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Computational Materials Science

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Mesoscopic study of thermal behavior, fluid dynamics and surfacemorphology during selective laser melting of Ti-based composites

Donghua Daia,b, Dongdong Gua,b,⁎, Qing Gea,b, Yanze Lia,b, Xinyu Shia,b, Yixuan Suna,b, Shuhui Lic

a College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics (NUAA), 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 (NUAA), Yudao Street 29, Nanjing 210016, Jiangsu Province, PR Chinac Shanghai Key Laboratory of Digital Manufacture for Thin-Walled Structures, Shanghai Jiao Tong University, 200240 Shanghai, PR China

A R T I C L E I N F O

Keywords:Selective laser meltingTitanium matrix compositeMesoscopic simulationSurface morphologyThermodynamics

A B S T R A C T

A mesoscopic simulation based on the randomly packed powder bed model was developed to study the thermalbehaviors during selective laser melting (SLM) of Ti-based composites. Effects of processing parameters on thethermal behavior, fluid dynamics and surface morphology evolution within the molten pool were investigated.The obtained results revealed that the operating temperature, cooling rate and melt lifetime were highly en-hanced as the laser power was increased. Meanwhile, the increased molten pool dimensions, the turbulent fluidflows, the improved escaping rate of the entrapped gas and the efficient rearrangement of reinforcing particleswithin the molten pool appeared at the application of the high laser power. At the optimized processing para-meters, the peak of the operating temperature profile located in the laser and powder interaction area wasapparently disappeared with the formation of the maximum temperature of 3300 K and, the mean operatingtemperature of the platform caused by the heat accumulation was as high as 1300 K. Moreover, the surfacemorphology of the molten pool predicted by the simulation showed a variation from continuous pores tofragments, then to the typical and regular liquid front, and finally to the turbulent liquid front and spatter andballing phenomenon as the laser power increased. At the laser power of 200 W and laser energy density of 140 J/m, the maximum velocity was located in the front and rear region and, the velocity vector located in the meltadvanced front pointed to the rear region of the molten pool, indicating that the melt from the irradiation regionwould complete the efficient melt supplement and avoid the formation of residual pores and therefore, a goodand flat surface with few spatters was obtained with the clear liquid front. The simulated surface morphologywas found to be consistent with the experimental measurements.

1. Introduction

Selective laser melting (SLM) is considered as one of the most pro-mising additive manufacturing (AM) technologies, which is capable offabricating complex geometrical components with excellent perfor-mances [1–3]. As a thriving and high-efficient manufacturing method,SLM has been adopted for the production of stainless steel, aluminum,titanium and nickel based alloys [4–7]. Due to the excellent propertieswith the high specific strength, remarkable corrosion resistance anddistinguished biocompatibility, titanium alloys are widely applied inthe aerospace, automotive and biomedical industries [7]. However, theurgent demand for the higher performance increases with the realiza-tion of the longer service life on harsh environmental conditions. Inorder to obtain better strength properties, the method of adding

ceramic reinforcement in titanium alloy matrix has been paid sustainedattention. TiB, TiB2, SiC, TiC and WC reinforcement materials werecommonly considered in Ti based composites [8,9]. Among these re-inforced materials, due to the excellent chemical stability and perfectcompatibility of TiC such as thermal expansion coefficient and density[10], the addition of the TiC reinforcement can considerably enhancethe strength of the titanium matrix composites, yielding the stablephysicochemical properties at elevated temperature. Previous workshave shown that the addition of the TiC reinforcement improved theelastic modulus and tensile strength of the SLM-fabricated parts[11,12]. While the addition of the ceramic particles was detrimental tothe spreading behavior, the densification behavior and surface quality,due to the formation of high viscosity and the limited flowability of themolten pool. Meanwhile, the physical mechanisms of the thermal

https://doi.org/10.1016/j.commatsci.2020.109598Received 9 October 2019; Received in revised form 10 February 2020; Accepted 10 February 2020

⁎ Corresponding author at: College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics (NUAA), Yudao Street 29, Nanjing210016, Jiangsu Province, PR China.

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

Computational Materials Science 177 (2020) 109598

0927-0256/ © 2020 Published by Elsevier B.V.

T

behavior, the densification behavior and the melt flow within themolten pool are generally complex and there is a large challenge tovisually monitor these physical phenomena.

In order to advance the research of SLM processed Ti-based com-posites, the mesoscopic simulation as an excellent research method waspromoted, which could provide insight into the physics of the laser-composite interaction and the thermal behavior during the SLM pro-cess. Some studies involved of the mesoscopic simulation methods havebeen reported. Martin et al. elucidated the mechanisms of residual poreformation and liquid-solid interface dynamics using in-situ X-ray ima-ging and multi-physics simulations, which showed the significance ofthe research on pore formation and thermodynamic behavior [13].Khairallah et al. developed a 3D thermo-fluid model based on an Ar-bitrary Lagrangian-Eulerian method (ALE3D) and identified three dis-tinct zones, called depression, transition and tail end in laser powderbed fusion of 316L stainless steel [14]. Panwisawas et al. studied thethermal fluid flow and the microstructural evolution of a set of laserscanned single tracks using a particle dropping model [15]. Mesoscopicsimulations have been widely applied in the study of SLM processedsingle powder system. Recently, mesoscopic simulation began to beused in the study of SLM processed composites. Gu et al. studied theinfluence of thermodynamics within the molten pool on the migrationdistribution state of reinforcement during SLM of AlN/AlSi10Mg com-posites [16]. Meanwhile, they established the mesoscopic powder bedmodel with the random distribution of reinforcements for WC/Inconel718 composite and, it was found that the reduction of operating tem-perature and the resultantly weak atoms diffusion rate contributed tothe thermodynamic driving force of dendrite growth [17]. The re-lationship of the thermal behavior and the microstructure evolution hasbeen established. However, it seemed that the effect of the thermalbehavior on the metallurgical behavior and the morphology evolutionusing the mesoscopic simulation method had not been studied. There-fore, it is significant to promote mesoscopic simulation on SLM pro-cessed composites and discover the variation trend within the moltenpool using different processing parameters.

In this work, the influences of laser power on the thermal behavior,fluid dynamics and surface morphology evolution of the SLM-processedTi based composites were studied using the computational fluid dy-namics (CFD). The physical mechanisms of the temperature fields,thermocapillary convection and surface morphology evolution in SLMprocessed composites were discussed. To further validate the simulationresults of mesoscopic powder bed model, the top surface morphologywas compared with the experimental results.

2. Numerical simulation and experiments

2.1. Model establishment

Generally, during the laser powder bed fusion process, the materialphase undergoes the continuous transformation from the powder ma-terial to the melt irradiated by the laser beam and terminally to thesolidified part as the laser far away from the irradiated region. The meltconvection and the attendant heat transition from the free surface to thebottom of the molten pool were predicted by the calculation of thesurface tension coefficient of the melt and the operating temperature.As a result, the surface tension of the melt within different regions ofthe molten pool was obtained, driving the melt convection and thesurface morphology evolution of the solidified melt. The definition ofthe state of the material, the surface tension coefficient, the surfacetension gradient of the molten pool and the melt convection driven bythe surface tension were calculated by the consideration of the thermalphysical properties of the applied materials. This work considered themelting and solidification, the phase transition, the thermal conduc-tion/convection and radiation between laser beam and powder material(Fig. 1a). The effect of evaporation on the molten pool was consideredas the recoil pressure, which added extra momentum source to the

liquid fluid that made a molten pool surface depression. A randomlypacked powder bed model was established in dimensions of300 × 150 × 65 μm3 (Fig. 1b), where the red and blue particles re-presented TiC and Ti6Al4V powder material, respectively. The directalgorithm compiled geometric model was used to generate the cartesiancoordinate information and the radius of powder particles in MATLABsoftware. The randomly packed Ti6Al4V powders with particle dia-meter ranging in 15–25 μm were generated and, the TiC powders withthe mean size of 6 μm were generated in the adherence with theTi6Al4V powder particles. The information was imported in simulationsoftware, and the correspondingly produced phase field consisted ofthree portions: the randomly packed Ti6Al4V powder, the TiC powderand the argon gas. The zigzag scanning strategy was also considered(Fig. 1b). The scanning speed, v, was settled as a constant of 1400 mm/s, and P were preset at 100 W, 150 W, 200 W and 250 W, respectively.Thus, four different laser energy densities, η, of 70 J/m, 105 J/m,140 J/m and 180 J/m were changed to vary the SLM processing con-ditions, which were defined by η = P/v.

Due to the effect of the recoil pressure and the surface tension actedon the free surface of the molten pool, the melt companied with the gasphase close to the free surface of the molten pool was appeared with theobvious fluctuation of the height of the molten pool. In order to in-vestigate the thermodynamic behavior during SLM process, the XYmonitoring plane located in the middle of the powder layer along thebuilding direction (Z = 15 μm), the YZ plane (X = 140 μm) and the XZplane (Y = 14 μm) was employed to record the temperature field andthe velocity field. The attendant cooling rate was correspondingly ob-tained at the monitoring point located in the scanning track(X = 30 μm, Y = 0 μm, Z = 15 μm).

2.2. Governing equations

To rationalize the simulation of thermal molten liquid flow, a vo-lume of fluid (VOF) methodology was applied to solve the coupling ofmass conservation equation, momentum equation and energy equation,calculating the evolution of the interface of different phases [13,14].

Mass conservation equation:

∂∂

+ ∇∙→

=ρt

ρV M( ) S (1)

Momentum equation:

∙⎛

⎝⎜

∂→

∂+

→∙∇

→⎞

⎠⎟ = ∇

→− ∇ + ∙

→+

→ρ V

tV V μ V p M V FS

2

(2)

Energy equation:

∙⎛

⎝⎜

∂→

∂+

→∙∇ ⎞

⎠⎟ = ∇∙ ∙∇ +ρ T

tV T k T S( ) H

(3)

where ρ, μ, k and p are the density, dynamic viscosity, thermalconductivity and pressure, respectively.

→V denotes the velocity of the

molten flow, MS indicates the mass of molten liquid,→F presents the

body force and momentum source term, SH is the source of the energyequation and can be written as:

= − ⎛⎝

∂∂

+ ∇∙→ ⎞

⎠+ρ

tH V H qS Δ ( Δ )H (4)

where HΔ represents the latent heat of the phase transform, q is thevolumetric laser heat source. The laser heat source with a volumetricGaussian distribution was amended from Xia’s research [18]:

=−

⎛

⎝⎜⎜

− + ⎞

⎠⎟⎟( ) ( )

q PA

R πHexp x y

R log

6

1

9( )

eHz

2 1

2 2

23 (5)

where P is laser power, R represents the diameter of laser beam, H is

D. Dai, et al. Computational Materials Science 177 (2020) 109598

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the penetrated depth of heat source, and A is the effective laser energyabsorption of material. The value of A is settled as 0.82 for TiC and 0.77for Ti6Al4V [19].

According to the previous research, the image captured by the high-speed camera showed a vivid fluid flow with the turbulent character-istic [20], which was caused by excessive evaporation, enhancedthermal conductivity and viscosity [15–18]. Therefore, the k-ε turbu-lent model was applied in the model and, the turbulence could be ex-pressed as follows:

⎜ ⎟

∂∂

+ ∂∂

= ∂∂

⎡⎣⎢

⎛⎝

+ ⎞⎠

∂∂

⎤⎦⎥ + + −

tρk

xρku

xμ

μσ

kx

G G ρε( ) ( )i

ij

t

k jk b

(5)

and

⎜ ⎟

∂∂

+ ∂∂

= ∂∂

⎡⎣⎢

⎛⎝

+ ⎞⎠

∂∂

⎤⎦⎥ + + −

tρε

xρεu

xμ

μσ

εx

C εk

G C G C ρ εk

( ) ( ) ( )i

ij

t

ε jε k ε b ε1 3 2

2

(6)

where Gk represents the generation of turbulence kinetic energy due tothe mean velocity gradients. C1ε (1.44), C2ε (1.92), C3ε (0.25) areconstants. σk (1.0) and σε (1.0) are the turbulent Prandtl numbers for kand ε, respectively.

The turbulent viscosity is obtained from:

=μ ρC kεt μ2

(7)

where Cμ is constant and equal to 0.09.The turbulence kinetic energy production term is computed from:

=G μ Sk t2 (8)

where S is the modulus of the mean rate-of-strain tensor, defined as≡S S S2 ij ij . Sij represents the mean strain rate given by

= +∂∂

∂∂( )Sij

ux

ux

12

ij

j

i.

The generation of turbulence due to buoyancy is obtained from

= − ∂∂

G βgμPr

Txb i

t

t i (9)

where Prt is the turbulent Prandtl number for energy (0.85) and gi is thecomponent of the gravitational vector in the ith direction.

2.3. Boundary conditions

Considering the radiation and convection, the boundary conditionat the top surface (Z = 55) is given by:

∂∂

+ − + − =Tn

h T T σε T T qK ( ) ( )c 04

04

(10)

where T0 is the ambient temperature, hc is the coefficient of the con-vective heat-transfer, σ is the Stefan-Boltzmann constant and ε indicates

the emissivity. The first term on the left represents conduction, thesecond represents convection and the third term represents radiation.

The side wall (X = 0, X = 300, Y = 150, Y = −150) and theunderside wall (Z = −10) were settled as the thermal convection withthe heat transfer coefficient equal to 10 W/(m2K), which described theheat transfer of the meso-scale domain within the powder bed.

2.4. Material property

To discover more details of fluid flow in the molten pool, the visc-osity [21], thermal conductivity [22] and surface tension [23] wereregarded as the temperature-dependent properties. Meanwhile, otherthermal physical properties of TiC [24] and Ti6Al4V [25,26] were as-sumed to be temperature independent, such as density, specific heat,melting heat, solidus temperature and liquidus temperature (Table 1).The interfacial forces have been taken into account and simulated, in-cluding the surface tension (capillary force), the Marangoni flow(thermo-capillary force) and the recoil pressure due to the solid/liquidtransition.

Based on the volume of fluid (VOF) model applied in this three-phase-system simulation, the volume-fraction-averaged material prop-erties are calculated by:

= + +−

α x α x α xx 1 1 2 2 3 3 (11)

+ + =α α α 11 2 3 (12)

And the volume fraction equation for i phase is

∂∂

+ →∙∇ =αt

v αSρ

ii

α

i

i

(13)

where αi represents the volume fraction of i phase.

2.5. Experimental procedures

The TiC powder with a mean particle size of 6 μm and the gasatomized Ti6Al4V powder with a spherical shape and an average

Fig. 1. Schematic of the physical model during selective laser melting process (a) and establishment of randomly packed powder bed model (b).

Table 1As-used material properties and physical modeling parameter.

Parameter Ti6Al4V TiC

Ambient temperature 300 KConvective heat-transfer coefficient 10 W/(m2K)The Stefan-Boltzmann constant 5.67 × 10−8 W/(m2K4)Viscosity (liquidus state) 0.005 kg/m·sDensity 4428 kg/m3 4940 kg/m3

Melting point 1928 K 3035 KAbsorptivity at 1.064 μm 0.77 0.82Melting heat 3.65 × 105 J/kg 1 × 106 J/kgSpecific Heat 700 J/kg·K 1000 J/kg·K

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particle diameter of 20 μm were used as the raw material with theweight ratio of 1:99. The composite powder was mechanically mixed ina planetary ball mill, using a ball-to-powder weight ratio of 1:1, a ro-tation speed of main disc of 200 revolutions per minute, and a mixingduration of 4 h. The SLM apparatus consisted of an YLR-500-SMYtterbium fiber laser with a power of ~500 W and a spot size of 70 μm(IPG Laser GmbH). For further validation of the accuracy of the physicalmodel, the experimental processing parameters were in a full ac-cordance with those applied for numerical simulation. The pressure ofinsert argon gas was settled as a constant of 0.5 kPa throughout theexperiments. The three-dimensional top surface morphologies of theSLM-processed components were observed by FEI Quanta 200 scanningelectron microscope (SEM).

3. Results and discussion

3.1. Temperature evolution of the molten pool

The temperature distribution profiles on the top surface of powderbed under various laser powers are depicted in Fig. 2. The volumetricGaussian distributed laser made a peak-shaped temperature profile withthe formation of the platform at the rear location along the laser scandirection, which was caused by the change of the irradiated materialfrom powder material to bulk part and the attendant heat accumulation

[27]. As the applied laser power was 100 W, the temperature dis-tribution was shown in the broad and lower-height pattern with themaximum operating temperature of 2200 K, indicating that the heataccumulation was relatively weak and, the mean operating temperatureobtained in the plateau was close to 1000 K (Fig. 2a), giving rise to theformation of the decreased cooling rate and the resultant thermal stress.As the laser power was elevated to 150 W, a sharp Gaussian distributionof the operating temperature was appeared with the maximum oper-ating temperature of 2860 K (Fig. 2b). The plateau of the heat accu-mulation in the rear part of the irradiation region was apparently dis-covered with the average temperature of 1100 K. It could be concludedthat the temperature gradient was increased. While for the applicationof the laser power of 200 W, the peak of the operating temperatureprofile located in the laser and powder interaction area was apparentlydisappeared with the formation of the maximum temperature of 3300 Kand, the mean operating temperature of the platform caused by the heataccumulation was as high as 1300 K (Fig. 2c), implying that the limitedheat dissipation behavior would play a positive role in the formation ofthe surface tension gradient, the cooling rate and the efficient spreadingbehavior of the melt. As the laser power was increased to 250 W, itseemed that the plateau pattern of the heat accumulated region wasstrictly eliminated, resulting in the generation of the considerablyhigher cooling rate and restricting the lifetime of the molten pool.Meanwhile, the maximum operating temperature of 3350 K, above the

Fig. 2. Temperature distribution profiles on the surface of powder bed using various processing parameters (v = 1400 mm/s): (a) P = 100 W, η = 70 J/m; (b)P = 150 W, η = 105 J/m; (c) P = 200 W, η = 140 J/m and (d) P = 250 W, η = 180 J/m.

D. Dai, et al. Computational Materials Science 177 (2020) 109598

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boiling temperature of Ti6Al4V material (3318 K), was appeared in thecenter of the laser irradiation area (Fig. 2d), giving rise to the formationof the material evaporation and the splashing of the melt. As a result,the fluctuation of the molten pool and the high tendency of the ballingphenomenon were severely produced, deteriorating the surface mor-phology of the solidified part, densification behavior and the terminalperformance of the solidified part.

In order to have a thorough understanding of the temperature his-tory of the SLM of TiC/Ti6Al4V composite, the calculated temperatureprofiles and the corresponding temperature gradient versus irradiationtime at the monitoring point are shown in Fig. 3. It was apparent thatthe operating temperature of the molten pool had a sharp arising trendas the region was irradiated by the laser beam and a slightly decreasingtrend as the laser beam was far away from the irradiated region. For thelaser power of 100 W, it was shown that the lifetime of the middle partof the molten pool was significantly restricted with the time of 18 μsdue to the insufficient laser energy input and the limited operatingtemperature (Fig. 3a) and therefore, the cooling rate of the molten poolwas 2.17 × 107 K/s (Fig. 3b). In this situation, there was not sufficienttime for the melt to complete the spreading process, resulting in theformation of the cavity between the powder particles and an inevitableporosity phenomenon. As the laser power was elevated to 150 W, thelifetime of the middle part of the molten pool was increased to 37 μs(Fig. 3a) while the cooling rate of the molten pool was slightly in-creased to 2.18 × 107 K/s (Fig. 3b), indicating that there was morelifespan for melt spreading and wetting process before the solidifica-tion. For the laser power of 200 W, the lifetime of the middle part of themolten pool was significantly enhanced with the time of 62 μs (Fig. 3a),predicting the effective elimination of residual pores and the attendantdensification behavior [28]. Meanwhile, the cooling rate of the moltenpool was elevated to 4.07 × 107 K/s (Fig. 3b), restricting the crystalgrowth and having a high tendency to generate cracks and delamina-tion [29]. On increasing the laser power to 250 W, the lifetime of themiddle part of the molten pool and the cooling rate was as high as 72 μsand 5.62 × 107 K/s (Fig. 3a and b), respectively, yielding the ex-cessively melt splashing from the molten pool caused by the long life-time of the melt. As a result, a poor solidified surface quality would beobtained due to the high instability of the molten pool, which wasdetrimental to the relative density in the terminal SLM-processed parts[16].

3.2. Thermodynamic behavior within the molten pool

Typical calculated temperature contours and velocity vectors withinthe molten pool are illustrated in Fig. 4. The convection within the

molten pool was highly sensitive to the processing parameters. Whenthe laser power was 100 W, the thermocapillary convection was uni-formly distributed within the Ti6Al4V liquidus temperature line,1928 K, at the free surface of the molten pool with the maximum ve-locity of 1.5 m/s (Fig. 4a). The main vectors located in the liquidusregions of the molten pool illustrated that the fluid flow and the at-tendant heat and mass transfer was limited due to the insufficient en-ergy input and the high melt viscosity. As the laser power was elevatedto 150 W, the slightly expanded dimensions of the molten pool and anapparent velocity vector difference within the molten pool were pro-duced, showing that the stronger dynamic field (v= 3m/s) was locatedat the center region while the restricted melt flow (v = 0.5 m/s) was atthe edge of the molten pool (Fig. 4b). It was shown that the typicalsurface tension gradient due to the temperature gradient tended to begenerated and, the disordered fluid flow was produced at the forepart ofthe molten pool, which had a negative effect on the uniform flow of themelt and resulted in the formation of the cavity on the free surfacederived from the limited melt spreading. As the laser power was 200 W,the molten pool was appeared in the stripe pattern with an obviousdissipation tail of the length of 130 μm (Fig. 4c). The maximum velocitywas located in the front and rear region and, the velocity vector locatedin the melt advanced front was pointed to the rear region of the moltenpool, indicating that the melt from the irradiation region would com-plete the efficient melt supplement and avoid the formation of residualpores. Meanwhile, the multi-vortex convection was appeared in thecenter of the molten pool with the uniform distribution of the velocity,implying the efficient heat and mass transfer and the attendant meltspreading process driven by the reverse motion (Fig. 4c). Besides, aclear mushy zone with a weak dynamic characteristic (v < 0.5 m/s)appeared at the tail of the molten pool under the elevated energydensity (Fig. 4c). As the laser power increased to 250 W, the intenseflow vector was located in the melt front with a velocity of 3.5 m/s.There existed two orientations of the convection flow located in themaximum velocity of the advanced region of the molten pool, in-dicating that the significantly turbulent and instable melt was produceddue to the different convection vectors. As a result, the instability of themolten pool and the high tendency of spatter phenomena were severelyproduced. Meanwhile, the mushy zone located at the tail of the stret-ched molten pool with the mean velocity of 0.4 m/s was produced,significantly decreasing the melt velocity due to the high viscosity andthe restricted Marangoni flow (Fig. 4d). Therefore, the considerablyfluctuated molten pool was obtained under the combined effect of themelt viscosity, the stretched length of the molten pool and the dis-tinguished difference in the melt convection pattern, which was detri-mental to the surface morphology obtained in the solidified part.

Fig. 3. The calculated temperature profiles (a) and corresponding temperature gradient (b) versus iteration time of the monitoring point (X = 30 μm, Y = 0 μm,Z = 10 μm) at the scan track using various processing parameters.

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In order to study the surface morphology evolution and the densi-fication behavior, the phase contour and the velocity vector within theXZ cross-sections of the SLM-processed TiC/Ti6Al4V composites usingdifferent processing parameters are shown in Fig. 5. When the appliedlaser power was 100 W, it was observed that some irregular pores werelocated in the bottom region of the molten pool, implying that themetallurgical bonding ability of the as-fabricated layers and the pre-vious layers was seriously poor due to the insufficient laser energy inputand the limited laser energy penetration depth (Fig. 5a). Due to themost laser energy absorbed on the free surface of the molten pool, anapparently fluctuated free surface of the molten pool was generated inthe combination with the cavity near the reinforcing particles. For thelaser power of 150 W, the free surface of the molten pool becameconsiderably smooth and, the large melt velocity was located in thebottom area of the molten pool (Fig. 5b). However, due to the down-ward orientation of the velocity vector of the melt and the long channelat the bottom of the molten pool, the gas located in the layer connectedregion had large difficulty in escaping, giving rise to the formation ofthe residual pores. As the laser power was elevated to 200 W, the

maximum velocity of the melt with the upward orientation was pro-duced in the bottom of the molten pool irradiated by the laser beamand, meanwhile, the free surface of the molten pool was relatively flatand smooth (Fig. 5c). In this situation, the gas driven by the thermo-capillary convection could be efficiently escaped, eliminating the re-sidual pores within the solidified part. While for the laser power of250 W, the height of the processed layer was seriously decreased due tothe formation of the material evaporation, resulting in the formation ofthe recoil pressure with the downward orientation of the velocity vectornear the free surface of the molten pool. Therefore, the entrapped gaswithin the molten pool could not be effectively escaped under the effectof the downward vector (Fig. 5d). Under the combined effect of thematerial evaporation and the downward vector, the surface mor-phology of the solidified part had a high tendency to generate the ad-hered melt splashing and the relative density within the solidified partwas limited.

Fig. 4. Temperature contours and velocity vectors within the molten pool of the XY cross-section of the SLM-processed TiC/Ti6Al4V composites (Z = 15 μm) usingvarious laser powers (v = 1400 mm/s): (a) P = 100 W, η = 70 J/m; (b) P = 150 W, η = 105 J/m; (c) P = 200 W, η = 140 J/m and (d) P = 250 W, η = 180 J/m.

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Fig. 5. Evolution of the morphology of molten pool within the SLM-processed TiC/Ti6Al4V parts on the XZ cross-section (Y = 14 μm): (a) P = 100 W, η = 70 J/m;(b) P = 150 W, η = 105 J/m; (c) P = 200 W, η = 140 J/m and (d) P = 250 W, η = 180 J/m.

Fig. 6. Temperature distribution and surface morphology evolution of the scan tracks of the SLM-processed TiC/Ti6Al4V composites using different processingparameters (v = 1400 mm/s): (a) P = 100 W, η = 70 J/m; (b) P = 150 W, η = 105 J/m; (c) P = 200 W, η = 140 J/m and (d) P = 250 W, η = 180 J/m.

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3.3. Surface morphology during the SLM process

The representative evolution of the calculated surface morphologiesof the molten pool irradiated by the laser beam at various processingparameters is depicted in Fig. 6. It was obvious that the surfacemorphologies were sensitive to the processing parameters. As the laserpower was 100 W, a narrow scan track with the appearance of thepartially melted particles and the residual pores was obtained (Fig. 6a),giving rise to the formation of the inter-track porosity and the resultantpoor densification behavior of the solidified part [30]. It was reason-ably attributed to the shrinkage behavior and the resultant cavitycaused by the insufficient energy input. Nevertheless, for the laserpower of 150 W, it could be found that the surface morphology of thescan track became flat and smooth and, the melt was completelyspreading with the powder material at the edge of the molten pool,leading to the fine wetting behavior. However, the fragmented tracksand some cavities were inevitably eliminated on the surface of the so-lidified tracks (Fig. 6b). Generally, the fragment was caused by theballing phenomenon due to the severe instability of the molten poolinduced by the high melt viscosity under a relatively low energy den-sity, decreasing the surface quality and densification behavior [27–29].As the laser power increased to 200 W, an integrated and smooth scantrack with a stable liquid front was obtained (Fig. 6c). The disappearedfragments of the scan melt were attributed to the application of theimproved energy density, leading to the formation of the deducedviscosity, reasonable liquid lifetime, the moderate surface tension andthe sufficient spreading of the molten pool [31,32]. In this condition,TiC reinforcing particles within the molten pool, driven by the ther-mocapillary convection, had sufficient time to complete the re-arrangement process, yielding an enhanced metallurgical behavior and

uniform distribution within the molten pool. Therefore, the uniformlydistributed viscosity within the whole molten pool made the melt fullyspreading and wetting with the partially melted particles, promotingthe densification behavior and smoothening the free surface of themolten pool. As the laser power was further increased to 250 W(Fig. 6d), the surface quality of the scan track was maintained, whilethe fluctuation of the molten pool was significantly enhanced with thedeterioration of the continuity and roughness of the edge of the moltenpool. In this situation, the material evaporation of the molten pool wasobserved (Fig. 5d), which could be attributed to the passive movementand fluctuation of the molten pool. Meanwhile, the increased intensityof the fluid flow within the molten pool generated a turbulent liquidfront and promoted the spatter and the overflow phenomenon, de-creasing the surface quality of the solidified part.

3.4. Physical mechanisms of the densification behavior and surfacemorphology evolution

The evolution of the temperature contours and the velocity vectorsin the YZ plane (X = 140 μm) under different iterated time is shown inFig. 7. When the laser spot was approaching to the plane (Fig. 7a), thehigh temperature was obtained at the irradiated surface of the powderlayer and, the main heat transfer direction was in a downward patternunder the combined effect of the refraction and reflection during theinteraction of the laser beam and the powder bed [33,34]. As the op-erating temperature was exceeded above the solidus temperature ofTi6Al4V, the surface of the powder particle began to collapse under theeffect of the dramatic decrease of viscosity. With the laser spot irra-diation proceeding (Fig. 7b–d), the significantly enhanced operatingtemperature was obtained and, to reduce the free surface energy of the

Fig. 7. Temperature contours and velocity vectors in the YZ plane (X = 140 μm) obtained in different iteration time (P = 200 W): (a) t = 7 μs, (b) t = 8 μs, (c)t = 9 μs, (d) t = 10 μs, (e) t = 11 μs, (f) t = 12 μs, (g) t = 13 μs, (h) t = 14 μs, (i) t = 15 μs.

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melt, the melt had a tendency to shrink into a spherical pattern. As theSLM process continued, the operating temperature was close to theboiling point, which brought about an intense upward evaporation withrecoil pressure forced on the free surface of the molten pool (Fig. 7c andd). As a result, the flow at the edge of the molten pool was prone toexpand, driving the escaping of gases from the cavity (Fig. 7d–f). As thelaser spot was far away from the irradiated region, the operating tem-perature gradually dropped to the liquidus temperature and entered themushy zone (Fig. 7g–i). The recoil pressure gradually disappeared andthe viscosity was recovered. As a result, the effect of the viscositydominated the fluid flow. In addition, the migration of reinforcementwithin the molten pool could also be observed in this model. Generally,the motion of TiC particles was driven by three forces within the moltenpool, including the propulsive force from the fluid flow, the gravity andthe buoyancy force. Under the mesoscopic scale, the gravity andbuoyancy could hardly make an impact on the motion of TiC particlescaused by the narrow difference in density between TiC and Ti6Al4V[14]. Because of the relatively high melting point of TiC (3035 K), thereinforcing particles were difficult to melt completely. Regardless of thechemical reaction between the reinforcement and the matrix, the re-inforcement was not static during the SLM process. The reinforcingparticle was located along the side of the molten pool at the initial time(Fig. 7). During the SLM process, the fluid flow interacted with thereinforcing particles and, the reinforcing particles would rearrange withthe fluid flow. On the other hand, the unmolten reinforcement wouldhinder the fluid flow (Fig. 7c and d), giving to the formation of multi-vortex within the molten pool.

3.5. Experimental validations

Top surface morphologies of the SLM-processed TiC/Ti6Al4V com-posites using different processing parameters are illustrated in Fig. 8.

For laser power of 100 W (Fig. 8a), the low-quality surface morphologywas obtained, showing the rough surface with the discontinuous scantracks and the continuous gaps between the overlaps. It was caused bythe limited amount of melt, the high viscosity and the limited spreadingbehavior within the molten pool. For the applied laser power of 150 W,the continuous scan tracks were obtained, while the partially meltedpowder particles were adhered to the edge of the scan track and theirregular pores were remained (Fig. 8b), implying the surface mor-phology tended to be improved. As the laser power was elevated to200 W, a flat and dense surface with few spatters was obtained with theclear liquid front (Fig. 8c), caused by the reasonable lifetime of themolten pool, the suitable viscosity and the efficient melt spreading. Asthe laser power increased to 250 W, the surface morphology of thesolidified part maintained the dense surface while, a large number ofmelt balls caused by the splashing were observed (Fig. 8d). The tur-bulent liquid front and a large amount of small-sized balls along thesolidified liquid front were mainly attributed to the low viscosity, thelong lifetime of the molten pool and the material evaporation producedwithin the molten pool, seriously decreasing the surface quality of thesolidified part.

4. Conclusions

The simulation of the molten pool temperature evolution, thermo-dynamics behavior and the surface morphology during selective lasermelting of the Ti-based composite powder system has been performed.The simulation results were validated by the experimental results,which were conducted under the same processing parameters as used inthe simulation. The physical mechanisms of the thermocapillary con-vection, the densification behavior and the surface morphology evolu-tion during SLM of TiC/Ti6Al4V composites were discussed.

Fig. 8. Typical surface morphologies of the SLM-processed TiC/Ti6Al4V composite parts using various processing parameters (v = 1400 mm/s): (a) P = 100 W,η = 70 J/m; (b) P = 150 W, η = 105 J/m; (c) P = 200 W, η = 140 J/m and (d) P = 250 W, η = 180 J/m.

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(1) The peak-shaped temperature profile with the formation of theplatform of the heat accumulation on the rear region of the moltenpool irradiated by the laser beam was generated. The operatingtemperature, melt lifetime and the cooling rate were highly en-hanced to the applied laser energy density.

(2) Thermodynamic behavior within the molten pool was significantlyaffected by the laser power. The laser energy input would play a keyrole in the formation of the turbulent thermocapillary convection,entrapped gas escaping behavior, material evaporation, meltspreading behavior and surface morphology.

(3) The surface morphology of the molten pool predicted by the si-mulation showed a variation from continuous pores to fragments,then to the typical and regular liquid front, and finally to the tur-bulent liquid front and spatter and balling phenomenon as the laserpower increased. However, excessively laser energy density wouldgive rise to a decreased surface quality due to the instability of themolten pool.

(4) During the SLM process, the evolution of the molten pool washighly sensitive to the melt viscosity, the surface tension and therecoil pressure. The existence of the particles hindered the efficientspreading of the melt flow, and the migration of the reinforcementgot more intense with the application of the high energy density.

CRediT authorship contribution statement

Donghua Dai: Methodology, Validation, Formal analysis,Investigation, Writing - original draft, Writing - review & editing.Dongdong Gu: Conceptualization, Formal analysis, Supervision,Funding acquisition, Project administration. Qing Ge: Methodology,Formal analysis, Visualization, Software, Writing - review & editing,Data curation. Yanze Li: Software, Validation. Xinyu Shi: Formalanalysis, Validation. Yixuan Sun: Software, Investigation. Shuhui Li:Visualization, Investigation.

Declaration of Competing Interest

The authors declare that they have no known competing financialinterests or personal relationships that could have appeared to influ-ence the work reported in this paper.

Acknowledgments

We are grateful for the financial support from the National NaturalScience Foundation of China (Nos. 51735005, 51790175); National KeyResearch and Development Program “Additive Manufacturing andLaser Manufacturing” (Nos. 2016YFB1100101, 2018YFB1106302);National Natural Science Foundation of China for Creative ResearchGroups (Grant No. 51921003); The 15th Batch of “Six Talents Peaks”Innovative Talents Team Program “Laser Precise AdditiveManufacturing of Structure-Performance Integrated Lightweight AlloyComponents” (No. TD-GDZB-001) (Jiangsu Provincial Department ofHuman Resources and Social Security of China); 2017 ExcellentScientific and Technological Innovation Teams of Universities inJiangsu “Laser Additive Manufacturing Technologies for MetallicComponents” (Jiangsu Provincial Department of Education of China).

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