International Journal of Refractory Metals and Hard Materials 96 (2021) 105490

Available online 23 January 20210263-4368/© 2021 Elsevier Ltd. All rights reserved.

Effects of processing parameters on the densification, microstructure and mechanical properties of pure tungsten fabricated by optimized selective laser melting: From single and multiple scan tracks to bulk parts

Xin Ren , Hailin Liu , Fayun Lu , Liming Huang , Xin Yi *

Department of Mechanics and Engineering Science, Beijing Innovation Center for Engineering Science and Advanced Technology, College of Engineering, Peking University, Beijing 100871, China

A R T I C L E I N F O

Keywords: Tungsten Selective laser melting (SLM) Single- and multi-track scan Bulk properties Microstructure characterization Processing parameter optimization

A B S T R A C T

In this study we focus on the manufacturing of pure tungsten materials by selective laser melting (SLM), and systematically investigate how processing parameters affect densification, microstructure, and mechanical properties of fabricated pure tungsten samples. Three key processing parameters are identified, the laser power, scan velocity, and overlap rate. An optimized power-velocity window for single-track scan is determined by characterizing the surface morphologies of tracks on a multi-layer powder bed. Based on that window, varying the hatch distance the overlap rate between adjacent scan tracks for multi-track scan is then optimized by assessing the optical metallographic microstructure. Moreover, bulk pure tungsten samples with size of 10 × 10 × 10 mm3 are fabricated with SLM, and their densification and mechanical properties are investigated. Our experimental results show that optimized processing parameters enable manufacturing of bulk pure tungsten samples with a high relative density up to 98.51%. After annealing, an excellent ultimate compressive strength of 1.007 GPa is obtained but with a volumetric energy density significantly lower than reported values in the literature.

1. Introduction

Safe use of nuclear fusion reaction is believed to be one of the most efficient ways to solve the world’s energy crisis. A key step is the se-lection of an appropriate armor material which could withstand neutron irradiation and high temperature as well as high heat-load owing to plasma, and serve as plasma-facing components (PFCs) in nuclear re-actors [1]. Tungsten (W) has been regarded as a priority candidate material for PFCs owing to its high melting point, high energy threshold for sputtering, high thermal stress resistance, high thermal conductivity, low swelling, and low tritium retention [2]. In comparison with stainless steels and high temperature alloys, the production of tungsten is significantly difficult using a conventional process such as casting and machining owing to the high melting point and brittle nature of tung-sten. So far the most popular ways of the production of tungsten and tungsten alloys include powder metallurgical techniques, physical vapor deposition, chemical vapor deposition for bulk materials, and plasma spraying for thin layer coatings [2]. However, these methods cannot be used to produce tungsten materials with complex net shapes such as

curved surface, channels, and grooves, which limits the flexibility in the development and design of PFCs to a certain extent.

Recently, the rapid development of metal additive manufacturing technology makes it possible to form near-net shaped parts of refractory alloy components like tungsten [3]. As one of these promising additive manufacturing approaches, selective laser melting (SLM) technology uses high intensity lasers as an energy source to melt and fuse selective regions layer by layer in a metal powder bed, which facilitates manufacturing bulk structures with novel and complex design [4,5].

Generally speaking, metals that can be welded could be fabricated by SLM. However, in practice only a limited number of commercial alloys such as steels, titanium alloys, nickel-base alloys, and a few types of casting aluminum alloys have been fabricated by SLM successfully with relatively good mechanical properties. A common feature of these ma-terials is that during the laser melting process they have a good balance in terms of suitable laser absorption, melting point, thermal conduc-tivity, surface tension and viscosity [6,7]. In contrast, tungsten as a re-fractory metal has an extremely high melting point (3695 K), and melted tungsten is of a high surface tension (2.361 N ⋅ m− 1) and high viscosity

* Corresponding author. E-mail address: xyi@pku.edu.cn (X. Yi).

Contents lists available at ScienceDirect

International Journal of Refractory Metals and Hard Materials

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https://doi.org/10.1016/j.ijrmhm.2021.105490 Received 3 December 2020; Received in revised form 14 January 2021; Accepted 17 January 2021

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(about 8 × 10− 3 Pa ⋅ s), and is sensitive to oxidation. These aspects make the production of pure tungsten by SLM very challenging, with unde-sired features of balling phenomena, low flow ability and wettability of the molten pool, easy crack formation due to grain boundary oxidation [6,8].

So far only few studies have focused on the production and analysis of pure tungsten by SLM, and among these processing parameters including the laser power, scan velocity, and hatch distance, only one or two parameters have been considered just for either single-track scan or fabrication of bulk tungsten samples. For example, it is reported that a high laser power can cause crack formation on the surface of the SLM tungsten parts, and a high scan velocity leads to shrinkage and irregular flow front of scan tracks [9]. Enneti et al. studied the effects of hatch distance and scan velocity on the densification of pure tungsten, and only a relatively low density around 70% was achieved [10]. Ivekovic et al. focused on melting and solidification behaviors of SLM pure tungsten and tungsten‑tantalum alloys, and a density of 97.1% was obtained for the pure tungsten [11]. Tan et al. [8] investigated the effect of linear energy, a ratio between the laser power and scan velocity, on the formation of pure tungsten by theoretical analysis and experimental studies, and a relatively high density of 19.01 g/cm3 was obtained. Guo et al. [12] systematically studied the role of volumetric energy density on densification, microstructure and mechanical properties of pure tungsten fabricated by SLM, and a maximum relative density of 98.4% with a ultimate compressive strength up to 902 MPa was obtained at a volumetric energy density of 1000 J/mm3.

It is known that in SLM the quality of fabricated parts strongly de-pends on the qualities of scan tracks [13]. Moreover, the processing parameter optimization based on the characterization of single and multiple scan tracks can significantly narrow the processing window for the fabrication of bulk samples and save the cost and energy. Therefore, to gain a better and more comprehensive understanding of the fabrica-tion of pure tungsten samples with SLM, an elucidation of the collective roles of the laser power, scan velocity, and hatch distance from single and multiple scan tracks to bulk samples is called for. Following the general SLM scheme in Fig. 1, here we optimize the processing param-eters from scan track evaluation, and then fabricate pure tungsten samples of high relative density and excellent ultimate compressive strength without balling phenomena and macrocrack formation. Our studies lead to a deep and systematic understanding of the full process of manufacturing pure tungsten samples by SLM, and provide rational guidance for the fabrication of refractory metal materials by additive manufacturing.

2. Materials and methods

2.1. Tungsten powder

The raw powder particles of pure tungsten used in this work are prepared using the radio frequency plasma spheroidization system, with particle diameter ranging from 5 μm to 25 μm (see Fig. 2). The spherical shapes of the tungsten microparticles are beneficial for the SLM process, in particular for our case with the usage of a powder bed equipment fed by a hopper [9]. To reduce the humidity and residual oxygen content, the powder particles are dried in a vacuum drying oven at 80 ◦C prior to the SLM process.

2.2. SLM process

The SLM® Solution 125HL system equipped with a gravity feed hopper is used with a fiber laser yielding a power of 400 W and a spot size of 70 μm, and a 316 stainless steel is selected as the substrate and fixed onto the system platform. The substrate is preheated to 200 ∘C, far below the melting point of pure tungsten and has little effect on reducing the residual thermal stress [11,14]. The SLM chamber is filled with argon gas to ensure a low-oxygen environment. Since an excessively thin powder layer could lead to serious damage to the fabricated parts during scraping [15], a powder layer thickness of 30 μm is selected here. A stripe scan pattern with an intra-layer misorientation of around 67◦ is

Fig. 1. A general SLM scheme linking processing parameters, evaluated products, and structural and morphological evolutions.

Fig. 2. SEM image of tungsten powder morphology.

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adopted to minimize residual stress. Fig. 3 shows the single-track scan in SLM on a multi-layer tungsten

powder bed with the laser power P varying between 250 W and 380 W and the scan velocity v in a range from 200 mm/s to 600 mm/s. Analyzing surface morphologies of the resulting single tracks and uni-formity of their widths can lead to a further narrowed viable ranges of P and v, which is discussed in the next section. At given P and v, the width w of a single scan track is determined. For multiple-track scan, the overlap rate φ is introduced and is a function of the width w of single scan track and the hatch distance h as

φ =w − h

w× 100%.

For a given width w, the hatch distance h or equivalently the overlap rate φ is optimized by evaluating porosity during a multiple-track multi-layer SLM process. Taking the optimized P and v (or w) from single-track scan and optimized h (or φ) from multiple-track scan, cubic bulk pure tung-sten samples with a size of 10 × 10 × 10 mm3 are fabricated (Fig. 4).

The bulk tungsten samples are fabricated by a typical layer-by-layer manner as follows. First, a thin tungsten powder layer on the substrate plate is laser scanned and melted together, then the platform is lowered a certain layer thickness and a new layer of fresh power is dispersed and scraped. The laser melts and fuses the new powder layer, and eventually, a bulk tungsten is fabricated layer-by-layer.

With knowledge of the thickness t of a single powder layer (t ~ 30 μm here), the volumetric energy density E for single-track scan can be estimated as

E =P

w × v × t,

which has been used in the fields of metal additive manufacturing to

estimate the laser energy input into the powder bed, and in some cases as a combined physical quantity to characterize the appropriate SLM pro-cessing window.

In the case of multiple-track scan with overlap rate φ, E is modified by a factor of w/h as

E =P

w × v × twh=

Ph × v × t

or

E =P

w × v × t1

1 − φ.

2.3. Characterization and tests

For single scan tracks, the surface morphologies as well as track widths are characterized by a TESCAN MAIA3 GMU scanning electron microscope (SEM). For multiple scan tracks, we analyze the sample microstructure using a Leica DM2700M optical microscope. The relative densities of as-fabricated cubic bulk pure tungsten samples are deter-mined using Archimedes principle at room temperature. The samples are then mechanically ground and electro polished using a 2% NaOH solution at voltage 20 V, and their microstructures are examined via JSM-7900F electron backscatter diffraction (EBSD). The densified sam-ples are annealed at 1100 ◦C for two hours in a vacuum environment to release the residual stress. Cylindrical tungsten samples with a length-to- diameter ratio of 1.5 are cut from the fabricated bulk samples, and two end faces are polished to ensure smooth surfaces. Compression tests of these annealed cylindrical tungsten samples are carried out at room temperature using an Instron 5585H universal testing equipment at a strain rate of 1.0 × 10− 3/s.

3. Results and discussion

3.1. Single-track scan

Surface instability of single scan tracks like balling and irregular track width can lead to the formation of pores, and deteriorate the sample quality. In view of this fact, several experimental studies and computational simulations have been performed to investigate the ef-fects of laser power and scan velocity on surface stability of single scan tracks [13,16–20]. However, most of these studies focus on the cases of substrate only or substrate with a single-layer powder bed. Recent computational simulations show that the thickness of the first few printed layers are non-uniform owing to the volume shrinkage of top melt layers and vaporization, and a uniform layer thickness can be achieved after sixth layers and onwards [21]. Therefore, the single scan tracks on substrate only or on substrate with a single-layer powder bed cause layer thickness deviation from the desired value and is far from the reality. A multi-layer powder bed is closer to the real situations. In

Fig. 3. Single-track scan in SLM on a multi-layer tungsten powder bed. (a) Schematic of the scan process, (b) an examined window of laser power P and scan velocity v. BD, building direction.

Fig. 4. Pure tungsten cubes of size 10 × 10 × 10mm3 fabricated with SLM.

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addition, it is still not clear how the presence of a multi-layer powder bed influences the actual manufacturing process, especially for tungsten fabrication with SLM. Therefore, a multi-layer powder bed is considered here.

Surface morphologies and track widths of single melt tracks at different values of the scan velocity v and laser power P are character-ized in Fig. 5a and Fig. 6, respectively. Depending on P and v, three types of scan tracks are observed, smooth and regular tracks with relatively uniform widths, irregular tracks with width values of a wide distribution (blue blocks in Fig. 5a), and tracks with melt droplets or balling (red blocks in Fig. 5a). As P increases, the mean track width increases at a given v; while the mean track width decreases as v increases at a given P

(Fig. 6). At a relatively low laser power (P = 250 W), the width of the melt tracks varies significantly at a scan velocity range from 200 mm/s to 500 mm/s, and irregular tracks are observed (Figs. 5a and 6a). This might be attributed to the incomplete melting of metal powder at low laser power [10]. Independent of laser power used here, balling phe-nomena are observed at relatively high scan velocity of 600 mm/s, since the excessive scan velocity can lead to incomplete wetting and spreading of melt pools or droplets [6,9]. Additionally, instabilities of the melt pool due to excessive laser energy density (E ~ v− 1) at low scan velocity can lead to an irregular track due to Marangoni effect [9] (see two blue blocks in the right column in Fig. 5a). These findings suggest that the laser power P and scan velocity v need to be well balanced to obtain

Fig. 6. Width w of single melt tracks at different v and P. Symbols with different colors refer to width measurements at different positions of scan tracks. Red and blue blocks denote the same cases as these in Fig. 5a.

Fig. 5. Surface morphologies of single melt tracks at different values of scan velocity v and laser power P (a) and corresponding P-v window (b). The dotted block in Fig. 5 is the further narrowed processing window suitable for bulk tungsten sample fabrication with SLM. Nine suitable cases in the proper window are further divided into five groups (G1 to G5) based on the value of mean single-track width (Table 1).

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smooth and regular tracks. Based on Fig. 5a, an appropriate P-v window of SLM processing is presented in Fig. 5b. As P increases, the range of v for smooth and regular tracks decreases. The processing parameters for the selected nine cases of (P,v) for the smooth and regular tracks are summarized in Table 1.

In the subsequent multiple-track scan and bulk sample fabrication, some of these nine suitable cases determined from the evaluation of single scan track are further screened out by the overlap rate (multiple- track scan) and densification rate (bulk sample evaluation). Eventually, a more narrowed suitable P-v window is determined for bulk tungsten sample fabrication (marked by the dotted region in Fig. 5b).

3.2. Multiple-track scan

Based on the values of mean single track widths (from around 120 μm to 170 μm, these nine suitable cases in Fig. 5 are divided into five groups with an interval of 10 μm. In each group only one case (case a if there are two or more cases in the same group) is considered for multiple-track scan owing to the similar mean width, which is not ex-pected to have negative influence on the choice of overlap rate φ or hatch distance h.

During multiple-track scan, tungsten in the overlap region undergoes partial re-melting. An appropriate overlap rate is key to the optimal densification of fabricated tungsten samples with SLM, and can be ob-tained by evaluating the porosity of scan tracks. Fig. 7 shows the optical metallographic microstructures of five selected sample cases in plots of the volumetric energy profiles E ~ 1/(1 − φ). The intertrack porosity of samples G-1a, G-2 and G-3a first decreases with increasing overlap rate φ, and then increases as φ increases, and an optimum overlap rate with lowest porosity is around 30% (Fig. 7a and b). At small φ or equivalently a large hatch distance h, there exists a considerable gap between two adjacent tracks which cannot be filled completely by the re-melt metal, leading to considerable intertrack porosity. On the other hand, at large φ or equivalently a small h, the re-melt area is large and causes significant height difference between the overlapped region and rest of the tracks, leading to non-uniform powder distribution and incomplete melting. Consequently, the microstructure is deteriorated with relatively high porosity [22]. Among these five samples, sample G-4a of an overlap rate 40% has the lowest porosity. For the sample G-5, no evident improve-ment in terms of porosity is found as φ varies, and a larger overlap rate φ(>40%) is required, which is limited by the laser beam size [23]. Therefore, the corresponding P-v set in sample G-5 is screened out for multiple-track scan. In a short summary for multiple-track scan, the

Fig. 7. Microstructure and volumetric energy density E ~ 1/(1 − φ) of five selected samples of different mean single-track widths. Solid dots with arrows indicate cases with the lowest porosities in each sample group.

Table 1 Suitable processing parameters for single-track scan of tungsten.

width range (μm)

number laser power P (W)

scan velocity v (mm/s)

mean width (μm)

Group 1 (160–170)

G-1a 350 300 167

G-1b 300 200 171 Group 2

(150–160) G-2 380 400 158

Group 3 (140–150)

G-3a 350 400 149

G-3b 300 300 148 G-3c 380 500 143

Group 4 (130–140)

G-4a 350 500 136

G-4b 300 400 136 Group 5

(120–130) G-5 300 500 126

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optimum overlap rate for tungsten samples G-1a, G-2 and G-3a is φ =30%, and φ = 40% for sample G-4a. We use the same optimized overlap rate φ for samples in the same group.

3.3. Characterization of bulk tungsten samples fabricated with SLM

3.3.1. Relative density Here we characterize the relative density of bulk tungsten samples

fabricated by SLM. The relative densities of the eight pure tungsten bulk samples versus the corresponding single-track width are illustrated in Fig. 8. Relative densities of over 98% are obtained for samples of single- track width 130 μm – 150 μm (all samples G-3 and G-4), with a maximum value of 98.51% for sample G-3b. For samples with a single- track width larger than 150 μm, the relative densities are less than 98%. According to Table 1, a high laser power with a relatively low scan velocity leads to wide single-track width, which is always accompanied by a wide denudation zone with few tungsten particles around the melt track. The available powder particles become fewer for nearby layers, which reduces the relative density of the fabricated samples [13,20]. Therefore, samples in groups 1 and 2 with a wide single-track width display lower densities compared to other samples in groups 3 and 4. The volumetric energy densities for samples of relative density over 98% (all samples G-3 and G-4) are between 253 J/mm3 and 322 J/mm3

(Fig. 9), with a feature that the relative density increases as the

volumetric energy density increases. However, the relative densities of samples G-1 and G-2 with comparable or even higher volumetric energy density are less than 98%. One possible reason might be due to the lower scan velocities for samples G-1 and G-2 in comparison with these for G-3 or G-4 at the same laser power, which causes relatively wider widths w of single scan tracks (see Table 1). Consequently, slightly larger hatch distances h = (1 − φ)w are applied for samples G-1 and G-2, leading to considerable gaps between adjacent tracks. As a result, samples G-1 and G-2 show slightly lower relative densities even they are fabricated at comparable or even higher volumetric energy density. From optical micrographs of polished horizontal cross-sections for the fabricated bulk tungsten samples in Fig. 10, one can see that samples with relative densities over 98% show similar microstructure and residual porosities. Few pores and micro-cracks are observed, which might be inevitable for pure tungsten fabricated by SLM, in agreement with reported work [12,14,24,25]. The pore formation might be associated with the trapped protective argon gas in melt pool during the SLM process.

In SLM, the cooling rate is very high, and could be on the order of thousands of Kelvin per second. As the local temperature becomes lower than the liquid-solid phase transition temperature of tungsten, a local shrinkage might occur which leads to a local tensile stress causing microcrack formation [12]. As the local temperature further decreases across the ductile-brittle transition temperature of tungsten (around 150 ◦C – 400 ◦C) [2], the brittle feature of tungsten might also results in microcrack formation and propagation.

3.3.2. Microstructure and mechanical properties In the SLM process, melting and subsequent rapid-cooling creates

significant thermal gradient which gives rise to residual stress [26]. To relieve the residual stress, we carry out annealing treatment for the SLM- built bulk pure tungsten samples. Fig. 11 illustrates EBSD (electron backscatter diffraction) figure maps of as-fabricated and annealed bulk samples from the top and front views, respectively. From the top views on the as-fabricated and annealed bulk tungsten samples (Fig. 11a, c), a periodicity perpendicular to the laser scan path in the checkerboard-like microstructure is observed, even after annealing treatment, consistent with the scan pattern. Along the building direction, epitaxial growth of columnar grains is observed (Fig. 11b, d). Slender grains and coarse grains coexist in all samples. The annealing treatment has only minor effect on the grain growth, which means the excellent thermal stability of the bulk pure tungsten samples fabricated with SLM.

Fig. 12a shows the compressive stress-strain curves of annealed bulk pure tungsten samples with SLM. Sample G-3b with the highest relative density exhibits the maximum ultimate compressive strength of 1.007 GPa. The maximum ultimate compressive strength of other samples with lower relative densities can achieve about 0.87 GPa-0.925 GPa (sum-marized in Fig. 12b). As the optimized volumetric energy density E in-creases from 253 J/mm3 to 322 J/mm3, the ultimate compressive strength increases. A similar trend has also been reported by Guo et al., but with a lower ultimate compressive strength of 0.92 GPa and a higher E(=1000 J/mm3) [12]. For a clear comparison, in Table 2 we list our results and reported results in literature with SLM and other fabrication approaches such as chemical vapor deposition (CVD), powder metal-lurgy, and spark plasma sintering (SPS). From fracture morphologies of the annealed sample G-3b, a few voids and micro-cracks can be observed (Fig. 13a). Numerous cleavage steps and facets also exist (Fig. 13b), which indicates a strong metallurgical bonding between melt tungsten in the same and adjacent layers.

4. Conclusions

Manufacturing pure tungsten by SLM is challenging owing to the intrinsic properties of tungsten including a high melting point and oxidation sensitivity for solid tungsten, high surface tension and high viscosity for melt tungsten. In this work pure tungsten samples of high relative density and superior ultimate compressive strength are

Fig. 9. Relative densities of fabricated bulk pure tungsten samples versus the volumetric energy density E.

Fig. 8. Relative densities of eight selected bulk pure tungsten samples versus the corresponding single-track width in Table 1. For samples in group 4, the overlap rate φ is 40%, and others groups φ=30%.

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Fig. 10. Optical micrographs of polished horizontal cross-sections (top view) for samples G-3a (a), G-3b (b), G-3c (c), G-4a (d), and G-4b (e). Scale bars, 200 μm.

Fig. 11. EBSD maps of as-fabricated and annealed bulk tungsten samples. Dotted lines indicate laser scan paths. Arrows indicate building directions (BD). Colors are just used to show the grains clearly, and do not distinguish grain orientations. Scale bars, 200 μm.

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successfully fabricated. Main conclusions are summarized as follows.

(1) Based on morphology and width evaluation of single scan tracks on a multi-layer tungsten powder bed, an optimized SLM

processing window for single-track scan is determined in terms of laser power and scan velocity. Within that window, smooth and regular single scan tracks are achieved.

(2) For multiple-track scan on multi-layer tungsten powder, the processing window for single-track scan is further narrowed by evaluating the relationship between the overlap rate and densi-fication. For different sets of laser power and scan velocity, optimal overlap rates are determined.

(3) Within the optimized SLM processing window, bulk pure tung-sten samples with high relative densities are obtained, with a maximum value of 98.51%. At 300 W laser power, 300 mm/s scan velocity and 30% overlap rate, a superior ultimate compressive strength of 1.007 GPa after annealing is achieved with a volumetric energy density of 322 J/mm3, which is significantly lower than the energy density for SLM of tungsten reported in the literature.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 12. (a) Compressive stress-strain curves for annealed pure tungsten samples G-3(a,b,c) and G-4(a,b) in Table 1. (b) Ultimate compressive strengths of annealed pure tungsten bulk samples with SLM at optimized energy densities.

Table 2 Ultimate compressive strength and relative density of pure tungsten bulk sam-ples fabricated with different approaches.

fabrication approach

ultimate compressive strength (GPa)

relative density (%)

volumetric energy density E (J/mm3)

reference

SLM G-3a 0.892 98.24 280 SLM G-3b 1.007 98.51 322 SLM G-3c 0.878 98.11 253 present

work SLM G-4a 0.887 98.20 285 SLM G-4b 0.929 98.37 305

NA 87.80 500 SLM by NA 89.10 667 [12] Guo et al 0.902 98.40 1000

NA 97.50 1167 SLM by Tan

et al 1.015 98.50 [8]

CVD 0.78–1.48 ≤99.79 [27] powder

metallurgy 1.00–1.20 ≤98.20 [28,29]

SPS 0.98 ≤96.30 [30]

Fig. 13. Typical fracture morphologies of the annealed tungsten sample G-3b in Table 1.

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Acknowledgements

We gratefully acknowledge support from the National Natural Sci-ence Foundation of China (Grants U1830121 and 11988102) and Na-tional Science and Technology Major Project (2017-VI-0003-0073).

References

[1] M. Merola, et al., ITER plasma-facing components, Fusion Eng. Des. 85 (2010) 2312–2322.

[2] V. Philipps, Tungsten as material for plasma-facing components in fusion devices, J. Nucl. Mater. 415 (2011) S2–S9.

[3] D. Zhang, Q. Cai, J. Liu, Formation of nanocrystalline tungsten by selective laser melting of tungsten powder, Mater. Manuf. Process. 27 (2012) 1267–1270.

[4] C.Y. Yap, C.K. Chua, Z.L. Dong, Z.H. Liu, D.Q. Zhang, L.E. Loh, S.L. Sing, Review of selective laser melting: Materials and applications, Appl. Phys. Rev. 2 (2015), 041101.

[5] W.E. Frazier, Metal additive manufacturing: A review, J. Mater. Eng. Perform. 23 (2014) 1917–1928.

[6] X. Zhou, X. Liu, D. Zhang, Z. Shen, W. Liu, Balling phenomena in selective laser melted tungsten, J. Mater. Process. Technol. 222 (2015) 33–42.

[7] I. Gibson, D. Rosen, B. Stucker, Additive Manufacturing Technologies: 3D Printing, Rapid Prototyping, and Direct Digital Manufacturing, 2nd edition, Springer, New York, 2015.

[8] C. Tan, K. Zhou, W. Ma, B. Attard, P. Zhang, T. Kuang, Selective laser melting of high-performance pure tungsten: parameter design, densification behavior and mechanical properties, Sci. Technol. Adv. Mater. 19 (2018) 370–380.

[9] M. Guo, D. Gu, L. Xi, L. Du, H. Zhang, J. Zhang, Formation of scanning tracks during Selective Laser Melting (SLM) of pure tungsten powder: Morphology, geometric features and forming mechanisms, Int. J. Refract. Met. H. 79 (2019) 37–46.

[10] R.K. Enneti, R. Morgan, S.V. Atre, Effect of process parameters on the Selective Laser Melting (SLM) of tungsten, Int. J. Refract. Met. H. 71 (2018) 315–319.

[11] A. Ivekovic, N. Omidvari, B. Vrancken, K. Lietaert, L. Thijs, K. Vanmeensel, J. Vleugels, J.-P. Kruth, Selective laser melting of tungsten and tungsten alloys, Int. J. Refract. Met. H. 72 (2018) 27–32.

[12] M. Guo, D. Gu, L. Xi, H. Zhang, J. Zhang, J. Yang, R. Wang, Selective laser melting additive manufacturing of pure tungsten: Role of volumetric energy density on densification, microstructure and mechanical properties, Int. J. Refract. Met. H. 84 (2019) 105025.

[13] A. Aversa, M. Moshiri, E. Librera, M. Hadi, G. Marchese, D. Manfredi, M. Lorusso, F. Calignano, S. Biamino, M. Lombardi, M. Pavese, Single scan track analyses on aluminium based powders, J. Mater. Process. Technol. 255 (2018) 17–25.

[14] A.v. Müller, G. Schlick, R. Neu, C. Anstatt, T. Klimkait, J. Lee, B. Pascher, M. Schmitt, C. Seidel, Additive manufacturing of pure tungsten by means of

selective laser beam melting with substrate preheating temperatures up to 1000 ◦C, Nucl. Mater. Energy 19 (2019) 184–188.

[15] Y.-j. Xie, H.-c. Yang, X.-b. Wang, L. Zhao, C.-j. Kuang, W. Han, Control of wall thickness and surface morphology of tungsten thin wall parts by adjusting selective laser melting parameters, J. Iron Steel Res. Int. 26 (2019) 182–190.

[16] J. Ciurana, L. Hernandez, J. Delgado, Energy density analysis on single tracks formed by selective laser melting with CoCrMo powder material, Int. J. Adv. Manuf. Technol. 68 (2013) 1103–1110.

[17] P. Wei, Z. Wei, Z. Chen, J. Du, Y. He, J. Li, Y. Zhou, The AlSi10Mg samples produced by selective laser melting: single track, densification, microstructure and mechanical behavior, Appl. Surf. Sci. 408 (2017) 38–50.

[18] I. Yadroitsev, A. Gusarov, I. Yadroitsava, I. Smurov, Single track formation in selective laser melting of metal powders, J. Mater. Process. Technol. 210 (2010) 1624–1631.

[19] N.T. Aboulkhair, I. Maskery, C. Tuck, I. Ashcroft, N.M. Everitt, On the formation of AlSi10Mg single tracks and layers in selective laser melting: Microstructure and nano-mechanical properties, J. Mater. Process. Technol. 230 (2016) 88–98.

[20] M.J. Matthews, G. Guss, S.A. Khairallah, A.M. Rubenchik, P.J. Depond, W.E. King, Denudation of metal powder layers in laser powder bed fusion processes, Acta Mater. 114 (2016) 33–42.

[21] Y. Li, K. Zhou, P. Tan, S.B. Tor, C.K. Chua, K.F. Leong, Modeling temperature and residual stress fields in selective laser melting, Int. J. Mech. Sci. 136 (2018) 24–35.

[22] Y. Guo, L. Jia, B. Kong, N. Wang, H. Zhang, Single track and single layer formation in selective laser melting of niobium solid solution alloy, Chin. J. Aeronaut. 31 (2018) 860–866.

[23] S. Bremen, W. Meiners, A. Diatlov, Selective laser melting. A manufacturing technology for the future? Laser Technik J. 9 (2012) 33–38.

[24] K. Li, D. Wang, L. Xing, Y. Wang, C. Yu, J. Chen, T. Zhang, J. Ma, W. Liu, Z. Shen, Crack suppression in additively manufactured tungsten by introducing secondary- phase nanoparticles into the matrix, Int. J. Refract. Met. H. 79 (2019) 158–163.

[25] Z. Hu, Y. Zhao, K. Guan, Z. Wang, Z. Ma, Pure tungsten and oxide dispersion strengthened tungsten manufactured by selective laser melting: Microstructure and cracking mechanism, Addit. Manuf. 36 (2020) 101579.

[26] J. Robinson, I. Ashton, P. Fox, E. Jones, C. Sutcliffe, Determination of the effect of scan strategy on residual stress in laser powder bed fusion additive manufacturing, Addit. Manuf. 23 (2018) 13–24.

[27] Y. Lü, X. Yu, C. Tan, T. Sun, K. Ma, H. Cai, Effect of the β phase on compressive mechanical property of CVD tungsten, Rare Metal Mater. Eng. 40 (2011) 1138–1140.

[28] X. Zhang, Q. Yan, S. Lang, Y. Wang, C. Ge, Preparation of pure tungsten via various rolling methods and their influence on macro-texture and mechanical properties, Mater. Des. 126 (2017) 1–11.

[29] A.M. Lennon, K.T. Ramesh, The thermoviscoplastic response of polycrystalline tungsten in compression, Mater. Sci. Eng. A 276 (2000) 9–21.

[30] J.-K. Lee, S.-Y. Kim, R.T. Ott, J.-Y. Kim, J. Eckert, M.-H. Lee, Effect of reinforcement phase on the mechanical property of tungsten nanocomposite synthesized by spark plasma sintering, Int. J. Refract. Met. H. 54 (2016) 14–18.

X. Ren et al.