Microstructure and mechanical properties of additive manufactured W-Ni-Fe-Co composite produced by selective laser melting
Release time:
2025-09-29
1. Introduction
Tungsten, as the highest melting point (3410 C) refractory metal,has many exceptional intrinsic properties, including high density, high tensile strength, high thermal conductivity and low thermal expansion coefficient. Tungsten and its alloys have attracted great attention for high-temperature and high heat flux components in radiation environments, such as collimators, plasma-facing components in future nuclear fusion reactors, and high-performance rocket nozzles [1–5].However, pure tungsten is hard to be consolidated to its theoretical density even with sintering temperatures close to the melting point,owing to its refractory attribute [1–4]. Consequently, tungsten powder is usually sintered into dense components by adding binder metals with relatively lower melting points [5].
The addition of the low melting point alloys results in liquid phase sintering of W composites and offer a unique combination of mechanical properties, easy machinability, high modulus of elasticity, good corrosion resistance, and high absorption capacity against X-rays and gamma-rays. These W composites often consist of bcc structured W phase and the surrounding fcc binder matrix. Properties of W composites are significantly influenced by the amount of tungsten versus the alloying additives. Fe, Ni and Cu are the most commonly used sintering additives for W composites. Recently, it was reported that small additions of Re, Mo and Co were found to benefit for restricting the tungstengrain growth and refine the grains during liquid phase sintering of the alloy, thereby, improving its strength and hardness [6–12]. The presence of cobalt offers solid solution strengthening of the binder matrix as well as strengthening the tungsten-matrix interface, thus, increasing tensile strength, impact strength and ductility of the W composites[11,12].
However, due to the high melting point and high ductile-brittle transition temperature of tungsten, W composites are generally processed through powder metallurgy (PM) techniques. Nevertheless, PM technology cannot yield complex shapes, although it can produce parts with very dense structures. Moreover, the conventional PM of tungsten alloys is mainly performed by liquid sintering with dissolving-precipitating and recrystallization mechanisms, whereas the tungsten particles are not melted [13]. To attain higher strength and hardness,these composites are subjected to post treatments such as swaging and aging [14–16]. Therefore, developing advanced processing methods for manufacturing complex-shaped W composites has huge demand.
Selective laser melting (SLM) is a type of additive manufacturing technique, through which it is possible to directly manufacture parts with complex geometries from powder in one single step [17]. This technology uses a laser power source to fuse powder materials to form functional parts directly based on computer aided design (CAD) files.The part to be built is first sliced into many thin and horizontal layers in the CAD file. Based on the subsequently applied computer-aided manufacturing program, powders are first deposited onto a substrate with the layer thickness carefully determined according to the slice thickness and particle size distribution, which are then melted selectively by a focused laser beam with high intensity. Rapid spreading and solidifi-cation of the melt forms a dense layer thereafter. By repeating this process layer by layer, a three-dimensional (3D) part is built up additively [ [18–20]].
Many previous works have demonstrated the feasibility of applying SLM to build complex metallic parts, including superalloys, copper alloys, titanium alloys and even refractory metals [21–25], which suggest that W-based complex structures have potential for the SLM process.Zhang et al. produced WeNi alloy by SLM and demonstrated the in-fluence of Ni content on microstructure of the alloy [26]. Gu et al. investigated the densification behavior, microstructural evolution and hardness performance of SLM-processed W alloy parts by changing laser energy density [27]. A good understanding of the effect of process parameters like laser power, laser scanning speed, hatch spacing and layer thickness on the densification of the W composites has been gained from these studies. However, very few studies have succeeded in fabricating near full density W composites with mechanical properties comparable to that produced by conventional press-sintering process.
In the present work, W-Ni-Fe-Co composite powders were consolidated by SLM using a high power fiber laser device. The addition of some Co element facilitates densification of the W composite. The effects of laser process parameters and chemical compositions on densi-fication, microstructures, phases, and tensile properties were characterized, and the corresponding metallurgical mechanisms were discussed.
2. Experimental
W powders (purity, 99.9%), gas atomized Ni powders (purity,99.9%), gas atomized Fe powders (purity, 99.9%), and gas atomized Co powders (purity, 99.9%) were used in this study. Hydrogen reduced tungsten powders were purchased from Zigong cemented carbide Co.,Ltd., China. Size distribution of raw powders was analyzed by laser particle size meter (Master Size R3000). The measured median particle diameter (D50) of W, Ni, Fe and Co raw powders was 18.1 μm, 21.2 μm, 29.7 μm, and 17.0 μm, respectively. The as-received powders were mixed in the required chemical composition of W-6Ni-2Fe-2Co wt% (W90), W-12Ni-4Fe-4Co wt% (W80), and W-18Ni-6Fe-6Co wt% (W70),respectively. The mixed powders were ball milled in a 10 L stainless steel tank for 12 h in Ar. The mass ratio of ball to powder was 2:1 and the rotation speed was selected as 50 r/min.
SLM processing was performed using a Farsoon 271 M instrument employing a standard alternating X/Y raster scanning strategy. Firstly,a thin layer of the mixed W-Ni-Fe-Co powder was spread out by a ceramic blade. Then, the mixed powders were selectively melted by an incident laser beam to be joined with the solidified areas of the parts underneath. After that, the building platform was lowered by one-layer thickness for the next powder deposition and laser exposure. Once layer n was completed, the bidirectional scanning of the next layer (n + 1)was performed after rotating by 67°. The chamber was evacuated and filled with Ar to protect the molten pool from oxidization. The processing parameters of the W-Ni-Fe-Co composites are described as follows. The laser power ranged from 325 to 425 W; the scan speed was 400 to 600 mm/s. The hatch spacing and layer thickness were fixed at 60 μm and 30 μm, respectively.
Cylinders with size of Φ15 × 10 mm were printed for density measurement and microstructure characterization. The size of tensile specimens is flat dog-bone-shape with a thickness of 3 mm and a gage length of 8 mm. The tensile tests were performed with a strain rate of 1.92 × 10−3 s−1. The equipment used in room temperature tensile test is the Instron 3369 double column desktop material testing machine produced by Instron company in USA. Three groups of tensile samples of the same parameter were measured in the experiment to ensure the accuracy of the results. The chemical composition of the SLM processed W-Ni-Fe-Co composites was analyzed by induction coupled plasma emission spectrometry (ICP). The densities were measured by Archimedes principle with deionized water. The surface morphologyand microstructure characteristics were investigated by optical microscope (OM) and scanning electron microscope (SEM). All the samples were initially ground using abrasive paper from 400 to 2000 mesh. The grinding was followed by cloth polishing using a disc polisher withdiamond paste. The polished samples were etched using a mixture of deionized water (40 mL), hydrogen peroxide solution (10 mL), and ammonia liquor (10 mL) to highlight the microstructural features. The photo micrographs of the W-alloy samples were obtained by a Leica 2700P with digital image acquisition capability. The micrographs with higher magnification were obtained by field emission SEM (FESEM, FEI250) with an energy dispersive spectrometer (EDS) system.
3. Results and discussions
3.1. Densification
On the basis of ICP analysis, the actual composition of W90, W80 and W70 was determined as 91.00 W-5.94Ni-1.66Fe-1.40Co wt%,82.10 W-10.90Ni-3.58Fe-3.42Co wt%, and 72.06 W-17.40Ni-5.76Fe-4.78Co wt%, respectively. It was considered that SLM process causesvaporization and losses of Ni, Fe and Co elements, because the melting point of W is higher than the boiling point of Fe, Co and Ni. The theoretical densities of the composites were calculated as ρT = ∑i3=0 ρ ωi i,where ρi is the theoretical density of each chemical composition, ωi is mass fraction of each chemical composition. Based on the actual compositions of the samples, theoretical density of W90, W80 and W70 is calculated to be 18. 3, 17.4 and 16.3 g/cm3, respectively. The relative density was calculated by dividing the measured value by theoretical value. The relative densities of W90, W80 and W70 samples prepared by SLM at different laser power and scanning speed are shown in Fig. 1.The relative density of all the W-Ni-Fe-Co composites (W90, W80 and W70) increases with laser power and decreases with laser speed. Besides, the relative density gradually increased with the decrease of the content of W. As shown in Fig. 1, the W70 alloy formed at 325 W and 600 mm/s rapidly undergoes densification and reaches a relative density of 91.4%. With the increase of laser power, the relative density of the W70 alloy shows a consistently increased trend and reaches 93.7%at 425 W and 600 mm/s. Similarly, the relative densities of both W80 and W90 increase with the increase of laser power. When slowing the laser speed, the relative density of all the samples increase remarkably.A maximum relative density of 96.1% was obtained in W70 with laser power of 400 W and laser speed of 400 mm/s. It was reported that appropriate sintering temperature was the most critical factor to obtain high relative density [28,29].
In the SLM process, high laser power gives rise to more liquid Ni-Fe-Co phase and results in a high densification level. In general, the densification process during SLM includes three stages. Firstly, W particles are rearranged driven by capillary forces from the liquid phase. Then,the W grain shape accommodation takes place by solution reprecipitation. Finally, the densification is completed by sintering of the skeletal structure due to solid-state bonding [30]. For the SLM processing of the W-Ni-Fe-Co composites, the melting of the Ni, Fe and Co promotes dissolution of W particles, and liquid films at the grain boundaries extend the time of grain shape accommodation and re-arrangement in the W-Ni-Fe-Co system. Thus, the increase of content of the liquid phase leads to the higher relative density of the composites.
3.2. Phases
The x-ray diffraction patterns of the composites are shown in Fig. 2.Strong diffraction peaks at 40.1°, 58.2° and 73.1° originate from the body-centered cubic tungsten phase, while the diffraction peaks at 43.2°and 50.2° originate from the NieFe solid solution phase. With the increase of the contents of Ni-Fe-Co, the intensity of the diffraction peaks of NieFe phase was enhanced. The presence of intermetallic Fe7W6 phase was also detected in W80 and W70 samples with the diffraction peak at 37.8° in Fig. 2.
3.3. Microstructures
Fig. 3 shows the SEM image of the top surface of the W-Ni-Fe-Co composites with different W contents fabricated by SLM. Defects like holes were observed in all the samples, and the number of holes increases with the increase of W contents. The EDS analysis shows that the white phase is W, while the surrounding grey zones are the Ni-Fe-Co binding phase. As shown in Fig. 3, unmelted polyhedral W particles were surrounded by a W-Ni-Fe-Co matrix with W dendrites. Because of the high melting point of W, the W particles were partially melted in the SLM process, leading to the presence of spherical unmelted W particles in the W-Ni-Fe-Co composites. Fig. 3a-b show the direct contact between the W particles in the W90 samples, indicating that the densification mechanism conforms to solid-state densification at this circumstance. A little Ni-Fe-Co bonding phase formed between the W particles.In the W80 samples, there is a homogeneous solids distribution, and they are almost free of the apparent aggregation of the W particles (Fig. 3c-d). Particle rearrangement seems to act as the main densification mechanism and the W particles are surrounded by a few dendrites in the W80 composites. In the W70 samples, there are many dendrites distributed among the W particles (Fig. 3e-f) and the quantity and size of the W particles are decreased.
In W90, the content of W is much higher than that in the W80 and W70 samples. The lesser amount of Ni-Fe-Co bonding phase results in the direct contact of many unmelted W particles. With the increase of the Ni-Fe-Co bonding phase, the ratio of dendrites increased significantly, as shown in the W70 sample. Many of the W particles were fully melted and solidified into the dendritic crystals in W70. The microstructure of the W70 presents a dendrite matrix (dendritic W with Ni-Fe-Co bonding phase) with dispersed unmelted W particles.
Fig. 4 shows the SEM image of the side view of the SLM-processed W-Ni-Fe-Co composites. As shown in Fig. 4(a-b), layered structures are observed in the side view of the W90 samples. It was clear that cracks appeared and extended along the building direction. Because of the brittleness of tungsten, the cracks are easy to appear in the tungsten layer under thermal stress during SLM process when the W particles are not coated by the Ni-Fe-Co bonding phase. With the increase of the content of the Ni-Fe-Co, the tungsten particles can be partially melted during the SLM process and the dendrites grow around the W particles in the W80 alloy. The bonding between W particles and Ni-Fe-Co phase becomes more compact. Macro cracks were seldom observed, but holes and micro cracks still exist in W80. In W70 samples, numerous dendrites were distributed around the W particles, and no obvious cracks were observed. Two kinds of dendrite zones (a fine dendrite zone and a coarse dendrite zone) are apparent in the SEM image (Fig. 4e-f). The fine dendrite zone contains thin W dendrites with few W particles,while the coarse dendrite zone consists of short W dendrites and many W particles.
Alternating layered fine dendrite and coarse dendrite zones are visible in the sectioned surface of the W70 samples, as can be seen in Fig. 5(a-b). The elemental contents illustrated in Fig. 5(e) show that the coarse dendrite zones consist of W particles and Ni-Fe-Co bonding phase. When laser beam irradiated the mixed powders, they absorbed the energy and the temperature rose quickly. Most of the W particles were fully melted and a temperature gradient formed in the molten pool. In the upper part of the molten pool, the temperature exceeded the melting point of tungsten, inducing the melting of W particles.However, in the regions adjacent to the solid formed parts some of theW particles were unmelted. Gravity induced segregation also result in more W particles in the bottom of the layer due to the abundant liquid phase during the SLM process, as reported by Gurwell [ [31]].
It is reported that the addition of Co in W-Ni-Fe ternary system attributes to the enhancement of W solubility in the matrix [32–34]. As shown in Fig. 5, the EDS analysis shows that the dissolved tungsten is about 27.7% in our SLM processed W70 samples (Spot 3 in Fig. 5(d) and the corresponding EDS results). Cobalt increases the solid solubility of tungsten in the matrix phase. This leads to increase in the volume fraction of the Ni-Fe-Co matrix phase, thus reducing the connectivity of the tungsten grains. Enhancement of W solubility in the matrix helps to increase the cohesive strength of tungsten and matrix and benefit ductility and impact toughness of the composite [32].
During solidification process, the low solid-liquid interface temperature in SLM resulted in a strong cooling of the adjacent layer,leading to a high driving force for nucleation, and the resultant cellular grain close to the unmelted tungsten acted as a heterogeneous nucleation site. Due to the existence of unmelted tungsten particles, a coarse dendrite zone was formed in the bottom of the molten pool and the Ni-Fe-Co bonding phase surrounded the coarse tungsten dendrites. In the upper part of the molten pool, the W particles are fully melted and solidified from the liquid. The solidified W dendrites are much thinner than the unmelted W particles, inducing a fine dendrite layer in the upper part of molten pool.
Some parts containing fine and homogenous dendritic crystals can be seen in the bottom of the molten pool when the 400 W laser power was utilized, as shown in Fig. 5(c-d). These delicate fine dendrite zones were caused by the remelting process under large laser power input.The energy spectrum analysis (Fig. 5(e)) shows that the elemental contents in the remelted zone (spot 4 in Fig. 5(d)) were close to the nominal composition of the W70 composite, indicating the uniform microstructure in the remelted zone. The remelting effects benefit densification of the composite and produce high relative density of W70(Fig. 1).
When the composite powders were irradiated under high laser power, the laser not only melted the mixed powders, but also caused a remelting zone on the formed parts, as illustrated in Fig. 6. Due to the fast heat dissipation along the formed part, the rapid solidification process occurred and fine dendrites solidified at the solid-liquid interface. As solidification proceeded, the cooling of the remaining liquid slowed due to the latent heat release of the solidification process,thus the supercooling degree in the liquid front was reduced. In this case, the fastest growing direction was perpendicular to the solid-liquid interface and the dendrites grew preferentially along the heat dissipation direction thereby pointing to the center of the molten pool, as shown in Fig. 5(d).
3.4. Tensile properties
Fig. 7 shows the tensile properties of W70 composites processed with different laser powers and scanning speeds. At the scanning speed of 600 mm/s, the ultimate tensile strength (UTS) of W70 composite increases monotonously with the increase of input energy density, as shown in Fig. 7(a). The UTS is only 950 MPa at an energy density of 300 J/mm3, while the UTS reaches to 1060 MPa when the energy density increases to 390 J/mm3. As discussed above, there are unmelted tungsten particles in W70 composite under laser power of 325 W and scanning speed of 600 mm/s. The large unmelted tungsten particles and coarse grains in the Ni-Fe-Co binding phase lead to the relatively low UTS. With the increase of input energy density, the tungsten powders can be fully melted and fine dendrites are formed. Thus, the UTS is greatly enhanced under energy density of 390 J/mm3 (425 W and 600 mm/s).
When the scanning speed decreases to 400 mm/s, the tensile performance of the W70 composite has been improved remarkably. The UTS increases in the initial stage but decreases thereafter with the increase of energy density. The UTS increases from 1030 MPa at 450 J/mm3 to 1198 MPa at 490 J/mm3, then decreases with further increase of energy density. As the scanning speed decreased from 600 to 400 mm/s, the mixed powders absorbed more energy. More tungsten particles can be melted and more fine dendrites were formed in the W70 composite. Fine dendrites produce grain refinement effects and more phase boundaries. The obstruction of dislocation movement by the phase boundaries results in the enhancement of tensile strength. However, further increase of laser power means excess energy input, which results in an unstable molten pool in the selective melting process. Bubbles and voids can form in composites processed under high energy input [23]. On the other hand, high energy input could promote the reaction between W and the redundant Fe elements at high temperature and form the Fe7W6 intermetallic phase (Fig. 2). The precipitation of Fe7W6 phase promotes cleavage fracture and thereby leads to degradation of mechanical properties [35]. As a result, the UTS of W70 composite decreases when the energy density exceeds 490 J/mm3.A maximum UTS of 1198 MPa was obtained at a laser power of 350 W and scanning speed of 400 mm/s. The elongation of W70 composite is in the range of 7% to 9.5%. It seems that high energy input also benefits the ductility of the W-Ni-Fe-Co composites, and higher elongation was achieved at lower scanning speed (400 mm/s).
The SEM micrographs of fractured surfaces of the W70 tensile tested specimens under different scanning speeds are shown in Fig. 8. The orphology of the fracture surface consists of tungsten particle cleavage and dendritic W-Ni-Fe-Co matrix. As can be seen, the SLMprocessed W70 composite shows the characteristic of ductile tearing of the dendritic matrix and transgranular cleavage fracture of the tungsten particle. Small dimple topography can be observed in the dendritic W-Ni-Fe-Co matrix in the sample processed under low scanning speed(400 mm/s). The transgranular cleavage in W particles means that there is good adhesion between matrix and tungsten particles, as shown in Fig. 8(a). This is consistent with the higher tensile strength obtained under high energy density. Tungsten exfoliation phenomena were observed in the sample processed under high scanning speed. These weak positions can serve as crack initiation sites under stress concentration,resulting in the relatively lower strength of samples processed under high scanning speed [36,37].
4. Conclusions
In this study, a novel W-Ni-Fe-Co composite material produced by milling of elemental powders has been successfully processed using the SLM additive manufacturing process. The phases, microstructure evolution, and mechanical performance have been characterized and the corresponding metallurgical mechanisms were illuminated.Conclusions are drawn as follows:
1) The density of the W-Ni-Fe-Co composite produced by SLM increased with the increase of laser power. Maximum relative densities of 93.9% for W-6Ni-2Fe-2Co, 94.2% for W-12Ni-4Fe-4Co and 96.1% for W-18Ni-6Fe-6Co were achieved. Cracks were visible in the W90 composite, while the W70 composite processed at 400 W exhibited near full density with absence of cracks and pores.
2) The XRD patterns indicated the main phases of SLM-processed W-NiFe-Co composite were W and NieFe solid solution phases regardless of the laser parameters. Besides, an intermetallic Fe7W6 phase were observed in the W80 and W70 composites with the increase of the contents of Ni-Fe-Co.
3) The typical microstructure of the W-Ni-Fe-Co composites consisted of unmelted polyhedral W particles and the surrounding W-Ni-Fe-Co matrix with W dendrites. Alternating layered fine dendrite and coarse dendrite zones were visible in side views of the composites.With the decrease of the W content, the unmelted W particles were reduced, and the fraction of the dendritic zones broadened. Delicate fine dendrite zones were observed at the bottom of the molten pool of W70 composite caused by remelting of the solid part under largelaser power input.
4) The tensile strength of the W70 composite has a pronounced improvement with the increase of energy density, although excess energy input may generate voids due to the unstable molten pool.The UTS of W70 processed under laser speed of 400 mm/s was much higher than that processed at 600 mm/s. Maximum UTS of 1198 MPa was obtained in W70 composite under laser power of 350 W and scanning speed of 400 mm/s. Laser parameters didn't exert obvious effect on elongation of the composite. Elongations in the range of 7% to 9.5% were achieved in the SLM processed W70 composites.
Paper Citation Information
International Journal of Refractory Metals & Hard Materials 86 (202o) 105111
Tungsten-Nickel-Iron (W-Ni-Fe)
Tungsten-Nickel-Iron is a tungsten-based heavy alloy, a powder metallurgy material produced through a specialized process using tungsten (W), nickel (Ni), and iron (Fe). It combines the high density and strength of tungsten with the toughness of the nickel-iron binder phase, making it an important structural material in the industrial sector.
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