WC-Co Powder Research Articles

Influence of heat treatment on properties of SD251-PH1 composite produced by additive SLM technology

This study evaluates how heat treatment affects the structure and properties of the engineered composite material SD251-PH1. This material was formed by mixing two powder blends in a weight ratio of 90 wt. % WC-Co powder SD251 with 10 wt. % of a PH1 precipitation hardening powder of the stainless steel. Samples were prepared from this mixture on an SLM additive device. The printed samples were then divided into groups. Some of them were left in the initial state and the rest were used for heat treatment based on the standard AMS 5659. Light and electron microscopy metallographic analysis together with X-ray diffraction analysis were then used to evaluate structural and phase changes in the prototype samples. The main attention was paid to changes in the phase composition of the samples. In addition, changes in the size and shape of the pores and tungsten carbide (WC) grains were studied. The metallographic analysis was supplemented by an evaluation of the changes in their mechanical properties and wear resistance. Vickers hardness measurements and the ball on disc test were used for this purpose. The experiments showed that the selected heat treatment processes precipitated new structural phases which differed from the original sample structure in terms of shape, size, chemical composition, and mechanical properties.

Microstructure and mechanical properties of laser-cladded WC–Co composite coatings on Ti–6Al–4V

WC-Co powder reinforced coatings were prepared on the Ti-6Al-4V substrate for improving the hardness and wear resistance by laser cladding technique. The effects of different laser power and Co content on the microstructure and properties of the cladding layer were studied. The results showed that when the laser power was too low, there were unmelted WC-Co powders on the surface of the cladding layer. In contrast, when the laser power was too high, the coating materials were prone to over-burning and evaporation, cracks and pores were aggravated. The WC-25Co coatings without cracks and pores could be formed when power is set at 1300 W, and its average micro-hardness was higher than other specimens. The addition of Co was helpful to the bonding of clad and Ti-6Al-4V substrate. New hard phases of TiC and CoTi2 were mainly distributed in the cladding coatings. The micro-hardness of the cladding coatings increased by 2–4 times than that of the Ti-6Al-4V substrate, and the maximum hardness reached 1349 HV. Under the same dry sliding wear condition, the wear resistance of the WC-25Co coatings was significantly improved and increased by 15.7–34.1 times compared with the substrate.

Additive manufacturing of WC-13%Co by selective electron beam melting: Achievements and challenges

Additive manufacturing is a powerful tool for rapid prototyping and fabricating metal articles having a complicated geometry. This method is known to be used almost solely for the manufacture of articles consisting of pure metals and alloys. In the present work the possibility of obtaining dense carbide articles by a single-step process of additive manufacturing based on selective electron beam melting was evaluated. A new technology for fabricating cemented carbide granules suitable for selective electron beam melting was developed. It includes conventional granulating WC-Co powders followed by solid-state pre-sintering and preliminary screening of the granules. After that their liquid-phase sintering and final screening are carried out to obtain a desired fraction needed for the additive manufacturing process. Results of experiments on selective electron beam melting at different scan rates and current values indicated that it was possible to obtain non-porous carbide articles of complex geometry from WC-Co granules initially containing 13 wt% Co. The selective electron beam melting process led to the evaporation of some liquid Co and very intense local WC grain growth resulting in peculiar microstructures of the cemented carbide articles comprising layers with medium-coarse and abnormally large WC grains. A near-surface layer of the cemented carbide articles obtained by additive manufacturing is characterized by a high roughness comparable with the mean size of the original WC-Co granules.

PROCESSABILITY OF WC-CO POWDER MIXTURES USING SLM ADDITIVE TECHNOLOGY

This contribution presents the findings from an experimental study which explored the possibility of processing WC-Co-based powders using selective laser melting (SLM) additive technology. Two commercially available WC-Co powder mixtures were studied. One is intended for thermally sprayed coatings and the other for isostatic pressing applications. Both were analysed using optical and scanning electron microscopy and then processed in an additive manufacturing (AM) machine. Process parameters were proposed on the basis of a literature search and metallographic characterization of the mixtures. Metallography was also employed for examining the prototype specimens. The mixtures were found to differ in morphology, technological properties and phase composition, which was also manifested during their processing. The powder mixture for thermally sprayed coatings proved completely inadequate for AM due to undesirable grain morphologies and phase composition. The powder mixture for isostatic pressing was found to be more suitable. Nevertheless, all the builds were highly porous which had an adverse impact on the product properties.

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.

Fabrication and properties of extrusion-based 3D-printed hardmetal and cermet components

Hardmetal and cermet bodies were printed by fused-filament fabrication (FFF) and composite-extrusion modelling (CEM) in an SDS (shaping – debinding – sintering) process. For FFF the filaments were prepared from hardmetal (WC-10Co) and cermet powder (Ti(C,N)-Co/Ni-based) and organic binder. The CEM feedstock consisted of WC-Co MIM powder. A 3D filament printer as well as a 3D printer working with a MIM granulate were employed to fabricate printed bodies by FFF and CEM, respectively. The solvent debinding process was performed in cyclohexane (FFF-printed bodies) or water (CEM-printed bodies). Thermal debinding of all parts was performed in a tube furnace up to a temperature of 800 °C. The pre-sintered parts were then subjected to vacuum sintering by application of conventional vacuum sintering profiles up to 1430 °C for hardmetals and up to 1480 °C for cermets. Dimensional and mass changes upon the various preparation steps as well as microstructure and porosity of the sintered bodies were investigated. While the microstructure is practically identical to that of conventionally prepared materials, some cavities were present from the printing process because of yet non-optimised printing strategy. By change of printing strategy the cavities could be minimised or even avoided. The study shows that with the applied 3D extrusion-printing techniques, hardmetal and cermet components with innovative geometries are accessible.

High-temperature oxidation and wear resistance of WC-Co based coatings with WB addition

A new type of WC-based coating with high oxidation- and wear-resistance at elevated temperature was fabricated by thermal spraying the pre-treated WC-Co powder doped with WB. Addition of WB led to in situ formation of WCoB, which acted as a substitute for Co in the powders and the resultant coatings. It was shown by thermal analysis that WCoB has obviously higher oxidation resistance at high temperatures than that of WC and Co. Thus, the oxidation of the WC-WCoB coating was mainly initiated from WC, rather than from Co in the conventional WC-Co coatings. Most of WCoB was preserved in the coating after high-temperature wear tests. Particularly, with an addition of 40 wt.% WB, the wear rates of the WC-Co coating were dramatically decreased by 90% and 77% at the room and elevated temperatures, respectively.

Study of the formation features of hard metal composites structure obtained from bimodal powder mixtures

A new method of forming dispersion-hardening oxide inclusions in the process of thermoreactive synthesis from aluminum nanoparticles introduced into WC-Co powder mixture is proposed. Such multiphase and fragmentarily nanostructured composite is characterized by additional heterogeneity, which is determined by the difference in sizes and elastic properties of the phases. In the structure of WC-Al2O3-Co composites a “barrier” effect is provided, the length of the carbide grains contacts is reduced. Carbide grains are separated by a thin cobalt layer with increased strength. In accordance with the results of numerical estimates using finite element analysis (FEA) method, the maximum stress intensity in tungsten carbide is 9.1 GPa (occurring at an external tension of 3 GPa), which is 30% less than the maximum stress intensity in the basic material. The strength improvement predicted by the FEA method in fragmented nanostructured hardmetal composite correlates with experimental results of measurements of fracture toughness according to Palmqvist data. In case of the optimal amount of additives, the increase in crack resistance is equal to 50% compared with the basic material.

Structure and Mechanical Properties of HVOF Sprayed (WC-Co+Co) Composite Coating on Ductile Cast Iron

An investigation was conducted to determine the role of Co particles in the WC-Co coating produced with the High Velocity Oxygen-Fuel (HVOF) spraying on microstructure, mechanical and wear properties in a system of type: WC-Co coating/ductlile cast iron. The microstructure of the thermal sprayed WC-Co+Co coating was characterized by scanning electron (SEM) and transmission electron (TEM) microscopes as well as the analysis of chemical and phase composition in microareas (EDS, XRD). For analysis of the quality and adhesion of coatings, the scratch–test was applied. It was found that as a result of the HVOF spray of WC-Co powders with the addition of Co particles, the coatings of low porosity, high hardness, a very good adhesion to the substrate, compact structure with partially molten Co particles and finely fragmented WC particles embedded in a cobalt matrix, coming to the size of nanocrystalline sizes were obtained. Moreover, the results were discussed in reference to examination of bending strength considering cracking and delamination in the system of (WC-Co+Co)/ductile cast iron as well as hardness and wear resistance of the coating. It was found that the addition of Co particles was significantly increase resistance to cracking and wear behaviour in the studied system.

Laser texturing as an alternative to grit blasting for improved coating adhesion on AZ91D magnesium alloy

ABSTRACTSubstrate preparation plays an important role in the performance of thermal spray coating, especially on softer materials like magnesium and aluminium alloys. Conventional substrate preparation methods such as grit blasting may not be the most suitable choice due to grit embedding, lower coating adhesion strength and environmental concerns. Laser texturing can be an attractive alternative to the grit blasting method for such materials. AZ91D substrate was prepared for thermal spray coating using grit blasting and laser texturing techniques. WC-12Co powder was thermally sprayed on AZ91D magnesium alloy using the high-velocity oxygen fuel technique. The adhesion strength of the coating, thus produced, was determined using the ASTM 633C adhesion strength test. Scanning electron microscopy was used to investigate substrate morphology and to qualitatively analyse substrate and coating interface. X-ray diffraction was used to identify phase compositions. The coating was characterised for roughness, porosity, micro-hardness and fracture toughness. Laser texturing as a substrate preparation technique has been able to produce well-adhered coatings, with adhesion strength of 45.6 MPa, and comparable coating characteristics with those of the grit blasting technique.

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.

High temperature wear resistance and thermal fatigue behavior of Stellite-6/WC coatings produced by laser cladding with Co-coated WC powder

The Stellite-6/WC composite coatings were produced on AISI H13 hot work tool steel by laser cladding with mixture of Co-coated WC (WC-12Co) particles and Stellite-6 powder. The phase composition, microstructural characterization, high temperature wear resistance and thermal fatigue behavior of Stellite-6/WC coatings were investigated and compared with the properties of the coatings produced from mixture of WC particles and Stellite-6 powder. The results showed that using the WC-12Co particles alleviated the decomposition of WC and resulted in the weaker intensity of W2C, CoCx and Co6W6C peaks in the X-Ray Diffraction (XRD) patterns. Compared with using the WC particles directly as the coating material, using the WC-12Co particles could further improve the wear resistance of coatings according to the relative lower width and depth of wear scars at the same WC content. In addition, fewer fatigue cracks were observed on the surface of coatings made by adding WC-12Co particles under the same thermal fatigue conditions, which indicates that using WC-12Co is beneficial to extend the life of Stellite-6/WC coatings.

Electroless nickel–phosphorus plating on WC–Co powders using HVOF feedstock

ABSTRACTIn this study, electroless nickel–phosphorus plating was optimised for WC–Co powder using different bath loads and then subjected to a high-velocity oxygen fuel process. The produced Ni–P-coated WC–12Co powders were subsequently used as HVOF feedstock materials. Based on the SEM results, an increase in the bath load resulted in a decrease in the thickness of the Ni–P coating formed on the WC–12Co powders. Moreover, applying the HVOF process led to a lower porosity and roughness and a higher thickness of nickel coating, whereas applying a suitable thickness of this coating can prevent the destructive W2C phase. Finally, a bath load of 16 g L–1 was determined as the optimum value to achieve a proper thickness of the Ni–P, extremely low porosity (0.3%), good surface roughness (5 μm), and high microhardness (1112 Hv) in the produced coating.

Mechanical, Anticorrosion, and Tribological Properties of Nanostructured WC-Co/Cr3C2-NiCr Multilayered Graded Coating on Aluminum Substrate

This paper examines the application of multilayered hard coatings on aluminum substrates to improve their mechanical, anticorrosion, and tribological properties. In the present study, graded coatings with alternate layers of chrome carbide–nickel chrome (75:25) and tungsten carbide–cobalt (88:12) are applied on aluminum substrates for improvement in mechanical and tribological properties. High-velocity liquid fuel (HVLF) spray technique is employed to coat the materials on the aluminum substrate. Nanosized Cr3C2-NiCr and WC-Co powders with particle sizes ranging from 300 to 1000 nm are employed for coating on aluminum substrates. The tribological properties are tested on a pin-on-disk tribometer under lubricated conditions. It is found that graded coating with alternate layers of Cr3C2-NiCr and WC-Co have shown good tribological properties. The hardness has improved, with coating strength being in acceptable range for application in automotive engines. The corrosion resistance of multilayered coating is excellent and in line with Cr3C2-NiCr coating. The effects of number of coating layers of graded coating on tribological and mechanical properties are found to be significant.

Consideration of the criteria for successful deposition in cermet kinetic spraying using WC-17Co powder

There have been several studies related to the cermet kinetic spraying, despite its difficulty of proper deposition. However, there is no specific deposition criteria for kinetic spraying of cermet due to the unclearness of deposition mechanism in cermet kinetic spraying. Thus, in this study, the criteria of the successful cermet kinetic spraying was considered using commercial WC-17Co powder by correlating the numerical and experimental results on the basis of the reported phenomena in other previous studies. Especially, in terms of numerical analysis, newly established impact model was applied to deduce thermomechanical response of the supersonic impacted WC-17Co particle. Firstly, alteration in the deposition behavior of WC-17Co powder according to the change of impact velocity was investigated and classified from the results of single particle impact tests. Afterward, by correlating the numerical and experimental results, the deposition criteria of cermet kinetic spraying was considered. As a result, two criteria were derived: high particle impact velocity i) enough to cause a plastic deformation of metallic matrix of WC-17Co particles to such an extent that temperature rise to melting point occurred around the interface region, ii) but not enough to make elastic recovery energy overly dominant than plastic deformation energy. When the practical kinetic spraying of WC-17Co powder was conducted after the choice of process condition compatible to such criteria, it was confirmed that the two deposition criteria mentioned above was fairly effective, although expected conditions were not the most optimized.

Comparative Study of Corrosion Performance of HVOF-Sprayed Coatings Produced Using Conventional and Suspension WC-Co Feedstock

Corrosion properties of nanostructured coatings deposited by suspension high-velocity oxy-fuel (S-HVOF) via an aqueous suspension of milled WC-Co powder were compared with conventional HVOF-sprayed coatings. Microstructural evaluations of these coatings included x-ray diffraction and scanning electron microscopy equipped with an energy-dispersive x-ray spectroscopy. The corrosion performance of AISI440C stainless steel substrate and the coatings was evaluated in a 3.5 wt.% NaCl aqueous solution at ~ 25 °C. The electrochemical properties of the samples were assessed experimentally, employing potentiodynamic polarization and electrochemical impedance spectroscopy. The potentiodynamic polarization results indicated that coatings produced by S-HVOF technique show lower corrosion resistance compared with the coatings produced by HVOF-JK (HVOF Jet Kote) and HVOF-JP (HVOF JP5000) techniques. Results are discussed in terms of corrosion mechanism, Bode and Nyquist plots, as well as equivalent circuit models of the coating–substrate system.

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.

Assessment of Usability of WC-Co Powder Mixtures for SLM

This paper aims to provide comprehensive information on the usability of WC-Co powder mixtures for the additive technology of selective laser melting (SLM). Three different WC-Co powder mixtures and a precipitation-hardenable steel powder, which is ordinarily used for this process, were compared. Metallographic analysis by means of optical and scanning electron microscopes was performed for evaluating their properties. Phase composition of the powders was studied using X-ray analysis. Results of these analyses enable the WC-Co powder mixtures to be ranked in the order of suitability for the SLM process.

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.

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.