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International Journal of Refractory Metals& Hard Materials

journal homepage: www.elsevier.com/locate/IJRMHM

Formation of scanning tracks during Selective Laser Melting (SLM) of puretungsten powder: Morphology, geometric features and forming mechanisms

Meng Guoa,b, Dongdong Gua,b,⁎, Lixia Xia,b, Lei Dua,b, Hongmei Zhanga,b, Jiayao Zhanga,b

a College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Yudao Street 29, Nanjing 210016, Jiangsu Province, PR Chinab Jiangsu Provincial Engineering Laboratory for Laser Additive Manufacturing of High-Performance Metallic Components, Nanjing University of Aeronautics andAstronautics, Yudao Street 29, Nanjing 210016, Jiangsu Province, PR China

A R T I C L E I N F O

Keywords:TungstenSelective laser melting (SLM)Scanning trackGeometrical characteristicsSurface morphologies

A B S T R A C T

Due to the high melting point and high heat conductivity, selective laser melting (SLM) of tungsten is stillchallenging. To have a better understanding of SLM tungsten parts, the effects of processing parameters such aslaser power and scanning speed on scanning tracks formation of pure tungsten powder were investigated. Aslinear energy increased with increasing laser power and decreasing scanning speed, the height and contact angleof scanning tracks gradually reduced, while the width and penetration depth increased. Owing to the goodwetting and spreading, the flow front of scanning tracks gradually became smooth and stable with the increasedlinear energy. However, the transverse cracks induced by large temperature gradient and high cooling rateappeared on the surface of the scanning tracks at linear energy of more than 1.75 J/mm. A maximum tem-perature of 4630.27 °C and high cooling rate of 8.6× 106 °C/s were obtained during SLM process of tungstenpowder when the linear energy was 1.75 J/mm. This work provides scientific guidance for SLM-processedtungsten parts.

1. Introduction

As a relatively new manufacturing technology, selective lasermelting (SLM) is extremely attractive for its ability to fabricate com-ponents with complex geometries. During SLM, a 3D model slicedthrough slicing software is built by a modeling software and a 3D part ismanufactured by selectively melting powders layer by layer. Comparedwith traditional manufacturing methods such as powder metallurgy andcasting, SLM has an irreplaceable advantage on convenient productionof complex structures as well as high densification level and dimen-sional accuracy of SLM-processed items. So far, a number of metalpowders have been manufactured by SLM, such as stainless steel, tita-nium alloys, aluminium alloys and nickel-base superalloy [1–8]. As afinal 3D component is essentially fabricated from melting powder ma-terials track by track, it is necessary to obtain scanning tracks that havegood metallurgical bonding with neighboring ones. During SLM processof scanning tracks, some phenomena may occur such as laser radiation,reflection, absorption and heat transfer, powder melting, phase trans-formation and melt flow within the molten pool driven by surfacetension gradients, and evaporation and mass transfer of materials.These phenomena significantly depend on the powder properties andprocessing parameters. As a laser beam irradiates on powder bed, there

exists an energy balance between radiation absorptivity and heat con-ductivity. This balance is supposed to control the melting mode duringan interaction of laser with powder. Appropriate modification of theenergy balance can cause either keyhole or conduction mode melting[9]. In view of scanning track formation as a basic constitution in SLMprocesses, investigations on scanning track formation of aluminiumalloys, copper alloys, stainless steel, and superalloys have been doneelsewhere [10–14]. Aversa et al. [11] investigated the effects of pro-cessing parameters and powder properties on the stability of scanningtracks. It demonstrated that scanning tracks were an effective methodfor studying the effects of the building parameters on the geometricalfeatures of the molten pool. Yadroitsev et al. [13] systematically studiedthe formation of scanning tracks of different metal powders (e.g. SSgrade 904L, 316L, tool steel H13, copper alloy CuNi10, superalloy In-conel 625) on SS grade 304L substrate. The results showed that thereexisted stable and instable zones governed by a threshold character.Instabilities appeared at low scanning speed in the form of distortionsand irregularities, while high scanning speed was prone to cause theballing effect. Later, he studied the influence of the laser scanning speedand preheating temperature on the geometry and microstructure of thescanning tracks fabricated from 316L powder [15]. The results showedthat the geometrical features of 316L stainless steel scanning tracks

https://doi.org/10.1016/j.ijrmhm.2018.11.003Received 8 August 2018; Accepted 7 November 2018

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

International Journal of Refractory Metals & Hard Materials 79 (2019) 37–46

Available online 09 November 20180263-4368/ © 2018 Published by Elsevier Ltd.

T

were determined by the input energy density. The scanning speedcontrolled the width and contact zone size of the tracks, while thepreheating temperature governed their contact angle and height.

The refractory metal tungsten is widely used in military applica-tions, aerospace industries and nuclear industry because of its highmelting point, good thermal conductivity and high strength [16,17].Traditional manufacturing technologies such as metal injectionmolding (MIM), hot isostatic pressing (HIP) and powder metallurgy(PM) [18–22] for production of the refractory metal tungsten, have acommon problem that near-net shape parts are impossibly manu-factured by these methods. Although SLM has processability of complexnet shaped parts, the work on refractory metals fabricated by SLM arevery limited [23–25]. Due to high melting point and good thermalconductivity, SLM of tungsten is still challenging and complex. There-fore, a good understanding of the influence of process parameters suchas laser power, scanning speed, hatch spacing and powder thickness onthe densification and microstructure evolution of SLM-processed tung-sten powder is vital for fabricating components with good perfor-mances. In this work, a study of scanning tracks was conducted tounderstand the effects of processing parameters like laser power andscanning speed on SLM process pure tungsten powder. The surfacemorphologies and geometrical features at cross section of scanningtracks were studied. Based on the numerical simulation by the ANSYSsoftware, the temperature gradient and cooling rate during SLM processof tungsten were discussed.

2. Experimental procedures

2.1. SLM process

The morphology of the pure tungsten powder was shown in Fig. 1a.The size range of tungsten particles was 5 μm to 25 μm with an averagediameter of ~17.3 μm. The tungsten particles had a good sphericity,which was beneficial to SLM processing. The SLM equipment used inthe present study was self-developed in Nanjing University of Aero-nautics and Astronautics, which was composed of a YLR-500 Ytterbiumfiber laser with a maximal power of 500W and a spot size of 70 μm (IPGLaser GmbH, Germany). When the samples were to be prepared, a thin304 L stainless steel block was fixed on the platform as substrate. TheSLM experiments were conducted under the protection of argon and theoxygen content must be very low to prevent oxidation of tungsten. Toreduce the thermal stress, the substrate was preheated to 200 °C. Fig. 1showed the formation process of scanning tracks during SLM-processedtungsten powder. A laser beam irradiated on the powder bed, thepowder particles simultaneously absorbed the energy and fused to formthe molten pool. With the movement of the laser beam, the molten

pools solidified and formed continuous scanning tracks along thescanning direction, as shown in Fig. 1b. Table 1 showed the processingparameters used to fabricate scanning tracks. The scanning tracks weremanufactured using laser power ranging from 250W to 450W incombination with scanning speed in the range of 200–500mm/s. Theeffect of the building parameters was studied mainly using the LinearEnergy Density (LED) calculated as P/v (J/mm), P represented laserpower and v represented scanning speed.

2.2. Characterization

The specimens were grinded and then polished by diamond sus-pensions according to the standard metallographic procedure. For theobservation and measurement of scanning tracks at cross sectionsclearly, the nitro-hydrochloric acid was used to etch the 304 L stainlesssteel. The images of scanning tracks at cross section were characterizedby PMG3 optical microscope (Olympus Corporation, Japan). The geo-metrical features of scanning tracks at cross section were shown inFig. 2. The width (W), the height of the tracks (H), the average value of

Fig. 1. Formation process of scanning tracks during SLM process.

Table 1SLM processing parameters of scanning tracks of tungsten powder.

Processing parameter Value

Laser power, P 250W, 300W, 350W, 400W, 450WScanning speed, ν 200mm/s, 300mm/s, 400mm/s, 500mm/sLaser beam spot size, D 70 μmLayer thickness, h 80 μmHatch spacing, H 50 μm

Fig. 2. Schematic of geometrical features of scanning tracks at cross section.

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the contact (α=(α1+ α2)/2) and the penetration depth (D) weremeasured, respectively. The depth to width aspect ratio was also de-fined to describe the morphology of molten pool of scanning tracks.Statistics were calculated at least for ten times to reduce the error. Thesurface morphologies of scanning tracks were characterized by a JEOLJSM-6360LV Scanning Electron Microscope.

2.3. Model for scanning tracks by SLM

As to obtain quantitative thermal data within the molten pool, thenumerical simulation was performed based on the ANSYS software.Fig. 3a showed the established three-dimensional finite element modeland laser scanning direction during the SLM process. The model con-sisted of a tungsten powder bed and a 304 L stainless steel block sub-strate. The tungsten powder bed had a length of 3mm, a width of

1.5 mm and a thickness of 0.08mm. The dimensions of the substratewere 4mm×2mm×0.8mm. To obtain computational precision andcalculation efficiency, the powder bed was divided into 70 hexahedronelements with the fine mesh 17.5 μm×17.5 μm×25 μm, while a re-latively coarse tetrahedron mesh was adopted in the substrate. Thethermo-physical parameters of tungsten and 304L stainless steel wereshown in Fig. 4 and Table 3 [26,27]. Fig. 3b showed the central positionof the surface profile of molten pool, where temperature distributionprofiles and cooling rate versus time were extracted.

The differential equation of 3D heat conduction in a domain D wasused to control the spatial and temporal distribution of the temperaturefield. The equation was expressed as [28]:

⎜ ⎟∂∂

= ∂∂

⎛⎝

∂∂

⎞⎠

+ ∂∂

⎛⎝

∂∂

⎞⎠

+ ∂∂

⎛⎝

∂∂

⎞⎠

+Tt x

k Tx y

k Ty z

k Tz

Qρc(1)

where ρ is the density of material, c represents the specific heat

Fig. 3. Finite element model of SLM-processed scanning tracks (a) and surface profile of molten pool (b).

Fig. 4. Heat conductivity and specific heat capacity of tungsten and 304Lstainless steel, respectively [26,27].

Fig. 5. Effects of laser power on the height and contact angle (a), and the width,penetration depth and depth to width aspect ratio (b) of scanning tracks at crosssection, the scanning speed is 200mm/s.

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capacity, T is the temperature of the powder bed, t represents the in-teraction time, k is the thermal conductivity, and Q represents the vo-lumetric heat generation.

The Gaussian heat source model was used as the heat load. The laserintensity distribution conformed to a Gaussian relationship, which canbe expressed as:

⎜ ⎟= ⎛⎝

− ⎞⎠

q APπR

exp rR

2 22

2

2 (2)

where P represents the laser power, R is the laser beam radius and r isthe radial distance from a position on the powder bed to the centralposition of the laser spot, and A is laser energy absorptance.

The initial temperature in the powder bed at the time t=0 can bedefined as:

∣ = ∈=T x y z t x y z D( , , , ) T ( , , )t 0 0 (3)

where T0 is the preheating temperature and is taken as 200 °C.The natural boundary condition can be defined by:

∂∂

− + + = ∈k Tn

q q q x y z S0 ( , , )c r (4)

where S is the surfaces, n represents the normal vector of surface S, theinput heat flux q is presented in Eq. (2), qc represents the heat con-vection, which can be expressed as:

=q h T Tc ( – 0) (5)

qr represents the heat radiation and can be defined as:

=q σε T Tr ( 4– 04) (6)

h represents the heat convection coefficient, σ is the Stefan–Boltzmannconstant and ε represents the emissivity.

3. Results and discussion

3.1. Geometric characteristics of scanning tracks at cross section

Fig. 5a shows the variation of height and contact angle of scanningtracks at cross section with laser power at a scanning speed of 200mm/s. A maximum height of ~116 μm and a contact angle of ~47° wasobtained under a laser power of 250W. With increasing laser power, aminimum height of ~50 μm and a contact angle of ~34° was obtainedunder the laser power of 450W. All the contact angles measured fromthe scanning tracks were less than 90°, implying a good wetting beha-vior during SLM of W powder. Fig. 5b shows the effects of laser poweron the width and penetration depth of scanning tracks at cross sectionat a scanning speed of 200mm/s. It was obvious that at a low laser

Fig. 6. Effects of scanning speeds on the height and contact angle (a), and thewidth, penetration depth and depth to width aspect ratio (b) of scanning tracksat cross section, the laser power is 450W.

Fig. 7. OM images showing morphologies of cross sections at different processing parameters, (a) Keyhole mode, (b) Conduction mode.

Table 2Variations of modes at different processing parameters.

Laser power(W)

Scanning speed(mm/s)

Linear energy density(J/mm)

Types of mode

250 200 1.25 Conduction300 200 1.5 Conduction350 200 1.75 Keyhole400 200 2 Keyhole450 200 2.5 Keyhole450 300 1.5 Conduction450 400 1.125 Conduction450 500 0.9 Conduction

Table 3Physical properties of tungsten and 304 L stainless steel used in the simulation.

Materials Density (g/cm3) Melting point (°C)

tungsten 19.3 3410304L stainless steel 7.9 1440

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power of 250W, the width and penetration depth had the lowest valuesof ~336 μm and ~236 μm, respectively. While the laser power in-creased to 450W, both of them reached the highest values of ~435 μmand ~566 μm, respectively. The value of depth to width aspect ratioalso increased with increasing laser power. The height above the sub-strate and the contact angle decreased with increasing laser power from250W to 450W. This implies that a further increase in laser power canlead to a reduction in height and an increase of contact between thetrack and the substrate material.

Due to high melting point and high thermal conductivity of tung-sten, the energy input into SLM-processed tungsten should be muchhigher than those of metals like aluminium alloy and stainless steel.Here, the height of scanning tracks reflects the melting behavior ofpowder particles; a low laser power such as 250W was not enough tomelt the W particles on powder bed with a thickness of 80 μm, thus itsheight under this condition reached a maximum value of ~116 μm.However, a high laser power of 450W can provide sufficient energy

input to completely melt the particles and spread well, thus the ob-tained height of ~50 μm was far lower than powder thickness of 80 μmAs observed by decreasing contact angle in Fig. 5a, it was obvious thatthe wettability of scanning tracks was gradually enhanced with theincreasing laser power. During SLM process, wettability significantlyinfluences the spreading of molten powder on the substrate and themetallurgical bonding with neighboring scanning tracks. High laserpower facilitates the melting of powder and spreading of the melt flow,resulting in a small contact angle and good metallurgical bonding be-tween scanning tracks. Conversely, low laser power can result in in-complete wetting and spreading of droplets, thus deteriorating themetallurgical bonding between scanning tracks. During SLM, smallpenetration depth caused by insufficient laser power can reduce thetransmission of energy, which is detrimental to the metallurgicalbonding with a previous layer. On the other hand, large penetrationdepth can cause large heat-affected zone (HAZ) and coarseness of themicrostructure [29], which can deteriorate the properties of final

Fig. 8. SEM images showing the typical surface morphologies of scanning tracks at differernt laser power (a) 250W, (b) 300W, (c) 350W, (d) 400W, (e) 450W, thescanning speed is 200mm/s.

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components. Therefore, laser power should be optimized to obtainapplicable penetration depth and contact angle, so that near full denseparts can be successfully produced.

Fig. 6a shows the variation of the height and contact angle ofscanning tracks at cross section with scanning speed at a laser power of450W. Obviously, when the scanning speed was 200mm/s, a minimumheight of ~50 μm and contact angle of ~34° was obtained. While thescanning speed was 500mm/s, a maximum value of the height of~106 μm and contact angle of ~43° was observed. Similarly, all thecontact angle measured from the scanning tracks were less than 90°.Fig. 6b shows the effects of scanning speed on the width and penetra-tion depth of scanning tracks at cross section at a laser power of 450W.When the scanning speed was 200mm/s, the width and penetrationdepth had the highest values of ~435 μm and ~566 μm, respectively.While the scanning speed was 500mm/s, both of them had the lowestvalues of ~322 μm and ~183 μm. The value of depth to width aspectratio decreased with the increasing scanning speed, which was oppositeto the results of laser power. The height and contact angle increased

with the increasing scanning speed from 200mm/s to 500mm/s, im-plying that a further increase in scanning speed can result in a lack ofcontact between the neighboring tracks and an increase in height.

According to the aforementioned analysis of laser power, similarphenomenon can be explained as follows. When the scanning speed isvery low, the energy absorbed by the powder particles can be enough tomelt the powder to form good wetting and spreading. Thus, the mea-sured value of the height and the contact angle is low. This result in-dicates that a low scanning speed can promote the wetting andspreading of droplets. When the scanning speed is low, the powderparticles have more time to fuse and the spreading behavior of meltflow can be facilitated. With the increasing scanning speed, the energyinput absorbed by powder was gradually reduced, leading to in-sufficient fusion and poor wetting. Thus, the measured value of heightand contact angle is relatively high, especially at a high scanning speedof 500mm/s, as shown in Fig. 6a. This suggests that high scanningspeed can lead to poor wetting and spreading on the substrate. As theincreasing scanning speed causes the gradual reduction of the laser

Fig. 9. SEM images showing the splash zones of scanning tracks at different laser power (a) 250W, (b) 300W, (c) 350W, (d) 400W, (e) 450W, the scanning speed is200mm/s.

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energy in unit time, the width and penetration depth of scanning trackssimultaneously decreases. Similar to laser power, appropriate penetra-tion depth caused by optimized scanning speed can offer good me-tallurgical bonding between layer and layer and instead a large pene-tration depth can lead to a large heat-affected zone (HAZ), which caninduce a coarse microstructure and poor mechanical properties of as-fabricated components [29]. Therefore, an optimized scanning speed isvital for SLM-processed scanning tracks of tungsten.

3.2. Morphologies of scanning tracks at cross section

Fig. 7 shows two kinds of morphologies at cross section under dif-ferent processing parameters, keyhole mode and conduction mode. Alarge depth with respect to width was characteristic for the keyholemode, as seen in Fig. 7a. Table 2 lists variations of modes at differentprocessing parameters. It was obvious that at a linear energy density ofmore than 1.5 J/mm, the morphology of molten pool was prone to formkeyhole mode. While the linear energy density was less than 1.5 J/mm,the morphology tended to form conduction mode. This suggests thatthere exist a critical value of linear energy to control the morphologyevolution of scanning tracks at cross section.

Madison et al. [30] pointed out that in keyhole mode of laserwelding, the laser energy density was sufficient to cause evaporation ofmetals, causing the formation of vapor cavities. To a certain degree,these vapor cavities increased the contact area with the laser beam, thusenhancing the laser absorption. Due to the evaporation of metals, thecollapse of vapor cavities caused large voids in the wake of laser beam.Notably, the keyhole mode is detrimental to the densification level,microstructure and the mechanical properties of as-fabricated mate-rials.

3.3. Surface morphologies of scanning tracks under different processingparameters

Fig. 8 shows the surface morphologies of scanning tracks at different

laser power under a scanning speed of 200mm/s. It can be found thatthree types of scanning tracks were distinguished from: irregular trackswithout cracks (Fig. 8a), regular tracks without cracks (Fig. 8b) andregular tracks with cracks (Fig. 8c–f). Under a low laser power of250W, the surface morphology of the scanning track was uneven, andan irregular flow front without cracks was observed (Fig. 8a). When thelaser power was 300W, the scanning track became stable with a regularflow front and smooth surface without visual cracks (Fig. 8b). With thelaser power increasing to more than 350W, the surface morphology ofthe scanning track remained stable and smooth with a regular flowfront, but transverse cracks were found on the surface of the scanningtracks (Fig. 8c–e). It has to be mentioned that the cracks became dis-tinct with large width and length with increasing laser power. Fig. 9shows the magnification of splash zones under different laser powers.At laser powers of 250–350W, unmelted tungsten particles werewrapped into the flow front and spatters were adhered to the border ofsplash zone (Fig. 9a–c). However, with the increasing laser power, theamount of unmelted particles and spatters gradually decreased. Whenthe laser increased to 400W, very limited unmelted particles remainedwrapped in the flow and the melt flow became regular and smooth, asshown in Fig. 9d. At a laser power of 450W, unmelted particles dis-appeared completely, but voids caused by the evaporation of materialswere found on the surface (see inset image in Fig. 9e). With the in-creasing laser power, the input energy would sufficiently melt thepowder particles and less unmelted particles remained.

Due to the influence of laser power, the scanning tracks show var-ious surface morphologies. When the laser power is low, the energyinput was insufficient to melt powder and form regular flow front. Withthe increasing laser power, the sufficient energy input can promote theformation of regular and smooth flow front, contributing to good sur-face quality. However, high laser power can promote the formation ofspatters formed during SLM. On the other hand, high energy input willreduce the melt viscosity and resultant spatters adhered to the surface.Therefore, the obtained splash zones without unmelted particles andspatters can facilitate the metallurgical bonding with neighboring

Fig. 10. Linear energy dependence of temperature distributions during SLM process, (a) 1.25 J/mm, (b) 1.75 J/mm, (c) 2.25 J/mm and temperature gradient in therange of laser spot diameter (d).

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tracks, thus improving the final densification level of the components.To reveal the forming mechanisms of the cracks, temperature dis-

tributions and cooling rate during SLM process were calculated. Fig. 10presents the temperature distributions of different linear energy and thetemperature gradient within a range of laser spot diameter. Fig. 11 isthe temperature distribution profiles and cooling rate versus time underdifferent linear energy, the temperature distribution and cooling ratewere taken from the central position of molten pool surface. When thelinear energy was 1.25 J/mm, a maximum temperature of 3565.75 °Cand a cooling rate of 6.68× 106 °C/s were obtained. With the in-creasing linear energy, the temperature and the cooling rate graduallyincreased. A maximum temperature of 4630.27 °C and a cooling rate of

8.6× 106 °C/s were obtained under a linear energy of 1.75 J/mm. Inthis case, the cracks formed. As the linear energy increased to 2.25 J/mm, a maximum temperature and a cooling rate was 5536.92 °C and10.25×106 °C/s, respectively. The simulation results (Fig. 10d) de-monstrated that the temperature gradient in the center of laser spot wasmuch smaller than that at the edge, and the temperature gradient andcooling rate gradually increased with the increasing linear energy.Large temperature gradient and high cooling rate can cause shrinkageand residual stress formed upon the solidification process [31]. As theincreasing linear energy, the temperature gradient and cooling rateincreased. In this case, large temperature gradient and high cooling ratein the wake of high linear energy input result in a more tendency ofshrinking during the cooling process, thus resulting in large residualstress at the border region of laser spot compared to that in the center.During the solidification process, the residual stress released, formingthe transverse cracks, as well seen in Fig. 8c–d.

Fig. 12 shows the typical surface morphologies of scanning tracks atdifferent scanning speeds under a laser power of 450W. It can be foundthat two types of scanning tracks were identified: stable tracks withregular flow front (Fig. 12a and b) and unstable tracks with irregularflow front (Fig. 12 c and d). Under a low scanning speed of 200mm/s,stable track with regular flow front and smooth surface was observed(Fig. 12a). When the scanning speed was 300mm/s, the track remainedstable with regular flow front and smooth surface (Fig. 12b). As thescanning speed increased to 400mm/s, the shrinkage phenomenon wasobserved on the surface and the flow front was irregular and uneven(Fig. 12c). When the scanning speed reached 500mm/s, the wrinkleswas apparently observed (as shown in Fig. 12d), implying remarkableshrinkage on the surface. Fig. 13 shows the magnified SEM images ofsplash zones. It was obvious that with the increasing scanning speed,the spatters caused by the recoil of laser apparently increased.

As a high scanning speed leads to a short duration of interactionbetween the laser and the powder bed, this results in an insufficientenergy input. Due to insufficient energy input for completely meltingthe powder under a high scanning speed, poor wettability, high visc-osity and balling phenomenon would occur during SLM, leading tounstable and wrinkled surface of the as-fabricated tracks.

A high laser power or a low scanning speed can provide sufficientenergy input, thus resulting in high temperature of molten pool andlong liquid lifetime. The high temperature in the molten pool effectivelydecreases the viscosity of molten metals and enhance the wettabilityand spreading on the substrate. This can contribute to regular flow frontand smooth surface of the as-fabricated scanning tracks. However, ex-cessive energy input can lead to evaporation of metals and the cracksinitiation. Therefore, laser power as well as scanning speed should bewell balanced to obtain good surface quality and wettability.

4. Conclusions

In this work, an investigation on the fabrication of scanning tracksof tungsten powder by SLM were conducted. The main conclusions areas follows:

(1) The geometrical features of scanning tracks at cross section weredominantly influenced by laser power and scanning speed. Whenthe laser power increased from 250W to 450W (the scanning speedwas 200mm/s), the contact angles decreased from ~47° to ~34°,while the penetration depth increased from ~236 μm to ~566 μm.When the scanning speed increased from 200mm/s to 500mm/s(the laser power was 450W), the contact angles increased from~34° to ~43°, while the penetration depth decreased from~566 μm to ~183 μm. The laser power and scanning speed showedan opposite tendency to the changes of geometrical features ofscanning tracks at cross section.

(2) Two molten pool morphologies of conduction mode and keyholemode were formed under different linear energy. When the linear

Fig. 11. Temperature distribution profiles and cooling rate versus time at thecentral position of molten pool surface under different linear energy (a) 1.25 J/mm, (b) 1.75 J/mm, (c) 2.25 J/mm.

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Fig. 12. SEM images showing the surface morphologies of scanning tracks at different scanning speed (a) 200mm/s, (b) 300mm/s, (c) 400mm/s, (d) 500mm/s, thelaser power is 450W.

Fig. 13. SEM images showing the splash zones of scanning tracks at different scanning speed (a) 200mm/s, (b) 300mm/s, (c) 400mm/s, (d) 500mm/s, the laserpower is 450W.

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energy input was less than the threshold value of 1.5 J/mm, con-duction mode was generated. While the linear energy input wasmore than 1.5 J/mm, the keyhole mode was formed and should beavoided during the SLM process.

(3) The laser power and scanning speed had a great influence on thesurface morphologies of scanning tracks. The high laser power cancause cracks formed on the surface, while the high scanning speedcan lead to shrinkage and irregular flow front of scanning tracks.The cracks formed on the surface of scanning tracks when the linearenergy exceeded a value of about 1.75 J/mm. The simulation re-sults demonstrated that the formation of cracks was owing to largeresidual stress caused by high temperature gradient and rapidcooling rate.

Through this study, laser power ranging from 250W to 350W andscanning speed ranging from 200mm/s to 400mm/s with a linearenergy density less than 1.5 J/mm were optimized for the tungstencomponents. This result demonstrated that the study of scanning trackwas an effective method to understand the effects of parameters forSLM- processed refractory and difficult-to-machine materials.

Acknowledgments

The authors gratefully acknowledge the financial support by ScienceChallenge Project (No. TZ2018006-0301-02 and No. TZ2018006-0303-03).

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