WC Grains Research Articles

WC Grain Growth Behavior During Selective Laser Melting of WC–Co Cemented Carbides

WC Grain Growth Behavior During Selective Laser Melting of WC–Co Cemented Carbides

WC-Based Cemented Carbides with High Entropy Alloyed Binders: A Review

Cemented carbides have belonged to the most important engineering materials since their invention in the 1920s. Commonly, they consist of hard WC grains embedded in a cobalt-based ductile binder. Recently, attempts have been made to substitute the cobalt using multicomponent alloys without a principal component (also known as high entropy alloys—HEAs). HEAs usually contain at least five components in more or less equal amounts. The substitution of a cobalt binder with HEAs can lead to the refinement of WC grains; it increases the hardness, fracture toughness, corrosion resistance and oxidation resistance of cemented carbides. For example, a hardness of 2358 HV, fracture toughness of 12.1 MPa.m1/2 and compression strength of 5420 MPa were reached for a WC-based cemented carbide with 20 wt.% of the equimolar AlFeCoNiCrTi HEA with a bcc lattice. The cemented carbide with 10 wt.% of the Co27.4Cr13.8Fe27.4Ni27.4Mo4 HEA with an fcc lattice had a hardness of 2141 HV and fracture toughness of 10.5 MPa.m1/2. These values are higher than those for the typical WC–10 wt.% Co composite. The substitution of Co with HEAs also influences the phase transitions in the binder (between the fcc, bcc and hcp phases). These phase transformations can be successfully used for the purposeful modifications of the properties of the WC-HEA cemented carbides. The shape of the WC/binder interfaces (e.g., their faceting–roughening) can influence the mechanical properties of cemented carbides. The most possible reason for such a behavior is the modification of conditions for dislocation glide as well as the development and growth of cracks at the last stages of deformation. Thus, the substitution of a cobalt binder with HEAs is very promising for the further development of cemented carbides.

Effect of grain size on oxidation resistance of WC-6wt%Co cemented carbide sintered by spark plasma sintering

In this study, two WC-6wt%Co cemented carbides of different fine grades were prepared via spark plasma sintering, and the average WC grain sizes in prepared WC/Co cemented carbides were 320.9 nm (ultrafine-grain) and 847.2 nm (fine-grain), respectively. The effect of grain size on the oxidation resistance of cemented carbide in the air at 600–800 °C was investigated. The results show that when the oxidation temperature is relatively low (600, 700 °C), grain refinement leads to an increase in the number of grain boundaries in the sample, which induce the formation of more fine cracks and pores in the oxide layer and adversely affect the oxidation resistance of ultrafine-grained (320.9 nm) cemented carbide. However, as the oxidation temperature increases to 800 °C, the antioxidant performance of the fine-grained (847.2 nm) cemented carbide decreases and is lower than that of the ultrafine-grained (320.9 nm) cemented carbide, with larger intergranular fracture and disintegration behavior of the oxide grains within the oxide layer being the main reasons for the deterioration of the antioxidant performance of the sample. In addition, the oxidation thermodynamics of WC/Co cemented carbide was discussed, and the macroscopic color inconsistency of the two samples was observed and explained. The apparent activation energies of WC-6wt%Co cemented carbides at 600–800 °C are calculated to be 162.78 kJ/mol and 229.69 kJ/mol, respectively for ultrafine- and fine-grained samples.

Effect of carbon nanotubes additives on mechanical properties of WC–6Co cemented carbide

AbstractIn order to increase the toughness of WC–6Co cemented carbide, different contents of carbon nanotubes (CNTs) were added to the WC–6Co alloy powder to prepare cemented carbide by low‐pressure sintering. The results showed that some of the CNTs were embedded between the grains of WC–6Co cemented carbide, which would hinder the growth of WC grain boundary, thus leading to grain refinement. In addition, CNTs inhibited the formation of decarbonized phase and guided the deflection and bridge of crack to hinder the crack extension. With the increase of CNTs content, the density increased at first and then decreased, and the transverse fracture strength increased at first and then decreased. When the content was 0.2 wt.%, the alloy had the best performance. The density of the alloy was 99.67%; the transverse fracture strength was up to 2937.5 MPa, which is about 100% higher than that of cemented carbide without CNTs. The fracture toughness was 9.84 MPa m1/2, and the hardness was 1924.8HV30.

An experimental and theoretical investigation on Ti-5553/WC–Co(6%) chemical interactions during machining and in diffusion couples

Chemical interactions that drive crater wear in turning are often studied using diffusion couples where the tool and workpiece are fixed. In contrast, in actual turning, there is a constant supply of new workpiece material at the tool-chip interface. In this work, diffusion simulations of a WC–Co(6%) and Ti–5Al–5V system were conducted, with constant replenishment of titanium at the interface (open system) and a fixed amount of material (closed system). The simulations showed that the formation of W(bcc), η-phase, and TiC is dependent on the activity of C and the permeability of Co and C in titanium. Scanning and transmission electron microscopy-based techniques were used to analyse a Ti–5Al–5V–5Mo–3Cr and WC–Co(6%) diffusion couple and a worn WC–Co(6%) insert. The sequence of phases in the closed system simulation was similar to that observed in the diffusion couple. The open system simulation indicated that W(bcc) can form at WC–WC boundaries (where Co is low) within the subsurface of a WC–Co(6%) that has adhered titanium, and at the WC/Ti interface. Additionally, high densities of stacking faults and dislocations were found within subsurface WC grains, indicating a significant reduction of the tool’s integrity.

Thermal properties of cemented carbides used for metal cutting

High temperatures, generated during machining, may have a detrimental effect on the cutting tool. Therefore, tool material's ability to effectively lead away the heat is an important property that needs to be understood and quantified. To accomplish that, thermal conductivity, diffusivity and specific heat of 20 cemented carbide grades were studied using light flash analysis at temperatures between 22 and 800 °C.The results demonstrate how thermal conductivity of cemented carbides increases with coarser WC grain size and lower Co content. Presence of cubic carbides in the material is shown to greatly reduce its thermal conductivity. Specific heat of investigated materials increases with higher Co and cubic carbides content, but it is not affected by WC grain size. Influences of temperature on thermal properties are illustrated and discussed.Most importantly, using regression analysis, an attempt is made to predict thermal conductivity based on reliable, readily available or easily measurable material characteristics. It is shown that thermal conductivity of cemented carbide grades could be estimated with good accuracy from the material's chemical composition and coercivity value.

Additive manufacturing of cemented carbide using analogous powder injection molding feedstock

In this study, WC-8%Co cemented carbide was fabricated by extrusion-based additive manufacturing process using analogous powder injection molding feedstock. Feedstocks with powder loading content of 51 vol% (PW51) and 57 vol% (PW57) were prepared based on thermoplastic binder system composed of paraffin wax, high density polyethylene and ethylene-vinyl acetate. The results show that the extrusion of feedstock is steady and easily controlled. The binder is efficiently removed by solvent leaching and thermal decomposition. After sintering at 1400 °C, densified samples with a relative density beyond 99% are obtained. Sintered PW57 presents less and more isotropic shrinkage below 17.0%, showing a great significance in print accuracy and deformation control. Sintered PW57 has larger transverse rupture strength and fracture toughness of 1695 MPa and 7.27 Pa·m0.5, respectively. Vickers hardness of sintered PW51 is slightly larger because of finer WC grain size. Mechanical properties, microstructure and surface roughness of sintered samples are all acceptable in actual application. Three-dimensional printing using an extrusion-based additive manufacturing with analogous powder injection molding feedstock is an extremely practicable method to fabricate cemented carbide parts with complicated shape.

Effect of Mo and C Additions on Eta Phase Evolution of WC-13Co Cemented Carbides

The WC-13Co (wt.%) cemented carbide was prepared by simple pressureless sintering, and the influence of Mo and C additions on the eta evolution and mechanical properties was analyzed by XRD, SEM, EDS, XPS, and Vickers hardness tester. The results show that the addition of Mo has an important influence on the composition, size, and distribution of the eta phase and Mo2C phase. When the Mo content increases from 0 to 2.5%, the Mo-enriched eta phase grows abnormally and the area fraction of the eta phase significantly increases to 40%, leading to an obvious increase in hardness from 1232 HV30 to 1321 HV30, and a decrease in fracture toughness from 12.5 MPa·m1/2 to 9.8 MPa·m1/2. The addition of carbon black effectively inhibits the formation of the eta phase in the samples with 2.5% Mo. Moreover, adding Mo can suppress WC coarsening in a high-carbon content, which is different from the obvious growth of WC grains in a high-carbon environment in traditional research. Finally, the mechanism of eta phase evolution during the sintering process of WC-Co cemented carbides containing Mo is discussed systematically.

Phase transformations in WC-Cr3C2-Ni thermal spray coatings under low, medium and high thermal input: Part 1 – Feedstock and as-sprayed coating characterisation

Conventionally, bulk WC and Cr3C2-based carbide compositions have been used independently of each other. However, recent investigations have begun to explore combining these carbides together within the same composite/hard metal coating system. This work explored the diversity of phases produced in the coatings from a 42 wt% WC-42 wt% Cr3C2–16 wt% Ni powder sprayed under “low”, “medium” and “high” thermal input conditions using high velocity oxygen-fuel (HVOF), an argon‑helium (ArHe) plasma and an argon‑hydrogen (ArH2) plasma respectively. During spraying, the Ni binder melted, leading to variations in the extent of carbide dissolution and peritectic decomposition as a function of thermal input. Low thermal input, typified by the HVOF technique, led to preferential dissolution/decomposition of the Cr3C2 grains into the melt, while retaining more of the higher melting point WC grains. The coating was dominated by a supersaturated Ni phase, along with the retained carbides. High thermal input, typified by the ArH2 plasma, was postulated to exceed the peritectic decomposition temperature of WC. The coating consisted of a supersaturated Ni alloy, along with the metastable phases WC1-X and β-W2C. No metastable Cr-carbides formed in any of the coatings. The mechanism of coating phase formation is discussed.

Cohesive failure and film adhesion of PVD coating: Cemented carbide substrate phase effect and its micro-mechanism

For a coated cutting tool, film adhesion and cohesive failure resistance are two equally important technical parameters. To systematically investigate the effect of the hard phase and binder phase on the two parameters, WC–Co cemented carbides with varying average WC grain sizes and Co contents were used as the substrates. A PVD-Ti0.94Si0.06N/TiAlSiN/Al0.52Ti0.48N multilayer coating was deposited on the substrates. The results of quantitative single-point scratch testing show that there is a positive correlation between the film adhesion and the substrate hardness. Nevertheless, at the condition of Co ≤ 6 wt% in the substrates, the cohesive failure resistance declines substantially. The results of high-resolution transmission electron microscopy observation and analysis disclose the micro-strengthening mechanism as follows: (1) the WC hard phase contributes to the enhancement of the film adhesion through the formation of coherent/semi-coherent WC/film interfaces; (2) the binder phase contributes to the enhancement of the cohesive failure resistance through plastic deformation constrained by WC hard phase, fcc to hcp martensitic transformation and formation of disordered atomic layer, which consume the strain energy of the coating. Nevertheless, due to the formation of conical convergence points by grain merging growth of the film on the binder phase, which can lead to stress concentration in the film, there is a suitable range of the cobalt mass fraction. The selection criteria for the substrates of PVD coatings are discussed.

Improved comprehensive mechanical properties of oscillatory pressure sintered WC–ZrO2–Al2O3 ceramics with VC/Cr3C2 addition

To optimize the microstructure and comprehensive mechanical properties of WC-based ceramics, the addition of grain growth inhibitors was an important and effective approach. The effect of VC/Cr3C2 addition on the densification behavior, microstructural evolution, comprehensive mechanical properties of WC-based ceramics fabricated by oscillatory pressure sintering was investigated in this work. The introduction of VC/Cr3C2 significantly controlled the size of WC grains and regulated the distributed uniformity of ZrO2 and Al2O3 grains in the WC matrix. When the added contents of VC and Cr3C2 were 0.2 wt% and 0.8 wt% respectively, WC-based ceramics with the best comprehensive mechanical properties were achieved, in which their Vickers hardness, fracture toughness and bending strength were up to 29.15 GPa, 8.37 MPa m1/2 and 1513 MPa, respectively. Furthermore, the enhancement of mechanical properties was mainly attributed to the grain refining strengthening, the transformation of fracture modes and dislocation toughening.

Manipulation of the thermal conductivity for two-phase WC-Ni composites through a microstructure-based model along with key experiments

Thermal conductivity, one of critical properties for composite materials, constitutes an indispensable part of high-efficient materials design. However, a quantitative model to describe the thermal conductivity of two-phase composites has yet to be developed. In this work, we proposed an effective evaluation framework of a microstructure-based thermal conductivity model with key inputs from CALPHAD-type approach. Two-phase WC-Ni composites with Ni–W solid-solution matrix were used as the target system to validate this framework combined with key thermal conductivity measurements. Calculated thermal conductivities agree well with experimental results within the experimental error of 10%. Applying this well-established framework, thermal conductivities depending on wide ranges of temperature, WC grain size (dWC), and phase constitution are predicted, suggesting some unique features. The thermal conductivity becomes less dependent on dWC when dWC is >3 μm. Additionally, the thermal conductivity of nano-sized WC-Ni shows a quite different temperature-dependent variation tendency in contrast to that of micron-sized WC-Ni. These correlations are found and quantitatively described in the present work. Furthermore, the thermal conductivity for WC-Ni carbide rolls is predicted by the established framework, demonstrating a high accuracy. The present work is expected to guide the design of composite materials with desirable thermal properties efficiently.

Co segregation on WC grain formed in an interfacial reaction between WC–Co cemented carbide and Ag–Cu eutectic brazing material

The interfacial structure between Cu and WC–Co cemented carbide after Ag–Cu vacuum brazing was evaluated. No Co was present on the surface of the WC grains before brazing. The vacuum-brazed specimen was heated at 850 °C for 0.5 h, and Ag and Cu were found to penetrate the grain boundary phase, which initially consisted of Co, over a distance of about 1 μm from the surface of the WC–Co cemented carbide. A Co-containing segregated layer with a thickness of about 2–7 nm formed at both the Cu–rich phase/WC grain interface and the Ag–rich phase/WC grain interface. In addition, a Cu-containing segregated layer with a thickness of about 2 nm formed on the Ag–rich phase side against the Co-containing segregated layer at the Ag–rich phase/WC grain interface. This suggests that the formation of the Co-containing segregated layer originated from the local interfacial reaction between the Co dissolved in molten Ag–Cu phase and the WC grains during brazing.

Additive manufacturing of cemented carbides inserts with high mechanical performance

The WC-Co cemented carbides with near fully-dense and crack-free microstructure were fabricated by the laser powder bed fusion (LPBF) process and a subsequent heat-treatment. The microstructures and mechanical properties of the as-printed and heat-treated cemented carbide specimens were investigated in detail. The formation mechanisms and control method of the microstructural defects in LPBF printed cemented carbides were proposed. A number of particles of 2.5 nm-2.0 μm formed in WC grains after heat-treatment. The nano-sized particles have a coherent interface with the WC matrix. The formation mechanisms of the particles and their effect on the enhancement of mechanical properties of the cemented carbides were demonstrated. Furthermore, the cutting performance of the LPBF printed cemented carbides inserts was tested under actual working conditions, which exhibited excellent service properties and promising application prospects.

Corrosion Behaviour of Cemented Carbides with Co- and Ni-Alloy Binders in the Presence of Abrasion

More and more often, cemented carbides are employed for the production of wear resistant components and have to face highly demanding service conditions that combine different damage mechanisms. A key example is the range of tetraphasic (sea water, sand, liquid and gaseous hydrocarbons) flows encountered in the Oil and Gas extraction industry. Notwithstanding the importance of operating regimes of this type, the availability of fundamental and quantitative information on the corrosion performance of cemented carbides in the presence of abrasion is still limited. In this paper, we report a systematic study of the corrosion behaviour of cemented tungsten carbide grades with binders containing different amounts of cobalt (Co), nickel (Ni), chromium (Cr) and noble metal additions, subjected to controlled mechanical abrasion, impacting the stability and nature of pseudopassivation films. In this work, special attention is devoted to Cr, a classical of additive that inhibits the Ostwald ripening of tungsten carbide (WC) particles and notably improves the corrosion resistance of grades with ultrafine-to-fine WC grain size and low-to-medium binder content. We assessed the impact of binder composition on the anodic behaviour by means of linear-sweep voltammetry and chronopotentiometry as well as on the mechanical properties. The application of controlled abrasion conditions under electrochemical control is carried out with an in-house modified ASTM B611 apparatus, equipped with a three-electrode system, enabling the systematic investigation of the synergy of electrochemical and mechanical damaging conditions. Increased corrosion resistance in environments without and with added chloride—both in the absence and in the presence of abrasion—was observed in all the Co- and Ni-based grades to which growing quantities of Cr were added. Moreover, doping with ruthenium (Ru) further enhances corrosion resistance. Regarding corrosion in the presence of abrasion, the addition of Cr and Ru increases the ability of regenerating the pseudopassivation film. The optimized compositions of the binder have been highlighted that open up attractive opportunities of improved service behaviour and deployment in new applications.

High-pressure synthesis of tungsten carbide–cubic boron nitride (WC–cBN) composites: Effect of cBN particle size and volume fraction on their microstructure and properties

WC-cBN composites without any additives were synthesized by High Pressure-High Temperature (HPHT) method. The effect of sintering temperature, cBN content, and particle size on the densification behavior, microstructure, and mechanical properties of WC-cBN composites were investigated. It was found that the low sinterability of cBN particles resulted in poor bonding between WC grains and difficulty in forming grain boundaries, leading to difficulties in synthesizing fully dense WC-cBN composites at a pressure of 5.5 GPa. Under high pressure conditions, the phase transformation from cBN to the low-hardness hexagonal hBN was inhibited. The low sintering temperature and the inhibition effect of WC grain growth by the large cBN particles can deteriorate the hardness and fracture toughness of the composite. However, the hardness and toughness of the WC-cBN composites increased significantly with increasing cBN content. Excellent mechanical properties can be obtained by sintering composites containing cBN with small particle size and large content at high temperature with the hardness of 32.8 GPa and fracture toughness of 9.95 MP·m1/2.

Effects of WC grain size on surface hardening of WC-10Co cemented carbides by pulsed electron beam irradiation

Effects of primary WC size (DWC) in the surface enhancing of WC-Co cemented carbides by high current pulsed electron beam irradiation (HCPEBI) were studied in this work by investigating the morphological/microstructural evolution, phase transformations and the variation of microhardness of samples (Co: 10 wt %, DWC: ∼1.27 μm, ∼0.87 μm and ∼0.42 μm) treated with a range of HCPEBI shots under the energy density of 6 J/cm2. Results showed, with accumulating HCPEBI, the surface roughness of coarse-grained samples increased more significantly. The effects of DWC were more prominent in phase transformation and microstructure change under lesser irradiation. For samples with larger DWC, although the decomposition rate of WC in the top surface region was the lower, the amount of graphite precipitates prevailed overall. The maximum microhardness was also found in the irradiated coarse-grained (DWC = ∼1.27 μm) samples 3128.5 HV, showing a ∼80% increase from the initial state ∼1735.8 HV. Whereas for the fine-grained samples, surface microhardness gained a mere ∼29% increase after 35 shots.

Effects of nano WC and Cr3C2 on grain growth, microstructure and mechanical properties of ultra-coarse cemented carbides

Ultra-coarse cemented carbide is widely used in construction and mining fields, but it is difficult to achieve good coordination between WC grain size and mechanical properties by traditional technology. In this paper, ultra-coarse cemented carbide was successfully prepared by the synergies of nano-powder activation and Cr3C2 grain growth inhibition. The specific effects of nano WC powder and Cr3C2 on WC grain growth at various stages and the microstructure and performances for ultra-coarse cemented carbide were investigated. The result shows that nano WC powder had no significant effect at the solid phase sintering stage, but could promote the WC grain growth at the liquid phase sintering stage. Cr3C2 was able to inhibit the WC grain growth at both the solid phase and liquid phase sintering stages, the WC grain size distribution of the alloy was more uniform. But excessive Cr3C2 would lead to the appearance of pores, weakening the mechanical properties of the alloy. The most suitable addition amount of Cr3C2 was 0.5 wt%. The simultaneous addition of nano WC and Cr3C2 guaranteed the ultra-coarse cemented carbide with uniform microstructure and excellent mechanical properties, its TRS, hardness, and average WC grain size were 2753 MPa, 1047 kg/mm2, and 6.63 μm.

Effect of Substrates Characteristics on Tribological Behaviors of AlTiN-Based Coated WC–Co Cemented Carbides

The wear resistance of coated tools is a key technical parameter, which is indirectly affected by the substrate phase characteristics. WC–Co cemented carbides with varied WC grain sizes (0.4, 0.7, 1.2 μm) and Co contents (3, 6, 10, 12 wt.%) were used as the substrates. Single-layer Al0.52Ti0.48N and multilayer Ti0.89Si0.11N/TiAlSiN/Al0.52Ti0.48N films were deposited on the substrates by DC magnetron sputtering. Reciprocating friction tests were carried out in the air medium and the 3.5 wt.% NaCl aqueous solution, respectively. In the air medium, the films on the fine and the submicron WC–Co substrates with the weaker carrying capacity became worn through earlier than those on the ultrafine substrates. In the NaCl solution medium, for the ultrafine-grained WC–10Co substrates with different Co contents, the friction coefficients (FS) of the film had a linear negative correlation with the hardness (HS) of the substrates. With the decrease in the WC grain sizes or the Co contents, the wear rates of the coated alloys decreased gradually (by 19.7% and 34.5%, respectively). The regular patterns obtained can provide a reference for the selection and design of the phase composition of the cemented carbide substrates.

Coarse cemented carbide produced via binder jetting 3D printing

Until recently, cemented carbide produced via Binder Jetting 3D Printing (BJ3DP) resulted in microstructures with mixed WC grain size. Those materials exhibit excellent hardness and wear resistance but have transverse rupture strength that is lower than the strength of cemented carbide with a uniform grain size. The present investigation details the development of BJ3DP coarse tungsten carbide (WC-12%Co and WC-10%Co) that displays a sintered microstructure with uniform WC grain size. The free flowable (Carney flow <12 s/200 g) spherical powders exhibit very good printability and can be sintered to full density under standard sinter-HIP conditions (temperature 1460 °C, pressure 50 bar). The sintered mechanical properties compare well against cemented carbides produced by powder metallurgy. Vickers hardness, transverse rupture strength, and fracture toughness (determined by the Palmquist method) are in the range HV30 1050–1119, 2231–2684 MPa, and 18.8–19.4 MPa·m1/2, respectively. The wear properties were evaluated under abrasion and erosion using the ASTM B611 and ASTM G65 standard testing procedures. The wear resistance of the coarse BJ3DP cemented carbides compares well against conventionally produced cemented carbide. Parts over a wide range of sizes (from <1 g to 9 kg) and geometric complexities have been successfully printed and sintered with the new materials.