Search

The effect of corrosion-induced changes on the contact fatigue response and associated damage in a medium-grained WC-6%wtCo hardmetal grade is investigated. Cyclic contact loading tests are conducted by means of Hertzian indentation techniques. Corrosion damage is introduced in a controlled way by previous immersion of specimens in a stirred acidic medium. Results reveal that corrosion significantly affects the contact fatigue behavior of the material under consideration, in terms of decreasing the effective load bearing capability of the material as well as earlier emergence and evolution of specific damage features, as compared to those discerned under monotonic loading and in a pristine hardmetal respectively. As cycle number increases, damage evolution in the corroded condition is more gradual and diffuse than for the uncorroded one. In general, it is dominated by existence of ill-defined radial cracks together with pronounced carbide grain pull-out which finally evolve into cohesive-like spallation in regions close to the periphery of indentation imprints. These findings are rationalized on the basis of phase assemblage changes linked to corrosion, particularly in terms of lessening of toughening capability and easiness of WC grain pull-out intrinsic to the loose and porous binderless WC skeleton remnant at the surface level, after exposure to the acid medium. Although some removal of carbide grains and discrete cohesive chipping is also observed in the pristine sample for very high number of cycles, such different response, as compared to monotonic loading, is linked to oxidation phenomena of the metallic binder for this reference condition.

21. CIRP Conference on Electro Physical and Chemical Machining, ISEM XXI, Zurich, Switzerland, 14 Jun 2022 - 17 Jun 2022; Procedia CIRP 113, 53-58 (2022). doi:10.1016/j.procir.2022.09.119 special issue: "21st CIRP Conference on Electro Physical and Chemical Machining, ISEM XXI June, 14 to 17, 2022 in Zurich / edited by Konrad Wegener, Stefan Fabbro, Paulo Matheus Borges", Published by Elsevier, Amsterdam [u.a.]

Laser-based additive manufacturing of cemented carbides (WC-Co) remains challenging due to crack forming and residual porosity. Reduction of temperature gradients by high preheating temperatures and successive thermal treatments are starting points for improving the quality of additively manufactured components. Thermal post-treatment is beneficial for the final densification and homogenization of the microstructure. In this study, hot isostatic pressing (HIP) and sinter-HIP are used for thermal post-treatment of additively manufactured parts of WC-12Co. Both methods use elevated temperatures and pressures for densification. Densification during the sinter-HIP process is achieved by liquid phase sintering, while in the HIP process, compaction is expected by plastic deformation of the material. The effect of these both methods for post-treatment on microstructure and fracture strength is investigated in this study. Subsequently, tungsten carbide grain size, as well as the composition of the binder phase, are related to these results.

Previous studies on Laser Powder Bed Fusion of tungsten carbide-cobalt (WC-Co) revealed several defects in the laser molten microstructures. To analyze the defect formation mechanisms and the fundamental impact of laser energy input on phase development and microstructure of WC- Co, single-pulse and single-track analogy experiments are conducted. Conventional WC-Co is used as substrate. Variations in cobalt content and processing conditions are investigated. It is found that a single laser exposure can be enough to induce decomposition of microstructure, WC grain growth, formation of non-equilibrium phases and thermal cracking. Based on the results, defect formation mechanisms and countermeasures are discussed.

Cemented carbide tools are often used for milling operations that cause high thermal and mechanical process loads, e.g. machining processes for titanium alloys. However, the disposal of those tools after one life cycle would significantly reduce their resource efficiency. Therefore, regrinding operations are crucial in order to recycle worn tools and ensure an economical as well as resource efficient manufacturing process. The main challenges during regrinding are the precise quantification of present defects and the subsequent determination of the grinding allowance. As it is, a worker performs both tasks using his individual estimations. Consequently, the estimated grinding allowance is often too low or too high. This either decreases the lifetime of the reground tools due to remaining defects or reduces the resource efficiency since more material than necessary is removed. This paper investigates the determination of the grinding allowance and the environmental impact of regrinding operations on the life cycle of the investigated tools. It is shown that about 12.5% percent of the worn tools are being unnecessarily disposed of. Furthermore, the resource efficiency of tools with small breakouts might be increased by 20% if the recommended allowance strategy is utilized. The tool wear of the grinding tools is also taken into consideration in order to further increase the resource efficiency of the whole life cycle, including milling tool and grinding wheel. The results show that small grain sizes and low grain concentrations are not suitable for efficient regrinding processes since higher wear and consequently higher geometrical inaccuracies of the reground tools occur.

A study was carried out to investigate the influence of the initial powder features on the microstructure, the phase composition, and on the mechanical properties of WC-12%Co samples produced by binder jetting and densified by sintering in vacuum at 1400 °C and by sinter-HIPing. The initial powder consisted of a mixture of fine-grained and coarse-grained WC-based particles, and W2C-based ones. The density of the printed samples was 97.4% and 99.3% after sintering and sinter-HIPing, respectively. Layer-oriented porosity was observed only in sintered samples. The microstructure of the samples consisted of a mixture of fine and coarse WC grains, which can be the result of the coalescence of grains from anomalous coarse grains in the powder particles. Vickers hardness and transverse rupture strength of sinter-HIPed samples are 1205 HV and 2257 MPa, respectively, which is coherent with the microstructural analysis and close to coarse-grained commercial products.

We present a new method for retarding the oxidation rate of hardmetals. By diffusion impregnating a WC-FeCr hardmetal with silicon, we manufacture two-layered silicide coatings consisting of an FeSi x outer crust and WSi 2 beneath. The structure results from a preferential reaction between silicon and the metallic binder. The FeSi x outer layer is crucial to providing oxidation resistance as when exposed to oxygen it passivates, forming a protective SiO 2 surface film – while simultaneously preventing exposure of the underlying WSi 2 , which is known to oxidise in an active manner. Our analysis shows the coating method is applicable to various hardmetals structures.

The influence of single and repetitive sudden changes of temperature on the mechanical integrity of cemented carbides was investigated as a function of their microstructure. Thermal shock resistance was assessed by testing the residual flexural strength of hardmetal beams after being subjected to thermal shock by water quenching. Results indicate that hard cemented carbides tend to exhibit a superior resistance to the nucleation of thermal shock damage but a lower resistance to the propagation of this damage mechanism than tough grades, and vice versa. These trends are in agreement with those expected from the evaluation of the thermal shock Hasselman’s parameters. The evidenced strength loss after thermal shock may be related to the subcritical growth of intrinsic flaws driven by localized microcracking surrounding them. Results also point out on Ni-base hardmetals to exhibit a slightly higher resistance to abrupt changes of temperature than Co-base ones.

In this study, the influence of corrosion on the mechanical response and damage induced under Hertzian indentation is assessed for three cemented carbides with metallic binders of different chemical nature. Corrosion degradation is introduced in a controlled way, before subsequent spherical indentation testing, by immersing specimens in a stirred acidic medium. Results reveal quite strong corrosion effects on indentation stress-strain response and contact damage scenario. Such detrimental influence is found to be dependent on both the ratio between indentation depth and thickness of the corroded layer as well as chemical nature of the binder. In this regard, critical loads for emergence and evolution of specific damage events (i.e. ring and radial cracks, and even specimen failure) are proposed as figures of merit for material selection under the combined action of corrosion and contact loads. Within this context, the hardmetal grade with Co-base binder and addition of Cr is found to be the best option, among the three cemented carbides studied in this investigation. It points out the consideration of the synergic interaction between corrosion resistance and hardness/toughness correlation for microstructural design optimization of hardmetals under service-like conditions. These statements are supported by the relevant corrosion-induced changes also observed, by means of advanced characterization techniques, in terms of deformation/failure micromechanisms at both surface and subsurface levels.

Cemented carbides, also known as hard metals (HM), are composite materials that are widely used in many technological applications requiring favourable mechanical properties, specifically a high wear resistance. In the steelmaking industry, these materials have been effectively used to produce rollers for both hot and cold rolling operations. In this study, failure analysis of a roller used for cold rolling was performed, starting from a detailed material investigation to a fracture surface characterisation using scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM + EDXS). A finite element (FE) analysis of the roller was performed to determine the Hertzian-type stress distribution in the contact area. To identify the most critical zone, the Crossland multiaxial criterion was applied, and the results were then compared with measurements obtained from a component failure analysis. An additional investigation of the possible causes of failure was performed by adopting a material model (the Murakami–Endo equation) to distinguish the actual causes of failure more clearly, mainly at the crack nucleation site. The failure analysis results showed that the roller failed primarily because of rolling fatigue, which started from an internal pore acting as a source of local stress concentration. The component reached the fatigue limit of the material in proximity to the pore, which behaved as a crack nucleation site.

Several finite element models of the morphology of WC-10 wt.% Co were employed to reproduce the build-up of thermal residual stresses as well as the phase specific strain during loading-unloading in compression. The different models differ only in their geometry of the interpenetrating skeletons of WC and Co. They all respect the given volume proportion of each phase. Thermoelasticity is considered for the brittle WC, while also plasticity is included to model the Co binder phase. We compare the predictions of our FEM models with phase specific strain measurements performed by in-situ neutron diffraction and discuss the model validation. (C) 2014 Elsevier Ltd. All rights reserved.

There is a major interest in replacing cobalt binder in hardmetals (cemented carbides) aiming for materials with similar or even improved properties at a lower price. Nickel is one of the materials most commonly used as a binder alternative to cobalt in these metal-ceramic composites. However, knowledge on mechanical properties and particularly on fatigue behavior of Ni-base cemented carbides is relatively scarce. In this study, the fatigue mechanics and mechanisms of a fine grained WC-Ni grade is assessed. In doing so, fatigue crack growth (FCG) behavior and fatigue limit are determined, and the attained results are compared to corresponding fracture toughness and flexural strength. An analysis of the results within a fatigue mechanics framework permits to validate FCG threshold as the effective fracture toughness under cyclic loading. Experimentally determined data are then used to analyze the fatigue susceptibility of the studied material. It is found that the fatigue sensitivity of the WC-Ni hardmetal investigated is close to that previously reported for Co-base cemented carbides with alike binder mean free path. Additionally, fracture modes under stable and unstable crack growth conditions are inspected. It is evidenced that stable crack growth under cyclic loading within the nickel binder exhibit faceted, crystallographic features. This microscopic failure mode is rationalized on the basis of the comparable sizes of the cyclic plastic zone ahead of the crack tip and the characteristic microstructure length scale where fatigue degradation phenomena take place in hardmetals, i.e. the binder mean free path. (C) 2014 Elsevier Ltd. All rights reserved.

For conventional structural and tool materials, in particular WC-Co cemented carbides, hardness and wear-resistance can usually be increased only at the expense of toughness and strength. For the first time we have achieved a dramatically increased combination of hardness, wear-resistance, fracture toughness and strength as a result of precipitation of extremely fine nanoparticles in the cobalt binder of cemented carbides. These nanoparticles are similar to 3 nm in size, coherent with the Co matrix and consist of a metastable phase. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Cemented carbides, referred to as hardmetals, are forefront engineering materials widely implemented in industry for chip-removal cutting tools and supporting parts. As a newly developed technology for surface modification with high precision, the application of short pulse laser may extend the utilization of cemented carbides. However, surface integrity of laser-treated materials may be affected during the ablation phenomena. These potential changes may also be relevant for subsequent coating deposition, a surface modification stage usually invoked in many cutting and forming tools. It is the objective of this work to study the influence of a previous laser treatment on the surface integrity of a cemented carbide grade, finally coated by a ceramic layer introduced by physical vapor deposition. In doing so, a nanosecond laser has been employed. Surface integrity is assessed in terms of roughness, hardness, and microstructural changes induced at the subsurface level. It is found that pulse laser can effectively remove the target material, resulting roughness being similar to that attained by abrasive grinding. Although some subsurface damage is observed, it is limited to a very shallow layer, this being thoroughly eliminated during sandblasting implemented before coating deposition. Relative hardness increase is larger for laser treated substrate than for just polished one, reason behind it being speculated to come from the sandblasting stage used for removing damaged layer.

In this study, the correlation existing among grinding, surface integrity, and flexural strength is investigated for WC-Co cemented carbides (hardmetals). A fine-grained WC-13 wt % Co grade and three different surface conditions: (1) ground, (2) mirror-like polished (reference), and (3) ground plus high-temperature annealed, are investigated. Surface integrity and mechanical characterization is complemented with fractography. The grinding strongly affects both surface integrity and flexural strength. During grinding, a damaged thin layer together with high compressive residual stresses is introduced. The layer results in considerable strength enhancement compared to the reference polished surface condition. Fractography reveals that the improved strength mainly stems from grinding-induced changes on effective location, from surface into subsurface levels, of the strength-controlling flaw.

Nanocrystalline tungsten carbide has been obtained by reduction/carburization at low temperature from precursors obtained by freeze-drying of aqueous solutions. Nanocrystalline WC powders with a adequate content of carbon were mixed with submicrometric Cobalt powder (12 wt.%), obtained by same synthesis method, and sintered in vacuum furnace. The cemented carbides fabricated from experimental powders were compared with both commercial ultrafine and nanocrystalline WC-12Co mixtures consolidated by the same route. The synthesised powders were characterized by X-ray powder diffraction, elemental analysis and scanning and high resolution transmission electron microscopy. On the other hand, density, microstructure, hardness and fracture toughness together with X-ray diffraction analysis of the sintered materials were evaluated. The cemented carbides obtained from synthesised powders exhibited a WC platelet-based homogeneous microstructure. This anisotropic growth might be due to the presence of stacking faults parallel to the basal plane in the starting WC powder, which would promote the defect-assisted preferential growth. These materials showed excellent mechanical properties, with a superior hardness/fracture toughness combination compared to materials prepared from commercial mixtures.

This study reports a determination of the polarity of WC facets in WC–Co alloys sintered at the liquid state. In these alloys, WC grains are delimited by basal facets and two sets of { 1 0 1 ¯ 0 } prismatic facets, one set of facets being much more developed than the other. A variation of the shape is observed as a function of the composition of the alloy. High resolution transmission electron microscopy is used to investigate the polarity of the prismatic facets owing to the typical triangular patterns appearing on the images. The effect of the composition is investigated using two alloys with different carbon potentials and one containing VC and Cr3C2 as grain growth inhibitors. The interpretation of the images shows that in all cases, the same set of prismatic planes is favoured.

WC-17Co coatings with Ni-5Al bonding layers were deposited on Al-7075 by HVOF spraying. The top-coat consisted of layers comprising tungsten carbide particles embedded in a Co(W,C) matrix of varied composition. The coated specimens were subjected to potentiodynamic polarization in 3.5% aqueous NaCl at 25, 35 and 45 degrees C. The coatings exhibited pseudopassivity caused by the oxidation of tungsten, carbon and possibly cobalt. Chronoamperometric measurements indicated that the inhomogeneous binder composition induced active corrosion processes taking place simultaneously with pseudopassivity. Cyclic polarization suggested that the coatings were not susceptible to pit corrosion in the temperature range of 25-45 degrees C. The likely "critical pitting" temperature of the coatings was 60 degrees C. Higher testing temperatures led to lower corrosion potentials and faster corrosion kinetics. (C) 2007 Elsevier B.V. All rights reserved. Thin Solid Films

WC-Co hard metals continue to gain importance for cutting, mining and chipless forming tools. Cobalt metal currently dominates the market as a binder because of its unique properties. However, the use of cobalt as a binder has several drawbacks related to its hexagonal close-packed structure and market price fluctuations. These issues pushed the development of pre-alloyed binder powders which contain less than 40 wt.% cobalt. In this paper we first report the results of extensive investigations of WC-Fe/Ni/Co hard metal sintering, surface pretreating and deposition of adherent diamond films by using an industrial hot filament chemical vapour deposition (HFCVD) reactor. In particular, CVD diamond was deposited onto WC-Fe/Ni/Co grades which exhibited the best mechanical properties. Prior to deposition, the substrates were submitted to surface roughening by Murakami's etching and to surface binder removal by aqua regia. The adhesion was evaluated by Rockwell indentation tests (20, 40, 60 and 100 kg) conducted with a Brale indenter and compared to the adhesion of diamond films grown onto Co-cemented tungsten carbide substrates, which were submitted to similar etching pretreatments and identical deposition conditions. The results showed that diamond films on medium-grained WC-6 wt.% Fe/Ni/Co substrates exhibited good adhesion levels, comparable to those obtained for HFCVD diamond on Co-cemented carbides with similar microstructure.

This paper concentrates on the morphology of the ground surface of cobalt tungsten carbide (WC) composite materials that belong to the category of so-called hardmetals. A deformed and detached surface layer was found on top of the specimens after surface grinding with a diamond wheel. In order to determine aspects of material removal, various routes were followed. Etching the surface layer revealed WC grains in the subsurface of the machined samples. Most of these grains were plastically deformed by prismatic slip in the hexagonal lattice of WC. Also cracks were found in these grains. Further, the increase in residual stress due to the grinding process was examined by X-ray diffraction (XRD). The stress increases to a maximum stress in the carbide grains before the relaxation occurs by slip and cracking. The surface roughness after grinding was measured by scanning confocal microscopy. Roughness exponents were calculated from the 3D profiles and the values of this process were in good comparison with values for grinding ceramics.