Preparation and properties of WC–Co textured components

Abstract

WC–Co components having one {0001} textured surface have been produced and tested. The textured surface has beenfound to be harder and more resistant to fracture and abrasive wear than standard surfaces. A method is proposed to produce WC–Co components having a bulk {0001} texture.

MST/1337     

Investigation of TiC–C Eutectic and WC–C Peritectic High-Temperature Fixed Points

TiC–C eutectic (2,761°C) and WC–C peritectic (2,749°C) fixed points were investigated to compare their potential as high-temperature thermometric reference points. Two TiC–C and three WC–C fixed-point cells were constructed, and the melting and freezing plateaux were evaluated by means of radiation thermometry. The repeatability of the TiC–C eutectic within a day was 60 mK with a melting range roughly 200 mK. The repeatability of the melting temperature of the WC–C peritectic within 1 day was 17 mK with a melting range of ∼70 mK. The repeatability of the freezing temperature of the WC–C peritectic was 21 mK with a freezing range less than 20 mK. One of the TiC–C cells was constructed from a TiC and graphite powder mixture. The filling showed the reaction with the graphite crucible was suppressed and the ingot contained less voids, although the lack of high-purity TiC powder poses a problem. The WC–C cells were easily constructed, like metal–carbon eutectic cells, without any evident reaction with the crucible. From these results, it is concluded that the WC–C peritectic has more potential than the TiC–C eutectic as a high-temperature reference point. The investigation of the purification of the TiC–C cell during filling and the plateau observation are also reported.     

Special WC/Co orientation relationships at basal facets of WC grains in WC–Co alloys

In WC–Co alloys prepared by liquid phase sintering WC grains are partly surrounded by the Co binder and are terminated by basal and prismatic facets. This work reports transmission electron microscopy (TEM) observations on basal WC/Co interfaces. Two kinds of preferred orientation relationships are found. The one most common corresponds to the joining of (0001)_WC with (111)_Co planes. It leads to a large parametric misfit of 15% and is observed for only small pools of Co, mainly located in corners formed by the contact between WC grains. The second type of interface corresponds to the joining of (0001)_WC with (001)_Co and is more rarely found. For random WC/Co orientation relationships a thin WC_1−x cubic layer about two atomic rows is revealed at the interface.     

Chemical processing of nanophase WC–Co composite powders

Abstract

The future development of advanced engineered materials for structural, electrical, magnetic, catalytic, and other applications will depend to an increasing extent on improved control of the size, distribution, and morphology of the constituent phases of the materials. In advanced materials systems, this development is in the direction of diminishing scale and increasing uniformity of the structure, extending into the nanoscale regime. The capabilities for synthesising novel nanophase structures are now becoming available in the laboratory and efforts are being made to scale up these processes to produce the quantities required for prototype development, field testing, and commercial applications. This paper addresses the scientific and technical issues relating to the chemical processing of nanophase WC–Co composite powders and the integration of the various processing steps into a new spray conversion processing technology. The new technology involves three coordinated steps: preparation and mixing of starting solutions; spray drying to form chemically homogeneous precursor powders; and fluid bed thermochemical conversion of the precursor powders to nanophase WC–Co powders. Both spray drying and fluid bed conversion are proven scalable technologies and offer the potential for producing bulk quantities of cemented carbide powders at lower manufacturing cost.

MST/1318     

Densification behavior and mechanical properties of spark plasma-sintered ZrC–TiC and ZrC–TiC–CNT composites

Titanium carbide (TiC) and carbon nanotubes (CNTs) were introduced into zirconium carbide (ZrC) ceramics to improve the fracture toughness. ZrC–TiC and ZrC–TiC–CNT composites containing 0–30 vol.% TiC and 0.25–1 mass% CNT were prepared by spark plasma sintering at temperatures of 1750–1850 °C for 300 s under a pressure of 40 MPa. Densification behavior, microstructure, and mechanical properties of the ZrC-based composites were investigated. Fully dense ZrC–TiC and ZrC–TiC–CNT composites with a relative density of more than 98 % were obtained. Vickers hardness of ZrC-based composites increased with increasing TiC content and the highest hardness was achieved with the addition of 20 vol.% TiC. Addition of CNTs up to 0.5 wt% significantly increased the fracture toughness of ZrC-based composites, whereas the addition of TiC did not have this effect.     

Investigation of WC–Co alloy properties based on thermodynamic calculation and Weibull distribution

The influence of alloy composition and sintering temperature on the mechanical properties and reliability of WC–Co cemented carbides was studied theoretically and experimentally. For the first time, through a hybrid approach of thermodynamic calculations and Weibull distribution, the comprehensive performance of ultrafine WC–Co cemented carbides with different C contents and inhibitor type was investigated in detail. The carbon content of WC–10?wt-% Co–0.5?wt-% Cr cemented carbides was carefully controlled within the range of 5.38?5.52?wt-%. The contents of Cr and V are chosen to be in the range of 0–1?wt-%. It is found that WC–10?wt-% Co–0.5?wt-% Cr alloys with 5.46?wt-% C or 5.5?wt-% C show excellent mechanical properties and high reliability. WC–10?wt-% Co alloys with 0.5?wt-% Cr and 0.4?wt-% Cr–0.2?wt-% V demonstrate high mechanical property and reliability. The results of this study can be used to design process parameters during the manufacture of WC–Co cemented carbides.     

Failure Analysis of WC–2Ni–1Co Rotor Part

This study deals with the analysis of an ink-producing machine rotor part composed of WC–2Ni–1Co which failed in brittle manner during service. The part was made by powder metallurgy techniques and is being used in ink-grinding machines due to its high hardness and wear resistance. Similar parts had worked satisfactorily for many ink compositions, but the part under investigation failed prematurely. Investigation was considered important because the part is expensive, and other identical components frequently failed after a short service life. Moreover, replacement of the part requires complete dismantling of the machine which reduces the production rate. Spectroscopic analysis, density, optical and scanning electron microscopy, SEM–EDS analysis, fractography, X-ray diffraction, and microhardness measurements were carried out on failed parts to find out the root causes of the failure. Results revealed that the part cracked due to combined effects of selective dissolution of metal binder-caused corrosive action of ink solution and hydrogen-induced deterioration of WC and Ni–Co phases. Localized removal of binder phase left the hard WC phase unsupported. Cracks were found initiating from the root of the machined slot which acted as a stress concentration point and resulted in brittle fracture.     

Influence of Laser Shock Processing on WC–Co Hardmetal

Hardmetals are widely used as machining tool material whose wear resistance is of vital importance. Using coatings as the primary tool strengthening method has certain limitations. On the other hand, laser shock processing (LSP) is an emerging technology which has been applied extensively in improving the properties of materials. In this paper, WC–Co hardmetal is treated with LSP to explore a new way of tool strengthening. Scanning electron microscope (SEM) and transmission electron microscope were used to observe the near-surface microstructure. Microhardness was measured and a friction-wear testing machine was used to test the friction and wear properties of the processed surface. Wear factor and friction coefficient were analyzed and optical microscope and SEM were applied to observre the morphologies of wear tracks. Results show that LSP gives rise to grain refinement and the dislocation density is augmented, thus increasing the surface hardness and improving the friction and wear behavior.     

Laser surface remelting of WC–12Co coating: finite element simulations and experimental analyses

A three-dimensional finite element model taking into account interfacial topography, porosity of plasma sprayed coatings, temperature dependent thermophysical parameters and phase change has been developed to simulate multipass laser remelting process. Temperature evolution, temperature gradient, melting pool shape and dimensions are simulated. The laser remelting experiments are carried out on the plasma sprayed WC–12Co coatings. The simulated results are in good agreement with the experimental measurements. After laser remelting, a denser and more homogenous coating is obtained. The microhardness of the coating is significantly enhanced owing to the dispersion strengthening, the fine grain strengthening and the solution strengthening, which is increased by three times compared with that of the plasma sprayed coatings.     

Investigation on diffusion bonding of functionally graded WC–Co/Ni composite and stainless steel

A functionally graded WC–Co/Ni composite (FGWC) and 410 stainless steel (410ss) were successfully bonded by diffusion bonding. With the bonding temperature or holding time increasing, the tensile strength of the joints increased firstly and then decreased. The maximum tensile strength of the FGWC/410ss joints was 195 MPa bonded at 950 °C for 80 min. A diffusion layer was formed between the Ni layer and the 410ss as a result of the interdiffusion of Ni and Fe. The Ni layer could release the residual stresses of the FGWC/410ss joints. The fracture of the FGWC/410ss joints occurred in the Ni layer by the way of ductile fracture.     

Influence of High Pressure and Temperature on the Structure and Properties of the WC–6Co Hard Alloy

The results are presented of the barothermal treatment studies of a WC–6Co hard alloy at high pressure (7 GPa). It is shown that such a treatment of hard alloy leads to the deterioration of the basic physico-mechanical characteristics, specifically the strength at the contact, and the coercive force, which is stipulated by the increase of the defects amount in the structure of the carbide phase of the hard alloy.     

Decarburization mechanisms of WC–Co during thermal spraying: Insights from controlled carbon loss and microstructure characterization

Decarburization behavior of WC–Co particles in terms of transformation of WC to W₂C and W, and formation of η and γ phases and microstructure evolution during plasma spraying have been systematically investigated in this study. The extent of the carbon loss of WC was tailored by either altering cooling conditions of substrate/pre-coating or spraying the particles into the media with different temperatures. It is revealed that loss of carbon of WC was alleviated by protection of Co during the coating formation stage. W₂C exhibits epitaxial growth on the WC substrates in perpendicular direction and forms a nearly complete shell around the WC particles. η phase was formed as a result of decarburization and diffusion of associated phases and is located around WC–Co splats with its crystals being in cross shape. The γ phase in rod-like shape with a size of 10–20 nm embeds within the binder Co and is clearly well separated from WC grains. Further decarburization-induced W was detected mainly in Co binder, being apart entirely from WC grains. The main advantage of Co for preventing decarburization in WC–Co particles is not associated with oxidation, but instead the diffusion-controlled carbon loss. These findings would facilitate fabrication of the WC-based cermet coatings with excellent mechanical properties in particular wear resistance for extreme wear applications.     

Experimental Investigation and Optimization of Machining Characteristics in Ultrasonic Machining of WC–Co Composite Using GRA Method

WC–Co composite material is highly demanded in manufacturing industries, because of its unique properties such as excellent hardness with toughness, higher mechanical strength, and good dimensional stability. The present investigation is aimed at studying the impact of different experimental conditions (by varying cobalt content, thickness of work piece, tool profile, tool material, abrasive grit size, and power rating) on responses of interest (material removal rate and tool wear rate) in ultrasonic drilling of WC–Co composite material. The experiments have been planned by using Taguchi's L-36 orthogonal array and grey relation analysis has been applied for optimization of multiple responses. Analysis of variance is also employed to find the significant factors. Significant effects are observed for process variables such as cobalt content, abrasive grain size, and power level. Tools with higher hardness delivered better machining performance.     

Characteristics of dissimilar laser-brazed joints of isotropic graphite to WC–Co alloy

The effect of Ti serving as an activator in a eutectic Ag–Cu alloy filler metal in dissimilar laser-brazed joints of isotropic graphite and a WC–Co alloy on the joint strength and the interface structure of the joint is investigated in this study. To evaluate the joint characteristics, the Ti content in the filler metal was increased from 0 to 2.8 mass%. The laser brazing was carried out by irradiating a laser beam selectively on the WC–Co alloy plate in Ar atmosphere. The threshold content of Ti required to join isotropic graphite to WC–Co alloy was 0.4 mass%. The shear strength at the brazed joint increased rapidly with increasing Ti content up to 1.7 mass%, and a higher Ti content was found to be likely to saturate the shear strength to a constant value of about 14 MPa. The isotropic graphite blocks also fractured at this content. The concentration of Ti observed at the interface between isotropic graphite and the filler metal indicates the formation of an intermetallic layer of TiC.     

On bending strength of superhard composite materials based on WC–Co hardmetals

We present analytical algorithms for computing the ultimate bending strength of superhard composite materials based on WC–Co hardmetals. The study is performed for fine-grained materials (where mean particle size of the dispersed superhard phase d _C and that of carbide grains d _WC are of the same order of magnitude) and coarse-grained materials (with d _C ≫ d _WC ). The strength of the composite is assumed to be governed by the strength of its hardmetal matrix. The stressed state of the matrix is assessed through volume-average microstresses for fine-grained materials and interface-average stresses for coarse-grained composites. The calculated results presented in the form of tables and graphs have been analyzed. The strength has been found to decrease drastically with increasing particle size of the superhard phase and its concentration in the composite.     

Investigation of solidification behavior and associate microstructures of Co–Cr–W and Co–Cr–Mo alloy systems using DSC technique

This article presents a study of solidification behavior and the corresponding microstructure of Co–Cr–W and Co–Cr–Mo alloy systems using the differential scanning calorimetry technique. The influence of main constituents on the solidification behavior and associate microstructures of these alloys are investigated. It is found that chemical composition influences significantly the solidification behavior of cobalt-based alloys. Solution-strengthened alloy has the highest solidification temperature and narrowest solidification range. Presence of carbon decreases the solidification temperature and increases the solidification range. Addition of boron greatly decreases the solidification temperature. Carbon content dominates the solidification behavior of cobalt-based alloys when the contents of the solution-strengthening elements Mo and Ni are within their saturation in the solution matrix. However, as these contents reach a certain level, formation of intermetallic compounds changes the solidification behavior of these alloys remarkably. Increase in the contents of solution-strengthening elements reduces the solid solution transformation temperature and the eutectic temperature when carbon content is constant.     

Characterization of tool wear when machining alloy 718 with high-pressure cooling using conventional and surface-modified WC–Co tools

Coolant supplied by high pressure into the cutting zone has shown the lower thermal loads on the tool when machining difficult-to-cut materials as the Alloy 718. In this study, we investigate how the combination of high-pressure cooling and tool–surface modifications can lead to further improvements regarding tool life. The general approach is to enhance the coolant–tool interaction by increasing the contact area. Therefore, we machined cooling features into flank and rake faces of commercially available cemented tungsten carbide inserts. In this way, the surface area was increased by ~ 12%. After the cutting tests, the tools were analyzed by scanning electron microscopy combined with energy-dispersive X-ray spectroscopy. Compared with conventional tools, the tool modifications reduced the flank wear by 45% for the investigated cutting parameters. Furthermore, we were able to significantly increase the cutting speed and feed rate without failure of the tool. The investigated surface modifications have great potential to enhance the productivity of metal cutting processes.     

Combustion synthesis and densification of large-scale TiC–xNi cermets

Large-scale TiC–xNi cermets with 240-mm diameter were fabricated by self-propagating high-temperature synthesis and combined with pseudo heat isostatic pressing. Combustion-synthesized products consisted of TiC phase and Ni binder phase. Spheroidal TiC particles were enveloped by nearly continuous Ni binder phases. Size of TiC particles decrease with Ni content increase. Synthesized products have excellent mechanical properties and the bending strength of TiC–20Ni and TiC–30Ni is close to K151A and K152B, respectively, produced by traditional powder metallurgy technology.     

浅析WC—Co硬质合金研究现状

我国一直以来是硬质合金的生产和消费的大国;硬质合金的产量从2003年开始一直稳居世界第一位,我国硬质合金产量达到了整个硬质合金市场产量的20％-30％。但是现阶段我国并不是硬质合金的生产强国,这主要是由于我国硬质合金产品方面的结构和技术含量以及一些附加值都落后于国外将近10年以上。这使得我国硬质合金生产主要集中在低档合金产品的生产,而高性能合金的技术和产品很少。这种情况也为我国发展超细晶硬质合金技术提出了严峻的挑战。