.Furthermore,they identified near net-shape building and finish machining on a singular platform as the mostsignificant feature ofWHASPs. Example mould parts manufactured by the proposedmethod aredisplayedinFigure6.Karunakaranetal.[67]highlightedthatheatmanagementduringthisprocessisessential topreventunwanteddistortionand residual thermalandmechanical stresses in finishedparts.Furthermore,thepossibilityofthermalspikesduringthematerialdeposition(welding)processshouldbeconsideredtopreventundesirabledamagetothemachinetools’controller.

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Figure6:Examplemanufactureofamouldcoreusingretro-fittedadditive(GMAW)andsubtractive(CNCmachining)[66]

TheSouthernMethodistUniversityinTexashavedevelopedtheirownWHASPcapabilityintheformoftheMULTIFABsystem[68],[69].TheMULTIFABsystemcomprisesamulti-axisrobotformaterialdepositionandweldingwhichissynergisticallyintegratedintoa5-axismachinetool[70].Thissystemcan accommodate both arc and laser-based directed energy deposition processes, using a 6-axisroboticmanipulatortoexecuteeitherlaser(1kW/2.5kWNd:YAG)ormicro-plasmaweldingfacilities.This robotic systemwasable toprocessmaterial that is fixturedwithina5-axismachine tool.TheMULTIFABcapabilityhasprimarilyfocusedontherepairofhigh-valuemetalcomponents,whichinturn necessitates integrated machine tool metrology to characterise and existing component’sgeometry.Scanningtechnologiesareusedtoachievethis,byenablingreverseengineeringand in-processinspectionofcomponentgeometries.

Thefollowingpointssummarisethedevelopmentsmadeinarc-baseddirectedenergydepositionandCNCmachiningWHASPs.Motionplatforms take the formof commercial three-axismachine tools,withretrofittedweldingfacilitiestodepositmaterial.Insomecases,theweldingheadisretractabletoavoidinterferencewiththeCNCmachiningoperations.Alternatively,industrialrobotshavebeenused to work collaboratively with commercial 5-axismachine tools. Process sequencing generallyalternatesbetweenlayerdepositionandplanarmilling.Therehasbeennoparticularfocusontheuseprofilemillingbetweendepositedlayers,whichcouldpotentiallymakefinishingofoverhangingandinternal features difficult. There are differing opinions on the mechanical and microstructuralpropertiesofthemanufacturedparts,withsomeresearchersclaimingcomparableperformance,andotherstipulatingtheneedforheattreatmentandcarefulavoidanceofthermalbuild-uptominimiseresidualstresses.

4.1.2. CNCmachiningwithlaser-baseddirectedenergydeposition

ThelimitationsofGTAW,suchaspooraccuracyandreliability,partdeformation,poorbondingandrestricted material choices, has resulted in an increased interests in using laser-based materialdeposition methods [54]. Laser-based directed energy deposition is similar to its arc-basedcounterpart; however, in these processes, a laser is used to create a localised melt-pool on thesubstratetowhichmaterial isthendeposited.Anotherwidelyadoptedtermfortheseprocesses is‘lasercladding.’

In 1996, Fessler et al. [71] described the installation of a laser-cladding head on a 4-axis roboticmanipulator.CNCmachiningwasusedachievedesirablegeometricalaccuracyandsurfacequalityin

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additivelymanufacturednear-netparts. Inthisresearch,theauthorsutiliseda2.4kWCWNd:YAGlaser(spotsizeof2.5mm)intheDEDprocess.Additiveandsubtractiveprocessingwerealternatedonalayer-wisebasis,usingseparatedepositionheadstodepositcoppersupportstructureandstainlesssteelpart features. The supportmaterialwas later removedbyacidetching.A comparisonof themechanicalpropertiesofthedepositedmaterialandwrought316Lstainlesssteelshowedcomparableperformance,withheightenedyieldstrength.Thesepropertieswerealsofoundtobecomparablefordifferentbuild-directions.

Theauthorshighlightedthatresidualstressesresultingfromthermalgradientscanleadtowarpingandalossofstrength.Tocircumventthisissue,experimentswithalternativebuildstrategies,inwhichtowerswerebuiltandthenthegapswerelatterlyfilledintopromoterelaxationthroughoutthebuild.Additionally, thenotionof thermally stable (INVAR) support structureswasconsidered.Apossiblesolutionwasproposedwherebysupportmaterialisprotectedbyabufferlayerthatissacrificial.Thisresearchalsoalludedtothefutureuseofmulti-materialdepositiontoproducefunctionalmaterialgradients(i.e.multi-materialdeposition)byexperimentallydepositingINVAR,stainlesssteel,copperand bronze on a single part. Apart from the thermal issues associated with laser deposition ofmaterialswithvariousthermalproperties,theauthorsidentifiedbuildinguponanexistingpartmayadverselyaffectexistingfeatures,surfacequalityormaterialmicrostructure[71].

In2003,Himmeretal.[72],describedtheprocessoflaserbuild-upwelding.Inthisresearch,alaser-claddingunitwasintegratedwithinacommercial3-axismachinetoolbymountingitadjacenttothemachiningspindle.Inthisway,themachinecouldaddmaterialtosupportlaminatedmoulddiesbybuildinganear-netrepresentationofthefinalgeometry,whichwaslatterlyrefinedusingsubtractiveCNCmachining.Theauthorshypothesisedaboutthefutureuseoffive-axismachinetoolstofacilitatefinishmachiningofmorecomplexgeometries.

KerschbaumerandErnst[73]publishedresearchonthedevelopmentofahybridlaser-claddingandCNCmachiningsystem.TheauthorsintegratedaNd:YAGlasercladdingnozzleandpowderfeedingsystemintoacommercialRöders5-axisCNCmachinetool(Figure7).Inthisimplementation,5-axismaterial deposition permitted multiple build directions, avoiding molten material flow along aninclined build surface, whilst significantly reducing the requirements for support structures. Theheighteneddexterityalsoledtoincreasedtoolaccessibilityduringmaterialremoval.Intheirprocess,theauthorsmachinedtheadditivelybuiltcomponentaftereveryfewlayerstoallowmachiningaccesswithsmalltoolsintothecomplexinternalgeometriesofthepart,potentiallyreducingtheneedfordie sinking EDM. This study identified that alternating laser cladding and machining operationsprohibitstheuseofcuttingfluidsduringmachining.Furthermore,theynotedthatsincecompleteheattreatmentoftheworkpieceaftermachiningisnotpossible,onlyverytoughhighstrengthalloysshouldbeused.Thishashighlightedthematerialcostsforthisprocessandtherequirementforspecialisedmillingprocesses,whichcanwithstandmachiningofadvancedalloysathightemperatures.

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Figure7:(Left)WHASPpresentedin[73],withadditiveheadpermanentlymounted

adjacenttothemachiningspindle,(Right–Top)Exampleofmulti-axisdepositioncapabilityofWHASP[73],(Right–Bottom)Componentafterfinish-machining[73]

The Laser-Assisted Manufacturing Process laboratory of the University of Missouri, Rolla, hasdevelopedtheLaser-AidedManufacturingProcess(LAMP).Eiamsa-ardetal.[54]describedahybridsystemwithlasercladdingandmachiningcapabilities.Theauthorsusedthissystemintherepairofmetalpartsbyfirstremoving(machining)materialsurroundingthedamagedzone,depositingnewmaterial and then finish-machining the deposited material to refine the geometry and surfacecharacteristics.Thisnotionofrepairbymaterialadditionandsubtractionsitsnaturallywithinsectorsthatproducelow-volume,high-complexitypartsthataresubjecttowearanddamagee.g.aerospace,military,medicalandmouldanddieindustries.Renetal.[74]describedtheintegrationofadditivelaser cladding capabilities on a FADAL 5-axis CNCmachine tool and extended Eiamsa-ard et al.’sresearchformulti-axissurfacepatchingofdamagedandworndietools.Theauthorsproposeda3-Dpatching method where the material is deposited on an existing feature and follows its surfacecontourasopposedto2-DmaterialdepositionasshowninFigure11.Theintegrationoflasercladdingintoa5-axisCNCmachinetoolmeantthatmaterialdepositionandfinishingcouldbeachievedwithasingle setup. This facilitated higher geometrical accuracy whilst minimising the time required forrepair,reducingassociatedcosts.

TheFraunhoferIPTinstitutedevelopedaWHASPthroughtheintegrationofawire-fedlaserdepositionheadona3-axishighspeedmillingcentre [64].This researchwas initiallyaimedat therepairandmodification of steel moulds. The layer-by-layer material deposition andmilling capability of thesystem allows machining to be carried out in-between building processes. Therefore, precisionfeatures could be manufactured using standard milling cutters, reducing the need for post EDMmachining. Synergistically utilising layered additive manufacturing techniques with conventionalmillingpermittedthemanufactureofengineeringfeatureswithhighaspectratios;however,this is

14

perhapsmoreapplicableto internal features,asexternal features,suchaspins(bosses),wouldbesusceptibletodamageduringmachining.

The‘RECLAIM’project[75]soughttodevelophybridmanufacturingtechnologiesviamulti-purposemachinetools[76].Thisresearchfocusedonthecombinationofadditive,subtractiveandinspectionprocessestofacilitatetheremanufactureofhigh-valueparts[77].AmoredetaileddescriptionofthisprocesscycleisillustratedinFigure8.TheinitialfocusofthisresearchwastorepairturbinebladesmadeofTi-6Al-4Valloywithtipdamageandwear. Inapreliminarycasestudy[76], timeandcostsaving could be achieved using this system. As shown in Figure 9, no significant microstructuralabnormalitiesweredetectedintherepairedturbineblades,whilstgoodfusiontobasematerialsandlowweldporositywerefound.However,Figure9doesshowaclearboundarybetweenthebaseandcladdedmaterials. The authors stated that further investigation on optimising the cladding headdesign and process parameters were necessary to guarantee part quality [76].Many commercialWHASPsnowadoptan identicalor similarapproach to this implementation,which isdiscussed inmoredetailinSection5.

Figure8:DescriptionoftheRECLAIMremanufacturingprocess[76]

Figure9:TransverseandlongitudinalsectionmicrographsofTi-6Al-4Vlasercladdedturbinebladetip[76]

InitialPartAlignment•ScanningTouchProbe

DefectCharacterisation•ScanningTouchProbe•Point-Cloud/Inspectionsoftware

AdaptiveToolpathGeneration•Point-CloudtoCADmodeltranslationSoftware

•CAMsoftware(ToolpathGeneration)

DefectRemoval•5-AxisCNCMachineTool

DefectRepair•DirectedEnergyDeposition(LaserCladding)

FinsihingandBlendMachining•5-AxisCNCMachineTool

15

4.1.3. Combinedselectivelasermeltingandselectivelasererosion

Producingfine(micro)featuresonmetaladditivepartsisasignificantchallenge.Externalfeaturesaretoodelicatetowithstandtheimpactfromthenextlayerofpowder,appliedusingthewiper.Internalfeaturesbecomepartiallyobscuredbytrappedorpartiallybondedparticulate.Traditionalmechanicalsubtractiveprocessesalsostruggletodealwiththistypeoffeature,especiallyifthefeaturehasahighaspectratio.

To address these issues, Yasa et al. [78] adopted a different approach to hybrid additive andsubtractiveprocessing.BytakingacommercialSelectiveLaserMelting(SLM)machineandoperatingtheNd:YAGlaserintwodifferentways,twodistinctmanufacturingprocessescouldbeexecuted.ThefirstoftheseisanadditiveSLMprocess,whichrequiresthelasertobeusedincontinuousmode.Thesecondprocess,SelectiveLaserErosion(SLE),utilisesapulsedlasermodetoevaporatematerialfromthe workpiece during or after SLM coalescence. Due to the non-contact nature of the process,cylindricalpinswithdiametersbetween50µmand350µmweproducedusingthisprocess.InternalfeatureswerealsoproducedusingSLEtodrillholesof126and120 µmdiameters.Itwasnotedthatthe laserwas commanded to follow the perimeter contour of the circle, rather than a stationaryprocessingpoint.

Usingthesamemachine,theauthorswerealsoabletoimprovesurfaceroughnessandreduceresidualporosity using laser re-melting. For planes normal to the build direction, the average surfaceroughness(Ra)wasreducedfromanaverageofRa12µmto1.5µm,whichiswithintherangethatisappropriateforcriticalapplicationse.g.aerospace[79].Thisprocesscouldbeundertakenlayer-wise,butalsoonside-profilesbyraisingthebuildpartoutofthepowderbedandblowingexcesspowderaway.Forinclinedplaneswithinclinationsof10and30degrees,a50%and75%reductioninsurfaceroughnessRacouldbeachieved.Usingarelativemetricderivedfromimageprocessing,theporosityofthelaserre-meltedspecimenhadamaterial-poreratioof0.036%,whereastheas-builtspecimenwas 0.77%, showing an improved density. This research represents the only example ofmachinereconfiguration via parameter change i.e. no hardware changes; offering a reminder that HASPprocessesarenotalwaysmanifestedviathephysicalconnectionofseparatehardwaremodules.

4.2. ThecontrollerlayerLiteraturereportingonthedevelopmentofdedicatedcontrollercapabilitiesforWHASPsandHASPsissparse.Thisisincontrasttoresearchaddressingadditivemanufacturingasadiscreteprocess.Inmetaladditivemanufacture(MAM),controlisbrokenoutintotwotranches:parameteroptimisation(open-loop),andclosed-loopcontrol.Closed-loopcontrolischallengingduetocomplexcorrelationsbetweenparametersandtheneedtousepenetrativemeasurementtechniquestogatherinformationaboutthebuildwithinapowderbed.

Therearenumerousexamplesofresearchintoclosed-loopcontrolofmetaladditivemanufacturingprocesses.Variousimagingtechniqueshavebeenusedtomeasuretheshapeandtemperatureofthemelt-poolinmetaladditiveprocesses.Themeltpoolgeometryandtemperaturehavebeenmeasuredwiththermalimaging[40],[80],andusingacombinationofahigh-speedcamerasandthermalimaging[22],[81].Othersystemsuseahigh-speedcameraandphotodiodetomeasuremeltpoolgeometry[82]–[84].Informationregardingtheshapeandtemperatureofthemeltpoolcanbeusedtofacilitatefeedbackcontrolofprocessparameters,suchaslaserpower.Researchhasalsoaddressedclosed-loopcontroloffeedstockmaterialflow-rate intheDEDprocessusinga laserdiodetomeasurematerial

16

throughput [80], [81]. Open loop control has also commanded the attention of several researchefforts. In additive processing, thismay be undertaken by optimising processing parameters.Oneexampleofthisisgivenin[85],wheretheauthorsexperimentwithdifferentlaserpowerprofilestocontrolheatingofthepowderedmaterial.Anotherformofcontroladjustsspatialaspectsofthebuild,compensating for shrinkageeffects in final part geometryby adjusting the commandedmelt poollocation[86]–[88].

Despitethisbodyofresearch,mostoftheliteratureonWHASPsandHASPsoptsforopen-loopcontrolstrategies,freezingparametersafterinitialoptimisation[62],[63].Jengetal.[89]identifiedlimitationsofopen-loopcontrolofDEDparameters,suchasexcessivematerialbuild-upincornerprofilesduetothe acceleration and deceleration phases whilst traversing the corner profile. Powder build-upresultingfromunsuitablepowderflow-ratesandamismatchbetweenmeltpoolandpowderstreamdiameterswerealsoinvestigated.Finally,theinabilitytodepositpowdereffectivelyoncetheprofileofpreviousdepositiontrackshadbecomepointedwasdiscussed.Researchinthisareaoftenexploitsthepresenceofasubtractiveprocesstocorrecterrorsgeometricalerrorsandpoorsurfacequalityarisingfromtheadditiveprocess[89].

Merzetal.[57]identifiedtheneedforclosed-loopcontrolofHASPparametersin1994.Karunakaranetal.[65]hasalsoexplicitlystatedtheneedforcontroloverseveralweldingparametersinordertoaffectachangeinasingleprocessoutput,suchasadditivelayerthickness.KerschbaumerandErnst[73]deviseanextendedCNCcontrol toaccommodate theadditiveprocess inacommercial5-axismachinetool.Thisresearchaddressedtheneedtoaccuratelycontrolmachinefeedratesandfeed-stockvolumetricflow-rates.Thelaserpowerwasrelatedtothefeedrateofthemachineviathirdorderpolynomialrelationshipcreatingaformofclosedloopcontrol.Choietal.[90]investigatedindividualwire-fedweldingparameterssuchastrackandlayerdimensions.Intuitively,anincreasedfeedrateforaconstantmaterialfeedrateresultedinareductiontrackwidthinthedeposition.Likewise,increasingthematerialfeedrate,withaconstantlaserpowerandtablefeedrate,resultedinanincreasedtrackwidth.Theworkof Jonesetal. [91]usesfixedadditiveparameters inanopen-loopsense,buthasprovision for inspection (tactile probing) of the workpiece to characterise the outcomes of bothadditive and subtractive processing. This makes it possible to operate closed-loop processing inaccordancewiththeinteractionsdescribedinFigure3.ThisresearchmakesuseofcommercialCNCcontrollersandCAD,CAMandCAIsoftwaretodeliverthisfacility.

Researchhasalsofocussedoncontrollingtheinterchangefromtheadditivetothesubtractiveprocessandviceversa.Theuseofavailablemachinecontrollertoolpreparatorycommands(GandM-codes)hasbeendiscusses[62],[63],[66],[67];particularlywhenaprocessiseitherinan‘on’or‘off’state(open-loop).

4.3. ThesoftwarelayerThe software layer of the WHASP architecture is largely concerned with three tasks, namely (i)Identifyingasuitablebuild-direction(partorientation),(ii)decomposingapartgeometryintoalayer-wiserepresentation,and(iii)Definingaprocesssequencetofacilitatethelayer-wisemanufactureofapart.Ineachcase,identifyingapreferablepartorientationandbuild-directioniskeytomaximisingtheeventualpartquality.

17

4.3.1. IdentifyingasuitablebuildorientationIdentifying a suitable build orientation is highly dependent on themanufacturing processes, partgeometryandhybridmanufacturingstrategyemployed.Forexample,DEDprocessesrequiresupportstructuresforfeaturesthatoverhangsignificantly,orthathavenocontactwithexistingstructures.The use of high degree-of-freedom motion platforms permits a change in build direction duringmanufacture.Thesequenceofmaterialdepositionandremovalalsochanges thebuildorientationrequirements. For example, planar milling of a deposited face will generally always be available;however,profilemillingofadepositedfeaturecanposetool-accessibilityissues.

KulkarniandDutta [62] identified thebuildorientationasan ‘essential’partof thehybridprocesschain.They identifiedconsiderationswhenchoosing thebuilddirectionaspartheight in thebuilddirection,theimplicationsonsurfaceroughnessduetothestaircaseeffect,theareaofthepartthatismountedto thebuild-plate, theeffectsonmechanicalproperties,partdistortionandvolumeofnecessarysupportmaterial.KulkarniandDutta[62]suggestedthatoptimisationofbuildorientationmaybeundertakenforeachofthesemetricsinisolation,oraspartofamulti-objectiveoptimisationproblem

Otherresearcheffortshaveidentifiedbuild-directionsviaoptimisation,withHuetal.[92]identifyingcandidate build directions that are assessed based on cutting tool accessibility, deposition time,machiningtime,numberofbridgedstructuresandthenumberofsupportstructures.Theauthorsuseaweightedcostfunctiontoallowuserstospecifytheirindividualrequirements.ZhangandLiou[56]alsosearchfortheoptimumbuild-directionbysettinganoptimisationproblem.Inthisresearch,build-directionsminimisingthetotalareaofoverhangingsurfacesorinaccessiblefeaturesarethetargetfortheoptimisationalgorithm.

4.3.2. Partdecomposition

Asadditiveprocessesbuildpartslayer-by-layer,researchfocusesonpartdecomposition.Forhybridadditiveandsubtractiveprocesses,thisisgenerallydividedintotwocategories,namely:planarslicingalgorithms and feature recognitionmethods. Planar slice thicknesses are either equally spaced oradaptivelychangedtosuitthepartgeometry.

TheworkofKulkarniandDutta[62]usesequallyspacedplanarslicesofthepart’sSTLfiletoidentifylayer-by-layerprocessplansandtool-paths.Thismethodisoftendescribedasa‘zeroth-order’edgeapproximation.Toreducethestaircaseeffect,coarseplanarslicesarefurtherdecomposedintofineslicestorepresentthepartgeometrytoasuitabledegreeofaccuracy.AkulaandKaranakuran[63]alsousedzeroth-orderedgeapproximationtocalculatetheslicethicknessandlayeringofthepartdesign. In each layer,materialwas built using either direction-parallel (zigzag) or contour-parallel(spiral)areafilling.Eachdepositedlayerwasface-milledtounifytheheightofthedepositedlayerandremovesurfacedefects.

Anadvancementofthefixed-thicknesspartslicingstrategyisthe‘adaptive’slicingstrategy.ZhangandLiou [56]were able to change theorientationof the slicingplane to alleviate thedependencyonsupportstructuresforoverhanginggeometries.Thisalgorithmfirstsearchesfortheoptimumslicingdirectionandthentool-accessibilityischeckedtoavoidcollisionandtoensurethatsuccessivelayersarewithinthelimitsofacceptableoverhang.Ruanetal.[93]furtheredthisworkbyintroducingnon-uniform layer building, where the thickness of the layer varies from point to point. Ruan et al.separatedthebuildprocessintotwostages,asshowninFigure10.Firstly,auniformlayerwith

18

Figure10:Partslicingusingnon-uniformlayerthicknessforusewithadditiveandsubtractiveprocesses[93]

constant thickness is deposited and then themachining capability is used to subtract the excessmaterialandformanon-uniformlayerwithatop-facethatisnormaltothepreferredbuilddirectionofthefollowinglayer.

Changetal.[94]decomposepartsbyidentifyingundercut,non-undercutandnon-monotonicsurfacesinanadditivebuild.Graphtheoryisthenemployedtoidentifyaminimalbuildsequence,inclusiveofmanufacturing precedents (e.g. surface B cannot be built before surface A). Furthermore,considerationtowardsavoidinginterferencebetweenthecuttingtoolandexistingpartstructuresisgiven.

OtherrelatedworksincludeHuandLee[95]andHuretal.[96].Bothofthesepublicationspresentpartdecompositionalgorithmsforpartsthataremadeviagluingfixed-thicknesssheetstogether,withinterimmachiningoftheassembledstructure.Althoughitcouldbearguedthatthisisahybridjoiningandsubtractiveprocess,thepartdecompositiontheoryremainsrelevant,asthelayerthicknesscouldsimply be adjusted to reflect the thickness of an additive layer. The algorithmemployed in thesepublicationsdividesapartintoslices,identifyingthesignoftheZ-componentofthesurfacenormalunitvectorforeachsurface.Positive(+)Z-componentsandnegative(-)Z-componentsareseparatedandtheirbuilddirectionchosenaccordingly.

4.3.3. ProcessplanningInHASPs,processplanningreferstotheidentificationofasequenceofoperationsthatwillleadtothemanufactureofthedesiredpart,alongwithanynecessarysupportstructure.Atthehighestlevel,thismaybebrokendown intosequencesofadditive, subtractiveandmetrology-basedprocesses.Atalower level, individual toolpaths and process parameters are defined. An important differencebetweenprocessplanningforHASPsandprocessplanningofaconventionalmanufacturingprocessisthefactthattheycanbebi-directional.Materialmaybeaddedandsubtracted,adinfinitum,untiladesirableoutcomehasbeenachieved.Thisnotioncangreatlyincreasethecomplexityoftheprocess-planningtask.

Forrepairing/remanufacturingprocesses,Eiamsa-ardetal.[54]andRenetal[74]identified4majorstepsforprocessplanning,namely:(i)definingtheworn/damagedfeature,(ii)generatingmachining

19

toolpathsforremovingthedamaged/wornfeature,(iii)generatingthedepositiontoolpathforre-buildingtheworn/damagedfeatureand(iv)post-processingthetoolpathsintomachiningcodes.In[54],toolpathsweredefinedbyusingMinkowskioperationsofdilationanderosiontooffsetthetool-centrepointfromthedesiredfeaturecontour.Later,[74]proposeda3-Dpatchingmethodwherethematerialisdepositedonanexistingfeatureandfollowsitssurfacecontourasopposedto2-Dmaterialdepositionasshownin.Theintegrationoflasercladdingintoa5-axisCNCmachinetoolmeantthatmaterial deposition and finishing could be achieved with a single setup. This facilitated highergeometricalaccuracywhilstminimisingthetimerequiredforrepair,reducingassociatedcosts.

Kerbratetal.[97]adoptanovelprocessassessmentandplanningapproach,whichisdrivenbytherelativecomplexityofmanufacturingafeatureeitheradditivelyorsubtractively.Complexity inthiscase is related towell understood process limitations, such as geometrical feasibility, diminishingstiffnessinstructuresandtoolswithhighaspectratios,andtoolaccessibility.Althoughnotexplicitlyappliedtothefieldofhybridprocesses,identifyingwhenitisadvantageoustomanufactureafeatureusingaparticularprocesscouldprovideavaluableinsightwhenprocessplanningforWHASPs.

The works of Zhu et al. [98]–[100] focus on process planning for hybrid additive and subtractivemanufacturing,includingtheuseofinspection.Theauthorsdecomposepartsinto‘manufacturable’sub-parts, eachwith their own build direction. Attention is given to the ability ‘promptly’ inspectfeaturesastheyareproduced.Inthisway,outoftolerancefeaturesmaybereworkedwhilsttheyarestillaccessible,whichavoidsunnecessarymaterialwastageandisessentialforinternaloroverhangingfeatures.Assuch,theprocessplanstartswithastaticsetofoperations,butquicklybecomesdynamicasfeaturesarecreated,measuredandreworked.

Figure11:Patchingofa2Dzigzagontocurvedsurface(top)andpartrepairusingmulti-axisadditivemanufacturing[74]

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5. Hybridadditiveandsubtractivemanufacturingprocesses–industrial

perspective

Since2003,additivelymanufacturedpartproductionhasincreasedfrom3.9%to34.7%ofallproductandservicerevenues[12].WithspecificreferencetometalAMprocessesthefuturemarketsizeandgrowthrateareexpectedtoexceedpolymeric-basedAM[101].Furthermore,focusliesincustomisedand reconfigurable manufacturing, the application of layered and other freeform manufacturingtechniquestofabricate intermediateandend-useproducts,andnear-netshapingthatreducestheneedforexcessivesurfacefinishing[102].Despitehavingbeenafertileresearchtopicsincethemid-late1990s,thecommercialisationofhybridmanufacturingprocesseshasbeengradual.Atthepresenttime,thepaceofdevelopmentforcommercialhybridmanufacturingmachinetoolsisaccelerating.

Trends suggest that the future manufacturing economy will rely heavily on reconfiguration andresponsiveness, with a migration away from production lines and towards highly capable singlemachinesthatareabletotransformrawmaterialintoafinishedpartinasinglemachinevisit.ThisnotionisparticularlywellalignedwiththeWHASPvision.TheproceedingsubsectionsgiveasummaryofcommerciallyavailableandcommerciallyannouncedWHASPs.

5.1. TheHardwareLayerTable 1 gives a summary of the commercially available products and publically announceddevelopmentalworkbeingundertakeninindustrywithregardstoWHASPs.DMGMoriSeikipossesstwohybridadditiveandsubtractivemachinetoolcapabilities,eachatdifferentstagesofdevelopment.The most developed of these is the LASERTEC 65 3D, integrating laser cladding and 5-axis CNCmachining [106].DMGMori Seiki hasalsoannounced thedevelopmentof theNT43003Dhybridmachinetool[107],whichutilisesaturn-millmachine.Intermsofmaterialreadiness,DMGMoriSeikiliststainlesssteel,Nickel-basedalloys(Inconel625,718),tungstencarbidematrixmaterials,bronzeandbrassalloys,chrome-cobalt-molybdenumalloys,stelliteandweldabletool-steelsasbeing‘triedandtested’[118].Obviousomissionsfromthislistincludetitaniumalloysandaluminiumalloys,whichappeartostillposeconsiderablecommercialissuesinDEDadditiveprocesses.

In2013,HamuelReichenbacherannouncedthedevelopmentoftheHYBRIDHSTM1000machinetool[119], [120], focusing largely on the repair of high-valueparts.Using an existingHamuel turn-millmachine, this offering combines high speedmilling, directed energy deposition via laser cladding,inspection, deburring / polishing and lasermarking. Particular focus is given to the integration ofinspection processes to close the loop between the additive and subtractive processes, and thedamaged part. Mazak Corporation has announced a hybrid multi-tasking machine, namely theINTEGREXi-400AM.ThismachineutilisestwoAmbitlasercladdingheads[108],coarseandfine,forhigh speed and high accuracy deposition, respectively. This WHASP is based on a multi-taskingmachining centre as a foundation, permitting the end-user tomill, turn and laser-mark additivelymanufacturedpartsusing5-axismotion.

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Table1:Announcedorcommerciallyavailablemachinetoolswithhybridmanufacturingprocesscapabilities.

Informationhasbeenpopulatedfromthepubliclyavailablereferences.(*ATC=AutomaticToolChange,(†)=

TerminologydefinedinASTMF2792standard[9])

Additive

Process

Product,Company Subtractive

Process

Additional

Capabilities

MotionPlatform Reconfig.

Mode

Ref.

Sheet

lamination(†)

Formation,Fabrisonic ü 3-AxisCNC

Machining

Dedicatedmachinetooldevelopment

ATC* [103]

Directed

Energy

Deposition(†)

AmbitLaserCladdingHead,HybridManufacturingTechonologies

- - - ATC* [104]

HYBRIDHSTM1000,HamuelReichenbacherLtd. ü 5-axisCNC

Machining

ü 3Dscanning,ü Inspection,ü Deburring/

Polishingü Lasermarking

HamuelReichebacherMill-Turn

ATC* [105]

LASERTEC653D,DMGMoriSeiki ü 5-axisCNC

Machining-

DMU65Monoblock5-axisMachiningCentre

ATC* [106]

NT43003D,DMGMoriSeiki ü 5-axisCNCMachining

ü Turning

-NT4300SZMTurn-Mill

ATC* [107]

INTEGREXi-400AM,MazakCorporation

ü 5–AxisCNCMachining,

ü Turning

ü LaserMarkingü Fine&Coarse

AdditiveNozzles

MazakINTEGREXi400Mill-TurnMachine

ATC* [108]

Replicator,CybamanTechnologies,traki-iskiLtd.

ü 6–AxisCNCMachining

ü Grinding

ü Robotweldingü 3Dscanningü Laser

Processing

Dedicatedmachinetooldevelopment

Automatedandmanual

[109],[110]

WFLMillturnTechnologies ü 5–AxisCNCMachining,

ü Turning

ü Laser-basedhardening

ü LaserWeldingü LaserCladding

WFLMillturnTechnologiesM80Turn-Mill

Unknown [111]

ZVH45/L1600ADD+PROCESS,Ibarmia ü 5-axisCNC

Machining-

Ibarmia5-axismachiningcentre

ATC* [112]

ColdSpraying MPA40,HermleAG ü 5–axisCNCMachining

ü Multi-metaldeposition

Hermle5-axismachiningcentre

Unknown[113],[114]

PowderBed

Fusion(†)

LumexAvance–25,MatsuuraMachineryCorp. ü 3–axisCNC

Machiningü Vision-based

monitoring

Dedicatedmachinetooldevelopment

Automated [115]

OPM250E,Sodick ü 3-AxisMachining

-DedicatedMachineToolDevelopment

Automated [116]

Material

Jetting(†)

SolidscapeProductlines,SolidscapeInc.(Stratasys) ü Planarmilling

-Dedicatedmachine

Automated [117]

Cybaman Technologies offer a comparatively compact solution [110], [121] built upon a 6-axismachinetool,whichmaybereconfiguredtodeliverCNCmilling,grinding,welding,laserprocessing,directedenergydeposition(additive)and3Ddigitising.Thesetechnologiesmaybecombinedtosuitend-userrequirements,oftenutilisingautomationforeaseofreconfiguration.

22

The year 2015 has also seen announcements from a consortium led by Optomec and backed byTechSolve,LockheedMartin,MachMotionandU.S.ArmyBenétLabsregardingthedevelopmentofalegacyCNCmachinetoolupgrade (retrofit) to includeadditivemanufacturingvia theLENSTM [122]DEDprocess[123].Thisresearch,undertakeninconjunctionwithAmericaMakes,aimstomakehybridmanufacturingmoreaccessibletomachinetoolownersbyfocussingonexistingmachineupgrades.Thistakestheformofamodular,permanentlymountedadditivehead,adjacenttothemachinetoolspindle. This is intended tobeamore ‘costeffective’meansbywhich tobridge thegapbetweenconventionalandhybridprocessing.Thisdevelopmentisoneofthefirstexplorationsoftheadjacentmountingconfigurationinacommercialsetting,havingbeenpopularisedintheresearch(SeeSection4.1andFigure12).

5.1.1. CNCmachiningwithadditivecoldsprayingprocesses

In this context, Cold Spraying refers to an additive process that propels powdered material at asubstrateatasufficientlyhighvelocitytocauseadhesionandmaterialbuild-up[10].Theuseoftheword‘cold’referstomaterialadhesionatatemperaturesignificantlylowerthanthematerial’smeltingpoint;although,uponcollision,localisedtemperaturesarehighasaresultofkineticenergytransfer[10].ThismethodcontrastswithotheradditiveprocessesconsideredforuseinWHASPs,asitoperatesat a comparatively low temperature. The only reference available for integration of cold sprayingprocesseswithasubtractiveprocesstoformaWHASPisbyHermle[124]–[126].

In2015,Hermlereleasedinformationpertainingtotheirhybridadditiveandsubtractivemachinetool[113], [114], [126].Throughvarying thecompositionof thepropelledmaterial, functionalmaterialgradientsmaybeadditivelyconstructed.ByintegratingHermle’s‘MetalPowderApplication’withinafive-axismachiningcentre,multi-metaldepositionmaybecombinedwith5-axisfinishmachiningtocreatepartsthatarebothgeometricallyandcompositionallycomplex(constituentmaterials).

5.1.2. CNCmachiningwithpowderbedfusion

Matsuura’sLumexAvance–25[115]offerscombinedlasersinteringandCNCmillingwithinasinglemachine tool. The technology is used to simplifymouldmanufacture by removingmould-splittingprocesses and including complex internal mould features such as conformal cooling channels. Incontrast to some of the other commercially available technologies, only three-axis machining isutilised. To avoid tool-accessibility issues, the machining process is sequentially interlaced withlayeredadditivemanufacturingtomachineinternalfeatureswhilsttheyarestillexposed.

AsimilarproducthasbeenreleasedbySodickviatheirOPM250Emachine[116].Primarilytargetingthemouldingmarket,thismachinecombineshigh-speedthree-axismillingwithpowder-bedfusioninwhatistermed‘afullyautomatic’fashion.Theadditiveprocessisdeliveredbya500WYbfiberlaser.Sodick describe their process, whereby ten layers are additively manufactured, before a singlemachiningpassismade.Thissequenceisthenrepeateduntilthebuildiscomplete.

5.1.3. CNCmachiningwithmaterialjetting

Solidscapeofferavarietyof3D-printingsolutions[117],allofwhichutilisematerialjetting(ink-jetting)toadditivelymanufacturepartgeometries.Avarietyofwax-blendsandwax-likeorganiccompoundsaremeltedtoallowhighfrequencydepositionofdropletsontoasubstrate[127].Betweenthelayers,thepartmaybe‘planarmilled’toprovideaflatbuildsurface,ataknownheight,forthenextlayertobebuiltupon.

23

5.2. ThecontrollerandsoftwarelayersWithaWHASP,materialmaybeadded,removedandalsomeasured.Assuch,itispossiblethattherewillbenowell-definedsequenceofoperationsasaHASPprocessmaybeadaptiveandreactive[99].Therefore,processplanningbecomessignificantlymorecomplexastherearepotentiallyaninfinitenumber of feasible process sequences to manufacture a part. Therefore, the need to updateinformationrelatingtothecurrentpartgeometryanddevelopnewprocessplansduringmanufactureisofgreatimportance.

Thefactthatsacrificialorsupportmaterialmaybeaddedtoa finishedpartmaynecessitatemoreinsightfulmetricssuchas:buildtime,materialusage,accuracyoffeatures,costetc.Inadditiontothis,lessobviousmetricsmayalsoplayan importantrole intheprocessplanningstage.Ahypotheticalexample of this might be the maximisation of tool-tip (or deposition head) access to a part’sengineeringfeaturesthroughoutthemanufacturingprocess;therebymaximisingtheopportunitytoreworkthesefeaturestomeetmanufacturingrequirements.Thistypeofmetricmaybecomecriticalin‘right-first-time’or‘zero-defect-manufacture’ofhigh-valueparts.

5.2.1. Commercialsolutionsinthecontrollerlayer

Table1canbedividedintothosethatutilisewell-establishedcontroller-vendorproducts,andthosethathavedevelopedtheirowncontrollercapabilities.Thesecontrollersareusedtoexactcontroloverthemachine’smotion,auxiliaryfunctionsandprocessparameters.IntermsofcommercialNCcontrolimplementations,theSiemens840D[128]hasbeenusedwith[129],[130]andFanuc31i[131]hasbeenusedwith[129].Thesecapabilitieshavebeenutilisedtocontrolbothadditive,subtractiveandinspectionprocessesduetotheirmulti-axisfunctionality,modularityandflexibility.Inadditiontotheapplication of general purpose NC control, dedicated NC control has been developed for specificmachines,suchaswiththeSodickOPM250E[116].

5.2.2. Commercialsolutionsinthesoftwarelayer

DuetothecomplexityofmanufacturingoperationsusingWHASPs,thereisoftenaneedtoincludeadditionalsoftwaretofacilitatemanipulationofthepartgeometryviaCAD,processplanningusingCAMandpotentiallycomputeraidedinspection(CAI)too.ForeachofthesolutionsinTable1,thereisanaccompanyingsoftwarecapability.Theavailability,complexityandbreadthofthesesoftwaresolutions can vary considerably. Traditionally, there is limited information available regarding theexact nature of proprietary process planning algorithms. Nevertheless, this subsection offers adescription of existing capabilities based on available information. A survey of publically availableinformationregardingtheuseofadditionalsoftwareproductshasbeenundertakenandthefindingsarelistinTable2.

Arecentadditiontothecommercialsoftware layer isaHybridManufacturingSimulationsoftware[136]. This software has been developed by MachineWorks Ltd. and offers a full-machine toolsimulation, including DED and CNCmachining capabilities. Although this is not a detailed processinteractionsimulation, itprovidesusefulvisualsimulationofthepartevolutionasmaterial isbothadded and subtracted. It also gives a clear indication of tool accessibility and collision risks. Thedeveloperssaythatthisdevelopmenthascomeinresponsetothe‘increasingnumberofhigh-profilemachinetoolmanufacturersthatarebringingtomarkethybridCNCmulti-taskingmachines[136].’

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Table2:ExamplesofCAD,CAMandCAIimplementationsincommercialWHASPs

CAD,CAMandCAISoftwarefromVendor

WHASP Software Description Ref.

HSTM1000,HamuelReichenbacher

• DelcampowerINSPECT

• DelcampowerSHAPE

• DelcampowerMILL

Softwaretocapturemeasurementdata(powerInspect),translatethisdataintoalignmentanddefectcharacterisation(powerSHAPE),anddeviseaprocessplan(powerMILL),includingbothadditiveandsubtractivetool-paths[132].Itshouldbenotedthattheuseofallthreeofthesesoftwareproductsisnotexplicitlystated;however,thesethreeproductsformDelcam’sadaptivemachiningcapability,whichwasusedintheRECLAIMproject,which–aprecursortotheHSTM1000[75].

[132]

LaserTec653D,DMGMoriSeiki • SiemensNX

PartsandprocessplansaredesignedthroughtheSiemensNXsoftwaresuite.

[130],[133]

Replicator,CybamanTechnologies

• hyperMILLAcommercialCAMpackageformulti-axistoolpathgenerationformachiningparts.

[134]

In-houseCAD/CAMSoftwareDevelopment

WHASP Controller Description Ref.

Formation,Fabrisonic • SonicCAM

SonicCAM imports a CAD model of the part and thenautomaticallygeneratesthetool-pathsandpartprogramsforthesheetlaminationandCNCmachiningoperations.

[121]

MPA40,Hermle• MPA-Studio

Thissoftwareundertakesalayer-by-layerassessmentofapart,resultinginthegenerationofprocessplans,includingtool-path. Included within this software suite will be asimulation environment to allow checking of processsequencesandqualityassuranceissues.

[126]

Solid-ScapeProducts

• 3ZWorks• 3ZAnalyser• 3ZOrganiser

Software is divided into self-contained units, which takeresponsibility for CAD file processing, motion planning,design of necessary support structures, simulation ofanalysis of part manufacture, and batch processing ofmultiplejobs.

[135]

Sodick(OPM250E)

• MARKS-MILL• OPM-

GenLaser• OPM-

Optimizer• OPM-Verify

Sodick have developed a suite of softwares to facilitatehybridmanufactureviatheircombinedhigh-speedmillingand powder-bed fusion. MARKS-MILL is a CAM system,chargedwith the generation of generation ofmachiningtool-paths. OPM-GenLaser assists in path planning(scanning strategy) for the powder-bed fusion process.OPM-Optimizer permits editing of the machining tool-pathsgeneratedinMARKS-MILL.Finally,OPM-Verifyoffersa simulation capability for both additive and subtractiveprocessing,actingasacheckingprocedure.

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6. Observations,emergingtrendsandfutureperspectives

Asaresultoftheliteraturesurveyundertakeninthisresearch,anumberofkeyobservationshavebeenmade,emergingtrendsidentifiedandfutureperspectivesforecasted.Thesearegroupedintosubsections addressing machining platforms and their structural elements, control systems andprocess planning software, metrology and the further integration of additive and subtractiveprocesses.Afinalsubsectionthenoutlinesthefuturevisionforthisresearcharea.

6.1. Machiningplatformsandstructuralelements

BysurveyingacademicresearchpertainingtothedevelopmentofWHASPs,somecommontraitshavebeenidentified.TheoverwhelmingmajorityofresearchconsiderstheintegrationofDirectedEnergyDeposition(DED)asanadditiveprocessandCNCmachiningasasubtractiveprocess.

Motionplatformsareeitherdevelopedin-housetoavoidtheinvestmentinunnecessarymachinetoolstructure and functions, or an existing machine tool is used as a foundation.When the latter isemployed,apopularconfigurationistopermanentlymountanadditiveheadadjacenttothemillingspindle,whichiscontrolledviatheNC(M-codes).Therearetwomorerecentexamplesofmachinetoolsthatarereconfiguredusingautomatedtool-changes.Thissolutionhasnowbecomeattractiveinthecommercialarena. Inaddition, thismore recent researchhasbegun toplaceemphasison theinclusionofin-processinspectiontoclosetheloopbetweentheadditiveandsubtractiveprocesses.

6.1.1. PopularhardwareconfigurationsAsaresultofthisliteraturesurvey,emergingtrendsinWHASPhardwareconfigurationshavebeenidentified. These configurations are illustrated in Figure 12. Figure 12a is a configuration that hasgainedtractioninindustry.Itcentresonthemodificationofacommercialmill-turnmachine,whereaworkpieceisheldinaspindle(rotaryaxis),whichrevolvestoachievedifferentpartorientations.Thetoolalsohasarotarydegreeoffreedom,toallowtool-accessthatisnormaltotheprocessedsurface.This configuration is well suited to hybrid processing of existing workpieces (e.g. part repair orreincarnation),astheworkpiecemaybeclampedateachendtoreduceunwanteddeflection.

Figure12brepresentstheadaptationofafive-axismachiningcentre,inwhichtheadditivecapabilityis interchangeableeithermanuallyorviaautomatictoolchange(ATC).Thisconfigurationhasbeenwidelyadopted inbothacademiaand industry.Unlike themill-turnconfiguration, thesemachineshavetheadvantageofaneasilyaccessiblebuild-plate,whichmakesthemwellsuitedtothehybridmanufacture of new parts. Figure 12c is similar to Figure 12b; however, the additive capability ispermanentlymountedtotheZ-axisofthemachinetool.Thissignificantlyreducesthecomplexityoftheintegration,astheadditiveheadistypicallyraisedandloweredusingavailableNCpreparatorycommands(G&M-codes).Thisconfigurationiswidelyutilisedinacademiabutisyettogainsignificantcommercialuptake.

Finally,Figure12disaconfigurationinwhichtheadditiveandsubtractivemanufacturingprocesseseachhavetheirownmotionplatform;typicallyanindustrialrobotandamachinetool.Inthisway,themachinesworkcollaboratively(notsimultaneously)toaddandsubtractmaterial.Despitebeinghighlydextrous, there is a significant requirement for additional investment in hardware, controllercapabilityandintegration.Thisconfigurationhasyettoseeindustrialuptake.Apossibleadvantageofthisconfigurationistheabilitytoundertakedifferingsimultaneousmotions,makingitpossibletoadd

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(a)Turn-Mill (b)5-AxiswithATC

(c)3/5-AxiswithPermanentMounting (d)3/5-AxiswithIndustrialRobot

Figure12:MachinetoolconfigurationsthatareemergingaspreferredmethodsofintegrationforCNCmachiningand

DEDprocesses

andsubtractmaterialsinunison.Tothebestoftheauthors’knowledge,thispossibilityisyettobeexplored.

Although machine configurations for other manufacturing processes have been explored, bothacademicandindustrialhardwaredevelopmentshavefocusedontheintegrationofDEDprocesses.This is largelydueto theability toaddmaterial tonewandexistingworkpieces,andthe fact thattransitioningfromadditivetosubtractiveprocessesisconsiderablymorestraightforwardwithoutapowderbed.DespitePBFprocessesbeingthemostabundant in industrialadditivemanufacturing,theirusageislimitedinHASPs/WHASPs.

The review of the published literature suggests that there has been approximately equal use ofWHASPs for new part manufacture, and existing part repair or modification. It is also highlyforeseeablethatHASPswillcreatediffering,ifnotconflicting,designrequirementsforanyeventualWHASP.Assuch,designerswillhaveachoice:(i)Identifythelimitingprocessrequirementsanddesigntomeetthese;(ii)Trytogenerateadesignthatmeetsbothsetsofrequirementssimultaneously.

Theformeroftheseapproachesmaybethoughtofaspessimisticandperhapssuboptimal.Thesecondapproachisconsiderablymorecomplexandrequiressignificantdesigneffort;potentiallyatthe

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Table3:AsummaryofthekeycharacteristicsofWHASPsdevelopedinaresearchenvironment(left)andthose

developedinacommercialsetting

AcademicPerspective IndustrialPerspective

Applications• Equalfocusonthemanufactureofnew

partsandtherepair/reincarnationofexistingcomponentse.g.moulddiesandturbineblades

• Equalfocusonthemanufactureofnewpartsandtherepair/reincarnationofexistingcomponentse.g.moulddiesandturbineblades

HardwareLayer

• SignificanttrendtowardstheintegrationofDEDandCNCmachiningprocesses

• Generallybuiltuponexistingmachinetoolstructures(3-axis&5-axis)

• Useofindustrialrobotsforparttransferbetweenprocessingstations

• Processinterchangeviapermanentmountingofadditivehead,adjacenttomachiningspindle

• SignificanttrendtowardstheintegrationofDEDandCNCmachiningprocesses

• Generallybuiltuponadapted5-axisandturn-millmachinetools

• ExamplesofPBFintegratedwithCNCmachining

• Automaticreconfiguration/interchangebetweenprocessesemergingaspopularchoice

• Metrologythroughtactileprobingand3Dscanning

ControllerLayer

• Researchfocusesprimarilyonintegratingadditivefunctionalitywithexistingcontrollersyntax(G&Mcodes)

• Controlofadditiveprocessispredominantlyopen-loop,whereparametersareoptimise,thenremainstatic

• Someexamplesofclosedloopprocessingfacilitatedbymetrology

• ProcesscontrolintegratedwithingeneralpurposecommercialNCcontrolandalsodedicatedcontrollerdevelopments

• Controlofadditiveprocessispredominantlyopen-loop,whereparametersareoptimise,thenremainstatic

• Someexamplesofclosedloopprocessingfacilitatedbymetrology

SoftwareLayer

• Softwaredevelopmentsfocusonpartdecompositionintolayers(zeroth-order&adaptiveslicing)

• Limitedexamplesofclosed-loopadditive-subtractiveprocessingfacilitatedbyCAD/CAM/CAI

• UsageofcommercialCAD/CAM/CAI• Morefocusonclosed-loopadditive-

subtractiveprocessingfacilitatedbyCAD/CAM/CAI

• Oneexampleofmachineandprocesssimulationsoftware

detrimentofdevelopmentcost.However,recentdevelopmentsinthemanufacturingcommunitymayprovideameansbywhichtocounteractconflictingmachinerequirements.Ultra-lightweightandhighlystiffstructuralcomponentsmayprovideameanstomeetthestiffnessrequirementsofsubtractiveprocesses,whilstalsomeetingthedynamicmotionrequirementsofadditiveandinspectionprocesses.Forinstance,dematerialisedmachinetoolsandnovelplatforms.

6.2. Controlsystemsandprocessplanningsoftware

The published research focuses predominantly software-based decisions regarding build-directionandplanarslicingofpartsintolayers.Particularattentionisgiventoundercut(overhanging)featuresand their implication on tool-accessibility. Some attention is given to other process planningconsiderations,suchastheintegrationofpartinspectiontoupdatetheprocessplan,andthedecisionprocessbehindwhethertoadditivelyorsubtractivelycreatepartgeometries.Basedonthefindingsofthispaper,processplanningisamajorresearchthemeforthefuture.Inparticular,itcouldbeofgreatbenefittointroduceadvancedcomputationandmathematicaltools,suchasmachinelearninganddecisionscience,todevelopresource-efficientprocessplanningtechniques.

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Muchoftheresearchrelatingtothecontrolofhardwareandassociatedprocessparametersisrigidin its implementation. Typically, experiments are undertaken to identify parameters that givedesirable outcomes (e.g. laser cladding parameters), and then these are left unchanged duringmanufacture.Hence,futureresearchopportunitiesexistinthedesignofadaptiveprocesscontrolthatis governed by closed-loop feedback using in-situ measurements. An important factor in thisdevelopmentwillbetheavailabilityofreliableprocessmodelsandtestdata.Furthermore,theexistingresearch is heavily based on optimising the process parameters for individual operations namely,additiveandmachiningoperations.Aholistic viewofWHASPs is required tooptimise theprocessparameterswheretheadditiveandsubtractiveprocessesinteractandareusedinterchangeably.Forinstance,theeffectofmachiningonthematerialpropertiesduringthebuildprocessandtheeffectsofbuildheatonthemachiningprocessarestillunknown.

CommercialWHASPshavevaryingcontrolandsoftware implementations.Earlydevelopmentssawtheinclusionofbothin-houseandcommercialNCcontrolandsoftware.However,recentmachinesthatuseexisting5-axisandturn-millmachinesasafoundationare increasinglyadoptinghigh-levelcommercialNCcontrolproducts.ThesearethenusedinconjunctionwithadvancedCAD,CAMandCAIsoftwarepackagestodevelopprocessplansthatarebaseduponCADmodelandinspectiondata.FutureopportunitieslieinenhancedintegrationandcommunicationbetweenNCcontrol,andCAD,CAMandCAIsoftware.Ifthisisachieved,partinspectionmaybeusedtoagreaterextenttodetermineandadaptsuitableprocessplanstrategies.Thismaygivemorecertaintyinpartqualityandmakemoreefficientuseofmachineandmaterialresources.

TheintroductionofWHASPsthatcanalternatelyaddmaterialontoandsubtractmaterialfromexistingparts,makedecisionmakingforprocessplanningamajorchallenge.Themajorityofexistingprocessplanning systems is for parts that are generally built by additivemanufacturing on a build plate.Therefore,themachiningprocessplansareeitherforfinishingtheadditivelymanufacturedpartsorforinprocessfinishingtoimprovethequalityofthebuiltsurface.Aholisticapproachfordecision-makingandprocessplanningisrequiredtoindicatetheshapeoftheinitialbuildblockandwhereandwhen material should be added or removed. Subsequently, material, resources and powerconsumption,manufacturingcarbonfootprint, lifecycleandcostsanalysis,andmaterialpropertiesarepotentialdriversforprocessplanning.

6.3. Processmonitoringandinspection

Aspart of this reviewof published research, observations have beenmade, trends identified andfuture perspectives derived for WHASP process monitoring and inspection. The dominanttechnologiesinWHASPinspectionare:tactileprobingforcharacterisationoffeaturesandworkpieceorientation,andscanningsystemsforreverseengineeringoffeatureandpartgeometries.

There isanoticeable lackof researchcoveringprocessmonitoring inHASPs.Onlyoneexampleofprocessmonitoringandcontrolhasbeenidentified[73],wherematerialdeliveryandlaserpoweraremonitoredandsubsequentlycontrolledviatheWHASP’snumericalcontrol.Therearestillsubstantialopportunities for development of further WHASP process monitoring capabilities, which may beintegratedwithinaclosed-loopcontrolsystemasafurtherdevelopment.

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Figure13:AroadmapforfutureresearchactivitiesandholisticdesignconsiderationsforWHASPs

7. ResearchchallengesandfuturevisionThisresearchscrutinisesemergingtrendsandtechnologiesandcurrentresearchchallengestoformwhattheauthorsbelievetobethefutureofWHASPresearch.Thesetrendsandchallengeshavebeencategorised and structured to form a roadmap for future lines of enquiry regarding research anddevelopment.ThisroadmapispresentedinFigure13,andselectedthemesareexpandeduponintheproceedingsubsections.

7.1. FurtheradditiveandsubtractivetechnicalchallengesSofar,bothresearchandindustrialcommunitieshavefocusedheavilyontherealisationofWHASPsthrough theamalgamationofDirectedEnergyDepositionandCNCmachiningprocesses.Althoughthesedevelopmentsareveryencouragingforthemanufacturingcommunity,theyonlyrepresentasmallsubsetofthelargerHASPandhybridmachinetoolfields.

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Regardingadditiveprocessesformetalparts,powderbedfusionandcoldsprayingprocesseshavebeenunder-exploredcomparedtoDEDprocesses.ThereisscopeforfurtherconsiderationoftheseprocessesascandidatesforWHASPs.Asnear-netshapingdevelopsfurther,machiningprocessesmaybesubstitutedbyothersubtractive,modificationandtransformativeoperations.Examplesoftheseoperationsarecleaning,heattreatment,surfaceengineering,grindingandpolishing.Forthistobeviable,near-netgeometriesmustbeacloserepresentationofthefinalgeometry,whichnecessitatescontinuedimprovementofthegeometricalaccuracyofadditiveprocesses;particularlywithdifficulttomachinematerials.

There are opportunities for further exploration of metal additive processes that have analogouscounterparts inpolymericadditiveprocesses.Whenconsidering theuseofmetals, theabsenceofloose powder and the potential avoidance of support structures make wire-fed / droplet basedprocessesviablecandidatesforenclosedgeometriesandalsocircumventsomematerialmanagementissues.Intermsofprocessingspeed,currentpolymericprocessessuchasvatphotopolymerisationarecapableofproducingpartsofahighresolutionwithcomparativelylowprocessingtimesowingtotheability to cure an entire layer by projecting an image. Equivalent developments inmetal additiveprocesseswouldgreatly increase thesuitabilityofhybridmanufacturingprocesses toan industrialsetting.

Inordertorealisecompleteintegrationofadditiveandsubtractiveprocesses,holisticconsiderationoftherequirementsofadditiveandsubtractiveprocessesiscrucial.Swarfmanagementsystemsarenecessary to prevent mixing and provide a sustainable recycling/disposal of micro scale additiveparticles and machining chips. Additionally, the effects of materials management on machinelongevity,andhealthandsafetyshouldbeexploredindetail.Furthermore,thereisanopportunitytoinvestigatetheeffectofsurfacequalityofthebuildplate/existingpartsonthequalityofthefinishedparts.Theheatingcapabilitiesof the laserheadmaybeusedtoheatworkpiecematerialsprior tomachining,furtherhybridisingtransformativeprocesseswithoutthenecessitytoaddanotherphysicalcomponenttothesystem.Ontheotherhand,theapplicationofWHASPsisdominantlyutilisedfordifficult-to-machinematerialssuchastitanium,nickelandstainlesssteelalloys.Machiningofthesealloysiscommonlyundertakenusingcuttingfluids.Duetothecontaminationissuesandtheresidualsleft on the parts, the use of cutting fluids inWHASPS should be eliminated orminimised for themachining process. This necessitates the requirements for further research into development ofadvancedmachiningstrategiesandtoolingtorealisedrymachining.

Finishmachiningofadditivepartsonasingleplatformeliminatestheheattreatmentstageforstressreleaseafterbuildprocessandpriortomachining.Itisknownthatheattreatmentsaffectthematerialpropertiesandgeometryofmaterials[137].Therearesignificantresearchopportunitiesinstudyingtheeffectsof(i)eliminatingtotalheattreatmentfromthemanufacturingprocess,(ii)heattreatment(postfinish-machining)onresidualstressesandpartgeometry,and(iii)partialheattreatmentduringthemanufacturingprocessusingWHASPs.

HASPshavetheabilitytocreateinternal,otherwiseinaccessible,andgeometricallycomplexfeatures.Assuch,inspectionchallengesrelatingtoworkpiecegeometry,alone,aresignificant.Thefactsthatas-builtsurfacesofmanyadditiveprocessesareequivalentlycomplexduetopartiallyadheredmetalpowderetc.,furthercompoundsthiscomplexity.Finally,theintroductionofhigh-temperatureheatsourcesgivesrisetonumeroustemperaturecontrolandmaterialpropertiesissues.Therefore,surface

31

non-destructive inspection of geometry and surfaces and financially viable thermalmeasurementtechniqueswillplayamajorroleinfutureprocessmonitoringandinspectioninWHASPs.TheNationalInstituteofStandardsandTechnology(NIST)issuedareportontheMeasurementScienceRoadmapforMetal-BasedAdditiveManufacturing[138].Thisdocumentishighlyrelevantintheforecastingofcurrent and futureprocessmonitoringand inspection challenges thatWHASPswill bring.Anothersignificant publication in the field of metrology issues relating to additive manufacture is theproceedingsfromthe2014AmericanSocietyforPrecisionEngineeringtopicalmeeting:‘DimensionalAccuracyandSurfaceFinishinAdditiveManufacturing’[139].

7.2. FuturevisionAs a result of this research, a future vision for the architecture ofWHASPs and their associatedcontrollerandsoftwarecapabilitieshasbeendefined.ThisisrepresenteddiagrammaticallyinFigure14. TheWHASP and its associated HASP are delivered through a machine tool that is inherentlyreconfigurableinaccordancewiththeabovedefinitions.Hardwareandsoftwarearebothmodularintheirarchitecture,withwelldefinedinterfacesfortheadditionofnewmodules.Thesemoduleseachdeliveraprocessor sensing (measurement) capability andnewmaterialsorproduction scalesareachievedviaintegrationofnewmodules.

Itisproposedthatallprocessingoftheworkpieceshouldformaclosedloop.Eachconstituentprocessshould be adaptive to tolerate a variety of material composition, processing conditions and partgeometries. Measurements of cutting forces and melt-pool conditions would be an essentialrequirementforsuchacapability.Onadifferentlevel,processingbetweendifferingmanufacturingprocesses should also be closed-loop in accordance with Figure 3. This will necessarily requireadequatemetrologycapabilitiestoinspecttheworkpiecebeforeinterchangingprocesses.

To generate an initial processplan and specificmanufacturing instructions, an advancedand fullyintegratedsoftwarelayerisrequired.TheidealpartisrepresentedintermsofitsgeometryandqualitycharacteristicsinCAD.Thesearethenpassedtoacomputer-aidedprocessplanning(CAPP)stagetodecompose the part into a sequenceof feasible sub-features that should result in successful partmanufacture.Inaccordancewiththequalityrequirements,computer-aidedinspection(CAI)interlacesmeasurementroutineswithintheprocessplan.Instructionsregardingthespecificprocessparametersandmotion profiles required to execute a givenmanufacturing process are developed via a CAMcapability.Theseinstructionsinformapredictionofmanufacturingoutcomesusingvirtualmodelsofmaterials, processes,machine tool and controller behaviour. The outcomes of this stage undergonegotiations with overarching manufacturing objectives relating to cost, resource efficiency,productivityandqualityetc.Iftheresultsofthevirtualmanufacturingphasesatisfythemanufacturingobjectivestowithinapredefinedacceptancelevel,manufactureofthepartmaycommence.Failureto meet the objectives results in an iteration of the process, thus far, to propose an alternativemanufacturingstrategy.

Afternegotiations, the instructionsarepassed to thecontroller layer.Thiscontroller isopen in itsarchitectureandcanbereconfiguredthroughtheadditionandomissionofmodules.Thecontrollercommunicateswithamachinetoolhardware,whichalsohasreconfigurablearchitecturetorespondtochangesinmanufacturingrequirements.Duringmanufacture,dataisfedbackfromthemetrologydomaintoupdatevirtualmodelsofmaterials,processes,machineandcontrollerbehaviour,suchthattheymimicwhatishappeningin-process.Furthertothis,measuredpartdataisfedbacktotheCAD

32

stage(viaCAI)toupdatetheperceivedpartgeometryandtomakeacomparisonwiththethenominalCADmodel andmanufacturing objectives. Interventions are put in place to correct discrepanciesthrough additional processing. In this sense, the process plan andmanufacturing instructions areadaptiveandandreactive.Thisloopofprocess-measure-reassess-processcouldberunad-infinitum.However,acrucialroleoftheoverarchingobjectivesistopreventexcessiveconsumptionofpower,materials and tooling. To realise this vision, significant developmentsmust bemade in regardingsupporting software, sensing and metrology capabilities, adaptive processing, and a generallyreconfigurablearchitectureforbothhardwareandcontrollerelements.

7.2.1. Designformachinetoolandcontrollerreconfiguration

It is the contention of this research that WHASPs are, by their very nature, reconfigurable.ReconfigurableMachine Tools (RMTs) have been a fertile research area since the late 1990s andthroughout the 2000s. A cross section of this research may be gleaned from [140]–[142]. Theunderlying research in this area has matured into with well-defined characteristics, and designmethodologiesandtools[143]–[148].SynonymouscontrollerarchitecturesexistintheformofOpen-Architecture Control Systems (OACS) [149]. WHASPs are closely aligned with these paradigms,exhibiting aspects of portability, extendibility, interoperability and scalability [149]. According to[150],‘modularity’requiresthatallmajorcomponentsaremodularindesign;‘convertability’requiresthattheoptimaloperatingmodeisachievedthroughreconfiguration,whichcanbeupdatedwithashortconversiontime;‘scalability’stipulatesthatnewscalesofproductionareachievablethroughtheaddition and reconfiguration of modules; ‘customisation’ provides permits flexibility within thedesiredpartandfeaturerange,whichmaylaterbechangedthroughreconfiguration;‘integrability’ensuresmodulesaredesignwithinterfacesforeaseofcomponentintegration;and‘diagnosability’isachievethroughtheabilitytorapidlyidentifytheperformanceofthecurrentconfiguration,andrelatepoorperformancetoagivenmoduleorinterfacewithinthesystem.

AlmostallofthecurrentWHASPimplementationsinbothacademiaandindustryhaveusedanexistingmachinetoolasafoundation.Althoughunderstandablefromafinancialstandpoint,thisnotionriskscontradicting RMT tool concepts, as the final WHASP solution should be equally sympathetic toadditiveandsubtractiveprocessing requirements,without incorporating redundantcapability. It issuggestedthatfutureresearchshouldconsiderthedesignofadedicatedmachinetoolstructurethatistailoredtobothprocesses,usingwell-definedinterfacestopermittheinclusionoffurthermodules.

Control systems development should follow a in a similar vein, taking on a modular and openarchitecture. It has become clear that bidirectional communication between the machine’s NCcontrollerandtheCAD,CAMandCAIsoftwareneedstobedetailedandfrequent.Thisislargelydueto theneed to regularlyacquire time-specific information relating to theworkpieceandhardwareinteractions,whichisthenusedthistoupdateadigitalrepresentationofthemanufacturingprocess.Atpresent,itisonlythroughtheuseofNCandadvancedPC-basedsoftwarethatthiscanbeachieved.It is, therefore, likely that therewillbea convergencebetween thecontrollerandPCworkstationunits,forminganintegratedsolutionwithsignificantlygreaterexchangeofmanufacturingdata.

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Figure14:ThefuturevisionforWHASParchitectureconsideringsoftware,controllerandhardware

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7.2.2. Processmonitoringandinspection

Asaresultofthisliteraturereview,theauthorshaveidentifiedmetrologyandprocessmonitoringasan area of vast potential for future research. It is expected that futuremetrology for HASPs andWHASPswillbegovernedbyafewcentralissues.Thereisaneedtodevelopfinanciallyviable,non-destructiveandpenetrativemeasurementtechniques(andtechnologies)toacquiredatapertainingtogeometry,surfacecharacteristicsandmaterialproperties.Atpresent,technologiessuchasx-raycomputedtomographyarestartingtoaddresssomeoftheseissues.

Itisenvisagedthatanymetrologysolutionthatissufficienttopermitqualitymanagementandprocesscontrol would, by its very nature, produce large quantities of data. Therefore, data processing,transmissionandstoragearelikelytobecomeimportantissuesintherealmofmetrologyforHASPsandWHASPs.

Thepresenceofcross-manufacturing-processinteractioninHASPswillnecessitatetheintegrationofmetrologywithprocessplanningandprocesscontrol.Issuessuchasthermalgradients,partdeflectionandchangingworkpiecegeometrieswillrequiremetrologysolutionsthatcandeliversalientmetrologyinformation, ina timeandcost controlledmanner. Inparticular,affordable thermalmeasurementsystemsandrapidpartgeometryscanningareburgeoningrequirements.

Finally,HASPsandWHASPsopenupnewpossibilitiesformetrologysolutions,asnewmanufacturingmetricsmaybecomesignificant.Theabilitytomeasurematerialandpartproperties,in-process,mayfacilitatecontrolovermaterialmicrostructure,porosity,materialinterfacecharacteristicsandmulti-material(functionalmaterialgradient)composition.

8. ConclusionsThedesignofhybridadditiveandsubtractiveprocesseshasbeenanactiveresearchthemesincethelate 1990s; however, the transition from research into the commercial arena has been gradual.Research has shown that HASPs may be used to manufacture geometrically and compositionallycomplexparts,whichwerepreviouslyconsideredtootimeconsumingorevenimpossible.Withtheexception of some early-adopters, the number of commercialWHASPs has increased significantlysincethelate2000s.

This researchhas surveyed literature fromboth academic and commercial sources, and identifiedcurrenttrends inthedesignofWHASPs.Thispredominantly includesthetendencytousedirectedenergydepositionasamanufacturingprocess,combinedwithahighlymobilemachinetool.TherehasbeenanequalapplicationofWHASPsinboththemanufactureofnewpartsandalsotherepair(remanufacture) of damaged components. The latter has clearly illustrated the need to uniteadvancedmetrology,CAD,CAMandCAIcapabilitiestoupdateanadaptiveprocessplanbaseduponin-situmeasurements.Theserequirementsalsotranslateintonewpartmanufacture,astheabilitytofreelyaddorsubtractmaterialpresentssignificantopportunitieshybridwork,measureandre-workprocessplanningstrategies,whichmayfacilitateastepchangeinqualitymanagement.

Amajorcontributionofthisresearchistheidentificationofresearchthemesthatarecurrentlyunder-explored,orthatcouldpresentsignificantopportunitiesandchallengesinthefuturedevelopmentofHASPsandWHASPs.Thisextendstothedesignofhighlyreconfigurablemachinetoolhardwareandcontrollerstoaccommodatetwo,ormore,manufacturingprocesseswithinthesamemachine.The

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needformultipleandvaried inspectioncapabilitiestoprovideclosed-loopfeedbackbetweeneachconstituentmanufacturingprocessandthemachinetool,aswellastheacquisitionofdataforthedevelopment of predictive feed-forward process and machine models, is discussed. Finally, largeopportunitiesinthedevelopmentofnovelprocessplanningtechniquesarerequired,asmanufacturemovesawayfromwell-definedsequencesofoperationsintoamorefluid‘crafting’ofpartsthatmeetmanufacturingrequirements.

The future vision of this research area is the emergence of highly capable hybrid machines thatcombinemanufacturingprocessesfromanumberofprocesscategoriestotransformnumerousrawmaterials into finished parts and even assemblies. These machines will intelligently, fluently andautomaticallyswitchmanufacturingprocessestowork,inspectandreworkmaterialuntilallnecessarymanufacturing requirements aremet. This is envisioned to be a largely unsupervised process, asintegratedsensorsandcomprehensivemetrologysolutionsprovideautomaticupdates toanever-changing process plan, thereby demonstrating ‘smartmachine’ characteristics.With the arrival ofthesetechnologies,manufacture,remanufactureandreincarnationofpartswillbecomepossiblewithasingleworkstationvisit.

Acknowledgements

TheauthorsarepleasedtoacknowledgeInnovateUKfortheirsupportinProjectFALCON(FinishingofAdditiveLayeredComponentsonaNovelPlatform–102183),andtheEngineeringandPhysicalSciences Research Council for their support in Project DHarMa (Design for HybridManufacture –EP/N005910/1).

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