Materials and Design 30 (2009) 2903–2910

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Materials and Design

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Balling phenomena in direct laser sintering of stainless steel powder:Metallurgical mechanisms and control methods

Dongdong Gu *, Yifu ShenCollege of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing 210016, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 18 October 2008Accepted 12 January 2009Available online 20 January 2009

Keywords:Ferrous metals and alloys (A)Sintering (C)Microstructure (F)

0261-3069/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.matdes.2009.01.013

* Corresponding author. Fax: +86 25 52112626.E-mail addresses: dongdonggu@nuaa.edu.cn, dongd

Balling effect, as an unfavorable defect associated with direct metal laser sintering (DMLS), is a complexphysical metallurgical process. In this work, two kinds of balling phenomena during DMLS of 316L stain-less steel powder were investigated and the metallurgical mechanisms of balling were elucidated. It wasfound that using a low laser power gave rise to the first kind of balling characterized by highly coarsenedballs possessing an interrupted dendritic structure in the surface layer of balls. A limited amount of liquidformation and a low undercooling degree of the melt due to a low laser input was responsible for its ini-tiation. The second kind of balling featured by a large amount of micrometer-scaled (�10 lm) balls onlaser sintered surface occurred at a high scan speed. Its formation was ascribed to laser-induced meltsplashes caused by a high capillary instability of the melt. Feasible control methods were proposed toalleviate balling phenomena. It showed that increasing the volumetric density of energy input, whichwas realized by increasing laser power, lowering scan speed, or decreasing powder layer thickness,decreased the tendency of balling. The addition of a trace amount of deoxidant (H3BO3 and KBF4) inthe powder yielded a smooth laser sintered surface free of balling.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Direct metal laser sintering (DMLS), as a typical rapid prototyp-ing (RP) technique, enables the quick production of complex-shaped three-dimensional (3D) components directly from metalpowder with no or minimal pre-processing and/or post-processingrequirements [1–5]. In this method, an object is created by selec-tively fusing thin layers of powder using a scanning laser beamaccording to CAD data of the part. DMLS, thus, exhibits a high po-tential for the net-shape fabrication of high performance engineer-ing metal parts having complicated configurations that cannot beeasily produced by other conventional processing methods [6–10].

Although recent efforts in DMLS have improved this techniqueconsiderably, there still exist many defects handicapping the suc-cessful fabrication of high-quality metal parts with favorablemicrostructures and properties, because of the complex nature ofDMLS in which both physical and chemical metallurgical phenom-ena may occur [11–15]. The typical process defects associated withDMLS include porosity, distortion, and delamination, but a moresubstantial problem is balling phenomenon [16–20]. Laser-inducedballing effect significantly influences the thermodynamic and ki-netic characteristics during sintering. Since DMLS is carried outline by line, balling phenomenon may result in the formation of

ll rights reserved.

onggu@hotmail.com (D. Gu).

discontinuous scan tracks. On the other hand, since DMLS is alayer-by-layer additive manufacturing process, balling phenome-non is a severe impediment to the uniform deposition of a freshpowder on the previously sintered layer and tends to cause poros-ity and even delamination induced by poor inter-layer bonding incombination with thermal stress [21–25]. Therefore, balling effectoccurred in laser sintered powder severely degrades the DMLSquality. Previous work performed by Niu and Chang [16], Tolochkoet al. [19], Das [20], and Simchi et al. [21] has provided some pre-liminary understanding of the balling problem. Nevertheless, ball-ing effect is a complex metallurgical process that is controlled byboth powder material properties and laser processing conditions.A further study and understanding of the physical nature of ballingeffect and the relevant control methods is, thus, regardednecessary.

The Cr element, as a primary alloying element in stainless steel(SS) materials, is a very active element to oxygen. Some degree ofoxidation, thus, cannot be avoided under normal DMLS processingconditions of SS powder [20]. Consequently, balling problem ismore likely to occur during DMLS of SS powder due to a contami-nation layer of oxide being present on the surfaces of SS melt. Inthe present paper, balling phenomena during direct laser sinteringof pre-alloyed 316L SS powder were investigated. The effects ofprocessing parameters (e.g., laser power, scan speed, and powderlayer thickness) and powder properties on the balling effect werehighlighted, with an aim to determine feasible control methodsto avoid balling phenomena.

Fig. 2. Process map for DMLS of Powder A for the estimation of parametric effects. Aseries of single line scan tracks are prepared using various processing parametersindicated by different symbols in the map. DMLS using the processing parametersindicated by the same symbols yields a similar mechanism of laser–powderinteraction. Fixed parameter is d = 0.25 mm.

2904 D. Gu, Y. Shen / Materials and Design 30 (2009) 2903–2910

2. Experimental procedures

2.1. Materials

Two kinds of 316L SS powders (supplier: Haining Feida Metal-lurgy Powder Co. Ltd., PR China) were used in the current study.Powder A had an irregular angular shape, with the particle size dis-tribution of 42–155 lm and the mean particle size of 75 lm(Fig. 1a). Powder B possessed a spherical shape, with the particlesize distribution of 6–38 lm and the average particle size of20 lm (Fig. 1b). Elemental compositions of SS powders were:16.9Cr, 13.2Ni, 2.7Mo, 1.6Mn, 0.019C, 0.23Si, Fe balance. The as-re-ceived Powder A was used for laser sintering without any furthertreatment. As to Powder B, however, small-sized particles weremuch easier to suffer oxidation than large particles. Thus, a traceamount of deoxidant (0.01 wt.%), which consisted of 70 wt.%H3BO3 and 30 wt.% KBF4, was uniformly mixed with Powder Busing a Fritsch Pulverisette 6 planetary mono-mill.

2.2. Processing

The DMLS system mainly consisted of a continuous waveGaussian CO2 (k = 10.6 lm) laser with a maximum output powerof 2000 W, an automatic powder delivery system, and a computersystem for process control.

Single line sintering experiments were primarily performedusing Powder A. Prior to laser process, a carbon steel substratewas placed on the building platform and leveled. A uniform0.25 mm thick powder layer was then deposited on the substrateby the roller. Afterwards, single line scan tests were carried outby scanning 50 mm long tracks across the powder bed. A 10 mmgap between adjacent lines was used to avoid any interaction be-tween neighboring sintering tracks.

Multiple layer (30 layers) rectangular samples with each layerdimensions of 50 mm � 10 mm were produced by repeatedly scan-ning Powder B in a layer-by-layer fashion. A simple linear rasterscan pattern with a long scan vector (50 mm) was utilized. Theprocessing parameters used were as follows: spot size (D)0.30 mm, laser power (P) 250–500 W, scan speed (v) 0.04–0.11 m/s, scan line spacing (h) 0.15 mm, and powder layer thick-ness (d) 0.10–0.25 mm.

2.3. Characterization

Photographs of the real-time laser sintering process were re-corded using a Kodak DX7590 digital camera. Samples for metallo-

Fig. 1. SEM images showing characteristic morphologies of SS powd

graphic examinations were prepared according to standardprocedures and etched in a solution containing 25 vol.% HNO3

and 75 vol.% HCl. Surface morphologies and microstructural fea-tures of samples were characterized using a Quanta 200 scanningelectron microscope (SEM).

3. Results

3.1. Process map

Fig. 2 depicts a parametric dependent overview of the mecha-nisms of single line laser scanning on powder bed. Over the entirerange of laser powers and scan speeds, the following four processwindows are defined to characterize the different laser–powderinteraction mechanisms:

I Tracks without consolidation. At a low laser power (6300 W),the energy delivered was insufficient to have any effect onthe powder. Powder particles were thus remained unsin-tered without any consolidation.

II Tracks with significantly coarsened balls. Laser sintering at arelatively high laser power (300–350 W) could produce aneffect on the powder, but was insufficient to cause the signif-

er: (a) irregular shape, Powder A; (b) spherical shape, Powder B.

D. Gu, Y. Shen / Materials and Design 30 (2009) 2903–2910 2905

icant melting of the powder, resulting in the formation ofconsiderably coarsened balls approximately the diameter ofthe laser beam.

III Tracks with small-sized balls and/or cracks. At an even highlaser power (P350 W) combined with a high scan speed(P0.07 m/s), a liquid scan track of cylindrical shape wasgenerated. A reduction in surface energy and a high instabil-ity of the melt produced rough tracks consisting of a num-ber of small-sized balls and/or microscopic cracks on theirsurfaces.

IV Continuous and smooth tracks. Using a laser power P400 Wand a scan speed 60.09 m/s led to a sufficient amount ofliquid formation with a reasonable liquid stability, yieldingcontinuous and smooth sintered tracks free of balling aftersolidification.

3.2. Microstructural characterizations of balling phenomena

The established process map (Fig. 2) revealed the diversity ofballing phenomena under different laser processing conditions. Inthis section, microstructural features of various kinds of ballingphenomena were further characterized.

Fig. 3 illustrates the characteristic microstructures of laser sin-tered tracks of Powder A with various laser powers. At a relativelylow laser power of 350 W, significantly coarsened metallic ballswith an average diameter of �350 lm were present. More seri-ously, no efficient inter-ball bonding was obtainable (Fig. 3a). Withincreasing laser power to 400 W, sintering necks were formed be-tween the neighboring spherical-shaped sintered agglomerates,leading to a stronger inter-agglomerate bonding (Fig. 3b).

As revealed in Fig. 3, the surfaces of the sintered tracks of Pow-der A were melted by the laser beam interaction and resolidified.The surface layer of the obtained sintered tracks was ground, pol-ished, and etched to perform the metallographic investigations,with the etched microstructures provided in Fig. 4. Laser sinteringat a lower laser power of 350 W produced discontinuous networksof disorderly dendrites (Fig. 4a). In this instance, the dendriticspacing was generally larger than 5 lm (Fig. 4b). At a higher laserpower of 400 W, well-developed dendrites were formed in lasersintered structure (Fig. 4c). A high-magnification SEM image re-vealed a unidirectional growth of dendrites along a certain pre-ferred orientation, with the primary dendritic spacing of �1.5 lm(Fig. 4d).

Fig. 3. SEM images showing typical microstructures of laser sintered tracks at differentd = 0.25 mm.

The real-time laser sintering processes of Powder A with vari-ous scan speeds are shown in Fig. 5. Laser sintering using a higherscan speed of 0.10 m/s produced a long thin melt pool, resulting ina significant splash of molten materials (Fig. 5a). Interestingly, at alower scan speed of 0.05 m/s, the powder was smoothly sintered,yielding a small-sized melt pool (Fig. 5b).

The influence of scan speed on typical morphologies of lasersintered tracks of Powder A is revealed in Fig. 6. At a lower scanspeed of 0.05 m/s, a continuous sintered track was formed viathe sufficient junction of inter-agglomerate sintering necks, show-ing no apparent balling effect (Fig. 6a). With increasing scan speedto 0.07 m/s, the surface of laser sintered track was considerablyrough, although the sintered track retained uninterrupted(Fig. 6b). A high-magnification SEM image showed that a largeamount of small-sized spherical balls with an average diameterof �10 lm were present around the sintered track (Fig. 6c). At aneven higher scan speed of 0.10 m/s, the similar micrometer-scaledmetallic balls were visible on the surface of laser sintered track(Fig. 6d and e). Also, narrow and deep longitudinal cracks, as selec-tively indicated in Fig. 6f, were formed in laser sintered track.

Fig. 7 shows the characteristic surface morphologies of lasersintered multi-layer samples of Powder B using different powderlayer thicknesses. At a relatively high layer thickness of 0.25 mm,a number of small-sized metal balls were visible among the highlycoarsened sintered agglomerates, resulting in a heterogeneous por-ous surface (Fig. 7a). As the layer thickness decreased to 0.15 mm, anear fully dense sintered surface consisting of a small amount ofdispersed metal balls was obtained (Fig. 7b). Interestingly, lasersintering using a 0.10 mm thick powder layer yielded a smoothand completely dense sintered surface free of any balling phenom-enon (Fig. 7c).

4. Discussion

4.1. First kind of balling: coarsened balls caused by limited liquidformation

During DMLS, the duration of the laser beam at any irradiatingregion depends on spot size and scan speed, and is typically lessthan 4 ms [1,2]. It is well known that under such a considerablyshort heating cycle, a rapid liquid phase sintering (LPS) is the onlyreasonable mechanism for a successful DMLS [3,4,21]. SS powder,as typical pre-alloyed powder, melts incongruently within a tem-

laser powers: (a) P = 350 W; (b) P = 400 W. Fixed parameters are v = 0.04 m/s and

Fig. 4. SEM images showing etched microstructures of laser sintered tracks at various laser powers: (a) P = 350 W; (c) P = 400 W; (b) and (d) are local magnifications of (a)and (c), respectively. Fixed parameters are v = 0.04 m/s and d = 0.25 mm.

Fig. 5. Photographs showing real-time laser sintering processes at different scan speeds: (a) v = 0.10 m/s; (b) v = 0.05 m/s.

2906 D. Gu, Y. Shen / Materials and Design 30 (2009) 2903–2910

perature range between solidus and liquidus temperatures. Thus, asupersolidus LPS mechanism is feasible for DMLS of SS powder.

On the other hand, laser sintering is performed line by line andthe energy is absorbed by powder particles through bulk-couplingand powder-coupling mechanisms [26]. Primarily, the energy isabsorbed in a narrow layer of individual powder particles deter-mined by the bulk properties of SS powder. This leads to a high

temperature rise of the surface of particles, thereby realizing liquidphase formation via surface melting of particles. The heat, subse-quently, flows mainly towards the center of the remained coresof particles until a local steady state of the temperature withinthe sintering system is reached.

The amount of liquid formation depends on the operatingtemperature of the sintering system, which is controlled by two

Fig. 6. SEM characterization of laser sintered tracks at different scan speeds: (a) v = 0.05 m/s; (b) v = 0.07 m/s; (d) v = 0.10 m/s; (c) is local magnification of (b); (e) and (f) arelocal magnifications of (d). P = 400 W and d = 0.25 mm are constant.

D. Gu, Y. Shen / Materials and Design 30 (2009) 2903–2910 2907

main parameters, i.e., laser power and scan speed, during a singleline scanning. For a given scan speed, the supersolidus sinteringtemperature decreases with lowering the laser power, resultingin a smaller amount of liquid formation. The viscosity of the li-quid–solid mixture, thus, becomes considerably high, handicap-ping liquid flow and particle rearrangement. This in turndecreases the overall rheological performance of the liquid inconjunction with solid particles [27]. Consequently, the moltenmaterials in each spot irradiating zone tend to aggregate into

an individual coarsened sphere approximately the diameter of la-ser beam (Fig. 3a). The initiation of this kind of balling phenom-enon is schematically depicted in Fig. 8. In this situation, noefficient bonding is obtained between the neighboring balls, sincethe limited amount of liquid prevents the sufficient growth ofsintering necks between metal agglomerates (Fig. 3a). On theother hand, a low laser power results in a limited undercoolingdegree of the melt [28–30]. Thus, coarsened and discontinuousdendritic structures are developed in surface layer of these solid-

Fig. 7. SEM images showing characteristic surface morphologies of laser sintered multi-layer samples at different powder layer thicknesses: (a) d = 0.25 mm; (b)d = 0.15 mm; (c) d = 0.10 mm. P = 375 W, v = 0.04 m/s, and h = 0.15 mm are fixed.

No sintering necks formed

Limited liquid formation;Balling occurrence

Sintering necks formed

Sufficient liquid formation;Continuous sintered track

Laser beam

Powder bed

Unmelted cores Liquid

Fig. 8. Schematic of first kind of balling phenomenon featured by coarsened spherical-shaped sintered agglomerates caused by limited liquid formation.

2908 D. Gu, Y. Shen / Materials and Design 30 (2009) 2903–2910

ified balls (Fig. 4a and b), producing an inherent weakness in la-ser sintered powder.

4.2. Second kind of balling: micrometer-scaled balls induced by meltsplashes

When a sufficient amount of liquid phase is generated by usinga relatively high laser power of 400 W, laser sintering at a highscan speed (typically larger than 0.07 m/s in the present study)

tends to shape the melt into a continuous cylindrical molten track(Fig. 5a), due to a considerably short dwelling time of laser spot oneach irradiating region. However, the present molten track is in ahighly unstable state. The surface energy of the liquid track willkeep decreasing, in order to obtain a final equilibrium state.According to Simchi’s results [30], the applied scan speed has a sig-nificant influence on the capillary instability of the liquid track.With increasing the scan speed used, the linear energy density oflaser input decreases, resulting in a decrease in the working tem-

D. Gu, Y. Shen / Materials and Design 30 (2009) 2903–2910 2909

perature and, accordingly, the diameter of cylindrical molten track.The melt instability, consequently, increases significantly. Underthis condition, a number of small-sized liquid droplets tend tosplash from the surface of the molten track, due to the reductionin the surface energy of liquid at short length scales. After solidifi-cation, a large amount of micrometer-scaled spherical splashes areformed around the sintered surface (Fig. 6b–e), resulting in the sec-ond of balling phenomenon, as schematically illustrated in Fig. 9. Itis noted that at an even higher scan speed (P0.10 m/s in thisstudy), a significantly elevated instability of the liquid cylindertends to alter its shape to reduce the surface energy, besides theoccurrence of the second kind of balling effect. This causes thebreaking up of the molten cylinder to reach a final equilibriumstate, thereby producing longitudinal cracks in the finally solidifiedtracks (Fig. 6d and f).

4.3. Control methods of balling effect

A close look at Figs. 3 and 4 reveals that the first kind of ballingeffect can be alleviated by means of increasing laser power, so as toimprove the structural refinement and continuity of laser pro-cessed materials. The second kind of balling effect can be elimi-nated by using a lower scan speed, as clearly indicated in Fig. 6.Actually, during a single line scanning, the combined effect of laser

Scanning laser beam

Molten sintering track

Powder bed

Scanning laser beam

Molten sintering track

Laser-induced melt splashes

Powder bed

Scanning laser beam

Laser-induced melt splashes

Cracks

Powder bed

Incr

easi

ng la

ser s

can

spee

d

Fig. 9. Schematic of second kind of balling phenomenon characterized by small-sized balls on sintered surface caused by melt splashes during laser scanning.

power (P) and scan speed (v) can be estimated by a single param-eter termed ‘‘linear energy density, LED” (k):

k ¼ Pv ð1Þ

Eq. (1) reveals that either an increase in laser power or a decrease inscan speed leads to a higher energy input and, accordingly, an ele-vated sintering temperature above the solidus of SS powder. Thepowder, thus, exhibits a higher degree of melting, yielding a largeramount of liquid phase possessing a lower viscosity. In this situation,the liquid is much easy to flow, spread, and wet the unmelted solidparticle cores, favoring a sufficient rearrangement of particles underthe action of capillary forces exerted on them by the wetting liquid.The spreading and flattening of the molten materials on the under-lying substrate is, thus, realized, leading to a significant increase inthe diameter of the molten track (Fig. 9). According to the perturba-tion theory [16], the capillary instability of the melt decreases due toa larger cylindrical melt formed under laser irradiation, leading tothe formation of coherently bonded track free of any balling phe-nomenon after sintering (Figs. 3b and 6a). Furthermore, an increasein laser energy intensity leads to a larger degree of undercooling ofthe melt [28–30], favoring the rapid growth of dendrites in a refinedmorphology during solidification (Fig. 4c and d). In this situation, theprimary dendritic spacing reaches �1.5 lm (Fig. 4d), from which itcan be estimated that the temperature gradient and the solidifica-tion rate are larger than 1.0 � 106 K/m and 6.0 � 103 K/min, respec-tively [21,31]. Therefore, an elevated undercooling degree and theresultant high solidification rate obtained at a higher laser energydensity is believed to be the transition mechanism in which 350 Wcauses discontinuous networks of disorderly dendrites while400 W creates well-developed dendrites (Fig. 4).

On the other hand, when a multi-layer sample is produced, ball-ing effect can be well eliminated via decreasing the applied powderlayer thickness (Fig. 7). Laser sintering of a bulk part involves an-other parameter termed ‘‘volumetric energy density, VED” (e).VED combines the influence of laser power (P), scan speed (v), scanline spacing (h), and powder layer thickness (d), and is defined by:

e ¼ Pvhd

ð2Þ

From the viewpoint of energy density, one can notice that a de-crease in the powder layer thickness can also enhance the laserintensity per melt volume, although the laser energy input is con-stant (Eq. (2)). For a relatively high layer thickness of 0.25 mm inthis study, the resultant low VED makes the operating temperature,particularly near the underlying surface, considerably low, therebygiving rise to a highly steep temperature gradient between the topand the bottom of the powder layer. A high working temperature onthe surface results in an overheating and attendant severe splash ofmolten liquid, incurring the occurrence of the second kind of balling(Fig. 7a). The powder at the bottom of the layer, whereas, cannot becompletely fused together due to a limited sintering temperature,producing a high amount of residual porosity (Fig. 7a). With lower-ing the layer thickness, a thinner layer of powder is easy to bemelted down, due to a sufficient penetration depth of the laserbeam possessing an elevated VED, yielding a dense sintered struc-ture free of any balling effect (Fig. 7c).

In pre-alloyed SS powder which contains active elements tooxygen such as Cr, the molten materials are much likely to sufferoxidation during sintering [21]. The oxidation of laser processedpowder, however, influences the contact angle and resultant wet-tability. Balling, thus, occurs when the laser melted powder doesnot wet the underlying substrate due to a contamination layer ofoxide being present on the surface of the melt [15]. The spreadingof the liquid and the resultant wetting of the solid by the liquid arerelated to the interfacial energies of the solid–vapor (csv), solid–li-

2910 D. Gu, Y. Shen / Materials and Design 30 (2009) 2903–2910

quid (csl), and liquid–vapor (clv) interfaces. The spreading coeffi-cient (S) of a liquid on a solid surface is defined by Das [20]:

S ¼ csv � csl � clv ð3Þ

It is known that a large positive S favors a spontaneous liquidspreading. For a metal oxide, however, the solid–vapor surface freeenergy (csv) is much lower than the corresponding liquid–vaporsurface free energy (clv) [32], resulting in a negative S (Eq. (3)). Un-der this condition, liquid metal does not spread on and then wet theoxide films on the surface of the melt, preferring to minimize sur-face area by balling instead.

In the present study, it is worth noting that the formation ofball-free laser sintered surface of Powder B (Fig. 7c) is also attrib-uted to the addition of a trace amount of deoxidant in the powdersystem, besides the optimization of laser processing conditions. Ata temperature above the solidus of SS powder, the deoxidant mate-rials possessing low decomposition temperatures (H3BO3 at�185 �C and KBF4 at �530 �C) undergo the following chemicalreactions [33]:

KBF4 ¼ KFþ BF3 ð4aÞ2BF3 þ Cr2O3 ¼ 2CrF3 þ B2O3 ð4bÞ4H3BO3 ¼ 2B2O3 þ 6H2 " þ3O2 " ð4cÞMeOþ B2O3 ¼MeO � B2O3 ð4dÞMe2O3 þ B2O3 ¼Me2O3 � B2O3 ð4eÞ

where MeO and Me2O3 represent the metal oxides. In the aid ofdeoxidizing agent, the oxidation films on the surface of the meltcan be sufficiently mitigated, thereby purifying the sintering sys-tem. Oxide cleanliness favors a sufficient spreading and flatteningthe melt on the previously sintered layer, ensuring good wettingcharacteristics and, thus, successful layer-by-layer consolidationof SS powder without any apparent balling effect.

5. Conclusions

A detailed study has been conducted to investigate balling phe-nomena and relevant control methods in DMLS of SS powder, andthe following conclusions can be drawn:

(1) Using a low laser power gave rise to the first kind of ballingphenomenon characterized by significantly coarsened ballspossessing inherent structural weakness. A limited amountof liquid formation and a low undercooling degree of themelt due to a low laser input was responsible for itsinitiation.

(2) The second kind of balling effect featured by a large amountof micrometer-scaled balls on the laser sintered surfaceoccurred at a high scan speed. Its formation was ascribedto the laser-induced melt splashes caused by a high capillaryinstability of the melt.

(3) Increasing the input energy density, which was realized byincreasing laser power, lowering scan speed, or decreasingpowder layer thickness, gave a high feasibility in alleviatingballing phenomena.

(4) Adding a trace amount of deoxidant in the form of H3BO3

and KBF4 in the powder system favored a better yield ofmulti-layer sintered parts free of any balling.

Acknowledgements

The present work is financially supported by the National Natu-ral Science Foundation of China (Grant No. 50775113) and the Sci-entific Research Foundation for Newly Employed Talents in Nanjing

University of Aeronautics and Astronautics (Grant No. S0806-061).One of the authors (Dongdong Gu) gratefully appreciates Dr. A. Sim-chi (Department of Materials Science and Engineering, Sharif Uni-versity of Technology, Tehran, Iran) for the fruitful discussions onthe balling effect during DMLS of iron-based alloys powder.

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