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Journal of Alloys and Compounds 432 (2007) 163–166

Balling phenomena during direct laser sintering ofmulti-component Cu-based metal powder

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

Received 12 May 2006; accepted 4 June 2006Available online 10 July 2006

bstract

The balling processes during direct laser sintering of a multi-component Cu-based metal powder, which consisted of a mixture of Cu, Cu–10Snnd Cu–8.4P, were investigated. Three kinds of balling mechanisms under different laser processing conditions were proposed. It shows that

canning the initial tracks onto a cold powder bed gives rise to the first line scan balling, due to the high thermal gradients imposed on the melt.sing a higher scan speed leads to the ‘shrinkage-induced balling’, because of a significant capillary instability effect. The ‘self-balling’ prevails

t a high laser power combined with a low scan speed, which is ascribed to an excessive liquid formation and a too long liquid lifetime.2006 Elsevier B.V. All rights reserved.

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eywords: Laser processing; Microstructure; Direct metal laser sintering; Cu-b

. Introduction

Direct metal laser sintering (DMLS) is one of a few rapidrototyping (RP) techniques which enable the quick produc-ion of complex shaped three-dimensional (3D) parts directlyrom metal powder with no or minimal pre-processing and/orost-processing requirements [1,2]. In this method, an objects created by selectively sintering and/or melting thin layersf powder using a scanning laser beam according to the CAData. Therefore, the DMLS process exhibits a high potential forhe net-shape fabrication of high performance engineering partsuch as functional prototypes and low volume tooling for injec-ion moulding and die casting [3–5].

Although recent advances in DMLS have improved this tech-ique considerably, there still exist many defects handicappinghe successful fabrication of high-quality metallic parts withavorable microstructures and properties, due to the complexature of DMLS in which both physical and chemical metallur-ical processes may occur [6]. These process defects include

orosity, distortion and delamination, but a more significantroblem is balling phenomenon [7–9]. Since DMLS is carriedut line by line, balling phenomenon may result in the forma-

∗ Corresponding author.E-mail address: dongdonggu@nuaa.edu.cn (D. Gu).

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925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.jallcom.2006.06.011

metal powder; Balling phenomena

ion of discontinuous scan tracks and poor inter-line bondingroperties. Furthermore, since DMLS is a layer-by-layer addi-ive manufacturing process, balling phenomenon is a severempediment to the uniform deposition of the fresh powder onhe previously sintered layer and tends to cause porosity andven delamination induced by poor inter-layer bonding in com-ination with thermal stresses. Therefore, balling phenomenaccurred in laser processed powder severely degrade materialroperties and part geometry. However, a review of existing lit-ratures reveals that not much previous work has been reportedn fundamentals of balling. In fact, balling effect is a complexrocess that requires significant further study and understand-ng, in terms of the mechanism of balling and the influencingactors.

In the present paper, the balling processes during direct laserintering of a multi-component Cu-based metal powder werenvestigated. The effects of processing parameters, e.g. laserower and scan speed, on the balling initiation were highlighted,ith an aim to determine an optimal process window to avoidalling phenomena.

. Experimental

Electrolytic 99% Cu powder with irregular structure and mean equiva-ent spherical diameter of 54 �m, water-atomized CuSn (10 wt.% Sn) powderith ellipsoidal shape and mean particle size of 28 �m, and gas-atomized CuP

164 D. Gu, Y. Shen / Journal of Alloys and Compounds 432 (2007) 163–166

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ig. 1. SEM image showing characteristic morphology of the starting powderixture.

8.4 wt.% P) powder with spherical morphology and average particle size of6 �m were used in this study. Phosphorus, taking as a fluxing agent, was addedn the form of pre-alloyed CuP to improve the wetting properties of the pow-er system. The three components were homogeneously mixed according touSn:Cu:CuP weight ratio of 30:60:10 in a rotary cylindrical vessel with aacuum-pumping system. The characteristic morphology of the mixed powders shown in Fig. 1.

The laser sintering apparatus used in this experiment, as schematically shownn Fig. 2, mainly consists of a continuous wave CO2 (λ = 10.6 �m) laser capablef providing a maximum output power of 2000 W, an automatic powder deliveryystem and a computer system for process control. The loose powder layer0.2 mm in thickness) was deposited across the powder bed and was leveledith a roller so as to obtain a flat powder surface. Laser sintering tests were

arried out by repeated scanning the powder bed surface using a simple linearaster scan pattern. The used laser processing parameters were: spot size of.30 mm, laser power of 300–500 W, scan speed of 0.03–0.07 m/s and scan linepacing of 0.15 mm. The entire sintering process was performed in ambient

tmosphere.

Photographs of the real-time laser sintering process were taken using a KodakX7590 digital camera. Surface morphologies of the laser sintered samples were

haracterized using a Quanta 200 scanning electron microscopy (SEM).

Fig. 2. Schematic of the direct laser sintering apparatus.

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ig. 3. Photographs of the real-time laser sintering process: (a) the first line scannd (b) the subsequent line scan.

. Results and discussion

Fig. 3 shows the real-time laser scan process during sintering.hen the first line scan where the whole laser spot interactedith the powder was processed, a large sparkle was observed in

he laser irradiating region (as indicated by the circle in Fig. 3a).nterestingly, in the subsequent line scan where the laser spotnteracted partially with the powder due to the overlap betweencan lines, the powder was smoothly sintered with a consider-bly small sparkle (as indicated by the circle in Fig. 3b). Fig. 4hows the characteristic microstructures of the first scan tracksith variation of preheating temperatures. When the laser sin-

ering was performed at room temperature (20 ◦C), significantlyoarsened balls were formed, between which a poor bond wasbtained. Furthermore, some unsintered starting powder parti-les were cohered to the surface of balls (Fig. 4a). At a preheatingemperature of 100 ◦C, although some metallic balls were stillnvisible, a relatively stronger inter-particle bond was achievedFig. 4b).

For the first scan track (Fig. 3a), scanning the whole laseream onto a cold powder bed leads to a pronounced increase in

bsorbed energy from the incident laser beam as compared withhe subsequent scans (Fig. 3b), thereby forming a high and steephermal gradient between the molten material and the unsintered

D. Gu, Y. Shen / Journal of Alloys and Compounds 432 (2007) 163–166 165

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Fig. 5. SEM images showing characteristic microstructures of scan tracks atdifferent laser powers and scan speeds: (a) 400 W, 0.04 m/s; (b) 400 W, 0.06 m/s;

ig. 4. SEM images showing characteristic morphologies of the first line scanalling at different preheating temperatures: (a) 20 ◦C and (b) 100 ◦C.

owder and, thus, significantly increasing surface tension forcesf the melt. Consequently, the cold powder bed tends to causehe melt to break up into metallic agglomerates of sphericalhape due to surface energy reduction (Fig. 4a). We termed thishenomenon ‘first line scan balling (FLSB)’. The protrusionf the initial discontinuous and coarse scan track induced byLSB will cause the movement of the previously sintered layerrom the predetermined position when depositing the fresh pow-er layer, thus disturbing the geometry precision of the sinteredart. Our results, as shown in Fig. 4b, reveal that increasing theowder bed temperature favors alleviating FLSB. As the surfaceension is a function of temperature [10], the presence of a rela-ively low temperature gradient in this instance leads to a lowerurface tension of the melt, so as to restrict the Marangoni flownd the capillary instability effect and, accordingly, decrease the

endency of FLSB.

Fig. 5 shows the characteristic morphologies of the single-ine scan tracks at different combinations of laser power and scanpeed. At a laser power of 400 W and a scan speed 0.04 m/s,

(c) 425 W, 0.04 m/s.

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66 D. Gu, Y. Shen / Journal of Alloys

continuous and smooth scan track was formed, showing nopparent balling phenomena (Fig. 5a). With increasing the scanpeed to 0.06 m/s, laser sintering at the same laser power resultedn the breaking up of the scan track into a series of coarsened

etallic balls with mean diameter of ∼400 �m (Fig. 5b). At anven higher laser power of 425 W and a lower scan speed of.04 m/s, a discontinuous and rough scan track was observed,round which a large amount of small balls with orders of tensf micrometers could be found (Fig. 5c).

In the powder system being investigated, the pre-alloyeduSn (10 wt.% Sn) powder melts incongruently and has a lower

olidus temperature (∼840 ◦C) and a liquidus one (∼1020 ◦C)han the melting point of the Cu powder (∼1083 ◦C). At theelting temperatures of the CuSn powder, the pre-alloyed CuP

8.4 wt.% P) powder is expected to be fully molten due to itsower eutectic temperature of ∼714 ◦C. Generally, the directaser sintering of this powder system is through the mechanismf liquid phase sintering involving full melting of the CuP pow-er, partial or complete melting of the CuSn powder (so-calledhe binder) and non-melting of Cu powder (so-called the struc-ural metal) [6]. Our results, as shown in Fig. 5a and b, revealhat a higher scan speed has a pronounced effect on enhance-

ent of the balling initiation. As the laser scanning is performedine-by-line and the laser energy causes melting along a row ofowder particles, the liquid is able to spread around solid struc-ural particles into a continuous film of cylindrical shape. Due ton enhanced capillary instability effect induced by a higher scanpeed in this instance, the liquid quickly collects at the con-acts between solid particles, where considerable stresses areenerated. Consequently, a significant transverse shrinkage dis-ortion tends to occur in inter-particle areas, because of a lowon-deformability of the liquid along the molten track, therebyesulting in the breakage of liquid film. Meanwhile, the radialontraction of the cylindrical shaped liquid track will also occur,hereby changing the direction of fluid flow from radially out-ard to radially inward and, eventually, causing the balling.hus, it is reasonable to consider that balling phenomena occur-

ing at a higher scan speed are caused by the excessive shrinkagef the liquid track in both transverse and radial directions. Weermed this kind of balling ‘shrinkage-induced balling’. On thether hand, our results, as shown in Fig. 5c, show the secondind of balling occurred at a high laser power combined with aow scan speed. Under this condition, a significantly enhancednergy input will lead to higher sintering temperatures and con-equently a larger amount of liquid formation. This is becausehe binder CuSn, which melts incongruently over a range ofemperature, exhibits a higher degree of melting as the temper-

ture above the solidus increases. However, an excessive liquidormation accompanied by a long liquid lifetime will lead toconsiderably lower melt viscosity, a higher degree of super-

eat of the low melting phase and an enhanced Marangoni effect, [

Compounds 432 (2007) 163–166

hereby forming a large amount of individual small balls with theiminishing surface energy. Here, we termed this kind of ballinghenomenon ‘self-balling’. As suggested in Fig. 5, in order tovoid the above-mentioned two kinds of balling phenomena anduitably bind these powder particles, the laser processing param-ters, especially the laser power and the scan speed, should bearefully optimized.

. Conclusions

Based on the experiments conducted, the following conclu-ions can be drawn:

1) The first line scan balling occurs in the initial scan track, dueto the high thermal gradients imposed on the melt. Usinga suitable preheating temperature of powder bed can wellalleviate the first line scan balling.

2) Using a higher scan speed gives rise to so-called ‘shrinkage-induced balling’, due to a significant capillary instabilityeffect.

3) The ‘self-balling’ prevails at the combination of a high laserpower and a low scan speed, because of an excessive liquidformation and a too long lifetime.

cknowledgements

The authors would like to thank the financial support fromhe Joint Fund of National Natural Science Foundation ofhina and China Academy of Engineering Physics (10276017),

he Aeronautical Science Foundation of China (04H52061)nd the Scientific Research Innovations Foundation of Nan-ing University of Aeronautics and Astronautics (S0403-061).he authors also acknowledge Prof. Yang Wang, Dr. JialinYangnd Dr. Xianfeng Shen (Institute of Machinery Manufac-uring Technology, China Academy of Engineering Physics,21900 Mianyang, PR China) for their helps in laser sinteringxperiments.

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