Potentials of nanostructured WC-Co hardmetal as reference material for Vickers hardness

The potential of nanostructured WC–Co hardmetal as reference material for Vickers hardness was researched in this paper. Preliminary reference Vickers hardness blocks were developed from a WC 9–Co mixture. The WC powder used was nanoscaled and had a grain size of 150 nm and a specific surface area of 2.5 m2/g. Hardness blocks were consolidated using the Sinter-HIP process. Properties of the sintered material were obtained by using standard measurement techniques like magnetic properties or the SEM analysis of microstructure. Special emphasis was placed on hardness uniformity of the test surface as it is the most important property placed on reference hardness blocks. Hardness uniformity was tested by analysis of variance, ANOVA, for single factor in order to determine if significant hardness variations across the block surface were present. From the conducted research it was concluded that hardness distribution over the whole test surface was uniform. The achieved superior properties of the material, such as full densification, nanosized homogeneous microstructure, high hardness and good fracture toughness, influenced the metrological characteristics of the reference Vickers hardness block. Hardness non-uniformity is nearly two times lower when compared to commercially available standard reference material made of ultra-fine WC–Co. Material response does not involve chipping, cracking or any other imperfections. The research opens a new area of application for nanostructured WC–Co hardmetals as reference material for Vickers hardness for high hardness range

Influence of cemented carbide composition on cutting temperatures and corresponding hot hardnesses

During metal cutting, high temperatures of several hundred-degree Celsius occur locally at the cutting edge, which greatly impacts tool wear and life. Not only the cutting parameters, but also the tool material’s properties influence the arising cutting temperature which in turn alters the mechanical properties of the tool. In this study, the hardness and thermal conductivity of cemented tungsten carbides were investigated in the range between room temperature and 1000 °C. The occurring temperatures close to the cutting edge were measured with two color pyrometry. The interactions between cemented carbide tool properties and cutting process parameters, including cutting edge rounding, are discussed. The results show that cemented carbides with higher thermal conductivities lead to lower temperatures during cutting. As a result, the effective hardness at the cutting edge can be strongly influenced by the thermal conductivity. The differences in hardness measured at room temperature can be equalized or evened out depending on the combination of hardness and thermal conductivity. This in turn has a direct influence on tool wear. Wear is also influenced by the softening of the workpiece, so that higher cutting temperatures can lead to less wear despite the same effective hardness

Testing length-scale considerations in mechanical characterization of WC-Co hardmetal produced via binder jetting 3D printing

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.Peer ReviewedPostprint (published version

Influence of printing direction on the mechanical properties at different length scales for WC-Co samples consolidated by Binder Jetting 3D printing

Additive Manufacturing (AM) is rapidly growing as a revolutionary technique. It provides an interesting ability to produce complex geometries, a key feature for enhancing performance and widening application fields of hardmetal components. Within this context, all the samples produced by AM [AMed] are expected to exhibit characteristics linked to the shaping route followed, which are also vital for defining their mechanical integrity. This work aims to study the correlation of the printing direction to the final microstructure, mechanical properties and layer assemblage at different length scales for a 12%wtCo–WC grade hardmetals of medium grain size consolidated by binder jetting 3DP and subsequent SinterHIP. Vickers macro- and micro-hardness as well as scratch tests, using different loads and indenter tips, are conducted. The results are analysed and discussed in terms of printing orientation effects on microstructural variability, mechanical response determined, intrinsic physical behaviour of the material and feedstock used.Postprint (published version

Gefügeausbildung und Eigenschaften von nanoskaligen binderfreien Hartmetallen

Gegenstand der hier vorgestellten Arbeit ist die Untersuchung der beim Sintern von nanoskaligen und metallbinderfreien Hartmetallen ablaufenden chemischen Prozesse und die sich dabei ändernde Mikrostruktur sowie die damit verbundenen Eigenschaften. Durch Versuche mit unterschiedlicher Zusammensetzung der Wolframcarbidmischungen konnte die relevanten Prozesse für die Herstellung von binderfreien Hartmetallen und technologische Vorgehensweisen für die Anpassung der Sintertemperatur, des Gefüges und der mechanischen Eigenschaften erarbeitet werden. Durch die herausragenden Eigenschaften des mittels konventioneller SinterHIP-Technik hergestellten binderfreien Hartmetalls mit einer Wolframcarbidkorngröße von 170 nm, einer Härte von etwa 2800 HV10 sowie einer Bruchzähigkeit von etwa 7,2 MPa m1/2 eignet sich das Material für eine breite Palette an Anwendungen im Bereich der Materialbearbeitung und als Verschleißmaterial

Grain growth inhibition of hardmetals during initial heat-up

Ultrafine and nanoscaled hardmetals show a significant grain growth already before reaching liquid phase sintering temperature. To limit grain growth during this stage of sintering the known grain growth inhibitors, mostly metal carbides like Cr3C2 or VC can be used. To investigate grain growth inhibition and dissolution of these carbides both the solid state sintering without Co (binderless tungsten carbide) and with 10 wt-% Co were investigated in the temperature range between 600 °C and 2000 °C and 1500 °C, respectively. It could be shown that e.g. the Cr3C2 starts to decompose already below 800 °C into a Cr-rich and C-rich phase. In both types of material with and without Co this leads to earlier reduction of W-oxides and the formation of Cr-based oxides which alter the chemical processes during sintering and change the grain growth behaviour. By using thermoanalytical methods as well as interrupted sintering experiments and their characterisation by FE-SEM and XRD, the direct influence of Co and grain growth inhibitors regarding the change in the chemical processes happening during sintering was studied

Manufacturing and properties of tungsten carbide-oxide composites

Conventional WC-Co hardmetals are widely used in various applications due to their excellent properties. High hardness can be achieved using compositions with little to no content of cobalt or nickel. These binder metals are hazardous to health, making a substitution not only desirable because of availability and cost reasons. A new possibility to manufacture such hard materials is the combination of tungsten carbide with oxides such as Al2O3 and ZrO2. In this way the binder metal content can be replaced. Furthermore the content of the also expensive WC can be reduced. Such metal carbide – oxide composites with oxide contents between 16 vol% and 40 vol% were manufactured. The completely dense composites feature high hardness values of 2000 HV10 to 2400 HV10 while also having an acceptable fracture toughness of up to 7 MPa⋅m1/2. The improved mechanical properties make the replacement of WC-Co hardmetals and binder free WC ceramics in special areas possible

Properties of Additive Manufactured Hardmetal Components produced by Fused Filament Fabrication (FFF)

Additive Manufacturing (AM) is experiencing an upswing in many sectors of industry for a broad variety of materials. Processes are mainly developed for polymers and metals. For ceramics and especially hardmetals there are only a few additive processes suitable to produce dense and defect free parts. Fused Filament Fabrication (FFF) is one of the suitable processes, which can be used for the fabrication of hardmetal parts via printing of green parts and consequent sintering. Within this study FFF of different hardmetal compositions representing different Co binder contents and different WC grain sizes are investigated. Main points of this investigation are the adjustment of debindering using different thermal and non-thermal steps and the adjustment of sintering regime. It is shown that FFF is a suitable AM technique for production of hardmetal parts with different compositions and microstructures. Dense parts with magnetic and mechanical properties as conventional produced hardmetals can be achieved. It is shown that with the Fused Filament Fabrication fully dense and defect free hardmetals parts can be produced. Because of the thermoplastic processability of the filaments which can be highly filled with particles this technique is interesting for manufacturing hardmetal parts