Review of the Methods for Production of Spherical Ti and TiAlloy Powder

PEI SUN,1 ZHIGANG ZAK FANG,1,3 YING ZHANG,1,2 and YANG XIA1

1.—Department of Metallurgical Engineering, University of Utah, Salt Lake City,UT 84112, USA. 2.—Institute of Process Engineering, Chinese Academy of Sciences, Beijing100190, People’s Republic of China. 3.—e-mail: zak.fang@utah.edu

Spherical titanium alloy powder is an important raw material for near-net-shape fabrication via a powder metallurgy (PM) manufacturing route, as wellas feedstock for powder injection molding, and additive manufacturing (AM).Nevertheless, the cost of Ti powder including spherical Ti alloy has been amajor hurdle that prevented PM Ti from being adopted for a wide range ofapplications. Especially with the increasing importance of powder-bed basedAM technologies, the demand for spherical Ti powder has brought renewedattention on properties and cost, as well as on powder-producing processes.The performance of Ti components manufactured from powder has a strongdependence on the quality of powder, and it is therefore crucial to understandthe properties and production methods of powder. This article aims to providea cursory review of the basic techniques of commercial and emerging methodsfor making spherical Ti powder. The advantages as well as limitations ofdifferent methods are discussed.

INTRODUCTION

In the most recent decade, with the advent ofadditive manufacturing (AM) technologies, themanufacturing of Ti components using selectivelaser melting (SLM), electron beam melting (EBM),and directed energy deposition (DED) techniquesemerged as one of the most important areas of Timanufacturing.1–4 One challenge for the develop-ment of these manufacturing technologies is to havehigh-quality and low-cost spherical Ti alloy powder.Other advanced near–net-shape (NNS) manufac-turing methods including metal injection molding(MIM) and hot isostatic pressing (HIP) also usespherical Ti or Ti alloy powders to make bulkmaterials and components. The critical characteris-tics of spherical Ti powder include particle size andsize distributions, flowability, and chemical compo-sitions, especially oxygen content. The require-ments of particle size distribution (PSD) vary withapplications: �45 lm for MIM, 20–45 lm for SLM,10–45 lm for cold spraying, and 45–106 lm forEBM, as shown in Fig. 1. Oxygen is a strongsolution strengthener for titanium material, butan excess will compromise ductility and fracturetoughness.5–8 To meet the oxygen requirement of

industrial standards for final manufactured compo-nents, which is less than 0.2 wt.%,9–11 most appli-cations require the oxygen content in Ti powder tobe less than 0.15 wt.%.

Commercial spherical Ti powder production meth-ods include gas atomization (GA), plasma atomization(PA), and plasma rotating electrode process (PREP).The PREP powder is widely recognized to have veryhigh purity and near-perfect spherical shape. Never-theless, the particle size of PREP powder is typicallycoarser (e.g. 50–350 lm),12 as shown in Fig. 2, which iscoarser than desired for SLM, EBM, or MIM applica-tions. The finer spherical powder can, however, beproduced via GA and PA methods. Typical particlesizes of GA and PA Ti alloy powders range from 10 lmto 300 lm.13,14 Although atomized powder can beclassified to produce desired size cuts, classificationreduces the yield of usable size cuts, further increasingthe cost of the material. The low yield of fine powderproduced by the current commercial methods is themain technical reason for the high cost of the powderused for the advanced NNS processes, especially forAM. Therefore, the recent R&D efforts have mostlybeen focused on improving the yield of fine powder(<45 lm) with an acceptable increase in operating andfeedstock cost, which is mainly driven by the boost in

JOM, Vol. 69, No. 10, 2017

DOI: 10.1007/s11837-017-2513-5� 2017 The Minerals, Metals & Materials Society

(Published online August 15, 2017) 1853

the market of the powder-bed-based additive manu-facturing. The ‘‘fine’’ powder used in this article refersto �325 mesh powder (<45 lm) unless otherwisenoted. The improvements in this regard will bediscussed in this article.

Needless to say, the quality and performance of Tialloy and components depend strongly on the qual-ity and cost of the Ti alloy powders used. The AMapplication set the stringent requirements of chem-ical compositions and physical properties: highpurity, high sphericity and flowability, and notrapped gas-bubble porosity. One challenge for theproduction of titanium powder is the control ofoxygen content in the powder, especially fine pow-der. In general, the oxygen content of Ti powder isinversely proportional to the particle sizes.15 Inother words, the smaller the particle size, the higherthe oxygen content as shown in Fig. 3.14 Addition-ally, most NNS methods mentioned earlier requirepowder to have excellent flowability. The powderflowability can be affected by a few factors includingpowder shape and size, interparticle friction, type ofmaterial, and environmental factors.16–18 In gen-eral, powder with good flowability should have aspherical shape and the particle sizes should bereasonably large. The flowability of the powderdecreases with decreasing particle size. Further-more, the powders must have good apparent densityand tap density, which also affect the density anduniformity of manufactured parts. In short, spher-ical Ti alloy powder with low oxygen and goodflowability is in high demand. Unfortunately, high-quality spherical Ti alloy powders that meet theserequirements, especially powders for AM Ti, are allvery costly and in short supply, which hinders thedevelopment of Ti for broad applications using AMand other advanced manufacturing techniques.Therefore, a strong need exists in the industry todevelop new methods for the production of low-costTi alloy powders that meet all requirements forchemical composition and physical properties.Therefore, in addition to reviewing the basic tech-niques, this article also examine the factors thathave significant effects on the cost of spherical Tialloy powder.

Atomization Techniques

Commercial spherical titanium powder in thecurrent market is almost all produced by atomiza-tion methods or plasma spheroidization. There are alarge variety of atomization techniques. Amongthem, GA, PA, and PREP are used commerciallyfor the production of spherical Ti alloy powders. Allatomization processes consist of three main inte-grated steps: melting, atomization, and solidifica-tion. Melting can be accomplished by techniquessuch as vacuum induction melting, plasma arcmelting, induction drip melting, or direct plasmaheating. Atomization is the process during whichliquid metal is broken into droplets, which solidifyduring flight in a cooling chamber under inert gasprotection. Atomization is normally accomplishedusing a high pressure gas to break up liquid streamthrough a nozzle. Droplets can also be formed by the

Fig. 1. Typical requirement of particle size distribution of spherical titanium powder for different applications.

Fig. 2. Curves of typifcal particle size distribution of Ti alloy powderproduced by free-fall gas atomization (FFGA),13 electrode inductiongas atomization (EIGA),31 plasma atomization (PA),14 and plasmarotating electrode process (PREP).12 Note: The PSD curves ofEIGA, PA, and PREP are for Ti-6Al-4V. Ref 13 did not specify whichTi alloy it was. Reported PSD is quoted from the correspondingreferences. They are not meant to be the limit of the methods.

Sun, Fang, Zhang, and Xia1854

spinning of a liquid stream off of a disc, causingmolten droplets to form and undergo centrifugalacceleration away from the center of spin. Thedroplet will subsequently solidify during flight. Thecommercial atomizing processes are usually con-ducted in ultra-high-purity inert gas (Ar or He) tominimize oxygen pick up, which can usually becontrolled within the range of 100–500 ppm depend-ing on the specific process and size of the powder.Each specific atomization technique varies in detailsfrom others, but they all share the same three mainsteps as described earlier. These different tech-niques are presented and compared with each otherbelow.

Gas Atomization

Gas atomization of titanium was originally devel-oped by Crucible Materials Corporation in the1980s.19,20 In this process, elemental raw materialsor pre-alloyed titanium alloy ingots or bars areinduction skull-melted in a water-cooled coppercrucible under vacuum. Once the composition ofalloy becomes homogenous after being held in themolten state for a certain period of time, the melt ispoured into a refractory metal nozzle in a tundishand then atomized by high-pressure streams of inletgas (Ar is usually chosen over helium for economicreasons). As the stream of molten metal falls freelyas a result of gravitational forces for a certaindistance before atomization, this technique is calledfree-fall gas atomization (FFGA). The basic config-uration of the FFGA process is shown in Fig. 4a.

The gas-to-metal ratio (G/M) is one of the impor-tant parameters that determines the PSD of thegas-atomized powder.21 Lubanska developed anempirical equation for the relation between the d50

(median particle size-the droplet size that corre-sponds to the 50% cumulative frequency) and the G/M:22

d50 ¼ KD

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

gm

ggW1 þ M

G

� �

s

where K is a constant, D is the diameter of the meltstream, gm and gg are the kinematic viscosity of themetal and gas, respectively. M and G are the metaland gas flow rate, respectively. W is the Weber

number (W ¼ V2qmDcm

, where m is the gas velocity at the

impact of the gas jets with the metal stream; qm, cm

are the density and surface tension of the melt,respectively). From the equation, it can be clearlyseen that with a fixed gas nozzle design and meltcomposition, the particle size is dominated by thegas-to-metal ratio (G/M). In the other words, ahigher G/M can result in a higher yield of finepowder. The G/M is thus indirectly related to theeconomics of atomization; it has to be optimized tominimize the production cost of fine powder. FFGAusually produces Ti powder in a wide size range (upto 500 lm).13 As shown in Fig. 2, the yield of finepowder is only around 15% with a reasonable G/M.13

The technique of close-coupled gas atomization(CCGA) was developed to increase the yield of finepowder.23 As shown in Fig. 4b, the melt is disinte-grated by the direct impact of high-pressure gasright below the tip of an extended melt guide tube.Compared with FFGA, CCGA is a more efficientmethod of producing fine spherical powder bymaximizing gas velocity and density in contact withmetal. The investigation on CCGA dates back to the1940s;24 nevertheless, it was only successfully usedfor making spherical titanium powder in the pastdecade25,26 because the choice of material for theguide tube is very limited. The inner surface of theguide tube has to be inert to the extremely reactivetitanium melt. Researchers at Ames Laboratoryfabricated composite pour tubes (Y2O3-W-YSZ, frominterior to exterior),27 which enabled the atomiza-tion of titanium with the CCGA technique. Ingeneral, compared with FFGA, CCGA can producepowders with the same PSD with a relatively lowerG/M as a result of its better atomization efficiency.The yield of fine powder by CCGA method can bemuch higher than that of the FFGA method.Nevertheless, there is no data of fine titaniumpowder yield of GGCA method in the archivedliterature to date.

As mentioned, molten titanium is very reactive tomost common metals and ceramics; electrode induc-tion gas atomization (EIGA) was developed by ALDVacuum Technologies to produce ‘‘ceramic-free’’powder, in which the melt is not in contact withany refractory metals or other ceramic componentsthat might introduce contamination.28,29 As shownin Fig. 4c, a prealloyed rod (25–70 mm) is rotated ata very slow speed and is melted in a conical

Fig. 3. Strong dependence of oxygen content in (plasma-atomized)spherical Ti-6Al-4V powder on the particle size.14 (Four curves rep-resent four runs of plasma atomization with different operating con-ditions.).

Review of the Methods for Production of Spherical Ti and Ti Alloy Powder 1855

induction coil;30 then the melt falls into a gas nozzleto be broken up into small droplets. The diameter ofthe electrode rod can be increased to up to 90 mm31

or 120 mm28 to increase productivity. To minimizethe possible pickup of contaminants during atomiza-tion, a gas-atomization apparatus with a Ti coatingon the inner wall of the atomization chamber andother components in the flow path was designed anddeveloped.32

Although gas atomization is a mature technology,there are a few issues worth noting. Fine particlesare flown back to collide with partially moltenparticles as a result of the circulation of gas in theatomizing chamber, causing the formation of satel-lite particles (see Fig. 5a). The satellite particleshave a negative influence on the free-flowing of theparticles, which are thus not desired for someapplications. Another issue with the gas atomiza-tion is that the high-pressure gas used for atomiza-tion may be trapped in the liquid metal, whichwould remain to become gas pores or gas bubbles inthe powder. These gas pores cannot be entirelyeliminated even by HIP,33 which is thus detrimentalto the mechanical properties, especially fatigueproperties.

Plasma Atomization

Plasma atomization (PA) was developed to pro-duce fine and spherical powder in 1996.34,35 In theplasma atomization process, as shown in Fig. 4d, a

pre-alloyed wire (e.g., 1/16’’ or 1/8’’) is fed into a hotzone (around 10,000 K) heated by plasma torches.The wire is melted and broken into droplets thatwould cool rapidly. A typical cooling rate is in therange of 10–1000�C/s. Besides the feed material,another major difference between plasma atomiza-tion and gas atomization is that in PA, wire ismelted and atomized by extremely high-tempera-ture plasma simultaneously, whereas in GA, metalis melted by an induction coil or other source andthen atomized by cold high-pressure gas. Theplasma-atomized Ti powder has high purity becausethe liquid metal does not contact any refractorymetals or other solid materials that may contami-nate the powder before solidification.

In general, the yield of fine powder using theplasma wire atomization technique is significantlyhigher than that of conventional gas atomizationprocesses. As shown in Fig. 2, the yield of finepowder from plasma atomization is greater than40% for Ti-6Al-4V.14 The yield of fine powder andthe capacity of plasma atomization can be adjustedby varying the diameter and the feed rate of thewire, the inlet gas pressure (or gas-to-metal ratio),the angle of attack between the wire and plasmajets, and the distance between the wire and theplasma outlet.36 It was reported that the yield offine Ti64 powder can be improved from 39.9% to59.6% by increasing the G/M from 8.7 to 12.9 andshortening the distance from the wire and theplasma outlet from 25 mm to 19 mm.37

Fig. 4. Basic configuration of (a) free-fall gas atomization (FFGA), (b) close-coupled gas atomization (CCGA), (c) electrode-induction gasatomization (EIGA), and (d) plasma atomization (PA) and plasma rotating electrode process (PREP).

Sun, Fang, Zhang, and Xia1856

Furthermore, the production rate can be signifi-cantly improved by adding an induction coil topreheat the wire before being fed into the plasma.36

As shown in Fig. 5b, the plasma-atomized powderhas very good sphericity, and fewer satellites par-ticles than the gas-atomized powder. Nevertheless,the issues of inner porosity resulting from trappedgas during atomization and satellite particles arestill concerns for plasma-atomized Ti powder. Themain drawback of this process is that the feedstockhas to be in the form of wires. In addition to the highcost of Ti wires, this technique cannot be used toproduce an alloy powder if the wire form of the alloyis not available (e.g., Ti3Al).

Plasma Rotating Electrode Process

The plasma rotating electrode process (PREP) is arefinement of a powder production method calledthe rotating electrode process (REP) that wasdeveloped by Nuclear Metals, Inc. in the 1960s.38

In the rotating electrode process, the metal elec-trode rod is melted by the arc from a tungsten-tipped cathode. The rod (usually with diameters of89 mm or 63.5 mm) spins at a speed 3000–15,000 rpm,39 so the liquid melt is spun off fromthe electrode surface to form droplets because ofcentrifugal force. After that, the droplets solidify toform solid spherical particles during flight. S.Abkowitz reported the production of spherical Tialloy (Ti-7Al-2Nb-1Ta) powder using REP in 1966,and the particle size was approximately 150 lm.40

Nevertheless, discrete tungsten particles were

found in hot-isostatic-pressed Ti64 from REP pow-der, which is detrimental to the fatigue properties.41

Later, the heat source was replaced with a trans-ferred arc plasma torch to avoid tungsten inclu-sion,12 as shown in Fig. 4e. Helium is preferredbecause of its improved heat transfer properties andelectric arc characteristics.12

The plasma rotating electrode process is one ofthe most recognized techniques for making spheri-cal Ti alloy powders, as a result of its advantageover other production methods. First, PREP Tipowder has high purity. As descried earlier, theliquid metal has no contact with other metals orceramics before solidification. Also, the pickup ofinterstitial impurities (i.e., O, N) during the processis minimal as a result of its relatively large particlesize or low specific surface area. Second, PREP Tipowder has no or minimal gas pores because themetal droplets are produced by centrifugal forcesrather than by high-pressure gas. Third, PREPpowder has fewer satellite particles compared withthe other productions methods using high-pressuregas. As discussed earlier, the satellite particlesformed during gas atomization are likely a result ofthe back flow of very fine particles to the sprayplume. In PREP, the droplets fly radially away fromthe metal surface in a centrifugal force; in otherwords, it moves in order, so the chance of collisionsof droplets and particles to form satellites is verylow.39

Nevertheless, PREP also has its challenges.PREP typically produces spherical Ti64 powder insizes ranging from 50 lm to 350 lm,12 as shown in

Fig. 5. SEM picture of Ti-6Al-4V powder produced by (a) gas atomization, (b) plasma atomization, (c) plasma rotating electrode process, (d)plasma spheroidization from �140 + 200 mesh HDH powder (reprinted with permission from Ref. 44), and (e) granulation-sintering-deoxy-genation.

Review of the Methods for Production of Spherical Ti and Ti Alloy Powder 1857

Fig. 2. This particle size range is suitable for HIPpowder metallurgy applications, whereas it is toocoarse for powder-bed-based additive manufactur-ing or powder injection molding applications. Thedependence of the mean particle size (d50) on therotation speed (S) and the electrode diameter (De) isexpressed in the following equation:12

d50 ¼ K

Sffiffiffiffiffiffi

De

p

where K is the materials constant determined bysurface tension and density of the material. Accord-ing to the equation, the yield of fine powder can beimproved by increasing rotation speed and thediameter of the electrode, which has also beenshown in experiments.42 It was reported that theyield of fine Ti alloy powder can be increased to�16% using an electrode rod with a diameter of100 mm and a rotating speed of 30,000 rpm.42 Itshould be noted that, with a bigger diameter for theelectrode and higher rotating speed, the require-ment of the precision of electrode dimensions ismore stringent to minimize out-of-balance forces.39

It is also worth mentioning that the PSD can beadjusted by the electric current applied to theplasma arc and the distance between the tip of theplasma gun and the end of the rod.42

OTHER METHODS

As discussed previously, the R&D efforts onconventional methods for producing low-cost spher-ical Ti powder were focused on increasing the yieldof fine powder by modifying the design and opti-mizing the processing parameters. Recently, a fewemerging technologies have been developed, aimingto produce more affordable spherical titaniumpowder.

Plasma Spheroidization

The plasma spheroidization (PS) of powders is arelatively new but popular technique. Plasmaspheroidization of powders has been applied to avariety of different powders, including refractorymetals such as tungsten.43

During plasma spheroidization, the metal powderis melted by a plasma torch and forms moltendroplets, which solidify to form spherical solidpowder before reaching the bottom of the reactorchamber.43 A unique characteristic of plasmaspheroidization is that the particle sizes do notchange during plasma processing. Plasma-spheroi-dized particles typically have the same nearlyperfect round shape as the other atomized powders(Fig. 5d).44 Feedstock materials can be hydride-dehydride (HDH) powder,44 or any irregular shapedTi powder made by a range of processes such asArmstrong process,45 and the FFC Cambridge pro-cess.46 Irregular-shape Ti powder by the HAMRprocess47,48 is expected to be able to be plasma-spheroidized as well. Another example is a contin-uous method during which low-cost Ti sponge fines,HDH powder, or electrolytically produced Ti andalloy powders are fed through a plasma transferredarc torch to make spherical alloy powder.49

The challenge for PS is to produce fine sphericalTi alloy powder with low oxygen at low cost. It wasreported that using Ti hydride powder as feedstockhelps to improve the yield of fine powder of plasmaspheroidization.50 The impurity level of the plasma-spheroidized Ti powder is largely determined by thefeed powder; however, the availability of low-oxygenand low-cost fine Ti powder is very limited andcostly currently. Another potential issue of the PSprocess for making Ti alloy powder is at risk oflosing the low-melting-point element (e.g., Al) as aresult of evaporation at the plasma temperatures.

Granulation-Sintering-Deoxygenation

Recently, a new approach, called granulation-sintering-deoxygenation (GSD), for making spheri-cal Ti powder was developed by the presentauthors.51,52 Figure 6 illustrates the key steps ofthis process. There are three main steps as indi-cated by its name: (I) Granulation-Ti alloy hydrideor Ti hydride with master alloy (hydrogenated fromTi sponge or Ti alloy scrap) was milled to fineparticles, and then granulated to spherical granulesin the desired size range using spray-drying. (II)Sintering—The spherical granules are sintered to

Fig. 6. Flow chart of granulation-sintering-deoxygenation (GSD) method. (Reprinted with permission from Ref. 52).

Sun, Fang, Zhang, and Xia1858

obtain dense Ti particles. (III) Deoxygenation—Thedensified spherical Ti powder with high oxygencontent is deoxygenated with Mg to meet industrystandards.

The spherical fine Ti alloy powder by GSD methodis low cost as it has much lower waste and is able touse low-cost starting powder. Most importantly, theyield from the GSD process is near 100%. Thepowder product from GSD process can be controlledin a very narrow PSD without much loss of yield asall the powder that is either under- or oversizedparticles can be recirculated through the process. Itshould be mentioned here that the de-oxygenationtechnology used in this process can also be used as astandalone process to reduce the oxygen content ofrecycled powder that may have oxygen contenthigher than 0.2 wt.% after repeated use through3D-printing processes.53

There are a number of key issues when using thisprocess to make spherical Ti powder. First, toproduce fine spherical powders, the particle sizesof the initial powder must be less than a fewmicrons. The finer the initial particle size, the betterthe granules will be with respect to sinter-ability.Nevertheless, the limiting factor is that the oxygencontents can increase with decreasing initial parti-cle size, which needs to be managed. Second,particles may bond to each other during sintering.Therefore, measures must be taken to prevent thesintering of particles to each other. Figure 5e showsthat the particles are discrete.

Spheroidization by Mechanical Means

In addition to spheroidizing or producing particlesin a molten state, there are reports of modifying theparticle shape in the solid state by mechanical

means.54,55 The flowability of irregularly shapedpowders was reportedly improved by removingsharp angles on the particles through high-speedblending or high sheer milling. Nonetheless, theparticles produced by this method are only quasi-spherical shaped, which may limit its applications.

CONCLUSION

The features of the commercial and emergingspherical Ti powder making methods are summa-rized in Table I.

Recently, many large companies including GE,GKN, Praxair, and Carpenter Technology enteredthe market of spherical Ti powder, motivated pri-marily by the rapid expansion of additive manufac-turing. The dominant commercial processes todayare gas atomization, plasma atomization, andplasma rotating electrode process. Spherical Tipowder produced by any one of these three methodshas its advantages as well as its disadvantages.Great progress has been made toward improvingthe yield of fine powder by these three relativelymature processes. Nevertheless, there is a strongmarket demand for further improvements to reducethe cost to make the fine powder affordable forbroader range of end-use applications. Newer pro-duction techniques such as plasma spheroidizationand the GSD process are yet to realize their marketpotential.

ACKNOWLEDGEMENTS

The authors acknowledge the funding support bythe Advanced Research Project Agency for Energy(ARPA-E) of the U.S. DOE (DE-AR0000420)through the Modern Electro/Thermochemical Ad-vances in Light-Metal Systems (METALS) program.

Table I. Key features of different spherical Ti powder-making methods

Methods Feed material Size range (lm) Advantage Disadvantage

FFGA Elemental/ingot/bar <300 Flexible alloy andfeedstock options

Satellites; possibleporosity in coarsepowder; possible

ceramic contaminationEIGA Bar <200 Relative high fine

powder yieldPossible porosity in

coarse powder; relativehigh Ar flow rate

PA Wire <300 Less satellite; relativehigh fine powder yield

Expensive feedstockwire; possible porosity

in coarse powder;limited feedstock options

PREP Bar 50–350 High purity; no satellites Low fine powder yieldPS Powder >5 (the same size with

feed powder)Relative low cost, high

fine powder yieldSubject to availability

of low-oxygen feedpowder

GSD Scrap/elemental 10–100 Very high fine powderyield; minimal satellites,

low cost

Not perfect sphericalshape; possible porosity

Review of the Methods for Production of Spherical Ti and Ti Alloy Powder 1859

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