Search

Binder jetting additive manufacturing offers a promising route to produce complex geometries in cemented carbides (WC-Co), but it may introduce direction-dependent microstructural variations potentially affecting wear resistance. This study investigates the influence of printing direction on the sliding contact response of 3D-printed and subsequently sintered (BJT) WC-12%Co. Prismatic specimens were printed along two orientations and subjected to single and repetitive scratch tests on three orthogonal faces. The microstructure, Vickers and scratch hardness, and wear rate were analyzed. The results showed a heterogeneous microstructure consisting of a matrix of fine carbides where several large particles where embedded. It was different from the homogenous microstructural scenarios exhibited by conventionally pressed and sintered fine- and coarse-grained hardmetals, used as reference for comparison purposes. The influence of printing direction on either the microstructure or mechanical properties of BJT specimens was found to be negligible. Interestingly, BJT samples exhibited superior wear resistance than the reference hardmetals, even though the hardness levels were alike for all the studied hardmetal grades. Such behavior is attributed to the co-existence of coarse and fine carbides within the microstructure, combining the energy absorption capability of the former with the inherent strength of the latter. These findings, together with the intrinsic flexibility and versatility advantages associated with additive manufacturing, highlight the potential of BJT hardmetals to be used in applications where contact load bearing or wear resistance are critical design parameters. Finally, the effectiveness of implementing an iterative sliding contact test for evaluating wear behavior in cemented carbides was also validated. [ABSTRACT FROM AUTHOR]

In this study, the effect of the number of wire-EDM passes on the surface integrity of WC-Co hardmetals was investigated. As-received doughnut-shaped WC-25 wt%Co samples were wire EDM'ed with one, two, three and five passes. The resulting surface conditions were investigated in terms of macroscopic and microscopic examinations. Surface roughness and hardness measurements were carried out. Detailed SEM investigations were coupled with EDS analyses. Based on the experimental findings, a detailed cutting strategy was offered for WC-Co hardmetals depending on the expectations of surface conditions, production route, cost and productivity. If there is at least one subsequent process including material removal, wire-EDM with one pass is suggested to decrease cost and increase productivity. However, heat-affected zone (HAZ), recast layer and lower surface hardness region should be removed at subsequent production steps. If there are no production steps to remove HAZ, recast layer and lower surface hardness region, the number of wire-EDM passes should be maximised. [ABSTRACT FROM AUTHOR]

Hot deformation of sintered cobalt during hot compression testing was investigated in the temperature range of 700–1000 °C and at strain rates ranging from 0.0005 to 0.1 s−1. Cobalt underwent considerable dynamic recrystallization (DRX) during hot deformation, with stress-strain flow curves exhibiting one or several peaks followed by significant flow softening and leading to a steady-state stress. Constitutive equations were used to derive the flow stress behaviour. A physically based model to describe the strain rate as a function of stress was suggested for temperatures ranging between 775 and 1000 °C. In this case, a creep exponent (n) of 5 indicated that the deformation mechanism was controlled by the glide and climb of dislocations. The activation energy coinciding with the one for self-diffusion of ferromagnetic cubic cobalt implied a diffusion-controlled mechanism and the presence of face-centered cubic (FCC) cobalt during deformation. Interestingly, the results at 700 °C could not be perfectly fitted to this model and exhibited a higher resistance to deformation. This revealed that the glide and climb deformation mode was close to the transition where glide mode was dominant, and thus mostly glide occurred at 700 °C, especially for the largest strain rates.

Binder jetting additive manufacturing offers a promising route to produce complex geometries in cemented carbides (WC-Co), but it may introduce direction-dependent microstructural variations potentially affecting wear resistance. This study investigates the influence of printing direction on the sliding contact response of 3D-printed and subsequently sintered (BJT) WC-12%Co. Prismatic specimens were printed along two orientations and subjected to single and repetitive scratch tests on three orthogonal faces. The microstructure, Vickers and scratch hardness, and wear rate were analyzed. The results showed a heterogeneous microstructure consisting of a matrix of fine carbides where several large particles where embedded. It was different from the homogenous microstructural scenarios exhibited by conventionally pressed and sintered fine- and coarse-grained hardmetals, used as reference for comparison purposes. The influence of printing direction on either the microstructure or mechanical properties of BJT specimens was found to be negligible. Interestingly, BJT samples exhibited superior wear resistance than the reference hardmetals, even though the hardness levels were alike for all the studied hardmetal grades. Such behavior is attributed to the co-existence of coarse and fine carbides within the microstructure, combining the energy absorption capability of the former with the inherent strength of the latter. These findings, together with the intrinsic flexibility and versatility advantages associated with additive manufacturing, highlight the potential of BJT hardmetals to be used in applications where contact load bearing or wear resistance are critical design parameters. Finally, the effectiveness of implementing an iterative sliding contact test for evaluating wear behavior in cemented carbides was also validated.

Hard materials, particularly tooling compositions based on WC/Co structures, are predominantly designed for applications that take advantage of their extremely good properties in compression. There is a standard test method for obtaining compression properties (strength and ductility), ISO 4506:1979, but it is rather expensive to use, needing high capacity testing machines and, in some cases, quite complex testpieces to avoid problems with premature failure from parasitic tensile stresses at the platen/testpiece interface. Consequently, for the most part, the compressive properties of hardmetals are estimated through measurements of hardness; using techniques such as Rockwell (ISO 3738: parts 1–1982 and 2–1988) or Vickers (ISO 3878:1983). This paper discusses the correlation of hardness values with compression data and reviews the testing background, advantages and disadvantages of the test methods, together with comments on the underlying science and comparative studies by complementary researchers. The adoption of stiff compressive testing rigs at NPL for the measurement of almost 40 grades of hardmetals with a comprehensive spread of microstructures is used to comment on good testing practice. • Issues associated with deformation metrologies. • Assessment of RT compression tests for WC-Co hardmetals. • Property mapping for WC-Co hardmetals. • Comparisons of data from different souces. [ABSTRACT FROM AUTHOR]

In conventional wire electrical discharge machining (WEDM), WC-Co machined pieces can stay for hours in the worktank. In the pH range of the dielectric Co actively dissolves and thus corrosion can occur affecting the mechanical properties and lifetime of those pieces. This study demonstrates how applying cathodic polarization through the implementation of auxiliary electrodes in the work zone helps reducing in-depth propagation of the corrosion attacks on WC-Co hardmetals during storage in the dielectric. Our experimental findings were supported by a prediction model that helps visualizing the electrical field distribution during machining and storage and identifying the critical engineering parameters. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

WC-Co hardmetals are widely used in wear-resistant parts, cutting tools, molds, and mining parts, owing to the combination of high hardness and high toughness. WC-Co hardmetal parts are usually produced by casting and powder metallurgy, which cannot manufacture parts with complex geometries and often require post-processing such as machining. Additive manufacturing (AM) technologies are able to fabricate parts with high geometric complexity and reduce post-processing. Therefore, additive manufacturing of WC-Co hardmetals has been widely studied in recent years. In this article, the current status of additive manufacturing of WC-Co hardmetals is reviewed. The advantages and disadvantages of different AM processes used for producing WC-Co parts, including selective laser melting (SLM), selective electron beam melting (SEBM), binder jet additive manufacturing (BJAM), 3D gel-printing (3DGP), and fused filament fabrication (FFF) are discussed. The studies on microstructures, defects, and mechanical properties of WC-Co parts manufactured by different AM processes are reviewed. Finally, the remaining challenges in additive manufacturing of WC-Co hardmetals are pointed out and suggestions on future research are discussed. [ABSTRACT FROM AUTHOR]

A new express method for measurements of WC grain sizes in WC-Co hardmetals was developed. This method is as precise as the EBSD grain size measurement method but much faster and thereby less expensive. The new measurement method is based on a peculiar procedure of hardmetal samples' preparation, recording microstructure images and their segmentation followed by determining WC mean grain sizes by use of a specially developed software; the measurement error is less than ±3%. Dependencies of coercivity and hardness on the WC mean grain size of WC-10 wt% Co hardmetal samples were established by the new express measurement method. With the aid of this method the dependencies reported in literature were updated as a result of more precisely determined values of the WC mean grain size compared to those established in literature by conventional methods based on SEM measurements. [Display omitted] • Novel WC grain size measurement method for WC-Co hardmetals is developed. • Correlation between WC grain sizes and coercivity of WC-10%Co hardmetals is established. • Correlation between WC grain sizes and hardness of WC-10% Co hardmetals is established. • Hall-Petch relationship for WC-10%Co hardmetals is obtained. [ABSTRACT FROM AUTHOR]

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 4000 MPa, 21.56 MPa m 1/2 and 1733 kg f/mm 2 , 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Hardmetal products are frequently fatigue loaded in service, such as e.g. cutting tools for milling or percussion drills. In the present work, the fatigue behaviour of hardmetals was investigated into the gigacycle range using ultrasonic resonance fatigue testing at 20 kHz in push-pull mode at R = − 1. Liquid cooling was afforded using water with addition of a corrosion inhibitor. Hourglass shaped specimens were prepared, the surface being ground and polished with subsequent stress-relieving anneal to remove the high compressive residual stresses introduced during grinding. S-N curves with fairly low scatter were obtained, which indicates microstructure-controlled and not defect-controlled failure. Low binder content as well as fine WC grains were found to improve the fatigue endurance strength. In no case, however, a horizontal branch of the S-N curve was observed, i.e. there is no fatigue “limit” at least up to 10 10 cycles. The initiation sites were in part difficult to identify; in such cases when the site was clearly visible, decohesion of the binder from large WC grains seems to have caused crack initiation. This further corroborates that microstructural features and not singular defects as e.g. inclusions are the initiation sites, which underlines the high purity of the hardmetal grades used. Based on fracture mechanical consideration a damage diagram was determined allowing to deduce critical defect sizes. [ABSTRACT FROM AUTHOR]

A new method has been developed for measuring residual stresses at the surface of hardmetal components with higher spatial resolution than standard X-ray diffraction methods. It is based on measuring the surface displacements produced when stresses are partially released by machining a thin slit perpendicularly to the tested surface. Slit machining is carried out by focused ion beam (FIB). Measurement of the displacement fields around the FIB slit are performed by applying an advanced digital image correlation algorithm based on Fourier analysis with sub-pixel resolution. This method compares SEM images of the same area of the hardmetal surface before and after slitting. The method has been successfully applied to as-ground and femto-laser textured surfaces showing good correlation with the standard sin2ψ XRD technique. It is concluded that texturing induced by laser pulses in the femtoseconds regime is not perfectly adiabatic, since residual stresses are reduced by 15%. • Residual stresses were measured on a hard metal subjected to surface laser treatments. • Novel FIB-DIC methods were considered for the local measurement of residual stresses. • The results were successfully compared with conventional X-Ray diffraction methods. • This work confirms that FIB-DIC methods can measure RS at the micrometre scale. [ABSTRACT FROM AUTHOR]

The extreme versatility of additive manufacturing (AM) as processing technology results in "AMed pieces" with intrinsic characteristics linked to the shaping route followed, which are also key for defining mechanical integrity. The latter requires validation by measuring the mechanical properties, at both macroscopic (global) and microscopic (local) levels; and thus, consideration of specific testing length-scale aspects. This work aims to study the correlation between microstructure and mechanical properties for a WC-12% wt. Co hardmetal grade produced via binder jetting 3D printing (BJT) and subsequent sintering. In doing so, macro- and micro- Vickers hardness as well as scratch tests, using different loads and indenter tips, are conducted. It is found that studied samples processed by means of BJT exhibit a microstructure consisting of a relatively wide carbide size distribution, including a significant volume fraction (higher than 15%) of carbides larger than 3 μm. This is a direct consequence of the relatively high sintering temperature needed for getting full dense specimens, when manufactured following this AM route. Meanwhile, mechanical properties are found to be isotropic, with hardness and scratch resistance values falling within ranges of those expected for hardmetals with similar binder content and mean carbide grain size. Very interesting, length-scale effects on testing are observed in terms of dispersion of measured hardness value as applied load decreases. These findings, together with similar ones linked to length-scale influence on scratch response, point out that effective selection of mechanical testing parameters become critical for studying and understanding phenomena such as elastic/plastic and deformation/fracture transitions in AMed hardmetals. • Hardmetal processed by Binder Jetting 3D printing exhibits an isotropic behavior. • WC-12% wt. Co done by BJT show a rather high fraction of carbides larger than 3 μm, mainly, by the high sintering temperature. • Dispersion of hardness values increases as characteristic lengths of hardness imprints and microstructure becomesimilar. • Scratch tests run using different tips allow to study elastic/plastic and deformation/fracture transitions in hardmetals. [ABSTRACT FROM AUTHOR]

Integrated Computational Materials Engineering (ICME) supports materials engineering by linking materials models at multiple length scales. Effective use of the ICME approach requires underlying data to fully enable a combination of appropriate models and software tools. In the Hardmetal world where materials are composites of hard carbides/nitrides with tough metal binders this requirement has many facets. This paper addresses one of these facets - the issue of the mechanical behaviour of typical Co based binder phases over a range of temperatures. Three Co-W-C alloys were manufactured to provide samples that could be tested at different temperatures and under various mechanical conditions. The principle of miniaturised testing was adopted to help generate as much information as possible from a restricted amount of original alloy. An important aspect was the phase content of the alloys since Co alloys can be FCC, HCP or HCP/FCC mixtures and resistivity measurements as a function of temperature were used to interpret the phase contents. Below the FCC/HCP phase transformation temperature the microstructure can be a mixture of HCP/FCC, dependent on composition and whether applied heating is increasing or decreasing (hysteresis in phase transformation kinetics). To fully characterise all these parameters and their effect on mechanical properties at different rates would be a significant effort in material manufacture, sample characterisation and testing resources. Consequently, an attempt was made to conserve resources by exploring these issues by completing multiple tests on a few samples of each of the three alloys, while in both the heating and cooling regimes. • Manufacture of three Co-W-C alloys with 4, 8 and 12 wt% tungsten; representative of WC/Co binder phases. • FCC/HCP phase transformation kinetics in three Co-W-C alloys representative of Hardmetal WC/Co alloys; obtained using resistivity measurements during temperature variations in miniature testpieces. • Measurements of the temperature variation of strength in the fully FCC structures at high temperature under both load and strain control tests. • Measurements of the temperature variation of strength in mixed FCC/HCP structures at low to medium temperatures under both load and strain control tests. [ABSTRACT FROM AUTHOR]

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. • Dual scale plate-like WC breaks trade-off between hardness and toughness of WC-Co. • Hardness and fracture toughness simultaneous increase to extremely high for WC-8Co. • Morphologic feature of WC is induced to modify predicting hardness and toughness models. • The modified predicting models of hardness and toughness are universal for hardmetals. [ABSTRACT FROM AUTHOR]

Abstract: The calculating algorithm for construction of the stress–strain curves of WC-Co hardmetals in simple tension and compression has been described. The algorithm is based on the mathematical means of mechanics of composite materials and in situ deformation properties of constituents. The mean field approach has been applied. The stress–strain behavior depends on such microstructural parameters as cobalt volume fraction, mean carbide grain size, and contiguity of carbide phase. The technological thermal residual microstresses have been taken into account. The wide comparison of theoretical and available experimental results has been done. It has been shown, that the given calculating algorithm has the good predictive capability. The analytical expressions of tensile and compressive strength of WC-Co hardmetals have been given. [Copyright &y& Elsevier]

Abstract: It is well known that the WC coarsening in WC–Co is suppressed in hardmetals having low carbon contents and consequently high concentrations of tungsten dissolved in the binder. However, the mechanism of this phenomenon is not completely understood yet. Heavily milled ultra-coarse WC powders in mixture with 10wt.% Co and either W metal or carbon black were employed to examine the WC coarsening of hardmetals with various carbon contents. The rate of WC coarsening is found to be strongly suppressed when decreasing the carbon content and consequently increasing the concentration of W dissolved in the binder. The apparent activation energy of WC coarsening in the alloys containing neither η-phase nor free carbon is found to be similar and vary from 163 to 177kcal/mol. These values are close to those obtained in the literature for dissolution of WC in liquid Co and WC coarsening, which is evidence that the limiting stage of WC coarsening in this case is related to interfacial reactions between WC and liquid binders. It can be supposed that the slowest reaction determining the process of WC dissolution as a whole is the carbon dissolution from WC grains to the liquid binder. The apparent activation energy of WC coarsening in the hardmetal with the lowest carbon content, the microstructure of which comprises η-phase, is found to be equal to 98kcal/mol, which is close to the activation energy of carbon self-diffusion in WC (88kcal/mol). Thus the mechanism of suppression of WC coarsening in WC–Co hardmetals with medium carbon contents when decreasing the carbon content is thought to be related to the carbon dissolution in the liquid binder. The limiting stage of WC coarsening in hardmetals with very low carbon contents corresponding to the formation of η-phase can be the carbon self-diffusion in WC grains towards the interface WC–liquid Co. It cannot be excluded also that the limiting stage of WC coarsening in this case might be related to the processes of W and C re-precipitation onto WC grains from the extremely W-rich and C-poor binder. [Copyright &y& Elsevier]

Abstract: The analytical expressions for minimal fracture toughness, toughness–hardness and coercivity–magnetic saturation relationships are developed on the basis of some published experimental data. [Copyright &y& Elsevier]

Abstract: Novel ultra-coarse WC–Co grades with a Co-based binder reinforced with nano-particles have been developed and implemented in industry on a large scale. The new grades with the brand name MASTER GRADES®, consist of rounded WC grains and the Co-based binder comprising nano-particles of the θ-phase (Co2W4C). The new hardmetal grades are characterised by a combination of higher hardness and transverse rupture strength as result of the binder hardening and reinforcement. Wear-resistance of the new grades in laboratory tests on abrasion, cutting of granite and abrasive concrete is found to be 2–3 times higher than that of standard grades with the same Co content and similar WC mean grain size. Life of mining and construction tools with the MASTER GRADES® is dramatically prolonged as a result of their enhanced wear-resistance and resistance to the initiation and propagation of thermal and fatigue cracks. [Copyright &y& Elsevier]

New impact-abrasion tests allowing one to evaluate performance of hardmetals operating in conditions of intensive abrasion, severe fatigue and high impact loads can be of great importance for many industrial applications. A new test for studying wear behaviour of hardmetals under high impact loads was developed and employed for evaluation of performance of different hardmetal grades in comparison with polycrystalline diamond (PCD). The wear behaviour of the same hardmetal grades and PCD was also examined in the standard ASTM B611 test, which was employed as a control. A significant difference between wear rates of near-nano and submicron hardmetals on the one hand and medium-coarse and ultra-coarse hardmetals on the other hand in the new impact-abrasion test was established. The wear of PCD in the impact-abrasion test was found to be close to zero. Examinations of wear surfaces of the tested hardmetal samples allowed wear mechanisms of the different hardmetal grades to be evaluated. The wear mechanism of the near-nano and submicron grades in the impact-abrasion test comprises phenomena of wear and flattening of WC grains, partial removal of Co from binder interlayers in a thin surface layer and formation of shallow holes on the worn surface as a result of detachment of relatively small WC-Co fragments. The wear mechanism of the medium-coarse and ultra-coarse grades in the impact-abrasion test includes phenomena of full removal of the binder phase from thick Co interlayers among WC grains leaving them unsupported. This leads to cracking, damage and breakage of the WC grains as well as detachment of large WC-Co fragments resulting in significantly higher wear rates of the medium-coarse and ultra-coarse grades in comparison with the near-nano and submicron grades. • New test for studying impact-abrasive wear was developed. • Significant difference among wear rates of different hardmetals was found. • Wear mechanism of fine-grain grades comprises detachments of WC-Co fragments. • Coarse-grain grades are subjected to removal of Co interlayers among WC grains. • Wear mechanism of coarse-grain grades includes damage of WC grains. [ABSTRACT FROM AUTHOR]