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Johnson Matthey Technol. Rev., 2015, 59, (3)http://dx.doi.org/10.1595/205651315X688686Stage 1OreHydrogenation anddehydrogenationExtractionFormingForming(billet, wire)IngotStage 2Plasma, PREP, REPor EIGA atomisationAtomisationWater atomisationGas or rocessingStage 3ValidationFig. 3. From ore to validated AM powder – powder production steps flow chartNone of the powder production routes actuallyproduce a 100% powder yield in the required sizefractions. Some post processing is therefore necessary.As a minimum, the as-produced metal powder must beclassified into a well-defined particle size distributionsuitable for the required process: typically 15–45 µm forSLM and 45–106 µm for electron beam melting (EBM).3.2.1 Water AtomisationAll atomisation processes begin with melting thefeedstock alloy. The melting process has a number ofvariations, but generally when atomising using water,the feedstock is first melted in a furnace before beingtransferred to a tundish (a crucible that regulates theflow rate of the melt into the atomiser). The liquid alloyenters the atomisation chamber from above; hereit is free to fall through the chamber. Water jets aresymmetrically positioned around the stream of liquidmetal, atomising and solidifying the particles. The final246powder exits at the bottom of the chamber, where it iscollected. Additional processing steps are then requiredto dry the powder. Metal powder produced in thisway is typically highly irregular in morphology whichreduces both packing properties and flow properties.Water atomisation is the main method of producing ironand steel powders and typically feeds into the pressand-sintered industries rather than the specialised AMindustry.3.2.2 Gas AtomisationThe gas atomisation process mimics water atomisation,with the differentiator being the use of gas insteadof water during processing. Air can be used as theatomising media, but it’s more likely that an inert gas(nitrogen or argon) will be used to reduce the risk ofoxidation and contamination of the metal. The processof melting the metal ingots can be the same as describedfor water atomisation, however for powders produced 2015 Johnson Matthey

Johnson Matthey Technol. Rev., 2015, 59, (3)http://dx.doi.org/10.1595/205651315X688686for high end applications such as aerospace, the needto control interstitial elements has led to increaseduse of vacuum induction melting (VIM) furnaces. AVIM furnace is typically installed directly above theatomisation chamber such that the molten stream ofliquid metal enters the atomisation chamber directlyfrom the furnace rather than through a tundish, similarto the set-up shown in Figure 4. The stream of liquidmetal is atomised by high pressure jets of gas. Due tothe lower heat capacity of the gas (compared to water)the metal droplets have an increased solidification timewhich results in comparatively more spherical powderparticles (i.e. droplet spheroidisation time is shorterthan the solidification time). Whilst it is not possibleto have complete control over the particle size of asatomised powder, the distribution can be influenced byvarying the ratio of the gas to melt flow rate. Researchin the field of gas atomisation has shown that even finerparticle size distributions can be achieved through theuse of hot gas atomisation (4).Although interstitial elements can be well controlledin gas atomised powders, there are still potentialcontamination risks. Contamination most pertinent tonon-static critical components, such as aero-engineparts, include refractory materials which can originatefrom the ceramic crucibles and atomising nozzlesused. One solution to this is to use electrode inductionmelting gas atomisation (EIGA). EIGA is a variation ofgas atomisation where the metal is fed into the atomiserin the form of a rod that is melted by an induction coilMeltGas sourceand pumpFig. 5. Schematic of the EIGA process for production of AMpowders (Courtesy of LPW Technology, UK)3.2.3 Plasma AtomisationPlasma atomisation is a method of producing highlyspherical particles. The feedstock used in the processcan either be in wire form such as the method used byAP&C Advanced Powders and Coatings Inc, Canada,or in powder form such as the method used by TeknaPlasma Systems Inc, Canada. The spool of wire orpowder feedstock is fed into the atomisation chamber,where it is simultaneously melted and atomised by coaxial plasma torches and gas jets, such as that shownin Figure 6.Plasma rotating electrode process (PREP) is avariation of plasma atomisation whereby a bar ofrotating feedstock is used instead of a wire feed. As therotating bar enters the atomisation chamber plasmatorches melt the end of the bar, ejecting material fromits surface. The melt solidifies before hitting the walls ofthe chamber.3.2.4 Hydride-Dehydride ProcessFine powderNozzleCollectionchamberFig. 4. Schematic of the gas atomisation process forproduction of AM powders (Courtesy of LPW Technology,UK)247just before entering the atomisation chamber, as shownin Figure 5. This application is used when processingreactive alloys, such as Ti-6Al-4V, minimising the risk ofcontamination from exposure of the molten titanium tothe crucible and the atmosphere (5).The hydride-dehydride (HDH) method (6) of powderproduction differs from the atomisation processesdescribed above due to the fact that it does not involvemelting of the metal feedstock. Instead, it involvescrushing, milling and screening to resize larger lumpsof metal feedstock into finer powder particles. TheHDH process relies on the brittle nature of certainmetals, such as titanium, when exposed to hydrogen.In the case of titanium, titanium hydrides are formed 2015 Johnson Matthey

Johnson Matthey Technol. Rev., 2015, 59, (3)http://dx.doi.org/10.1595/205651315X6886863.2.6 Summary of Powder Production RoutesTitaniumspoolPlasmatorchesEach of these processes yields powder with varyingcharacteristics, a summary of which can be seenin Table I. A series of micrographs highlighting thevarious particle morphologies obtained from eachmanufacturing route is shown in Figure 7.3.3 Powder Key Process VariablesVacuumpumpCollectionchamberFig. 6. Schematic of the plasma atomisation process forproduction of AM powders (Courtesy of LPW Technology,UK)in a hydride unit by introducing hydrogen and heat.The brittle lumps can then be crushed and screenedinto the required particle size distribution (PSD). Thepowder is then returned to the hydride unit to removethe excess hydrogen from the metal powder particles.Powder particles produced using HDH are typicallyhighly irregular. HDH powders are typically used eitherin their as-made condition or used as the powderfeedstock for plasma atomisation.3.2.5 TiROTM ProcessThe TiROTM process (7, 8) is a relatively newmethod for the production of pure titanium powderdeveloped by CSIRO, Australia. The TiRO TMprocess is a two stage continuous productionmethod in which titanium tetrachloride (TiCl 4) isfirst thermally reduced to an MgCl 2/Ti compositeunder the presence of magnesium in a fluidised bedreactor. The MgCl 2/Ti composite is then separatedusing vacuum distillation to produce high purity Tipowder. The as-processed Ti powder is unsuitablefor AM processes due to the particle size rangeprimarily being 150–600 µm. As such it is necessarythat the powder is modified using a high shearmilling process in a controlled environment to resizethe powder to a range suitable for AM.248The quality of a component built in an SLM processis assessed based on part density, dimensionalaccuracy, surface finish, build rate and mechanicalproperties. In order to achieve predictable andconsistent component qualities it is desirable thatthe characteristics of the powder bed and theparameters of the machine are maintained at aconstant level since the powder bed and machineparameters are closely correlated. In order tomaintain a constant powder bed during eachSLM build process it is important to understandand control characteristics such as powder bedtemperature and density. These characteristics aregoverned by the KPVs of the starting powder. Due tothe complex nature of powders, characterising theirperformance is not a trivial task. A list of variables(and analysis techniques) that may be consideredto have an impact on performance is provided inTable II.3.3.1 Particle MorphologyParticle morphology will have a significant impact onthe bulk packing and flow properties of a powder batch.Spherical or regular, equiaxed particles are likely toarrange and pack more efficiently than irregular particles(9). Research into the effect of particle morphology onthe AM process has shown that morphology can have asignificant influence on the powder bed packing densityand consequently on the final component density(10–12), where the more irregular the particlemorphology the lower the final density. As aconsequence of this highly spherical particles tend tobe favoured in the AM process. This limits the use ofpotentially cheaper powder production routes such aswater atomisation and HDH. Furthermore as can beobserved in Figure 7(b) gas atomised powders areonly nominally spherical. In the case of the titaniumalloy Ti-6Al-4V, this has led to widespread adoption ofplasma atomised powder. Plasma atomised powder istypically highly spherical, but is currently produced by asingle source – AP&C, Canada. 2015 Johnson Matthey

Johnson Matthey Technol. Rev., 2015, 59, (3)http://dx.doi.org/10.1595/205651315X688686Table I Summary of Powder Characteristics by Manufacturing ProcessManufacturingProcessParticle size, µmAdvantagesDisadvantagesCommon usesWateratomisation0–500High throughputRange of particlesizesOnly requiresfeedstock in ingotformPost processingrequired to removewaterIrregular particlemorphologySatellites presentWide PSDLow yield of powderbetween 20–150 μmNon-reactiveGas atomisation(inc. EIGA)0–500Wide range of alloysavailableSuitable for reactivealloysOnly requiresfeedstock in ingotformHigh throughputRange of particlesizesUse of EIGA allowsfor reactive powdersto be processedSpherical particlesSatellites presentWide PSDLow yield of powderbetween 20–150 μmNi, Co, Fe, Ti (EIGA),AlPlasmaatomisation0–200Extremely sphericalparticlesRequires feedstock toeither be in wire form orpowder formHigh costTi (Ti64 mostcommon)Plasma rotatingelectrode process0–100High purity powdersHighly sphericalpowderLow productivityHigh costTiExoticsCentrifugalatomisation0–600Wide range of particlesizes with very narrowPSDDifficult to makeextremely fine powderunless very high speedcan be achievedSolder pastes, Zinc ofalkaline batteries, Tiand steel shotHydride–dehydrideprocess45–500Low cost optionIrregular particlemorphologyHigh interstitial content(H, O)Ti6/4Limited to metalswhich form a brittlehydride3.3.2 Particle Size DistributionCharacterisation of PSD in a batch of powder ensuresthat the optimum range of particles, by size, are usedin each process. In general, EBM uses a nominalPSD between 45–106 µm, whilst SLM uses a finerPSD between 15–45 µm. PSD will have an obviousimpact on both the minimum layer thickness andthe resolution of the finest detail in the component.An inappropriate combination of PSD and layerthickness can potentially lead to in situ segregationdue to the mechanical re-coater pushing coarserparticles away from the bed (13), segregation in thissense could lead to variation in build quality in thevertical direction. It is generally well reported thatusing powders with a wide PSD and a high fine249content produce components with a higher fractionaldensity (13, 14). However, the use of fine materialsincreases the risk of health and safety issues. This isparticularly true when processing reactive materialssuch as titanium where finer particulates are likely tobe more flammable and explosive.3.3.3 Bulk Packing and Flow PropertiesPowder flowability is one of the most importanttechnological requirements for powders used in AM.The density homogeneity of the final part depends onthe layer-by-layer melting being performed on thin anduniform layers that are accurately deposited by thefeeding device. Cohesive powders which exhibit poor 2015 Johnson Matthey

Johnson Matthey Technol. Rev., 2015, 59, )(c)(d)Fig. 7. Example SEM micrographs of typical particle morphologies obtained using: (a) HDH process; (b) gas atomisation; (c)plasma atomisation; and (d) plasma rotating electrode process. In all micrographs the powder shown is Ti-6Al-4V and imageswere taken using a Hitachi TM3000 SEMTable II Powder KPVs and Techniques that Could be Used for their MeasurementParticulate propertiesBulk propertiesPowder propertyAssessment techniquePowder propertyAssessment techniqueParticle shape (morphology)SEMOptical microscopyApparent densityHall flowFreeman FT4Tap densityTapped density testerFlowabilityHall flowDynamic flow testing (e.g.revolution, Freeman FT4)Shear cellAngle of reposeCohesivenessFreeman FT4Surface AreaBET surface area analysisChemical compositionICP-OESXRDInert gas fusionCombustion infrareddetectionParticle size and particle sizedistributionParticle Porosity250SieveLaser diffractionOptical microscopyParticle polishing and opticalmicroscope 2015 Johnson Matthey

http://dx.doi.org/10.1595/205651315X688686flow properties are likely to be more problematic in termsof obtaining homogenous density layers throughoutthe build than powders which are comparatively morefree flowing. Powder flow is difficult to relate to anyone given parameter of a powder but there are somegeneral rules which can typically be applied (15, 16):(a) Spherical particles are generally more free flowingthan irregular or angular particles(b) Particle size has a significant influence on flow– larger particles are generally more free flowingthan smaller particles(c) Moisture content in powders can reduce flow dueto capillary forces acting between particles(d) Flow properties often show a dependency on thepacking density at the time of measurement –powders with a higher packing density are less freeflowing than powders with a lower packing density(e) Short range attractive forces such as van der Waalsforces and electrostatic forces can adversely affectpowder flow and may cause particle agglomeration(short range forces have a bigger impact on finerparticles).3.3.4 Chemical CompositionThe laser sintering behaviour of a metal powder willnot only depend on the physical properties, it will ofcourse also depend on the chemical properties.Powder chemical composition for AM should ideallybe optimised for the machine or application. Validatingchemical composition helps to ensure that themanufactured component has homogenous materialproperties.As well as the bulk alloying chemistry, it is important tounderstand the effect of interstitial elements, such as Oand N, since component properties will depend on theamount of interstitial elements present. For example,it is well known that the tensile strength and ductilityproperties of Ti-6Al-4V are influenced by O contentwhereby an increase in O results in an increase in tensilestrength and a subsequent decrease in elongation (17).Research has also shown that interstitial elementscan influence the melting kinetics of the powder byinterfering with the surface tension of the melt poolresulting in Marangoni flow (18). Marangoni flow canhave a negative influence on the porosity of the finalcomponent (11, 19).3.4 Powder RecyclingIt has been mentioned previously that for a powder bedAM process to be economically viable it is necessary251Johnson Matthey Technol. Rev., 2015, 59, (3)to recycle the large amount of unused powder. Theeffect of continuous powder reuse on the KPVs isan area that has up until recently received relativelylittle scientific attention. A handful of researchers haveinvestigated the effect of continuous reuse of powders(2, 3) for example, however further study is requiredto fully understand the impact of recycling on processperformance.4. Procurement OptionsOnce a suitable atomisation route has been selected,the AM user then faces more decisions aroundpowder supply. The market opportunity for metal AMpowder supply has not gone unnoticed, and threemain options for powder supply have emerged. Firstly,users can choose to procure powder directly from theAM machine provider. Secondly, users could chooseto procure powder from third party companies, whooffer AM machine ‘validated’ powders. Finally powdercan be sourced direct from an atomisation company.Indeed several powder manufacturers are now offeringAM specific powders as part of their product portfolio(the largest of these, and the alloys they provide arelisted in Table III). A summary of the advantages anddisadvantages of each procurement option is presentedin Table IV.At present the majority of powder sales are throughAM machine manufacturers or third party suppliers.The powder provided by these suppliers has beenoptimised for each additive process, also known asbeing ‘validated’.By validating a powder, the supplier is ensuring that thepowder given to the customer is of suitable quality sothat, when being processed, it will behave as intended,leading to a successfully built part that will adhere to thechemical composition and the mechanical properties ofthe given metal or alloy. Simply put, the machine willsuccessfully build with that material, thus de-riskingpowder supply for the end user.Machine suppliers can validate powder as they willdevelop processing parameters on their machines foreach specific material. Once the machine repeatedlybuilds reliable, mechanically suitable parts then theparameters will be stored and sent out with batchesof that material to users. The powder being usedwill be characterised using some of the techniquesdiscussed in Section 3.3 and all subsequent batcheswill undertake the same testing to ensure that theyadhere to the same specification. This ensures that the 2015 Johnson Matthey

Johnson Matthey Technol. Rev., 2015, 59, (3)http://dx.doi.org/10.1595/205651315X688686Table III Supplier List of Powders Specified for AMaPPHöganäsSweden ABEUPPSandvikMa

Introduction to the Additive Manufacturing Powder Metallurgy Supply Chain Exploring the production and supply of metal powders for AM processes By Jason Dawes*, Robert Bowerman and Ross Trepleton Component Technologies, Manufacturing Technology Centre, Pilot Way, Ansty Business Pa