WC-Co Composite Powders Research Articles

Laser powder bed fusion of cemented carbides by developing a new type of Co coated WC composite powder

The study addresses the issue of the absence of high-quality WC-Co composite powder for laser powder bed fusion and assessing the printability of the new powder in terms of densification, microstructure and mechanical properties. The fluidized bed chemical vapor deposition (FBCVD) combined with electroless plating process was adopted to make core-shell structured WC-Co composite powder. The excellent uniformity of Co and good powder flowability were achieved by tuning the size and distribution of Co catalyst during FBCVD and regulating their relationship to the subsequent electroless plating behavior. The newly developed composite powder exhibited excellent printability. The densification mechanism was largely dependent on the laser energy density. The liquid formed by the melting of Co was responsible for the densification at low laser energy density, while the Co-W-C ternary liquid resulted from the dissolution of WC into Co melt dominated the densification at high laser energy density. Microstructurally, the morphology and size of WC grains were insignificantly changed because of its non-participation with liquid formation at low laser energy density. The rectangular/triangular WC grain morphology similar to the traditional sintered was formed at high laser energy density because of the precipitation of WC from the Co-W-C ternary liquid. Affected by the different liquid formation processes, increasing laser energy density increased the relative density, promoted the W 2 C phase formation and decreased WC grain size, which remarkably improved the hardness and tribological properties of the printed cemented carbide.

Facile synthesis of tungsten carbide-cobalt mixtures with clean surface and controllable carbon content

Tungsten carbide-cobalt (WC-Co) mixtures is the most critical material for cemented carbide. However, traditional preparation methods by using various carbon sources (e. g. carbon black, glucose, methane, carbon monoxide) cause severe carbon deposition. Consequently, traditional WC-Co mixtures exhibits uncontrollable carbon content. To address the challenge, we investigate the in-situ synthesis of WC-Co mixtures by supplying successive liquid methanol as a carbon source and nitrogen gas as a diluent. The as-prepared WC-Co mixture possesses clean surface and controllable carbon content, which are no need for removing excess carbon deposition by washing with water. The carbon nanolayers derived from methanol cracking gas appear on the surface of the intermediates, which can effectively prevent particle aggregation, efficiently increase the intimate contact probability of WC particles and Co particles, and greatly reduce the appearance of excess carbon deposition during subsequent carbonization processes. The present study offers insights on employing quantitative methanol cracking gas as reductive and carbonaceous atmosphere to mediate carbon content in the WC-Co composite powder, which presents a more economical and efficient pathway for large-scale synthesis of high-quality WC-Co mixtures with clean surface and controllable carbon content.

Effect of binder saturation and drying time on microstructure and resulting properties of sinter-HIP binder-jet 3D-printed WC-Co composites

Binder-jet 3D-printing, a non-beam-based additive manufacturing method, is a process of selectively joining layers of powder using a polymeric binder. In this study, the effects of print processing parameters such as binder saturation (60–250%) and drying time (30–45 s) on 3D-printed WC-Co composite powder is studied. Densification is attained by sintering under a vacuum atmosphere to reach the maximum temperature of 1435 ℃ followed by hot isostatic pressing under a pressure of 6.1 MPa. Although the green density of the 3D printed samples was ~21–23%, solid volume fraction of 97% to nearly 100% and near theoretical densities of 14.10–14.35 g/cm3 were achieved depending on the print processing parameters. Microstructural characterization of the final parts reveals potential spreading defects appearing as remnant pores formed during printing at a higher binder saturation level. Additionally, Co-rich pools also appear in the final parts. X-ray diffraction shows WC and Co as the main phases and a potential η-phase formation depending on the binder saturation level. Parts with WC grain size of 1.6 ± 0.7 µm, average Vickers hardness of 1270–1320 HV60 and fracture toughness of 17–22 MPa m1/2 are attained, similar to conventionally produced WC-Co with medium grain size. Here, binder-jet 3D-printing is used to fabricate WC-Co parts with uniform microstructure and mechanical properties.

Core-shell structural iron based metal matrix composite powder for laser cladding

WC is considered as a good choice for the reinforced phase of many composite alloys because of its excellent hardness and good wear resistance. In the past, micro WC particles physically mixed with micro metal particles were also widely used in laser cladding, however, micron WC particles often sink down to the bottom and may cause uneven element distribution, holes, cracks, and other defects in the cladding layer due to its high density and high laser absorption rate. Here, we prepare a core-shell structural 420-nano WC-Co composite powder by high energy ball milling. The composite powder has high circularity and high fluidity, which is suitable for laser cladding. The cladding layer with no holes or cracks can be attained using the core-shell structural composite powders, moreover, the nano WC-Co particles are evenly distributed in the cladding layer and the microhardness of the cladding layer has enhanced significantly.

Effect of Re addition on the microstructure and mechanical properties of WC-10Co cemented carbides fabricated by chemical coating method

WC-Co composite powder with nano‑cobalt coatings doped with rhenium (Re) varying from 0 to 3 wt% were prepared by a novel sol-gel method. The effects of Re on the microstructure and mechanical properties of WC-10(Co, xRe) cemented carbides fabricated by composite powder were studied by XRD, SEM and TEM. The results indicated that C4H6CoO4·4H2O with or without NH4ReO4 was homogenously distributed on WC powder particles. After reduction in H2 atmosphere, nano‑cobalt with Re was coated on WC particles with a uniform distribution in composite powder. WC-10(Co, xRe) cemented carbide containing Re exhibited homogeneous grain distribution. Moreover, no stabilization effect for hexagonal close-packed (hcp) cobalt by Re addition was found mainly due to the minor additive amount and the unique preparation method. WC grains were refined and binder phases were strengthened due to the dissolved Re in binder phases. With increasing the content of Re from 0 to 3 wt%, an improved microstructure led to better mechanical properties, the hardness increased from 87.8 ± 0.1 HRA to 89.4 ± 0.2 HRA, transverse rupture strength (TRS) increased from 2810 ± 89 MPa to 3410 ± 113 MPa.

Preparation of high-purity and ultrafine WC-Co composite powder by a simple two-step process

In this study, high-purity and ultrafine WC-6%Co composite crystallites was synthesized by a simple two-step process consisting of the precursor-formation of the mixture of blue tungsten oxide (WO2.9) and cobaltic oxide (Co2O3) and the following deep reduction and carburization with CH4-H2 mixed gases. The experimental results revealed that after the first carbothermic reduction stage at 1050 or 1150 °C, a mixture of W, WO2 and Co7W6 was obtained, which was further carburized to the WC and Co phases by CH4-H2 mixed gases at 900 °C. With the increase of C/WO2.9 molar ratio, the particle sizes of first-stage precursor and carbonized product were both decreased. The particle sizes of final products are mainly determined by C/WO2.9 ratio and reaction temperature at the first stage. When the C/WO2.9 ratio was in the range of 2.3–2.7, the high-purity WC-6%Co composite powder with the average particle sizes of 160–410 nm could be obtained.

Effects of ultrafine WC on the densification behavior and microstructural evolution of coarse-grained WC-5Co cemented carbides

The effect of ultrafine WC on the macroscopic shrinkage behavior and microstructural evolution of coarse-grained WC-Co cemented carbides during sintering was studied. WC-Co composite powders containing mixture of ultrafine (300 nm) and coarse (6.0 μm) WC powders were prepared using a novel sol-gel process. The results indicate that ultrafine WC addition enhanced shrinkage in the composite powder compacts during solid state sintering, while its influence on densification process in liquid-phase sintering can be ignored. The microstructural evolution confirmed that ultrafine WC addition promote the spreading of cobalt binder and the formation of WC-Co agglomerate in initial state of sintering. And, ultrafine WC addition resulted in a more uniform densification in the solid state sintering. Mechanisms of the enhanced shrinkage include the accelerated spreading of Co due to shortened diffusion distances and the increased surface diffusion velocity, and the activated sintering of WC skeleton due to grain boundary sliding.

Low-energy grain boundaries in WC-Co cemented carbides

The influencing factors, distribution characteristics and evolution mechanisms of the typical low-energy grain boundaries in the WC-Co cemented carbides were demonstrated. Based on characterizations from various angles, the correlations of grain shape aspect ratio, WC contiguity and distribution of low-energy grain boundaries were analyzed for cemented carbides with addition of different grain growth inhibitors. It was found that the ∑2 grain boundaries have higher fraction in the cemented carbides with stable complexions, finer grain size and larger WC contiguity. The ∑13a grain boundaries are likely to form in the cemented carbides with anisotropic surface energy by coalescence of WC platelets with large shape aspect ratio. The results suggest that the fraction and distribution of low-energy grain boundaries in the cemented carbides can be modulated by matching grain growth inhibitor and sintering temperature of the initial WC-Co composite powder.

Effect of Co on morphology and preparation of in situ synthesis of WC-Co composite powders

The WC and WC-Co composite powders were fabricated through spray-drying, calcinations as well as by reduction and carbonization treatment using the precursors, AMT, C6H12O6, Co(CH3COO)2 · 4H2O and H2O with different Co content (0, 4, 6, 8, 12 wt%). The results show that the obtained WC by the spray-drying method were nearly spherical with a smooth surface and maintaining the spherical structure of AMT. Also, the loose cobalt oxide powders prepared by the calcinations of precursors had uniform structures with good dispersivity, and higher degree of sphericity when Co content was greater than 6 wt%. Moreover, aggregates with original sheet structure exist in the WC and WC composite powders prepared by reduction and carbonization, and also higher the Co content, the more obvious the agglomeration. Using XRD, EDS and XPS, activated carbon, WO3 and Co3O4 with a small content of WC were detected during the pretreatment of calcined products. Furthermore, the preparation temperature of pure WC powders was relatively high (about 1200 °C) compared to those contained with Co. The preparation temperature of WC-Co composite powders gradually decreased with an increase in the Co content. When the Co content increased to 12 wt%, the temperatures for reduction and carbonation remained at 1050 °C.

Synthesis of WC-Co composite powders with two-step carbonization and sintering performance study

In this study, WC-Co composite powder was synthesized by two-step carbonization method using W, Co and C as raw materials. X-ray diffraction (XRD) showed that the η phase (Co6W6C) was kept at 1100 °C for 1 h under vacuum, and it could be completely carbonized into WC-Co composite powders. The surface morphology of WC-Co composite powders was analyzed by scanning electron microscope (SEM). The effects of η phase and second phase (W phase) on WC morphology and Co phase distribution were investigated. Electron backscattered diffraction (EBSD) was used to analyze WC-10 wt% Co cemented carbide particle distribution. Comparison of transverse rupture strength, hardness and fracture toughness of two kinds of WC-10 wt% Co cemented carbides synthesized by WC-Co composite powders + WC and WC + Co respectively, the cemented carbide of composite powders + WC increases the fracture toughness from 11.4 ± 0.3 MPa·m1/2 to 12.4 ± 0.3 MPa·m1/2.

Low-Energy Grain Boundaries in WC-Co Cemented Carbides

The influencing factors, distribution rules and formation mechanisms of the typical low-energy grain boundaries in the WC-Co cemented carbides were studied. A number of characterizations were performed on the grain morphology and the features of low-energy grain boundaries in the cemented carbides, and the correlations of the grain shape aspect ratio, WC contiguity and distribution of the low-energy grain boundaries were analyzed. It was found that the ∑2 grain boundaries have a high fraction in the cemented carbide with stable complexions which lead to fine grain size and high WC contiguity. The ∑13a grain boundaries are likely to form in the cemented carbide with anisotropic surface energy by coalescence of WC platelets with a large shape aspect ratio. The present results suggest that the low-energy grain boundaries in the cemented carbides can be tailored by matching the grain growth inhibitor and the sintering temperature of the initial WC-Co composite powder.

The factors leading to abnormal grain growth during sintering of hard alloy

This work investigated the sintering of the nanocrystalline tungsten carbide and cobalt composite powder. Vacuum sintering and spark plasma sintering (SPS) were used to consolidate powders into bulk nanocrystalline materials. In addition, two methods of the cobalt introduction were developed for making WC/Co compositions. The first one uses mechanical mixing in ball milling machine. The second one is based on a precipitation and reduction of the cobalt salt on the WC particle surface. The oxygen was always present in the produced powders. Therefore, a control of common carbon is necessary to produce bulk materials with correct chemistry. When vacuum sintering or SPS process are used for consolidation, the addition of grain growth inhibitor is necessary to minimize grain growth. Sintering hard alloys using the nanocrystalline WC-Co composite powders has unique properties and potential for commercial applications.

Investigation on reduction and carbonization process of WC-Co composite powder obtained by In situ synthesis

WC-Co composite powders with WC grain size of about 170 nm were synthesized by spray conversion process and in situ synthesis at 1173 K, which is much lower than the traditional carbonization temperature. The reaction process of in situ synthesis process was investigated by thermodynamic calculation, XRD, XPS, SEM and TEM. Based on the results of thermodynamic calculation, XRD and XPS, it is found that Co3O4 is firstly reduced to CoO, which is further reduced to Co. WO3 is also reduced to low valent tungsten oxide firstly in the range of low temperature, and then reduced to W at higher temperature. In addition, the effects of synthetic temperature on the phase and morphology of composite powders were investigated systematically in the range of 873 K–1273 K. The thermodynamic analysis and experimental verification results obtained in the present work are significant to control and optimize the preparation processes of in situ synthesizing WC-Co composite powder.

Effect of cobalt on the synthesis and sintering of WC-Co composite powders

With the in-situ addition of cobalt, not only the carbothermal reduction of WC‒Co composite powder, but also its sintering was greatly promoted. It was found that the formation of CoWO4 could reduce the oxygen pressure in the system to 10−20 atm, which significantly accelerates the reduction process. Co3W3C, which was obtained from CoWO4 and WO2, enhanced the carburization reaction of W2C through the dissolution and precipitation of the grains. The sintering of the resulting WC‒Co composite powder was completed within only 360 s by solid state sintering at under 1300 °C. The excellent sinterability of the powder is attributed to a uniform distribution of thin Co film between WC grains.

Effect of cobalt content on the microstructure and mechanical properties of coarse grained WC-Co cemented carbides fabricated from chemically coated composite powder

Coarse grained WC-Co cemented carbides with Co content varying from 3 wt% to 12 wt% were fabricated from composite powder. The composite powder was prepared through chemical coating method that cobalt encased on the surface of WC particles. Effect of Co content on microstructure of the WC-Co cemented carbide was investigated by XRD, SEM and TEM. The experimental results indicate that the nanostructured cobalt coating exhibits spherical shape with a particle size ranging from 200 to 400 nm, and the WC-Co composite powder represents unique feature of WC particle encased by cobalt. With increasing Co content from 3 wt% to 12 wt%, the gradual increase of WC grain size from 5.23 μm to 6.67 μm, fracture toughness from 24.66 MPa m1/2 to 28.83 MPa m1/2 and transverse rupture strength from 2367.05 MPa to 2849.22 MPa of the WC-Co cemented carbides were resulted. The enhanced overall performance was attributed mainly to the equilibrium shape of WC caused by the well-encasing of cobalt on WC grain. This may give insights on the application of other coarse grained WC-based cemented carbides fabricated by chemically coated composite powder.

Synthesis and Characterization of WC-6Co Nanocrystalline Composite Powder

WC-6Co nanocrystalline composite powders without excess C and decarburization phases were synthesized via a facile route including spraying conversion, calcination and in situ reduction-carbonization processes. The phase constituents obtained by XRD show that the as-prepared powders after each step of the preparation are amorphous, WO3 and Co3O4 phases, WC and Co phases, respectively. Pure WC-Co composite powders could be obtained by a reduction-carbonization process heat treated at 900 °C for 1 h under a hydrogen atmosphere due to the catalytic effect of Co on the carbonation reaction. The effects of heat treatment temperature on phase constituents of powders were investigated in a range of 700∼900 °C. Morphology and microstructure of powders were observed by SEM and HRTEM. The results indicate that the morphology of powder is spherical. The grain size of WC is about 0.36 μm, and the crystalline size of WC is about 56 nm, which indicates that the WC grain is polycrystalline and composed of many crystallites. It is also found that the individual particles of WC are bonded together under the action of Co. Furthermore, a few sintering necks could be observed in the composite powders due to the contacts between WC particles. The stoichiometry of WC-Co composite powders could be adjusted easily. Moreover, the formation process and mechanism of the sphere structure were also discussed in this paper.

Synthesis of WC composite powder with nano-cobalt coatings and its application in WC-4Co cemented carbide

Composite powder of tungsten carbide with nano-cobalt coatings was successful manufactured via a novel sol-gel process, and coarse grained WC-4 wt% Co cemented carbide was produced from the cobalt-coated WC powder. Phase composition and microstructure evaluation were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The results revealed that the WC particles were completely encased in the CoC2O4 layers by the sol-gel process. Core-shell structured WC-4 wt% Co composite powder was subsequently prepared by reducing the WC-CoC2O4 precursor powder in hydrogen atmosphere. The morphology and crystal structure of the cobalt-coating were found to depend on the reduction temperature. Chemical analyses revealed that the WC-Co composite powders were non-contaminated and composition-controllable. WC-4 wt% Co cemented carbide fabricated from the cobalt-coated WC composite powder showed a refined and uniform microstructure, and therefore improved mechanical properties, compared to the cemented carbide made by ball-milled WC-Co powder. The transverse rupture strength was 2249 MPa in the cemented carbide from the cobalt-coated WC powder, compared to 1871 MPa of the carbide made by the ball-milling powder.

Fabrication of WC-Co cemented carbides with gradient distribution of WC grain size and Co composition by lamination pressing and microwave sintering

WC-Co composite powders with different particle sizes and Co contents were prepared by ball milling WC and Co powder mixtures for different durations. Functionally graded WC-Co cemented carbides with both Co content and WC grain size gradient were prepared by lamination pressing different WC-xCo (x = 10, 15, 20) powder mixtures and microwave sintering the layered compacts. The WC-xCo powder mixtures were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM), and the phase composition and microstructure of the functionally graded cemented carbides (FGCCs) were investigated by XRD, and FE-SEM coupled with energy dispersive spectroscopy (EDS). Mechanical behaviors of the layered WC-Co materials were measured and compared with those of WC-Co cemented carbides with single composition. The results showed that increasing the milling time from 6 to 24 h, results in the decrease of the particle size of WC-Co composite powders from 0.31 to 0.11 µm. After lamination pressing and microwave sintering, the WC-Co samples show nearly complete densification with a relative density higher than 99.7% and no ƞ-phase was detected in the FGCCs. The Co content and WC grain size in FGCCs decrease from the core to the surface. Homogenization of Co has hardly occurred and no cracks have formed between the layers in the sintered samples. In the inner layer, the mean WC grain size is 529 nm, while in the outer layer it is only 274 nm. Because of the difference in Co content and WC grain size, FGCCs have a Rockwell hardness of 90.75 HRA at the surface, which decreases to 86.75 HRA in the core. However the fracture toughness increases from 11.53 at the surface to 18.12 MPa m−1/2 in the core. The present results show that FGCCs with high outer layer hardness and high inner layer toughness were successfully prepared.

Wear resistance mechanisms of near-nanostructured WC-Co coatings

Dense near-nanostructured WC-Co coatings with hardness above 1400HV0.3 were fabricated by high velocity oxy-fuel spraying using the in situ synthesized WC-Co composite powders containing VC and Cr3C2. The carbide additives inhibited WC coarsening effectively and facilitated formation of the clustered structures consisting of WC grains and amorphous Co. Owing to the dragging effect of clusters, the connected WC particles are much more difficult to be peeled off than the isolated WC particles. Therefore, the wear resistance of the near-nanostructured coating was significantly enhanced. The mechanisms proposed the importance of interfacial characteristics for the wear resistance behavior of the cermet coatings.

Acceleration effect of cobalt on carburization of tungsten at low temperature

The acceleration effect of cobalt on carburization of tungsten at low temperatures was studied by a series of experiments of W, C and Co reactions and theoretical evaluations. Due to Co addition, the formation temperature of WC was greatly reduced (e.g. a decrease from 1300 °C to 850 °C by addition of 1.0 wt% Co). Below the formation temperature of the ternary phase, the converted fraction of W carburization increases with increased Co. Modeling of reaction kinetics revealed that the process of W carburization is dominated by carbon diffusion, and the reaction rate constant is increased much with the increase of Co. The mechanism was proposed that Co acts as the medium for rapid diffusion of C atoms and promotes W carburization by accelerating reaction diffusion kinetics. The present findings will facilitate to develop methods for synthesis of WC-Co composite powders at low temperatures.