Powder Technology 254 (2014) 235–240

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Influence of process parameters on the characteristics of TiAl alloyedpowders by fluidized bed jet milling

Xin Lu a,b, Cheng-cheng Liu b, Lang-ping Zhu c, Xuan-hui Qu a,b,⁎a State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, Chinab School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, Chinac Beijing Institute of Aeronautical Materials, Beijing 100095, China

⁎ Corresponding author at: State Key Laboratory for AUniversity of Science and Technology Beijing, Beijing 1002700; fax: +86 10 6233 4311.

E-mail address: quxh@ustb.edu.cn (X. Qu).

0032-5910/$ – see front matter © 2014 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.powtec.2013.11.013

a b s t r a c t

a r t i c l e i n f o

Article history:Received 18 July 2013Received in revised form 25 October 2013Accepted 8 November 2013Available online 30 December 2013

Keywords:High Nb containing TiAl alloysPowder preparationFluidized bed jet millingPowder characteristicsProcess parametersBreakage mechanism

Micro-fine high Nb containing TiAl alloyed powders were prepared by using fluidized bed jet milling from thepre-crushed Ti–45Al–8.5Nb–0.2W–0.2B–0.02Y(at.%) ingot. The effects of process parameters on the powdercharacteristics were investigated. The results show that the particle characteristics are mainly determined byclassifier frequency. At or above 5600 rpm, the ground powders are composed of coarse block-shaped particlesand finer flaky debris, with particle number-average diameter lower than 18 μm. With the increase of classifierfrequency and the decrease of grinding gas pressure, the amount of flaky debris formed under abrasion mecha-nism gradually increases, which causes the obvious decrease of the powder number-average diameter, distribu-tion uniformity, and sphericity degree. With the classifier frequency of 8400 rpm and grinding gas pressure of0.55 MPa, the obtained powders possess the minimum particle size of 5.32 μm, with a uniformity level of 0.38and a Wadell's sphericity factor of 0.820.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

With low density, high strength and corrosion resistance at elevatedtemperatures, high Nb containing TiAl alloys have attracted consider-able attention as candidate materials for high-temperature structuralapplication in aerospace, automotive and gas turbine industries [1–6].However, the low room-temperature ductility and workability are stillthe major obstacles to their wide application. In order to overcomethe bottlenecks, many efforts have been devoted to the research ofsuitable fabrication techniques for the alloys. Comparedwith ingotmet-allurgy (IM), powder metallurgy (PM) has unique advantages in pro-cessing complex shaped parts with less or even without furthermachining for energy and resource saving, and PM parts always exhibitfiner grains and uniform microstructure. Therefore, PM techniques willbe one of the major ways of promoting the application of high Nb con-taining TiAl based alloys [7,8].

The preparation of TiAl based alloyed powders with high propertiesis the basis for improving the PM products performance. So far, TiAlbased alloy powders are usually processed by plasma rotating electrodeatomization or inert gas atomization with non-crucible melting. How-ever, the powders obtained usually contain many pores and coarseparticles, and especially the proportion of particles under 30 μm is

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extremely low. As one of ultra-fine comminution technology, fluidizedbed jet milling has significant advantages in preparingmicro-sized par-ticles with narrow size distribution. The particles are accelerated in aninert gas jet stream and reduced in size during the interparticle collisionor impact against a solid surface, which make the process free of con-tamination [9–11]. The powder preparation technique has been widelyused to refinemineral, chemical materials, and some ceramic andmetalpowders [12–14], but the open literatures on fabrication of TiAl basedalloys are still very limited.

In the present work, micro-fine high Nb containing TiAl alloyedpowders were processed by fluidized bed jet milling from alloy ingot.The effects of the process parameters on powder characteristics andbreakage mechanism were investigated.

2. Experimental

High Nb containing TiAl alloy ingot was produced by a water-cooledcopper crucible induction melting furnace with a nominal compositionof Ti–45Al–8.5Nb–(W, B, Y). The ingot was chipped into scrapes withsize lower than 300 μm in an inert atmosphere. And as the feed, thecoarse particles were comminuted by QLMR-150T fluidized bed jetmill equipped with a forced vortex classifier. The grinding process wasdetermined by the operational parameters such as classifier frequency,grinding gas pressure, and feed quantity. In this grinding work, thefeed mass was kept constant at 2 kg, the classifier frequency was setat the range of 4200–8400 rpm, and high-purity argon was used asthe grinding gas with pressure range of 0.55–0.70 MPa.

236 X. Lu et al. / Powder Technology 254 (2014) 235–240

The phase constitutions of the feed and the ground powders werecharacterized by a Siemens D 5000 X-ray diffraction meter using Cu ra-diation. The powder microstructural observations were performed by aLWO1450 scanning electron microscopy (SEM). The oxygen andnitrogen contents were measured by the inert gas pulses-infrared ther-mal conductivity method, and carbon content by high-frequencycombustion-infrared measurement. The particle size and distributionwas examined by using a LMS-30 laser particle size analyzer. The pa-rameters of number-average diameter (Dn), size distribution uniformity(U), and sphericity

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffidi−1 � didi ¼

pfactor (Ψ) were chosen to denote the

particle size and shape characteristics of the powder specimens. Eachof them can be respectively derived from the detected particle size dis-tribution data. The value of Dn (D(1,0)) can be calculated by Eq. (1),where (di means detected particle diameter), and ni (%) indicates thepercentage of the particles with the diameter of di (μm).

D 1;0ð Þ ¼X

nidiXni

ð1Þ

The value ofU can be used to evaluate the uniformity level of particlesize distribution, and is calculated by using Eq. (2). A higher value of Uindicates a better uniformity of particle size distribution. In general, Uvalue is less than 1, and it is equal to 1 only for the powders consistingof mono-sized particles.

U ¼ D 1;0ð ÞD 4;3ð Þ ¼

Xnidi �

Xnid

3iX

ni �X

nid4i

ð2Þ

Wadell's sphericity factor, denoted asΨ, is defined as the ratio of thesurface area of a sphere with the same volume as a particle to the sur-face area of this particle [15–17]. When dealing with particle group, Ψis evolved as the ratio of the specific surface area per unit volume ofspheres with same particle number as a group to the actual specific sur-face area per unit volume of this particle group. Then Ψ value can be

Fig. 1. Particle size distributions of TiAl powders at various process param

used to quantify the particle shape of powders, and can be deduced byEq. (3), where Sv is the specific surface area per unit volume of powdersample. Assuming there is a groupofmono-sized sphere particleswhichhas the same total volume and number as the powder sample men-tioned above, S0 is the specific surface area per unit of the group, andD(3,0) indicates the diameter of the sphere particle. Ψ is generallylower than 1, and it has the highest value of 1 for the powders onlyconsisting of particles with ideal spherical shape.

Ψ ¼ S0Sv

¼ 6D 3;0ð Þ � Sv

¼ 6 �X

ni

Sv �X

nid3i

ð3Þ

3. Results

3.1. Particle size and distribution

Fig. 1 shows the particle size distributions of the powders ground atdifferent classifier frequencies and grinding gas pressures. It is clear thatthe process parameters of jet mill have a great impact on the particlesize distribution of the ground produces. Furthermore, the influence ofclassifier frequency takes a leading position compared with that ofgrinding gas pressure. At a lower frequency of 4200 rpm, the particlesize distribution is very narrow, and shows symmetric unimodal withthe logarithm of particle size. With the increase of classifier frequency,theproportion of thefineparticles in the size range of 1–10 μmobvious-ly increases, which leads to a wide bimodal particle size distribution. Inaddition, it is noted that the peak corresponding to the size range of 1–10 μm is basically fixed at 4.2 μm.Whereas the other distribution peakfor the part of particles larger than 10 μm is obviously shifted left to-wards lower values, and the corresponding distribution span for thesize range becomes narrow with classifier frequency increasing. Tillthe frequency rises above 7000 rpm, the second size distribution peaktends to fade out.

eters: (a) 4200 rpm; (b) 5600 rpm; (c) 7000 rpm; (d) 8400 rpm.

237X. Lu et al. / Powder Technology 254 (2014) 235–240

Grinding gas pressures also show a slight influence on the particlesize distribution. For the powder ground at a lower frequency with par-ticle size distribution of single peak feature, the distribution peak has asmall left shift from 38.9 μm to 32.8 μm as the grinding gas pressure in-creasing from 0.55 MPa to 0.70 MPa, as shown in Fig. 1(a). On the con-trary, at or above 5600 rpm, two peaks of the bimodal particle sizedistribution curve both gradually shift left as grinding pressure decreas-ing, Fig. 1(b)–(d). Moreover, the peak for the particle size range of 1–10 μm continues to grow up, while the peak for the part of particleslarger than 10 μm is gradually lowered.

Fig. 2 shows the characteristic parameter values of the powdersground at various classifier frequencies and grinding gas pressures,which are number-average diameter (Dn), particle size distribution uni-formity (U), and sphericity factor (Ψ), respectively. It is noted that thefeed powder can be efficiently refined by jet milling, and the powdercharacteristics are mainly controlled by classifier frequency. Thenumber-average diameter, D(1,0), sharply decreases with the rise offrequency at same condition of grinding gas pressure, Fig. 2(a). At4200 rpm, D(1,0) rapidly reduces to about 34 μm from feed of 200–300 μm, and then further decreased to about 6 μm for 8400 rpm.While at different level of classifier frequency, the influence of grindinggas pressure on average particle size is different. At a lower level of4200 rpm, the D(1,0) value gradually decreases with the rise of grind-ing pressure. However, at or above 5600 rpm, it tends to increasewith the value lower than 18 μm.

The particle size distribution uniformity (U) of the ground powdersis very sensitive to the process parameters, Fig. 2(b). In general, the Uvalue rapidly decreases with the rise of classifier frequency before7000 rpm, and then it shows little rebound at 8400 rpm. The powdersground at 4200 rpm present the highest U value of about 0.80, whichdoes not give rise to a clear change with varying grinding pressure.This is corresponding to the narrow particle size distribution, asshown in Fig. 1(a). At a higher classifier frequency, the decrease of the

Fig. 2. Characteristic parameters of ground powders at various process parameters: (a) numberfactor.

U value indicates the worse of size distribution uniformity, which isconsistent with the bimodal features of the distribution curves,Figs. 1(b)–(d). At 5600 rpm, theU value has a distinct increase as grind-ing pressure rising up, but the variation becomes gentle for the furtherhigher classifier frequency.

The change of particle sphericity factor (Ψ) with process parametersfollows the similar tendency as that of distribution uniformity, Fig. 2(c).The powders ground at 4200 rpm possess the highest Ψ value, whichslightly increases with the decrease of the grinding gas pressure. TheΨ value of the powders ground at 7000 rpm is lowest, and does notshow any variation with grinding gas pressure. However, at 8400 rpm,the value is obviously enhanced, and slightly decreases with the grind-ing gas pressure increasing.

3.2. Morphological changes

Fig. 3 presents the morphological variations of the alloyed powdersground at different process parameters. It is clear that the particle sizeobviously declines with the increase of classifier frequency. The pow-ders ground at 4200 rpm mainly consist of uniform block or plate-shaped particles, Fig. 3(a)–(c). Whereas at or above 5600 rpm, theground powders are composed of two different sizes of particles: coarseblock-shaped particles and some much finer flaky debris, as shown inFig. 3(d)–(l). And with frequency increasing, the block-shaped particlesize significantly decreases, while the proportion of the debris graduallyincreases. By grinding at 8400 rpm, the powders are mainly composedof finer debris with a small number of coarse particles scatteredamong them. This is consistent to the change of the particle distributioncurves, as shown in Fig. 1. In addition, it is noted that, at the same con-dition of classifier frequency, higher level of grinding gas pressure yieldsirregular-shaped particles with sharp edges. And as grinding gas pres-sure decreasing, the block-shaped particles tend to be smooth androunded, meanwhile the number of the finer flaky debris increases.

-average particle diameter; (b) particle size distribution uniformity; (c) particle sphericity

Fig. 3. SEM images of particles ground at various process parameters.

Fig. 4. XRD patterns of the feed and the powders ground at different classifier frequencies(0.65 MPa).

238 X. Lu et al. / Powder Technology 254 (2014) 235–240

3.3. Phase constitution

The phase constitution analysis results show that, at the same classi-fier frequency, there is no distinct difference for the powder samplesground at different grinding pressures. Then Fig. 4 just lists the XRD pat-terns of the feed powder and the powders ground at different classifierfrequencies but same grinding pressure of 0.65 MPa. The clear diffrac-tion peaks suggest that the ground powders mainly consists of γ-TiAlwith a small amount of α2-Ti3Al, similar to the feed. With the meansize of the ground powders decreasing, the diffraction peaks changeslightly, and don't broaden as obviously as those of ball milled powders[18]. The difference ismainly caused bybreakagemechanism. For the jetmill grinding, the breakage is mainly caused by the impact of the colli-sion among particles. Plastic deformation of the particles is seldom

239X. Lu et al. / Powder Technology 254 (2014) 235–240

occurred, and the lattice distortion is not severe compared with thoseoccurring during ball milling processing.

3.4. Impurity contents

The oxygen, carbon and nitrogen contents of the alloyed powdersduring the fabrication process are listed in Table 1. It is clear that, com-pared with the feedstock ingot material, the impurity contents of thejet milled powders inevitably increase. The oxygen content strongly de-pends on the particle size. The oxygen content of the scraps chipped fromthe ingot is 1.2 wt.‰, and after jetmilling, the content shapely increases.The oxygen content is about 2.0 wt.‰ for the ground powders withaverage-number particle size of 34 μm; 2.9 wt.‰ for the 16 μm pow-ders, and up to 4.3 wt.‰ for 6 μmpowders ground at 8400 rpm. The in-crease of oxygen content is caused by the larger specific surface areaand higher activation of finer particles. The nitrogen content of theground powders is about 0.25 wt.‰, and changes slightly with the var-iation of the particle size. In addition, there is about 0.06 wt.‰ of carbonin the ground powders, whichmay comes from the machining process.

4. Discussion

In general, jet milling system mainly consists of grinding chamberand classifying chamber. After entering grinding chamber, the raw par-ticles are accelerated by means of a jet stream of carrier gas, and me-chanical impact collision drastically occurs among inter-particles andrigid obstacles with extremely high energy. As a result, the powder par-ticles can be efficiently smashed and refined. After grinding, the parti-cles are carried out by gas and flow up to the classifier chamber withrotating rotor, where classification takes place. In the chamber, the par-ticles are simultaneously subjected to gas flow drag force and rotationcentrifugal force. The centrifugal force is proportional to particle weightand the rotating speed of rotor. If the centrifugal force is greater than thedrag force, particles will be sent back into the grinding chamber for fur-ther grinding.When theparticles are reduced in size, and the centrifugalforce is smaller than the drag force, particles will be transported by gasflow through the rotor and collected. The particle size for which centrif-ugal force equals drag force is called cut size. The value of cut size, xc, canbe calculated by using Eq. 4 [19].

xc ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi18ηvrrρsv

2φ

sð4Þ

where r is rotor radius, vφ is the circumferential velocity, vr is the radialvelocity, η is the dynamic viscosity, and ρs is the density of solids.

The breakagemechanism of the TiAl based alloy powders during thejet milling is shown in Fig. 5. At the initial stage of grinding, stress valuegenerated by the collision is much higher than the critical stress of brit-tle fracture for materials, leading to the occurrence of primary destruc-tive breakage for coarse particles. For the raw coarse TiAl based alloyedpowders, the microstructure consists of α2/γ lamellar colonies, andunder impact the microcracks prefer to nucleate and propagate via aninterlamellar mode with relatively low resistance[20]. When the crackencounters nearly perpendicular lamella within a grain, translamellarfracture first occurs. Therefore, the fracture mode of powder is

Table 1Impurity contents of alloyed powders during fabrication process (wt.‰).

Impurity Alloy ingot Scraps

O 0.8 1.2N 0.08 0.14C – 0.03

interlamellar and slightly deviated translamellar during primarycrushing. As grinding progress continues, the particles are refined high-ly. Smaller particles are more difficult to break owning to the fewerflaws, which makes abrasion and fatigue breakages tend to play a lead-ing role, as shown in Fig. 5. By abrasion breakage, particles becomesmooth with sharp edges trimmed off, and the outer layers and edgesseparated from particle surface form flaky debris with much finer size.Meanwhile, repeated impact collision makes the microcracks initiatedand propagated again within particles, and then the particles suffereda brittle fracture due to fatigue. In summary, the breakage mechanismof the jet milled TiAl alloy powders includes primary destructive, abra-sion, and fatigue breakage, depending onmilling conditions and particlecharacteristics.

The milling conditions which directly influence the breakage mech-anism are controlled by the operating parameters. Among them, classi-fier frequency represents the rotation speed of wheel classifiers. FromEq. (4), it can be deduced that the particle size of ground powders isdominated by classifier frequency. At a lower frequency correspondingto a higher theoretical particle cut size, the particles are prone to sufferprimary destructive breakage in grinding chamber, which make thepowders composed of uniform block-shaped particles. Therefore, thepowders ground at 4200 rpm exhibit the highest average particle size,uniformity level and Wadell's sphericity factor. Besides, grinding gaspressure also has an impact on the breakage mechanism and particlecharacteristics. Higher grinding pressure makes particles get higher ac-celeration and higher collision energy, which raise the odds of destruc-tive breakage. Consequently, in the case of 4200 rpm with mechanismdominated by destructive breakage, the average particle size tends todecrease with the rise of grinding pressure. Meanwhile, the action ofabrasion breakages declines, leading to a slight reduction of particlesphericity factor.

Destructive breakage brings the efficient reduction in particle size.Hence, when the classifier frequency rises from 4200 rpm to5600 rpm, the size distribution peak rapidly shifts left towards thelower values. The decrease of particle size makes abrasion and fatiguebreakages tend to play an important role, which produces more flakydebris among block-shaped particles. With the further increase of clas-sifier frequency, the proportion of finer debris gradually grows up,which causes the simultaneous decline of average particle diameter,uniformity level and Wadell's sphericity factor. In addition, decreasinggrinding gas pressure also contributes to the rise of the flaky debris pro-portion, as a result, the powder ground at 0.55 MPa possesses the low-est values of three powder characteristic parameters above. Tillclassifier frequency is up to 8400 rpm, finer debris is in the vast major-ity, and the concentrated particle size distribution causes the slightpicking up of the uniformity level. And the repeated abrasion amongfiner debris makes edges smoother, and then the sphericity factor ofground powders rises again. At this case, the sphericity factor increaseswith the decrease of grinding gas pressure, mainly because droppingthe grinding gas pressure also brings to the enhancement of the abra-sion among finer debris.

5. Conclusions

The characterization of high Nb containing TiAl alloyed powders byfluidized bed jet milling was investigated. The following conclusionscan be drawn from the discussion:

Jet milled powders with different average particle sizes

34 μm 16 μm 6 μm

2.0 2.9 4.30.25 0.27 0.250.06 0.06 0.08

Fig. 5. Breakage mechanism of TiAl powder during jet milling.

240 X. Lu et al. / Powder Technology 254 (2014) 235–240

(1). The micro-fine high Nb containing TiAl alloyed powders withcontrolled average particle size can be efficiently prepared inquantity by fluidized bed jet milling from the pre-crushed alloyingot.

(2). The particle characteristics are mainly determined by classifierfrequency. At or above 5600 rpm, the powders are composed ofcoarse block-shaped particles and finer flaky debris, with particlenumber-average diameter lower than 18 μm. With the furtherrise of classifier frequency, the amount of debris gradually in-creases, leading to the rapid decrease of average diameter, uni-formity level and Wadell's sphericity factor. Till 7000 rpm, finerdebris is in the vast majority, and sphericity factor begins torise again.

(3). Grinding gas pressure has a certain impact on particle character-istics and breakagemechanism.Decreasing grinding gas pressurecauses the enhancement of abrasion breakage, leading to the riseof the flaky debris proportion.

(4). The breakagemechanism of the jetmilled TiAl alloy powders de-pends on milling conditions and particle characteristics. Coarseparticles are dominated by destructive breakage, with fracturemode of interlamellar and slightly deviated translamellar. Asthe grinding process continues, the particles are efficiently re-fined. Abrasion and fatigue breakage tend to play a leading role,which producesmoreflaky debris amongblock-shaped particles.

(5). With the classifier frequency of 8400 rpm and the grinding gaspressure of 0.55 MPa, the obtained powders possess the mini-mum particle size of 5.32 μm with a uniformity level of 0.38and a Wadell's sphericity factor of 0.820.

Acknowledgments

This research was sponsored by the National Natural Science Foun-dation of China (No. 51204015) and the Fundamental Research Fundsfor the Central Universities (FRF-TP-12-149A).

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