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Journal of Alloys and Compounds 747 (2018) 684e695

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Journal of Alloys and Compounds

journal homepage: http: / /www.elsevier .com/locate/ ja lcom

Microstructure evolution, mechanical response and underlyingthermodynamic mechanism of multi-phase strengthening WC/Inconel718 composites using selective laser melting

Mujian Xia a, b, Dongdong Gu a, b, *, Chenglong Ma a, b, Hongyu Chen a, b,Hongmei Zhang a, b

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

a r t i c l e i n f o

Article history:Received 17 January 2018Received in revised form2 March 2018Accepted 5 March 2018Available online 6 March 2018

Keywords:Selective laser meltingMulti-phaseMesoscopic simulationCombined strengtheningThermodynamics

* Corresponding author. College of Materials ScienUniversity of Aeronautics and Astronautics, YudaoJiangsu Province, PR China.

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

https://doi.org/10.1016/j.jallcom.2018.03.0490925-8388/© 2018 Elsevier B.V. All rights reserved.

a b s t r a c t

For further understanding the underlying relations of microstructure evolution on mechanical propertiesof Inconel 718 composites reinforced by WC particles using selective laser melting (SLM), the influence oflaser scanning speed on microstructure growth, evolution mechanism and mechanical properties wasanalyzed combining with experiments and mesoscopic simulations. The obtained results apparentlyreveal that the Ni2W4C primary dendrites exhibit with a reduced trunk length as well as the decreasinglength and spacing of dendrite arms following an increasing scanning speed according to the combininganalysis of X-ray diffraction spectrum and EDS, due to the significant reduction of operating temperatureand the resultantly weak atoms diffusion rate and thermodynamic driving force of dendrite growth.Meanwhile, the (Nb,M)C carbides (M representing Ni, Cr, W, Fe, Ti) generated in g-Ni matrix are inverselyrefined as elevating the laser scanning speed. Both the experimental microhardness and ultimate tensilestrength of SLM-processed WC/Inconel 718 composite is, therefore, evidently enhanced with a slightreduction of elongation as successively increasing the scanning speed, attributing to the combinedstrengthening effects of refined multi-phase of Ni2W4C primary dendrite and granular (Nb, M)C carbides.Furthermore, the underlying evolution mechanism of composite microstructure with variable processingconditions is discussed.

© 2018 Elsevier B.V. All rights reserved.

1. Introduction

Nickel-based superalloys have experienced extensive develop-ment and attracted enormous attentions in the last decades andthus were widely applied in variable applications (e.g., industrialgas turbine, aircraft engine and hot end components, guide vanes,turbine disks, combustion chambers, etc), due to its excellentproperties including corrosion resistance, oxidation resistance andappropriate strength at high temperatures [1]. In particular, underthe combining activities of solid-solution strengthening, dispersionstrengthening and fine grain strengthening, the gradually

ce and Technology, NanjingStreet 29, Nanjing, 210016,

.

developed Inconel 718 alloy was considered as one of thecommercially used nickel-based superalloys and hence wasextensively used in various fields, attributing to its high ability ofretaining mechanical stability at high-temperature applications upto 700 �C [2e4]. Nevertheless, the limited mechanical perfor-mances of Inconel 718 alloy remained a serious concern for theapplication environments, where the severe abrasive and erosionphenomena commonly existed [5]. Normally, it was well knownthat the additions of ceramic particles incorporated into nickel-based matrix composites (CPNMCs) made a great effect on thestrength, ductility and wear resistance, which also performed aconsiderable potential for commercial applications. Numerousresearch attempts, therefore, have concentrated on the applicationof various preparation techniques to produce CPNMCs parts forfurther improving its mechanical performances. Inconel 718 su-peralloy and its CPNMCs have been recently developed and the

M. Xia et al. / Journal of Alloys and Compounds 747 (2018) 684e695 685

bulk-form components were commercially realized throughwrought, cast powder metallurgy techniques. Although the ulti-mately as-fabricated parts using these conventional methods havedemonstrated the desirable microstructure and mechanical prop-erties, the CPNMCs parts built through the conventional methodsusually appeared with insufficient densification response andinhomogeneous microstructure. Meanwhile, since Inconel 718 andCPNMCs were prone to be considerably strengthened in a broadrange of temperatures caused by solid-solution strengthening andprecipitation strengthening, the resultantly high hardness and lowthermal conductivity make it impossible to use the conventionalmachining methods due to the excessive tool wear and poorworkpiece surface integrity [6,7]. Moreover, the Inconel 718 su-peralloy and CPNMCs components highly demanded for geometry-complex shapes with mazy inner chambers or overhangs in theapplications of aeronautics and astronautics, and thus wereimpossible to produce by a single processing method. As a result,the conventional processing techniques were not efficient andcapable to produce the Inconel 718 parts with complex configura-tions and high performances.

Selective laser melting (SLM), which was well recognized as apromising additive manufacturing (AM) technology, enabled thequick fabrication of three-dimensional parts with any complexshapes directly from metallic powders using high-energy laserbeam [8,9]. It can built the parts directly from the digital data ofgeometrymodels, using a computer controlled scanning laser beamas the energy source to melt the pre-spread powders selectively ina layer-by-layer fashion, which also presented a high flexibility infeedstock and shapes [10]. On the other hand, the geometry-complex components with a high dimensional precision andexcellent surface integrity can be fabricated precisely using SLMtechnique nearly without any subsequent process requirements incomparison to the conventional techniques [11]. Furthermore, un-der the activities of high-energy laser beam, the ceramic particleswere partially and/or completely melted within pool and hence ithad a more strengthen bonding to matrix. Therefore, SLM tech-nique provided a prospective potential to fabricate the high-performance composite parts with efficient and cost-savingcharacters.

Recently, a considerable number of research efforts primarilyfocused on the densification, wear resistance and thermodynamicsof Inconel 718 composite reinforced by carbides through SLMtechnique. For instance, according to the favorable wetting abilityof WC particles and Inconel 718 alloy, the WC/Inconel 718 com-posite was successfully fabricated with variable processing pa-rameters using SLM technique by Rong et al. [5]. The result revealedthat the graded interfacial layer with a composition of (W, M)C3(M¼Ni, Cr, Fe) was formed in SLM-ed WC/Inconel 718 composite,due to the sufficient diffusion of Ni, Nb, W and C elements sur-rounding the WC reinforcements. Simultaneously, the properlygradient interface and diffusion layer of SLM-processedWC/Inconel718 composite generated using a reasonable laser scanning speedof 450mm/s, and the worn composite part was dominate by ad-hesivewearwith a significantly low friction coefficient of 0.35 and areduced wear rate of 2.5� 10�4mm3N-1m-1 [12]. Shi et al. [13]adopted a three-dimensional finite element model of TiC/Inconel718 composite using ANSYS multiphysics finite element package toinvestigate the thermodynamic behavior within pool and theevolution of the pore defects. Jia and his co-authors preparedInconel 718 composite parts reinforced by TiC particles using SLMtechnique and declared that the laser energy density played aremarkable role on the densification, microstructure and wearresistance of composite parts [14]. Nevertheless, SLM techniqueinvolved series of non-equilibrium, rapid melting and solidificationprocess caused by a high-energy laser beam, resulting in a

considerable evolution of the temperature gradient, chemicalconcentration and microstructure growth within pool of the com-posite. In particular, the growth of microstructure was highly sen-sitive to the variations of temperature within molten pool undervariable processing conditions during SLM. In case of this, there arestill some exiting challenges concerning the microstructure evo-lution, types of formed carbides and its growth mechanism of SLM-processed composite reinforced by ceramic particles, leading to theresultant variations of mechanical performances. To date, fewliterature focused on the aforementioned problems and thusconsiderable efforts were greatly acquired to give a comprehensiveunderstanding of the microstructure evolution and its underlyingmechanisms of the SLM-processed composite parts.

In this study, the SLM-processed WC/Inconel 718 compositewith novel microstructural characteristics with various scanningspeed were performed. Meanwhile, the influence of processingconditions on the microstructure evolution and mechanical prop-erties of SLM-processed WC/Inconel 718 composite parts was sys-tematically analyzed. To further give a unique insight of theevolution mechanism of obtained microstructure of composite, themesoscopic model with random distribution of WC reinforcementswithin the powder-bed was newly established to investigate thethermodynamic behavior within molten pool, and the corre-sponding mechanism of microstructure evolution was thusconcluded.

2. Experimental procedures

2.1. Preparations of samples

The gas-atomized and spherical pre-alloy Inconel 718 powderwas used with a purity of 99.7% and an average diameter of 30 mm.The irregular-shaped WC ceramic particles were applied as thereinforcements of composite with a purity of 99.7% and anapproximately average size of 10 mm. The WC/Inconel 718 com-posite powder was homogeneously mixed using a Fritsch Pulveri-sette 6 planetary ball mill (Fritsch GmbH, Germany) with thereinforcement weight ratio of 20wt% and a ball-milling duration of4 h.

2.2. SLM processing

The SLM system (SLM-150, China), which mainly contained anIPG Photonics Ytterbium YLR-500-SM fiber laser (Stuttgart, Ger-many) with amaximal power of 500Wand a focused spot diameterof 70 mm, an automatic powder layering apparatus, an inert argongas protection system and an automatic processing controllingsystem, was employed to build the WC/Inconel 718 compositecomponents. Based on the previous optimization of processingcondition, the laser power was optimized as a constant of 121Wand the variable laser scanning speed was properly settled as400mm/s, 500mm/s, 600mm/s and 700mm/s.

2.3. Microstructure characterization

The as-built WC/Inconel 718 composite parts, which werefabricated with various scanning speed, were cut with a three-dimensional dimension of 5mm� 5mm� 6mm. Subsequently,the obtained specimens were ultrasonically rinsed with ethanoland then dried. According to the standard procedures for thepreparation of metallographic specimen, the composite sampleswere ground and polished using diamond polishing media. Then,the polished parts were etched using a mixed etching solution ofHCl (10ml) and H2O2 (3ml). The Quanta FEG 250 field emissionscanning electronmicroscope (FE-SEM)was utilized to characterize

M. Xia et al. / Journal of Alloys and Compounds 747 (2018) 684e695686

the microstructure morphologies of etched samples. Simulta-neously, the chemical composition of generated dendrite and car-bides were identified using the Energy dispersive X-rayspectroscope (EDX; Oxford, UK).

3. Results and discussions

3.1. Phase constitution

Typically, the X-ray spectrum is employed to identify the con-sisting phases of WC reinforced Inconel 718 composite using SLMtechnique with a constant laser scanning speed of 400mm/s, asdepicted in Fig. 1. It was apparent that the as-built WC/Inconel 718composite was mainly composed of g-Ni, Ni2W4C, NbC and theresidual WC particles. Despite of employing a high-energy laserbeam, the incorporatedWC particles were insufficient to be meltedcompletely, due to its high melting temperature exceeding 3000 K.Owing to the limited diffraction intensity of Ni2W4C and NbCphases, the diffraction peaks plotted at a range of 34�e41� wereperformed (Fig. 1b). It was worth noting that the diffraction peaks,which were identities as the generating NbC phases, were slightlyshifted with a small diffraction angle in comparison to those of thestandard PDF card (marked by red dashed lines), indicating a visiblelattice deformation of the generating NbC. According to the prin-ciple of Bragg's law, the slightly shifting to a low angle of diffractionpeaks (namely, decreasing the 2q) primarily attributed to the solidsolution of Ni matrix with large-scale W atoms, which directlyderived from the incorporating WC particle.

3.2. Growth behavior of microstructure

The traditional FE-SEM images display the characteristic mor-phologies of WC reinforcing Inconel 718 composite using SLM withvariations of scanning speed, as shown in Fig. 2. It was visiblyapparent that themicrostructure of SLM-processedWC/Inconel 718composite exhibited with variable characteristics as varying thelaser scanning speed. The EDS analysis apparently indicated thatthe primary dendrite was mainly composed of W, Ni and C with anapproximate atomic ratio of 4:2:1. With the combing analysis ofEDS and XRD spectrum, it was revealed that reactions of W, C andNi atoms deriving from WC particles and Inconel 718 matrixoccurred with a reasonable operating temperature during SLM,leading to the in-situ formation of Ni2W4C primary dendrites. Thelength of primary Ni2W4C dendrite arms of SLM-processed WC/Inconel 718 compositewas evidently reduced ranging from 20.8 mmto 12.2 mm as successively increasing the laser scanning speed from

Fig. 1. The X-ray spectrum characterizing the generating phases of WC reinforced Inconel 7range of 30�e100� (a); the characterized diffraction peaks of formed Ni2W4C and NbC pha

400mm/s to 700mm/s. When a low laser scanning speed of400mm/s was employed, a large-scale dendrite arms were ob-tained with an approximate length of 20.8 mm (Fig. 4a) and theprimary dendrites grew with stagger features. On the contrary, theprimary dendrite arms were formed with distinctly reduced lengthand distributed dispersedly in the composite as further increasingthe laser scanning speed (Fig. 4b, c, d). Normally, the operatingtemperature gradually decreased and the resultant abilities atomsdiffusion reduced as elevating the laser scanning speed. It is,therefore, reasonable to deduce that the primary dendrite armsinsufficiently grewwith a relatively small length, due to the limitedpenetration of laser energy.

The high-magnification morphologies of microstructure revealits specific length and spacing of primary dendrite arms preparedusing various laser scanning speed, as illustrated in Fig. 3. It wasapparently observed that the primary dendrites appeared withvariable morphologies under various processing conditions. For theapplication of a relatively low laser scanning speed of 400mm/s,the dominating primary dendrite arms presented a large lengthand spacing of 5.52 mm and 0.63 mm, respectively (Fig. 3a). Asincreasing the laser scanning speed to 500mm/s, the primarydendrite arms were evidently observed with regular morphologiesand its length was significantly reduced to a certain dimension of4.51 mm with an average arm spacing of 0.54 mm (Fig. 3b). As evenelevating the laser scanning speed to 600mm/s, the reduced pri-mary dendrite arms were evidently dominated with an approxi-mate length and spacing of 3.32 mm and 0.47 mm, respectively(Fig. 3c), revealing a distinct refinement of primary dendrite arms.On further increasing the laser scanning speed to 700mm/s, theprimary dendrite arms with a small-size length of 2.28 mm and alow arm spacing of 0.31 mm were obviously obtained (Fig. 3d). Asthe laser beam scanned over the powder-bed under a giving laserpower, the laser energy penetrated into powder-bed was notablydecreased as increasing the scanning speed, which significantlyreduced the resultant operating temperature within pool. In thissituation, the limited energy was unfavorable for the coarsening ofdendrite arms and thus the resultant dendrite were obviouslyrefined with a small-size dimension. Hence, it was believable toconclude that the Ni2W4C primary dendrites of SLM-processedWC/Inconel 718 composite parts were gradually refined as successivelyelevating the laser scanning speed ranging from 400mm/s to700mm/s.

Fig. 4 elucidates the high-magnification features and its chem-ical concentrations of Nb-rich carbides generating in SLM-processed WC/Inconel 718 composite with various laser scanningspeed. The carbides particles prominently dispersed in all the

18 composites using SLM technique with a laser scanning speed of 500mm/s in a wideses with a narrow range of 34�e41�(b).

Fig. 2. FE-SEM images showing the characteristic morphologies of SLM-ed WC/Inconel 718 composites with variation of scanning speed: (a) 400mm/s; (b) 500mm/s; (c) 600mm/s; (d) 700mm/s; (e) displaying the chemical composite and its concentration of the primary dendrite; (f) presenting the length of primary dendrite trunk under various processingconditions.

M. Xia et al. / Journal of Alloys and Compounds 747 (2018) 684e695 687

composites with consistently granular morphologies. The particle,which was marked with red point, were mainly consisted of aconsiderably high concentration of Nb elements and with less traceof Ni, Cr, W, Fe, Ti elements, according to the analysis of EDSspectrum. Hence, it was inferred that the granular-shaped particlesdispersing in the composite were identified as NbC carbides withless solid solution of Ni, Cr, W, Fe, Ti elements, namely, (Nb, M)Ccarbides (where M¼Ni, Cr, W, Fe, Ti), according to the combininganalysis of aforementioned XRD spectrum and EDS. Xiao et al. [15]experimentally revealed that the Nb diffusion was strongly reliedon the processing conditions during SLM Inconel 718 alloy. Spe-cifically, a relatively high operating temperature and a lowercooling rate also provided the sufficient time for Nb atoms todiffuse from the solid phase to the liquid phase. Selective lasermelting of WC/Inconel 718 composite at a relatively low scanningspeed of 400mm/s, the sufficient penetration of laser energy wasbeneficial for the adequate growth of (Nb, M)C carbides, indicatinga remarkably coarsening morphology and a correspondinglyaverage dimension of 1.11 mm (Fig. 4a). As an increasing laserscanning speed of 500mm/s was employed, the generated (Nb,M)C

carbides presented with a reduced dimension of 0.89 mm (Fig. 4b),ascribing to the limited energy penetration and resultant insuffi-cient growth. As elevating the laser scanning speed to 600mm/s, acertain number of (Nb, M)C carbides produced in the compositewith a continuously reduced average dimension of 0.57 mm(Fig. 4c). As further increasing the laser scanning speed to 700mm/s, a considerable number of (Nb,M)C carbides distributed discretelyin matrix with a significantly refined dimension of 0.28 mm(Fig. 4d). Hence, it can infer that the granular-shape (Nb, M)C car-bides are prone to be refined as increasing the laser scanning speed,attributing to the limiting diffusion of atoms and the resultantlyinsufficient growth during SLM.

3.3. Mesoscopic simulation and thermodynamic behavior withinpools

To investigate the thermodynamic behavior within molten poolduring SLM under various laser scanning speed, the newly meso-scopic powder-bed was proposed based on an improved volume offluid (VOF) methodology, as shown in Fig. 5a. The WC

Fig. 3. FE-SEM images presenting the high-magnification morphologies of primary dendrite prepared using various laser scanning speed: (a) 400mm/s; (b) 500mm/s; (c) 600mm/s; (d) 700mm/s; and (e) exhibiting the specific length and spacing of primary dendrite arm under various processing conditions.

M. Xia et al. / Journal of Alloys and Compounds 747 (2018) 684e695688

reinforcements of WC/Inconel 718 composite model were assumedto be sphere-like and were initialized randomly with a constantdiameter of 10 mm and a weight ratio of 20wt% within matrix tosimplify the calculation (Fig. 5b). The generating WC particles wererandomly distributed into the physical powder-bed, consideringthe essential physical aspects, including phase transitions, melting/solidification, interfacial interactions of liquid/gas and WC/Inconel718, thermal conductivity, etc. Considering the requirements ofnumerical simulation and the calculated ability of central pro-cessing unit (CPU) equipped in the computer workstation, a novelmesoscopic powder-scale FVMmodel was establishedwith a three-dimensional dimension of 400� 300� 60 mm3. According to pre-vious literature, the molten liquid motioned within pool primarilyfollowed the three basic physical conservation laws, i.e., the con-servation of mass, momentum and energy [16,17]. A commercialcomputational fluid dynamics (CFD) calculation has been employedto give a quantitative analysis concerning the thermodynamicswithin molten pool of SLM-processed WC/Inconel 718 compositeunder variable laser scanning speed, which was in a full accordanceto experimental processing conditions.

The specific temperature counters of longitudinal section alongscanning direction of SLM-processed WC/Inconel 718 composite

fabricated with various scanning speed are depicted in Fig. 6. It wasapparently observed that the operating temperature and profiles ofmolten pool were significantly reduced as increasing the laserscanning speed ranging from 400mm/s to 700mm/s. This phe-nomenon primarily attributed to that the increasing laser scanningspeed also had a obvious tendency to reduce the laser interactiontime of powder-bed and laser beam, and the less energy was thuspenetrated into the pool. Normally, the regions, where the oper-ating temperature exceeded the melting line, were defined as themolten pool. It was worth noting that the profiles leftward thecenters of pools were visibly larger in comparison to thosedistributed along the right centers of pools, as well as the similarlyasymmetric feature in SLM-processed Cu-based matrix alloys andInconel 718 alloy [18,19]. It was mainly ascribed to the difference ofthermo-physical properties of the latter raw powder and the pre-vious solidified regions of the pool. Simultaneously, the thermalaccumulations of the solidification rearwards the laser beam werealso contributed to the asymmetric feature of pool.

Fig. 7 illustrates the calculated temperature profiles and corre-sponding temperature gradient distributing along the molten poolduring SLM-processed WC/Inconel 718 composite with variousscanning speed. According to the variable temperature distribution

Fig. 4. FE-SEM photos showing the high-magnitude morphologies of Nb-rich carbide with various laser scanning speed: (a) 400mm/s; (b) 500mm/s; (c) 600mm/s; (d) 700mm/s;(e) the chemical elements and its concentration of the Nb-rich carbide; and (f) the average size of Nb-rich carbide under various processing conditions.

Fig. 5. The mesoscopic SLM powder-bed model with random distribution of WC reinforcements established for the quantitative analysis of microstructure evolution during SLM-processed WC/Inconel 718 composite (a); and (b) representing the cross-sectional view of the established physical model.

M. Xia et al. / Journal of Alloys and Compounds 747 (2018) 684e695 689

profiles, it was distinctly apparent that the operating temperatureapproached to it maximum at the center of pool, due to theapplication of laser beam with Gaussian energy density

distribution. Similarly, the corresponding temperature gradientreduced from 4.36� 107 K/m to 3.55� 107 K/m as successivelyincreasing the laser scanning speed from 400mm/s to 700mm/s.

Fig. 6. The temperature counters of longitudinal section distributed along the scanning direction of SLM-processed WC/Inconel 718 composite with a constant laser power andvaluable scanning speed: (a) 400mm/s; (b) 500mm/s; (c) 600mm/s; (d) 700mm/s.

Fig. 7. The calculated temperature profiles and corresponding temperature gradientdistributed along the molten pool during SLM-processed WC/Inconel 718 compositewith a constant laser power and various laser scanning speed of (a) 400mm/s; (b)500mm/s; (c) 600mm/s; (d) 700mm/s.

M. Xia et al. / Journal of Alloys and Compounds 747 (2018) 684e695690

The temperature gradients were in the same order of magnitude incomparison to those of SLM-processed Inconel 718 alloy [20].Generally, acquiring a relatively high temperature and temperaturegradient were prone to result in a sufficient melting of the laser-irradiated powder particles and the resultant thermo-capillaryforce within pool was significantly enhanced, favoring theadequate migration of particles within pool [21]. As a result, an

intense thermodynamics was obtained by mean of using a rela-tively high laser energy density.

3.4. Evolution mechanism of microstructure

Generally, when the high-energy laser beam continuouslystroke over the powder-bed, the powders absorbed the laser en-ergy and thenweremeltedwith a liquid temperature exceeding themelting point through powder coupling mechanism [22]. Duringthe laser beam moved over the powder-bed, the energy wasinitially absorbed by the surface of individual particle and themelting depth was primarily determined by the thermo-physicalproperties of the materials, leading to a high temperaturegradient on the surfaces of powder. As the thermalization of theenergy processed, the powders surface melted due to the accu-mulation of energy and the heat migrated mainly towards thecenter of particles until a local steady state of the temperaturewithin the powder was obtained. As shown in Figs. 6 and 7, all thecalculated temperature profiles within molten pool was lower than2800 K and hence the Inconel 718 powders were completely mel-ted with a relatively low melting point of 1633 K, while the WCparticles cannot melt directly into the melt pool with a highmelting point of 3143 K. Consequently, the Inconel 718 powderswere preferentially melted to produce a molten pool and then theWC powders dissolved in pool with a certain thickness inwards,which provided a beneficial thermodynamic condition to synthe-size various phases between the dissolved elements.

As shown in Fig. 8a, under the activities of high-energy laserbeam, the dissolution of incorporating WC particles released free

Fig. 8. Schematics illustrating the evolution mechanism of microstructure of WC/Inconel 718 composite with variable scanning speed during SLM: (a) the diffusion behavior ofcarbon and tungsten atoms surrounding the incorporating WC particles during the slight surface melting; (b) large magnification of diffusion regions of atoms; (c) the developmentof primary dendrite and (Nb, M)C carbides within molten pool as increasing the laser scanning speed during solidification.

M. Xia et al. / Journal of Alloys and Compounds 747 (2018) 684e695 691

carbon and tungsten atoms into themolten Inconel 718 alloy liquid.The elements chemical potential gradient was primarily consideredas the driving force of atomdiffusion, whichwas strongly dependedon the operating temperature under variable processing condi-tions. The chemical potential mi and the carbon activity in sub-stoichiometric carbide as ai can be described as:

Fi ¼ �vmivx

(1)

where Fi denotes the driving force of atom diffusion, and thesubscript i stands for either one of W, Ni, and C atoms, ui is thechemical potential of elements. Simultaneously, the elementschemical potential can be expressed as:

vmi ¼ kTvðln aiÞ (2)

where k is a constant, T represents the operating temperature, ai isthe chemical activity of elements.

According to Kikuchi et al.’s results [23], the carbon activity aC ina ternary W-Ni-C systemwas generally controlled by the operatingtemperature T and was followed by the equation:

ln aC ¼ ln�

VC

1� VC

�þ��1:07þ 6806

T

�þ�13:51þ 9554

T

�VC

þ�1:21þ 9010

TC

�VW

(3)

where Vi¼ Xi/(1-XC), Xi represents the atom fraction and subscript idenotes as either one of W, Ni and C atoms. According to the Eqs.

(1)e(3), the driving force of diffusion with a constant compositionwas apparently proportional to the thermodynamic temperature Twithin pool. The working temperature during SLM was associatedto the penetrating energy of the powder-bed and was mainlyevaluated by the scanning speed in this study.

On the other hand, according to the “Arrhenius equation”, thekinetic description of diffusion ability at constant composition wasgiven as:

D ¼ D0exp��QkT

�(4)

where D denotes the diffusion rate of element, D0 is the diffusioncoefficient at standard temperature of 293 K and Q is the so-calledactivation energy of diffusion. It was evidently revealed that thediffusion rate can be significantly enhanced as increasing theoperating temperature. In general, the lower energy penetrationwas obtained as elevating the applied laser scan speed v, whichaccordingly reduced the operating temperature T [24,25]. In thiscase, the driving force of diffusion and diffusion rate of carbonwithin the molten liquid was thus increased, as revealed from Eq.(4). Simultaneously, the diffusion of carbon was considerablyenhanced at a constant temperature in companion to other ele-ments (e.g., Ni, W, Nb, etc), due to its variations of physical prop-erties. Therefore, the atom diffusion zones surrounding theincorporating WC particles can be divided into two sequential re-gions (Fig. 8b): (i) the regions closely neighboring WC particleswere abundant with W, C and Ni atoms, providing the thermody-namic formation condition of Ni2W4C phases; (ii) the diffusionregions outwards the above-motioned W-rich regions were pri-marily dominated with C, Nb and with less amount of Ni atoms,

M. Xia et al. / Journal of Alloys and Compounds 747 (2018) 684e695692

favoring the formation of NbC carbides with solid solution ofWandNi elements. As revealed from Figs. 6 and 7, the operating tem-perature within pool was obviously reduced as increasing the laserscanning speed, which significantly decreased the solubility of el-ements and the dimensions of fore-mentioned diffusion regionswith various atoms.

During the subsequent solidification of SLM-processed WC/Inconel 718 composite, the diffusion regions surrounding theincorporating WC particles presented with variable concentrationsand dimensions under various processing conditions. Fig. 8c sche-matically depicted the growth morphologies of Ni2W4C primarydendrite and granular (Nb, Ni)C carbides with various laser scan-ning speed. Selective laser melting of WC/Inconel 718 compositewith various laser scanning speed ranging from 400mm/s to700mm/s, the calculated operating temperature varied with areduction from 2800 K to 2200 K, whichwas favorable to generate alarge dimension of diffusion regions and tended to acquire aconsiderable amount of elements. As expressed from Eq. (4), thediffusion ability of atoms was extremely sensitive to the variationsof operating temperature and the diffusion rate distinctly reducedas decreasing the operating temperature. Meanwhile, the corre-sponding temperature gradient was reduced from 4.36� 107 K/mto 3.55� 107 K/m as elevating the applied scanning speed. Thereduction of temperature gradient was normally related to thevariations of chemical potential gradient and the resultantly ther-modynamic driving force was visibly decreased, as revealed fromEqs. (1)-3). With the combing reductions of diffusion rate andthermodynamic driving force, the length of Ni2W4C primarydendrite trunks was accordingly decreased as elevating the laserscanning speed during solidification (Fig. 2). Moreover, based onthe Bouchard et al.’s results, the spacing l of dendrite arm wasrelated to the tip growth rate VL and was determined byRefs. [26,27]:

l ¼ 2pa

4G

C0ð1� k0Þ2TF

�DL

VL

�2!1=3

(5)

where a denotes the dendrite arm-calibrating factor, which de-pends on the alloy composition and TF is the fusion temperature ofthe solvent, G stands for the Gibbs-Thomson coefficient, DL is thesolute chemical diffusivity in liquid, k0 is the partition coefficient, C0is the alloy composition. The tip growth rate VL was controlled bylaser beam scan speed v and can be defined by Liu et al. [28]:

VL ¼ v cos q (6)

where q is the angle between the vectors VL and v. A decrease in theapplied laser scanning speed also resulted in a higher operatingtemperature at the tips of dendrite arms and thus a resultantlylarger amount of heat was accordingly accumulated at the tips,where were generally regarded as the most instable areas, therebyproviding significant internal energy and thermodynamic poten-tials for the growth of tips. As a result, the tip growth rates VL andthe resultant length of dendrite arms appeared with an obviouslyreduced length as increasing the laser scanning speed. In addition,combining the analysis of Eqs. (5) and (6), it was apparentlyrevealed that a successively increasing of the applied scanningspeed was prone to decrease the dendrite armpacing l, as depictedin Fig. 3.

Generally, the granular (Nb, M)C were formed through a disso-lution precipitation mechanism by means of the heterogeneousnucleationrate and subsequent nucleus growth. During SLM, thenucleation rate was primarily referred to the thermodynamicsnucleation rate and kinetics nucleation rate. The former was mainly

influenced by the variation of free energy, while the latter wasprimarily depended on the transfer capacity of solute atoms. As aresult, the nucleation rate N can be determined by Ref. [29]:

N ¼ Kexp�� DG*

kAT

�� exp

�� QkBT

�(7)

where K is a constant, DG* represents the critical nucleation Gibbsfree energy, kA and kB are the Boltzmann constant, Q denotes theactivation energy for transferring through the solid/liquid interface,and T is the operating temperature. It was apparent that thenucleation rate N was greatly enhanced as increasing the scanningspeed.

Meanwhile, the nucleus growth rate of carbides precipitated inSLM-processedW-Ni-C ternary systemwas related to the operatingtemperature and was evaluated by Ref. [30]:

Tt ¼ TM þmC*Li �

RT2MDHf

,Vt

V0(8)

where TM represents the melting point of the pure component, Tt isthe operating temperature of tips, m is the liquids slope, C*

Li is theliquid solute concentration at the solid-liquid interface, Hf is thelatent heat of the material, V0 is the kinetic constant, Vt expressesthe growth rate of tips and is tailored by the applied laser scanningspeed. Combined with the analysis of Eqs. (7) and (8), the nucle-ation rate of (Nb, M)C carbides was significantly enhanced and thenucleus growth rate was inversely reduced as elevating the laserscanning speed, thereby generating a granular carbides withrefined features, as shown in Fig. 4.

3.5. Mechanical properties of multi-phase reinforced composite

Fig. 9 illustrates the microhardness and its characteristic mor-phologies of indentations of SLM-processed WC/Inconel 718 com-positewith variable laser scanning speed. It was obviously apparentthat the microhardness was increasingly reduced strengthenedfrom 385.6 HV0.2 to 475.2 HV0.2 with gradually decreased size ofindentions as successively increasing the laser scanning speed. Acomparison of characteristic morphologies of indentationsrevealed that the SLM-processed WC/Inconel 718 composite partsexperienced a larger deformation as increasing the laser scanningspeed. Since the gradually refined Ni2W4C primary dendrite withsmall-size trunks and granular (Nb, M)C homogeneously dispersedwithin the composite with an increasing laser scanning speed, theresistance capacity to deformation was considerably strengthenedduring indentations, which was primarily responsible for theenhancement of measured microhardness. It was, therefore,reasonable to conclude that the combining strengthen of refinedNi2W4C primary dendrite and granular (Nb, M)C homogeneouslydispersed in the composite was mainly attributed to the signifi-cantly enhanced microhardness of SLM-processed WC/Inconel 718composite using a comparatively high laser scanning speed.

Fig. 10 depicts the specific relations between strain and stressduring tensile process of SLM-ed WC/Inconel 718 composite usingvarious scanning speed. It was obviously revealed that the ultimatetensile strength of SLM-processed WC/Inconel 718 composite partswas highly enhanced ranging from 1299.6MPa to 1464.6MPa assuccessively increasing the laser scanning speed from 400mm/s to700mm/s, indicating a remarkable enhancement of the ultimatetensile strength. The as-built WC/Inconel 718 composite partsperformed a visibly high ultimate tensile strength in comparison tothe experimental ultimate tensile strength of SLM-processed 718alloy in Refs. [8,31], distinctly showing a significant strength of

Fig. 9. FE-SEM images displaying the characteristic morphologies of microhardness indentations of SLM-processed WC/Inconel 718 composite using various scanning speed of (a)400mm/s; (b) 500mm/s; (c) 600mm/s; (d) 700mm/s; and (e) presenting the specific hardness of composite.

Fig. 10. The tensile strain-stress curves of SLM-processed WC/Inconel 718 composite using various scanning speed (a); and (b) displaying the experimentally ultimate tensilestrength and elongation of the composite fabricated with variable processing conditions.

M. Xia et al. / Journal of Alloys and Compounds 747 (2018) 684e695 693

composite. It was worth noting that the resultantly measuredelongation was inversely decreased from 22.12% to 19.74%, leadingto a slight reduction of the plasticity of composite part. According to

the literature, the ultimate tensile strength of SLM-processed 718alloy part was normally enhanced with a considerable reduction ofelongation [8,31]. However, the as-built WC/Inconel 718 composite

M. Xia et al. / Journal of Alloys and Compounds 747 (2018) 684e695694

performed an apparently increasing of ultimate tensile strengthwith a slight reduction of elongation as elevating the scanningspeed, attributing to the incorporation of WC reinforcements. Sincethe reinforcing multi-phase of Ni2W4C primary dendrite andgranular (Nb, M)C generated during SLM acted as barriers to themovement of dislocations, the resultant strength was enhancedunder the combined activities of reinforcements according to theprinciple of Orowan strengthening [32]. Moreover, the multi-phaseof Ni2W4C and (Nb,M)C were significantly refined as increasing thescanning speed, which dramatically strengthened the hinderingeffects on the slipping of dislocation during deformation and theresultant ultimate tensile strength [33]. In addition, when itreferred to the slight reduction of elongation, the compositedeformed uniformly and the stress was weakened during tensileprocessing under the synergistic effect of the relatively large-sizeNi2W4C primary dendrite with several micrometers and small-size (Nb, M)C carbides with submicron characters. It was, there-fore, believable that the ultimate tensile strength of SLM-processedWC/Inconel 718 composite components was enhanced with anincreasing enhancement as elevating the laser scanning speed, dueto the prominent effects of combined refining Ni2W4C primarydendrite and (Nb, M)C carbides.

4. Conclusions

In present study, theWC reinforced Inconel 718 composite partswere achievable to be fabricated using SLM with variable laserscanning speed. Combing with the experimental characterizationsof microstructure and the mesoscopic numerical simulations, themicrostructure growth behavior, mechanical properties and itsevolution mechanism were discussed and the main conclusionswere drawn as followings:

(1) Combined the analysis of X-ray diffraction spectrum andEDS, the Ni2W4C primary dendrites and (Nb, M)C carbideswere formed with apparent variations of morphologies asincreasing the applied laser scanning speed.

(2) Due to the combing enhancements of diffusion rate andthermodynamic driving force with a high operating tem-perature and large temperature gradient using a relativelylow scanning speed, the primary Ni2W4C dendrite exhibitedwith an average trunk length of 20.8 mm, and appeared witha mean length and spacing of dendrite arms of 5.52 mm and0.63 mm, respectively. On the contrary, when an elevatedscanning speed of 700mm/s was applied, the primaryNi2W4C dendrite presented an average trunk length of 12.2mm and observed with a reduced length and spacing ofdendrite arms of 2.28 mm and 0.31 mm, respectively.

(3) The granular (Nb, M)C carbides were evidently refinedranging from 1.11 mm to 0.28 mm as elevating the laserscanning speed from 400mm/s to 700mm/s, due toconsiderably increasing nucleation rate and reduced growthrate as decreasing the operating temperature from 2800 K to2200 K.

(4) The measured microhardness and ultimate tensile strengthof SLM-processed WC/Inconel 718 composite were appar-ently enhanced with a slight reduction of elongation asincreasing laser scanning speed, primarily attributing to thecombined strengthening of refined multi-phase of primaryNi2W4C dendrite and granular (Nb, M)C carbides.

Acknowledgments

We are grateful for the financial support from the NationalNatural Science Foundation of China (No. 51575267); the National

Key Research and Development Program “Additive Manufacturingand Laser Manufacturing” (No. 2016YFB1100101); the Key Researchand Development Program of Jiangsu Provincial Department ofScience and Technology of China (No. BE2016181); and the PriorityAcademic Program Development of Jiangsu Higher Education In-stitutions, and greatly thanks financial support from Funding ofJiangsu Innovation Program for Graduate Education(No.KYLX16_0345).

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