Highly precise pulsed selective laser sintering of metallic powders

48 Laser Phys. Lett. 2, No. 1 , 48–55 (2005) / DOI 10.1002/lapl.200410118

Abstract: We studied the influence of the laser parameters onthe material properties of selectively laser sintered Titanium andPlatinum-alloyed powders, which are both of paramount inter-est in modern technology. In this article, we show that with anappropriate energy deposition in the metallic powder layer, thematerial properties of the selectively laser sintered parts can lo-cally be tailored to the requirements of the finished work piece.By adapting the laser parameters of a Q-switched Nd:YAG laser,notably pulse duration and local intensity, the degree of poros-ity, density and even the crystalline microstructure can be con-trolled. Pulsed interaction allows in addition to minimize the av-erage power needed for consolidation of the metallic powder, andleads to less residual thermal stresses. With laser post processing,the surface can achieve bulk-like density.We also demonstrate for the first time to our knowledge thehighly precise selective laser sintering of steel micro powder witha lateral accuracy of less than 10 micrometers by using a mod-elocked Nd:YAG laser. Furthermore, we present the possibilityof forming metallic glass components by sintering amorphousmetallic powders. SEM Micrograph of the unsintered Titanium powder

c© 2005 by Astro Ltd.Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA

Highly precise pulsed selective laser sintering ofmetallic powders

P. Fischer, 1,∗ V. Romano, 1 A. Blatter, 2 and H.P. Weber 1

1 Institute of Applied Physics, Sidlerstrasse 5, University of Bern, 3012 Bern, Switzerland2 PX Holding SA, Boulevard des Eplatures 42, 2304 La Chaux-de-Fonds, Switzerland

Received: 25 July 2004, Accepted: 3 August 2004Published online: 30 August 2004

Key words: pulsed selective laser sintering; metallic powder; amorphous metals

PACS: 42.62.-b, 81.05.Kf, 81.20.Ev

1. Introduction

Selective laser sintering (SLS) was invented in 1990 andrepresents a highly innovative rapid prototyping techniquethe potential of which has by no means been exploited yet[1]. The basic principle is the layer-by-layer local sinteringof the grains of a powder by a focussed laser beam.

The consolidation process of the powder by heatingunder protective atmosphere with a heat source can beclassified, according to the process temperature, into solidstate and liquid state sintering [2,3].

Compared to heating with a furnace, the laser as a heatsource provides the advantage of localized energy deposi-tion. The powder layer is selectively irradiated and locallysintered, while the non-irradiated part of the layer staysunconsolidated. This laser-powder interaction manner canbe applied for the generative selective laser sintering pro-cesses. In this process of layered manufacturing, any de-sired shape can be produced within a suitable process time(rapid prototyping, rapid manufacturing) with a minimisa-tion of wasted material [4].

The selective laser sintering process is usually per-formed with a continuous wave CO2 laser with average


∗ Corresponding author: e-mail: [email protected]

Laser Phys. Lett. 2, No. 1 (2005) / www.lphys.org 49

timeRT

Tm

timeRT

Tm

timeRT

Tm

Figure 1 Temperature evolution upon different laser parameters at selective laser sintering: (a) continuous wave sintering: the averagetemperature overcomes the melting temperature, (b) and (c) pulsed sintering: The peaks represent the temperature rise of the absorbingskin layer of the grains upon the interaction of the single laser pulses. This skin temperature overcomes the melting temperatureTm, whereas the average temperature (dashed line) of the inner parts and the surrounding powder remains clearly below the meltingtemperature

Nd:YAG

M3

Telescope

Focusing lens

Powder-bed M1

M2

Filter

Figure 2 (online color at www.lphys.org) Setup for the selective laser sintering

powers of several 100 Watts up to several Kilowatts [5,6].The laser spot is scanned over the surface of the pow-der. The achievable precision is limited by both the laserspot size and the powder grain size. Typically, the powdergrains are in the range of several 10 micrometers and thespot size on the surface of the powder bed, related to thebeam quality of the CO2 laser, is in the range of several100 micrometers [5]. In that case, the whole interactingpowder is heated over melting temperature Tm, as shownin Fig. 1a.

The advantages of scanning with pulsed radiation andshorter wavelength have recently been demonstrated withan Nd:YAG laser. Firstly, it allows enhanced lateral pre-cision, because focusability scales directly with the wave-length of the emitted radiation, which is one order of mag-

nitude smaller for Nd:YAG compared to the CO2 radiation.Secondly, the average power required for the sintering ofpure metallic powder is reduced to a few 10 Watts usingpulsed laser radiation, as only a thin layer of the individ-ual grains which absorbs the laser radiation is heated overthe melting temperature while the inner parts of the grainsas well as the surrounding stays at an average temperatureclearly below the melting point (see Figs. 1a and 1b). Therepetition rate of the laser pulses at a given average laserpower directly influences the porosity of the out comingsintered sample. [6,7,8].

The ability to control locally the porosity by onlychanging laser parameters represents another particularityof pulsed laser sintering that potentially allows the gener-ation of locally tailored functional materials.


50 P. Fischer, V. Romano, et al.: Highly precise pulsed selective laser

Figure 3 SEM Micrograph of the unsintered Titanium powder

Here we report on a further possibility offered bypulsed laser beam interaction: Depending on the thermaltransients during the process, controlled by the laser pa-rameters, the sintered part resulting from an amorphousPtCuNiP-powder may be either amorphous or crystalline.

2. Experimental setup

2.1. Laser equipment

Selective laser sintering was carried out using either a Q-switched or mode-locked Nd:YAG laser, emitting pulsesof 150 ns duration at repetition rates of 1-30 kHz or 150 psat 80 MHz, and average powers up to 10 Watts in the fun-damental mode. A filter set was used to reduce and controlthe average power while operating the laser within the sta-bility range of the resonator.

The beam was focused on the powder-bed after beingexpanded with a telescope. The laser spot size on the sur-face of the powder-bed was always chosen to be largerthan the grain size; 100 µm for powders of diameters upto 30 µm and 10 µm for the micro powder. The schematicsetup is depicted in Fig. 2.

For the laser irradiation, the powder-bed was protectedwith an argon atmosphere and placed on a computer con-trolled x-y table with 1 µm precision and a scan speed of1 mm/s.

2.2. Powders

Different powders were consolidated in the selective lasersintering process. Apart from the well investigated CP Ti-tanium powder, we used a steel micro powder (316 L, grainsizes < 5 micrometers) and an amorphous Platinum alloy(PtCuNiP).

30.0 µm

30.0 µm

30.0 µm

(a)

(b)

(c)

Figure 4 Sintered Titanium plates with different porosities: a)as sintered b) enhanced porosity by mixing with 50 vol.% NaCl,that was washed out after sintering, c) surface densification bypost processing with a Nd:Glass laser



Acc.V 3.00 kV

Spot 3.0

Magn 6885x

Det SE

WD 10.0

Exp 1

5 µm

Figure 5 SEM Micrograph of the unsintered steel powder

10 µm

10 µm

Figure 6 (online color at www.lphys.org) Consolidated traces ofsteel micro powder, sintered with a mode-locked Nd:YAG laser(150 ps pulses at 80 MHz). a) 195 mW and b) 275 mW averagepower

20 µm

Figure 7 SEM Micrograph of the unsintered Platinum alloypowder

3. Titanium sintering: control of porosity

3.1. CP Titanium

Commercially pure (CP) Titanium powder was providedby Pyrogenesis Co. The grains are perfectly spherical (seeFig. 3) and the grain sizes are between 1 and 30 microme-ters. The most frequent diameter is 8 micrometers.

A more detailed description of the Titanium powdercan be found in [8].

3.2. Titanium: experimental results

The porosity of the sintered specimen can locally beadapted in several ways. By variation of the repetition rate,the degree of sintering can be changed while keeping theaverage power the same [6,7,8]. A typical picture is givenin Fig. 4a.

The porosity can be significantly enhanced further,when using a blend of metallic powder and transparentspacers, which are removable after the sintering (Fig. 4b).We obtained solid sintered plates by using up to 50 vol.%NaCl as spacer material. This spacer was washed out aftersintering, leaving no traces of NaCl as confirmed by EDXanalysis.

The surface can be post-densified in a second process-ing step by using a Nd:Glass laser with a flat beam profile,a pulse duaration of 10 ns, a repetition rate of 10 Hz, anda pulse energy of 200 mJ. The beam was focused to a spotsize of about 2 mm in diameter (Fig. 4c).



20 30 40 50 60 70 80 90

2θ (degree)

arb.

uni

ts

Figure 8 XRD pattern of the unsintered Platinum alloy powder

3.3. Titanium: discussion

The possibility to adapt the porosity locally to a desiredvalue offers a great potential for several applications. Highporosity allows creating huge surface to volume ratios,what might be interesting for catalytic materials.

A generated sample consisting of a denser (i.e. lessporous) core, surrounded by a structure of higher porositywould provide a light-weight structure with yet enhancedmechanical stability and/or functionality. The strength andelasticity of the part can be selectively tailored by control-ling the local densification during the sintering process.

As an example for tailored functionality, the thermalproperties of a sintered structure strongly depend on thedegree of sintering: A very fine connecting bridge be-tween two grains will conduct heat less good than a thickerbridge. The control of the connecting bridges therefore en-ables a local control of thermal conductivity. For instance,one can conceive the integration of “heat channels” in asintered part, which guide the heat due to their higher ther-mal conductivity.

A more detailed description of the sintering of this Ti-tanium powder can be found in [6,7,8].

4. Steel micropowder sintering: highprecision

4.1. Steel micro powder

The 316 L stainless steel powder was provided by Osprey.The chemical composition in wt.% is 64.0% Fe,: 17.5%Cr, 13.5% Ni, 2.9% Mo, 1.4% Mn, and 0.7% Si.

50% of the grains have a diameter smaller than 3.3 mi-crometers and 90% smaller than 5 micrometers. The mostfrequent grain size is 4 micrometers. The grains are per-fectly spherical, as can be seen in Fig. 5.

4.2. Steel micro powder: experimental results

To demonstrate the high precision of the laser sintering,a powder with such small grains was consolidated againwith a focused Nd:YAG laser beam. To avoid completemelting of the grains, a mode-locked laser emitting 150 pspulses at a repetition rate of 80 MHz was employed. Twodifferent average powers were applied; 195 mW (Fig. 6a)and 275 mW (Fig. 6b).

As seen in Fig. 6a, the lower power produces a 10 mi-crometer thick consolidated trace, with the single grainsstill being visible. At the higher power, as seen in Fig. 6b,a trace of 10 micrometers is completely molten and sur-rounded by darker borders of about 5 micrometers.

4.3. Steel micro powder: discussion

The micro powder has a different optical penetration depthof the near infrared radiation in the powder bed due to itssmaller granulometry. The employed “scraper” depositionmethod produces a very dense powder bed and no men-tionable transmission could be measured. Therefore, anadequate description of the energy deposition is very dif-ficult. However, two different consolidation mechanismscould be observed:- At sufficient low average power, the energy is depositedin a very thin surface layer of the single grains that willmelt only partially. Neighbouring grains only will bridgetogether by the liquid surface layer that immediately reso-lidifies very rapidly. In this interaction regime, a homo-geneous sintering can be achieved while preserving thegrainy character of the part.- At higher average power, the whole interaction zone ismolten. Individual grains are no longer visible. Due to heatdiffusion, the average temperature in the surrounding area



50 µm

50 µm

50 µm

Figure 9 SEM Micrographs of sintered PtCuNiP samples at (a)1 W, 5 kHz (b) 800 mW, 5 kHz, and (c) 200 mW, 1.5 kHz averagepower

Pt Cu Ni Pnominal 57.3 14.7 5.3 22.7

X-ray fluorescence 57.45 14.84 5.83 21.87

Table 1 Nominal composition of the pre-alloy and powder com-position as measured by X-ray fluorescence. All values are inatomic percent

becomes high enough for solid state sintering. Within thisconsolidation regime, the achievable precision is reduced.

5. Amorphous powder sintering: control ofmicrostructure

5.1. Amorphous PtCuNiP powder

The amorphous Pt-Cu-Ni-P powder was obtained bywater-atomization of a pre-alloyed ingot. The compositionis given in Table 1.

The composition of the resulting powder differs onlymarginally from the nominal composition, as measured byX-ray fluorescence.

The powder was sieved to obtain a grain size distribu-tion limited to < 40 micrometers. 50% of the grains aresmaller than 19 micrometers and the most frequent grainsize is 26 micrometers.

As seen in Fig. 7, most of the grains are perfectlyspherical, yet a small fraction comes with lemon or pol-lywog shapes.

The X-ray diffraction pattern shown in Fig. 8 is typ-ical of an amorphous solid. With the help of the Scherrerformula, one can estimate the crystallite size from the mea-sured width of a peak in the diffraction curve [9]. The firsthalo centered at 40◦ returns a crystallite size of 10 A, andthe second order halo around 76◦ a size of 7.5 A. Differ-ential scanning calorimeter analysis revealed a glass tran-sition at 550 K, the onset of crystallisation at 575 K andthe melting point at 770 K.

5.2. Amorphous PtCuNiP powder: experimentalresults

The powder was irradiated with Q-switched 150 ns pulsesat average laser powers between 200 mW and 1 W at dif-ferent repetition rates. The resulting pulse energies were,respectively, 133 µJ (average power 200 mW, repetitionrate 1.5 kHz), 160 µJ (800 mW, 5 kHz), and 200 µJ (1 W,5 kHz). The average temperature rise of the powder-bedduring sintering roughly scales with the average power andtherefore is five times higher for the 1 W processing ascompared to 200 mW.

Fig. 9 shows the surfaces of the different sintered sam-ples at the same magnification. The micrographs were



2θ (degree)

40 50 60 70

arb.

uni

ts200 mW, 1.5 kHz

800 mW, 5 kHz

Figure 10 XRD pattern of the sintered PtCuNiP alloy

taken using backscattered electron detection. This detec-tion increases the chemical contrast and thus the contrastbetween different phases.

Fig. 9a shows the sample sintered at the highest av-erage power of 1 W. The powder grains were appar-ently melted and spread to completely merge together withneighbouring grains. The intergranular “empty” volumecollapsed into sphere-like pores. The resulting structureis crystalline as evidenced by the needle-like crystals dis-cernible in Fig. 9a. The XRD pattern in figure 8 confirmsthe crystalline structure by the presence of a prominentdiffraction peak at around 41◦ (2θ). A very similar pic-ture was obtained when interacting with continuous waveradiation.

Upon lowering the average power (Figs. 9b,c), thepowder grains still melt, but the non-spherical form ofthe pores and the ever finer interconnects imply a lowerspreading. In other words, the transient time in the lowviscous liquid state shortens because of less heating.

Simultaneously, the frequency and size of the crystalspresent in Fig. 9a decrease (Fig. 9b) and eventually dis-appear completely (Fig. 9c). XRD confirms the visual im-pression: the crystalline peak progressively broadens andthen vanishes. The only feature left at 200 mW is a broadhalo associated with an amorphous structure (Fig. 10).

5.3. Amorphous PtCuNiP powder: discussion

The transition from crystalline to amorphous solidifica-tion is related to the thermal transients during laser sin-

tering. Glass formation, i.e. the freezing of the disor-dered liquid configuration during solidification, requiresbypassing crystallization by quenching the melt sufficientrapidly to below the glass transition temperature TG. Be-low TG, crystallization kinetics is in fact so slow that themetastable, glassy structure remains stable over practicaltime scales. Our PtCuNiP alloy is an easy glass former,meaning that cooling at a few ◦C/s is sufficient to avoidcrystallization - values that are by far exceeded in transientlaser melting. In laser sintering, crystallization will occur,however, when the laser power is sufficient to raise the av-erage sample temperature to above TG (the crystallizationtemperature), as pictured in Fig. 11.

In comparison to metallic (microcrystalline) media,consolidation of amorphous PtCuNiP powder through se-lective laser sintering requires much less average powerand pulse energy. The reason is that an amorphous mate-rial already is in a thermodynamic liquid state and there-fore does not require any heat of transition for melting. Acomprehensive discussion of the much lower laser energyrequired for the sintering of amorphous PtCuNiP powderin comparison to the sintering of Titanium can be found in[10].

6. Conclusions

Pulsed laser radiation in selective laser sintering allows theconsolidation of metallic powders at moderate laser pow-ers of less than 10 Watts due to the fact that only a verynarrow layer of the single grains are molten whereas most



timeRT

Tm

TG

timeRT

Tm

TG

Figure 11 Scheme of the temperature evolution upon the in-teraction with a flat top beam. The peak temperatures upon thepulsed interaction overcome the melting temperature in both thecases. (a) The average temperature Tav (dashed curve) stays be-low the glass temperature TG, the sintered part remains amor-phous (b) Tav overcomes the TG, the sintered part will becomecrystalline

of the volume remains at much lower temperature. This isa much lower value than the several 100 W for continuouswave sintering. For instance, 3 Watts were sufficient forthe pulsed laser sintering of the Titanium powder with aspot size of 100 µm and a scan speed of 1 mm/s.

Pulsed laser radiation also provides a high potentialfor sintering with enhanced lateral precision as balling ef-fects are minimized. As reported earlier, the repetition rate,which determines the energy per pulse at a given powerlevel, is a crucial parameter for controlling the porosity ofthe sintered part. An increased porosity can be achieved bymixing the metallic powder with a removable spacer ma-terial such as NaCl, with the resulting pore size being di-rectly determined by the size of the NaCl crystals. A den-sification or surface sealing can be achieved by a secondprocess step with a Nd:Glass laser with shorter pulse du-ration, flat beam profile and higher pulse energy. With 40pulses from such a laser, the surface of the sintered platewas fully densified over a thickness of 40 µm.

The achievable precision in pulsed selective laser sin-tering is by now limited by the size of the powder grains.

The high potential of micro powder for high precision se-lective laser sintering has been demonstrated using a stain-less steel powder. Lines of a width of 10 micrometers havebeen consolidated. Pico-second laser pulses had to be em-ployed for this purpose, to avoid the melting of the entiregrain. Further investigations on this topic are in progress.

We also have demonstrated the consolidation of anamorphous PtCuNiP powder by the technique of pulsedlaser sintering. Because of the very low melting point ofthe powder together with the not required heat of crystalli-sation, the consolidation is achieved at much lower laserpulse energies as compared to crystalline powder. At lowaverage laser power, such that the average temperature ofthe powder-bed does not rise over TG, the structure of thesintered part can be kept amorphous. At higher averagelaser power, when the elevated average temperature pre-vents amorphous resolidification, a crystalline sinter partis formed. Pulse duration, scan speed and focus determinethe melt life-time and thereby the porosity (or densifica-tion) of the resulting sinter part.

In summary, pulsed selective laser sintering provides aversatile technique useful not only for the generative builtup of a work piece of any desired shape, but also to im-part to this piece functional micro- or mesoscopic physi-cal properties such as controlled porosity, density and, asshown in this article, microstructure.

Acknowledgements The authors acknowledge BernardBertheville, HEVs - Haute Ecole Valaisanne, Sion, Switzerlandand Martin Locher, IAP University of Bern for the XRDmeasurements. Furthermore we acknowledge Cedric Andre,Serguej Kolossov and Eric Boillat from the EPF Lausanne fortheir collaboration.

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Highly precise pulsed selective laser sintering of metallic powders