Plate-like WC Grains Research Articles

Comparative analysis of the tool life of submicron hard alloy WC-10Co sintered from powder obtained by electro discharge in oil

In this paper, comparative studies of the microstructure, mechanical characteristics and service life of a cutting insert made of submicron cemented carbide WC-10Co, obtained as a result of recycling VK10 cemented carbide by Electro Discharge Erosion (EDE) in oil, were carried out. The specific energy consumption directly for the formation of pulses in the process of EDE is to a relatively small value (5.7 kW·h/kg). Excess carbon formed as a result of oil pyrolysis during EDE was removed by heat treatment. The granulometric composition of the obtained powder and the microstructure of the particles were studied. It is shown that the particles of the resulting powder consist of plate-like WC grains with an average diameter of 0.46 pm and interlayers of cobalt. The WC-10Co cutting insert, obtained by sintering this powder in vacuum, was used for a comparative analysis of service life during fine turning of aluminum alloy D16T. As objects for comparative analysis, cutters equipped with blades made of industrial alloys VK8 and VK6OM of a similar design were used. The microstructure and mechanical characteristics of the experimental alloy and the analogues presented have been studied. On the basis of studies of the microstructure and chemical composition of the back surface of the cutting inserts, the wear mechanism of the cutters was analyzed. It is shown that the hardness of the alloys has the main effect on their wear resistance when cutting an aluminum alloy with the presented cutters. The influence of the hardness of the cutters on the surface roughness of the resulting part was also studied. The obtained submicron cemented carbide WC-10Co exhibits the highest hardness (1590 HV) and wear resistance of the presented samples due to the smallest WC grain diameter (0.59 pm).

Preparation of powder by electrical discharge erosion and sintering of ultrafine WC-5TiC-10Co alloy with high hardness

In this work, studies were carried out on obtaining a powder by electrical discharge erosion (EDE) of a WC-5TiC-10Co cemented carbide, sintering an ultrafine-grained alloy from the obtained powder, and studying the microstructures and properties of the obtained particles and alloys. We have established that with an increase in the pulse energy from 0.5 J to 10 J, the EDE productivity increases linearly. At the same time, energy consumption decreases and tends towards a constant value (3.4–4.2 kWt*h/kg). It has been established that an undesirable change in the carbon content in the resulting powder from the liquid pyrolysis is proportional to the product of the specific energy consumption and the difference in the mole fractions of carbon and oxygen in the liquid. Excess carbon was removed by heat treatment in the appropriate volume of CO2. It has been established that EDE results in a complete dissolution of tungsten in cubic carbide (Ti,W)C, which is retained in the formed particles. During further heat treatment and sintering, new plate-like ultrafine WC grains grow from a supersaturated (Ti,W)C solution. An increased concentration of tungsten is retained during sintering in ultrafine-grained grains of a supersaturated (Ti,W)C solid solution. By reducing the average diameter of the WC grains from 1.45 μm to 0.42 μm and increasing the carbide content (Ti,W)C, the hardness of the obtained alloys increases from 1410 HV in the original alloy to 1650 HV in the sintered alloy. Fracture toughness increases due to formation of lamellar WC grains from 12.9 mPa*m1/2 to 14.1 MPa*m1/2.

Breaking the hardness–toughness trade-off in WC–Co hardmetals: Introduction of dual-scale plate-like WC and modification of prediction models

The hardness–toughness trade-off relationship is an essential challenge when creating high-performance WC–Co hardmetals. In this work, by combining plasma milling and low-pressure sintering, a dual-scale plate-like WC–8Co hardmetal is fabricated. The hardmetal simultaneously exhibits ultra-high strength (1701 kgf/mm2) and fracture toughness (21.77 MPa*m1/2), with bimodal grain size distributions of plate-like WC with average diameter of 1.1 μm and 1.4 μm, respectively. This breaks the normal trade-off relationship between the strength and the fracture toughness, which is because of the enhancement of crack propagation resistance owing to the synergetic effect of the plate-like WC and the bimodal grain structure. Existing models for hardness and fracture toughness prediction are invalid for WC–Co hardmetals with oriented plate-like WC grains. Modified models, based on the Lee-Gurland hardness model and the Ravichandran fracture toughness model, are developed by inducing factors R, which is the proportion of the WC(0001) plane in the indented plane, and can be determined by XRD analysis, to represent morphology features of WC. The modified models are universal and can reliably predict the hardness and fracture toughness of WC–Co hardmetals with both equiaxial and plate-like WC grains.

Development of a method for producing submicron cemented carbide from a powder obtained by electrical discharge erosion of scrap in oil

The work is devoted to the production of powder by electrical discharge erosion (EDE) in oil from WC-10Co cemented carbide scrap, post processing of the obtained powder and sintering of a submicron alloy from it. As a result of EDE of WC-10Co alloy in oil, a powder with an average particle diameter of 12.7 μm, was obtained. Pyrolysis of the oil prevented the loss of carbon by tungsten carbide and led to an increase in the carbon content of the powder from 5.5% to 8.6%. A technique of heat treatment of the obtained powder in CO2, for the removal of excess carbon (3.1%) was developed. On sintering the powder at 1370 °C, a WC-10Co alloy with a stoichiometric carbon content (5.5%) was obtained, in which plate-like and prismatic WC grains an average diameter of 0.58 μm were formed. The alloy combines high hardness (1520 HV) and fracture toughness (14.6 MPa√m).

Effects of fine WC particle size on the microstructure and mechanical properties of WC-8Co cemented carbides with dual-scale and dual-morphology WC grains

Dual-scale and dual-morphology WC grained WC-8Co cemented carbides comprising triangular or hexagonal fine WC grains and plate-like coarse WC grains were synthesized by vacuum sintering using Co, flaky graphite, WC, and coarse W as the starting materials. The effects of fine WC particle sizes on microstructure, relative densities, and mechanical properties of the dual-scale and dual-morphology WC grained cemented carbides were investigated. The results revealed that the growth of plate-like coarse WC grains was further promoted with the decrease in the particle size of the added fine WC; hence, their aspect ratio increased. In addition, added fine WC led to the separation of plate-like coarse WC grains so as to break their oriented arrangement and prevent their face contact; hence, plate-like coarse WC grains were completely covered by the Co binder phase. Moreover, the addition of smaller particle size of fine WC contributed to more uniform Co binder phase. When 0.4-μm WC powders was added, the aspect ratio of plate-like coarse WC grains was greater than that of plate-like WC grained cemented carbides without the addition of fine WC. The dual-scale and dual-morphology WC grained cemented carbides by adding 0.4-μm fine WC exhibited good comprehensive mechanical properties, with a transverse rupture strength of 3645 MPa, a Rockwell hardness of 91.5 HRA, and a fracture toughness of 12.3 MPa∙m1/2.

Fabrication of Functionally Graded WC–Co Cemented Carbides with Plate-Like WC Grains

Functionally graded WC–Co cemented carbides (FG WC–Co) with plate-like WC grains were fabricated by carburizing of cemented carbides with no η phase. The phase composition and microstructure of FG WC–Co were investigated by XRD, SEM and EDX. The results showed that a cobalt-deficient layer was formed due to difference in affinity among carbon and other alloying elements. WC grain size increased while the ratio of side length to thickness decreased slightly in the surface zone due to difference of growth rate between different planes of WC grains. Moreover, FG WC–Co possessed an 80 μm deep cobalt-deficient layer, and showed excellent mechanical properties with a surface hardness of 1748 HV and fracture toughness of 9.6 MPa · m1/2 in the surface zone.

Effect of carbon content on microstructure and mechanical properties of WC-10Co cemented carbides with plate-like WC grain

Four sets of WC-10Co cemented carbides with different carbon content were prepared by adding the ultrafine WC powders as seeds during the in-situ sintering reaction among W, Co and C. The effect of carbon content on microstructure and mechanical properties were studied. The results show that the microstructure, phase composition and mechanical properties of WC-10Co cemented carbides with plate-like WC grains were seriously affected by the carbon content. The fast growth of WC grains with high carbon content could proceed the prismatic plane preferentially along the <1 0 1(−) 0> directions, resulting in the high content of plate-like WC grains. The density increased with the increment of the carbon content and reached the maximum value, then, followed by a decline. The hardness and the transverse rupture strength of the alloy in the two-phase zone with carbon content of 5.91 wt% reached the maximum value. The existence of plate-like WC grains could impede the propagation of the cracks due to the decrease of the weakest carbide regions and the increase of the basal facets of broken WC crystals. In this case, more fracture energy was required to crack propagation and further improved the transverse rupture strength. Additionally, the plate-like WC was benefit to reduce the wear volume and bring about a better wear resistance. Thus, the alloy with the appropriate proportion of carbon content can obtain higher mechanical properties and wear resistance.

Effect of particle size and high-energy ball milling time on microstructure and mechanical properties of WC-10Co cemented carbides with plate-like WC grains

The effect of particle size and high-energy ball milling time on microstructure and mechanical properties of WC-10Co cemented carbides with plate-like WC grains were studied using a scanning electron microscope, x-ray diffraction and mechanical properties tests. The results show that coarsening W particle size seemed to be beneficial for the increase of the plate-like WC content due to the easy acquirement of the thin W flake with high-density defects. Additionally, increasing the ball milling time could first increase the content of plate-like WC grains, and then decrease with 96 h ball milling. The high amount of defects and activity of fine W particles from fracturing with long ball milling time could enhance the formation of plate-like WC grains. However, the oxygen content could be increased with the excess ball milling time, which reduced the content of liquid Co and thus impeded the formation of plate-like WC grains. As the content of plate-like WC grains increased, the intergranular fracture gradually transformed into the main fracture mode. Additionally, crack bridging and crack path deflection could be seen in the alloy with plate-like WC grains. The high fracture surface area and energy required for crack propagation could improve the strength of WC-10Co cemented carbides with plate-like WC.

Fabricating Ultrathin Plate-Like WC Grains in WC–8Co Hardmetals by Increasing Discharge Intensity During Plasma-Assisted Ball Milling

The effects of plasma discharge intensity on the microstructure evolution of ball-milled tungsten (W)–carbon (C)–cobalt (Co) mixtures and the formation mechanism of ultrathin plate-like tungsten carbide (WC) grains prepared by ball milling with and without plasma discharge were investigated. It was found that increasing the plasma discharge intensity during ball milling obviously promoted the formation of a thin flake-like W phase because of the electroplasticity effect and simultaneously lowered the carburization temperature between W and C. A combination of high hardness and transverse rupture strength of 92.9 HRA and 3659 MPa, respectively, was obtained for the WC–8Co alloy fabricated by plasma milling at a gas pressure of 5 × 103 Pa with a dielectric barrier discharge layer thickness of 3 mm. These properties were mainly attributed to the markedly lowered activation energy of the WC phase and generation of highly oriented ultrathin plate-like WC grains by plasma milling. The combination of the flake-like structure of the plasma-milled W aggregate and high specific interfacial area and short diffusion distance of W/C were readily inherited by the ultrathin plate-like WC grains in the sintered WC–Co hardmetals.

Fabrication and mechanical properties of WC-10Co cemented carbides with plate-like WC grains

WC-10Co cemented carbides with plate-like WC grains were fabricated using ultrafine WC as seeds via the reaction among tungsten, cobalt and carbon. The effects of ultrafine WC contents on the microstructural characteristics and mechanical properties were investigated. Because the density of W is higher than that of complex carbide particles (CoxWyCz), some cracks could be existed in the thin flake W particles owing to the volume expansion, bringing about the formation of flake WC nuclei. Some flake WC nuclei aggregates could be seen in the alloys without the addition of ultrafine WC. The addition of ultrafine WC could bring about the gradually disappearance of flake WC nuclei aggregates and accelerate the formation of plate-like WC grains. The high content of plate-like WC could bring about high hardness. Furthermore, the palte-like WC grains could efficiently hinder the propagation of the cracks as the crack path deflection and crack bridging, and inevitably improve the transverse rupture strength.

Influence of Cu shell on the anomalous WC grain growth in Ni-base brazed cladding

In vacuum infiltration of Ni-based self-fluxing alloy (NiCrBSi) cladding, reinforced with ultrafine WC particles with average particle size (APS) of 0.2–0.4 μm, several plate-like anomalous WC grains were observed with size of greater than 20 μm. To prevent this anomalous grain growth, Cu shells were coated on WC particles. The very low solid state solubility of WC in copper phase at high temperature can lead to no anomalous WC grain growth. Cu shell with a thickness about 30–50 nm was deposited by means of electroless plating on ultrafine WC particles. The microstructural features of Cu-coated carbides and the resultant brazed claddings were characterized by field emission scanning electron microscopy (FESEM) equipped with energy dispersive X-ray spectroscopy (EDS). The evaluation of the brazed cladding revealed a thick ultrafine grained-microstructure with homogeneous grain size distribution, in contrast to the bare WC-based cermet cladding. The hardness of this novel composite cladding was nearly increased up to 1500 HV, which was 40% more than that of the un-coated WC reinforced cladding. The results demonstrated that the resistance against crack initiation and propagation considerably improved by using Cu-coated WC particles in the sintered media.

Achieving combination of high hardness and toughness for WC-8Co hardmetals by creating dual scale structured plate-like WC

The dual scale structured WC-8Co hardmetals with plate-like WC grains were fabricated by sintering the mixed W-C-Co elemental composite powders prepared by plasma milling. The SEM and XRD were used to characterize the microstructure, the degree of the preferential orientation of plate-like WC and the morphology of the sintered WC-8Co hardmetals. The mechanical properties, including hardness, toughness and transverse rupture strength (TRS) of WC-8Co were measured and investigated. The result shows that by creating the dual scale plate-like WC structure, the TRS and fracture toughness of WC-8Co hardmetal can be increased distinctly while maintaining high hardness, reached to about 4000MPa, 21.56MPam1/2 and 1733kgf/mm2, respectively. Through investigating the cracks propagation, the toughening mechanism is proposed. It is suggested that the dual scale structure of plate-like WC not only triggers lots of crack deflection, crack bridging and crack branching, but also increases the amount of occurrence of the fracture through plate-like WC. This synergetic effect of dual scale structure and the plate-like WC morphology increase strongly the resistance to crack propagation.

Achieving high transverse rupture strength of WC-8Co hardmetals through forming plate-like WC grains by plasma assisted milling

WC-8Co hardmetals with prismatic WC grains and plate-like WC grains were respectively produced through sintering the W-C-8Co elemental powder mixture, prepared by dielectric barrier discharge plasma assisted milling (denoted as DBDP-milling). The results show that the microstructure, hardness and transverse rupture strength (TRS) of the sintered hardmetals are related to the morphology and size of milled W, which is dependent on the DBDP-milling time. When using W of different particle sizes as raw materials, prismatic WC grains of corresponded different size tend to form in the DBDP-1h milled W-C-Co powders contain granular W particles. While the DBDP-3h milled powders contains flake-like W flakelets that generate plate-like WC grains with different degree of preferential orientation when the size of raw W materials are different. Compared with triangular prismatic WC, a suitable range of the degree of preferential orientated plate-like WC in hardmetals has the advantage for improving the TRS while maintaining excellent hardness.

Effect of Mo2C addition on the microstructures and mechanical properties of WC–SiC ceramics

Mo2C was added to WC–4.85mol% SiC ceramics and sintered at 1600°C using a resistance-heated hot-pressing machine. Dense ceramics containing 0–3mol% Mo2C were obtained. The reaction products (W, Mo)5Si3 and (W, Mo)Si2 were produced during sintering. The relative amounts of (W, Mo)Si2 decreased and (W, Mo)5Si3 increased with increasing Mo2C amount. The abnormally-grown plate-like WC grains formed for WC–4.85mol% SiC ceramics below 1.5mol% Mo2C. Above 2mol% Mo2C, the ceramics had a fine, equiaxed granular structure. The average WC grain size decreased from 0.77μm without Mo2C, to 0.50μm above 2mol% Mo2C. The Vickers hardness increased with increasing Mo2C amount, reaching 21.4GPa at 2mol% Mo2C. The hardness of the WC–4.85mol% SiC ceramics correlated with the inverse square root of the WC grain size. The fracture toughness for the Mo2C-added WC–4.85mol% SiC ceramics ranged from 7.1 to 7.8MPam0.5.

Properties of WC–8Co hardmetals with plate-like WC grains prepared by plasma-assisted milling

WC–8Co hardmetals with different proportions of prismatic WC grains and plate-like WC grains were directly produced through sintering the W–C–8Co elemental powder mixture which was fabricated by dielectric barrier discharge plasma (DBDP)-assisted milling. The morphology of prepared WC–8Co hardmetals, geometry and the preferential orientation of plate-like WC were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis. The results demonstrate that the microstructure and mechanical properties of the sintered hardmetals are related to the morphology of W grain which is dependent on DBDP-milling time. The DBDP for 1 h (DBDP-1 h)-milled W–C–Co powder contains granular W particles that tend to form prismatic WC grains, while the DBDP for 3 h (DBDP-3 h)-milled powder contains lamellar W particles that generate plate-like WC grains. By adjusting the weight ratio of DBDP-1 h powder and DBDP-3 h powder in W–C–8Co mixture, the proportion of plate-like WC in the hardmetals can be controlled, and relatively high transverse rupture strength (TRS) is obtained as the proportion of plate-like WC grain in the hardmetals is about 35 % in present experimental condition. The DBDP-1 h-milled W–C–8Co powder (Sample A) contains granular W particles that tend to form prismatic WC grains, while the DBDP-3 h-milled powder (Sample B) contains lamellar W particles that generate plate-like WC grains. When the weight ratio of Samples A to B powder is 1:1, Sample A1B1 has relatively high transverse rupture strength (TRS) along both Sections V and P, with proportion of plate-like WC grain of ~35 %.

Abrasion wear behavior of WC–10Ni3Al cermet with plate-like triangular prismatic WC grains

The abrasion wear behavior of WC–10Ni3Al cermet with plate-like triangular prismatic WC grains has been evaluated using a ball-on-flat tribometer. WC–8Co has also been tested under the same conditions for the purpose of comparison. The results showed that WC–10Ni3Al has lower friction coefficient and wear loss than WC–8Co under the same contact load. Comparing with WC–8Co, neither binder phase extrusion nor surface delamination is observed on the worn surface of WC–10Ni3Al under the examined loads. The higher abrasion wear resistance of WC–10Ni3Al is attributed to the synergistic mechanisms of its high hardness and sub-surface crack resistance induced by the intermetallic alloy Ni3Al binder and the existence of a large amount of plate-like triangular prismatic WC grains.

Growth Behavior of WC Grains in Ni&lt;sub&gt;3&lt;/sub&gt;Al Matrix during Liquid Phase Sintering

Growth behavior of WC grains with two different sizes of 10 μm and 4 μm in the same liquid matrix has been studied in the WC–40vol.%Ni3Al system. The samples were sintered under a carbon saturated condition. Surfaces of the coarse WC grains are strongly micro-faceted and stepped. With the increase of sintering time, the average size of the fine WC grain increases continuously and very large plate-like triangular prismatic WC grains appear. The present investigation thus shows that the growth behavior of fine WC grain is affected by lower solubility of W in Ni3Al matrix and the abnormal growth of WC grains is suppressed. However, the growth behavior of coarse WC grain is governed only by the difference in energy between the facets.

Effects of carbide grain growth inhibitors on the microstructures and mechanical properties of WC–SiC–Mo2C hard ceramics

Cr3C2, NbC, or ZrC was added to WC–4.85mol% SiC–1mol% Mo2C ceramics and sintered at 1600°C using a resistance-heated hot-pressing machine. Dense ceramics containing 0–0.3mol% Cr3C2, 0–3mol% NbC, and 0–10mol% ZrC were obtained. An additive carbide-based solid solution was observed only for NbC and ZrC additions. The coarse plate-like WC grains disappeared with the addition of Cr3C2, NbC, or ZrC and the ceramics consisted of fine equiaxed WC grains. Higher hardnesses, 19.4–21.8GPa, were measured for the carbide-added WC–SiC–Mo2C ceramics than for the WC–4.85mol% SiC ceramics because of the disappearance of coarse plate-like grains. The addition of a small amount of ZrC improved the fracture toughness without reducing the hardness.

Preparation and mechanical properties of WC-10 Ni3Al cemented carbides with plate-like triangular prismatic WC grains

The elemental powder mixtures of Ni, Al, Fe, Cr, Zr and B in the proportion Ni-10.50 Al-10.90 Fe-8.12 Cr-0.89 Zr-0.21 B(wt.%) were synthesized into Ni3Al by high-energy ball milling. The alloyed Ni3Al powders were then added to WC powders with a nominal stoichiometry of WC-10wt.% Ni3Al and underwent further milling. Finally the milled WC-10wt.% Ni3Al powders were fabricated into unusual alloys with obviously oriented plate-like triangular prismatic WC grains by spark plasma sintering (SPS) in the sintering temperature range of 1300–1400°C. The effect of sintering temperature on the microstructures and mechanical properties was also investigated. The results indicate that a proper temperature of 1350°C will help to obtain obviously oriented plate-like triangular prismatic WC grains with a high axial ratio. The obtained bulk WC-10wt.% Ni3Al possesses a high hardness (HV10), transverse rupture strength and fracture toughness of 17.76GPa, 2092MPa, 21.56MPam1/2, respectively.

Hardening and Toughening Mechanisms of Cemented Carbides with Plate-Like WC Grains Prepared by Seeding

WC-10Co (mass pct.) cemented carbides containing plate-like WC grains were prepared by using plate-like WC single crystals as seeds. With the increase of seed addition amount, more and bigger plate-like WC grains are formed, the hardness and transverse rupture strength (TRS) increase first and then decrease. When 2.5 mass% seeds are added in WC-10Co, the density is hardly affected, whereas the hardness and TRS increase simultaneously, especially TRS increases by 12.8%. The hardening and toughening mechanisms of WC-10Co cemented carbides with plate-like WC grains were discussed.