Cobalt capping: Why is sintered hardmetal sometimes covered with binder?

Some sintered hardmetal grades come out of the furnace covered with a thin layer of binder at the surface, while others do not. The layer can be beneficial for subsequent brazing, but it must be removed by grinding or etching if the hardmetal is to be coated. It would be beneficial to be able to control this “cobalt capping” during sintering by applying suitable sintering conditions. For this purpose, the mechanism of cobalt capping was investigated. The capping occurs on the cooling ramp, at the eutectic temperature of the binder. If the binder solidifies first in the interior, the contraction caused by the phase transition squeezes melt out of the exterior zones. The contraction of the specimen was verified with dilatometry. Conditions leading to cobalt capping are: higher carbon content in the surface than in the bulk (decrease of melting point), high binder content, especially in the surface zone, and a low cooling rate.     

Diffusion parameters of grain-growth inhibitors in WC based hardmetals with Co,Fe/Ni and Fe/Co/Ni binder alloys

The diffusion behaviour of the grain-growth inhibitors (GGI) Cr and V during early sintering stages from 950 to 1150 °C was investigated by means of diffusion couples of the type WC-GGI-binder/WC-binder. Besides Co, also alternative Fe/Ni and Fe/Co/Ni binder alloys were investigated. It was found that the diffusion in green bodies differs significantly from sintered hardmetals. Diffusivities in the binder phase were determined from diffusion couples prepared from model alloys and were found to be almost equal for Co and alternative binder alloys. The diffusion parameters determined from green bodies allowed to estimate the GGI distribution in a hardmetal during heat up. This was subsequently used to estimate an appropriate grain size of VC and Cr₃C₂ in hardmetals, which is required to ensure a sufficient GGI distribution during sintering before WC grain-growth initiates.     

Fabrication of cemented tungsten carbide components by micro-powder injection moulding

Micro-powder injection moulding (micro-PIM) is an advanced net-shaping process for the fabrication of metal and ceramic complex micro-components. Cemented tungsten carbide (WC–Co) hardmetal is known for its high hardness and wear resistance in various applications. Micro-PIM is a new alternative manufacturing technique for hardmetal micro-parts. In this work, the fabrication of WC–Co components via a micro-PIM process was studied. A fine WC–10Co–0.8VC (wt.%) powder was mixed with a binder system consisting of paraffin wax, low density polyethylene and stearic acid. A micro-component was injected at low pressure using a semi-automatic injection moulding machine. The injection temperature was determined from the rheological investigation of the feedstock. The binder extraction was carried out in solvent and thermal debinding methods under an argon atmosphere. Thermal gravimetric analysis was used to confirm the removal of the soluble binder from the green part. The sintering process has been performed within a temperature range of 1330–1450 °C under vacuum. After sintering, a density of 94.5% theoretical density was obtained, which is a reasonable value. The micro-components showed length shrinkage between 16 and 22% and good surface quality and hardness values when compared with conventional powder metallurgy. This research shows that micro-PIM is able to produce small WC–Co components with properties comparable to conventional powder metallurgy.     

Influence of microstructure on hardness and thermal conductivity of hardmetals

Hardmetals or cemented carbides are used in a wide range of applications due to their excellent mechanical properties. WC-Co hardmetals with the same room temperature hardness can be obtained by different combinations of the WC grain size and cobalt content. However, the thermal conductivity of such hardmetal grades is not equal. Applications such as cutting may require a certain combination of hardness and thermal conductivity, which means that a targeted adjustment is desirable. In this study a wide range of hardmetal grades was studied in respect of microstructure, hardness and thermal conductivity in the temperature range between 20 °C and 1000 °C. Results show that thermal conductivity is considerably influenced by Co content, WC grain size and Cr₃C₂ content. Furthermore, hardmetal grades with the same hardness at room temperature retain hardness very differently at elevated temperatures. For the selection of hardmetal grades for high temperature applications these findings help to choose the right composition in regard to Co content and WC grain size.     

Effect of substrate on the 3 body abrasion wear of HVOF WC-17 wt.% Co coatings

WC-17 wt.% Co coatings were deposited using high velocity oxy-fuel (HVOF) spraying onto four different substrate materials, namely aluminium, brass, 304 stainless steel and super-invar. These substrates have different coefficients of thermal expansion which have been shown to influence the final coating microstructural properties. The abrasive wear properties of the coatings were characterised using an ASTM-G65 three body abrasive wear machine with silica sand as the abrasive. The highest mass loss was recorded for the coating on the aluminium substrate whilst the coated 304 stainless steel showed the lowest mass loss. The coatings on brass and super invar experienced similar mass losses. SEM studies of the worn surfaces showed preferential removal of the Co binder phase as well as cracking and rounding of the carbide grains. The differences in wear behaviour may be attributed to the presence of residual stresses where the highest compressive residual stress led to the highest wear rate. The coatings deposited onto brass showed compressive stresses whilst those deposited onto super-invar had tensile stresses, yet these two coatings had similar wear rates. Thus further study is required to provide conclusive evidence of the role of residual stresses on the abrasion resistance of these coatings.     

The influence of Johnson–Cook material constants on finite element simulation of machining of AISI 316L steel

In literature, five different sets of work material constants used in the Johnson–Cook's (J–C) constitutive equation are implemented in a numerical model to describe the behaviour of AISI 316L steel. The aim of this research is to study the effects of five different sets of material constants of the J–C constitutive equation in finite-element modelling of orthogonal cutting of AISI 316L on the experimental and predicted cutting forces, chip morphology, temperature distributions and residual stresses. Several experimental equipments were used to estimate the experimental results, such as piezoelectric dynamometer for cutting forces measurements, thermal imaging system for temperature measurements and X-ray diffraction technique for residual stresses determination on the machined surfaces; while an elastic–viscoplastic FEM formulation was implemented to predict the local and global variables involved in this research. It has been observed that all the considered process output and, in particular the residual stresses are very sensitive to the J–C's material constants.     

In-situ observation of hardmetal deformation processes by transmission electron microscopy. Part II: Deformation caused by tensile loads

In the 1st part of this article, hardmetal deformation processes caused by bending loads were examined in-situ by transmission electron microscopy. The major objective of this work is to examine hardmetal deformation processes in special thin hardmetal samples as a result of applying tensile loads in-situ directly in a transmission electron microscope with the aid of the push-to-pull method. Applying tensile loads to the samples results in the plastic deformation of the Co-based binder phase leading to the formation of different crystal lattice defects in the binder. Force-time and displacement-time curves recorded when loading the samples and maintaining the loads provide evidence for continuous processes of the formation and movement of crystal lattice defects, presumably dislocations, in the WC phase and Co-based binder leading to a high rate of the binder plastic deformation. After increasing the tensile loads up to a certain level leading to the severe plastic deformation of the binder phase, the samples suddenly fail as a result of the crack initiation and propagation at WC-Co interfaces. Presence of cobalt on the WC surface after the cracking suggests that the cracks propagate through the binder region adjacent to the interface rather than through the interface itself.     

Wear behaviour and wear mechanisms of different hardmetal grades in comparison with polycrystalline diamond in a new impact-abrasion test

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.     

Microstructure and properties of HVOF-sprayed chromium alloyed WC-Co and WC-Ni coatings

In this paper the influence of the type of binder metal (nickel or cobalt) and chromium as an additional alloying element on the microstructure, mechanical properties and wear resistance of high velocity oxy-fuel (HVOF)-sprayed WC-based hardmetal coatings was studied. Plain WC-Co and WC-Ni as well as five chromium alloyed compositions were sprayed with a liquid-fueled HVOF-spray process from commercial and experimental agglomerated and sintered feedstock powders. The coating characterization included optical microscopy and SEM of metallographically prepared cross-sections, hardness measurements, determination of the Young's modulus and phase composition by X-ray diffraction. Erosion and dry oscillating sliding wear were studied. The resistance to erosive wear was found to be improved when cobalt was used as binder metal. A dependence on the chromium content was not detected. For the oscillating sliding wear against a hardmetal counterbody there is no dependence of the wear rate on the type of binder metal or the amount of chromium.     

Modeling of Residual Stress in Oxide Scales

The magnitude of the residual stress in an oxidescale, and how this varies with temperature, is of majorimportance in understanding the failure mechanisms ofoxide scales. This stress encompasses both growth stresses introduced at the oxidationtemperature and thermal-expansion-mismatch stressesinduced on heating and cooling, as well as anyexternally applied stresses or stress relaxation whichtakes place in the scale/substrate system. Althoughsome of these components are reasonably well understood(e.g., thermal stresses), growth stresses and therelaxation of the total scale stress by creep orfracture processes are much less well understood. Inthis study a model has been developed to predict stressgeneration and relaxation in oxide scales as a functionof time and temperature for both isothermal exposure and cooling to room temperature. The modeldetermines growth stress and thermal-stress generationin the scale and how this is balanced by stresses in thesubstrate. The substrate stresses are then allowed to relax by creep and the scale stressesrecalculated. This model accurately predicts theroom-temperature scale stresses for a range ofscale/alloy systems. The model can be used to show howthe scale stress depends on oxidation temperature,cooling rate, substrate, and scale thickness. The modelpredictions are discussed in light of experimentalobservations for alumina scales on FeCrAlY.     

On the influence of binder content and binder composition on the mechanical properties of hardmetals

In order to achieve an understanding of damage mechanisms and the role of the binder phase in the fatigue behaviour of hardmetals, two series of hardmetals with 7, 15 and 27 wt% of Co and CoNiFe binder phases were investigated in a reversed bending stress apparatus. Microstructural investigations were carried out using SEM and TEM. Finite element simulations based on real microstructure cut-outs were used to analyse the stress and strain states. While both series of hardmetals exhibit inert strengths which depend strongly on the binder content, their fatigue behaviour is totally different. For the WC–Co grades, no significant influence of the binder content on the fatigue behaviour was found. In contrast to this, the fatigue behaviour of the WC–CoNiFe grades showed a clear dependence on the binder content. The microstructural investigations revealed that the mechanism of stacking fault formation and phase transformation in the Co binder phase, together with a stress concentration in the binder pools are responsible for the fatigue behaviour of the WC–Co grades. In WC–CoNiFe grades, no stacking faults or transformed zones were observed. In addition, the CoNiFe binder phase reveals a very ductile flow nature, which results in large plastically deformed zones in the binder phase.     

Finite Element Prediction of Residual Stress in Rhombic Dodecahedron Ti-6Al-4V Titanium Alloy Additively Manufactured by Electron Beam Melting

In this work, a three-dimensional nonlinear transient thermo-mechanically coupled finite element model (FEM) is established to investigate the variation in temperature and stress fields during electron beam melting (EBM) of rhombic dodecahedron Ti-6Al-4V alloy. The influence of the processing parameters on the temperature and residual stress evolutions was predicted and verified against existing literature data. The calculated results indicate that the interlayer cooling time has very little effect on both the temperature and stress evolutions, indicating that the interlayer cooling time can be set up as short as possible to reduce manufacturing time. It is presented that the residual stress of the intersection is higher than that of non-intersection. With increasing preheating temperature, the residual stress decreases continuously, which is about 20%-30% for every 50 °C rise in temperature. The temperature and stress fields repeated every four layers with the complex periodic scanning strategy. Both x and y-component residual stresses are tensile stresses, while z-component stress is weak compressive or tensile stress in typical paths. It is proposed that the interlayer cooling is necessary to obtain a rhombic dodecahedron with low residual stress. These results can bring insights into the understanding of the residual stress during EBM.     

In situ neutron diffraction study of residual stress development in MgO/SiC ceramic nanocomposites during thermal cycling

Ceramic nanocomposites often contain large residual stresses due to differing thermal contraction between phases upon cooling from processing temperatures. Their role in affecting the mechanical properties is not fully understood, but is certainly of importance. This investigation used neutron diffraction to quantify the residual stresses in MgO/SiC nanocomposites throughout a thermal cycle to 1550 °C. The results showed that average stresses in 10 vol.% SiC samples at 100 °C approached −4 GPa in the particles and were +560 MPa in the matrix. The stresses showed good agreement with an elastic model with a stress-free temperature of 1600 °C. A small amount of inelastic relaxation (15%) was observed after cooling back to room temperature. Modelling suggested that this was due to relaxation of the stresses in grain boundary particles at a rate limited by diffusional processes in the MgO/SiC interface. The effect of particle size on stress level is discussed.     

Fracture toughness of cemented carbides obtained by electrical resistance sintering

The unique combination of hardness, toughness and wear resistance exhibited by WC-Co cemented carbides (hardmetals) has made them a preeminent material choice for extremely demanding applications, such as metal cutting/forming tools or mining bits, in which improved and consistent performance together with high reliability are required. The high fracture toughness values exhibited by hardmetals are mainly due to ductile ligament bridging and crack deflection (intrinsic to carbides). In this work two WC-Co grades obtained by using the electric resistance sintering technique are studied. The relationships between the process parameters (cobalt volume fraction, sintering current and time, die materials, etc.), the microstructural characteristics (porosity, cobalt volume fraction, carbide grain size, binder thickness and carbide contiguity) and mechanical properties (Vickers hardness and fracture toughness) are established and discussed. Also the presence of microstructural anisotropy and residual stresses is studied. The sintering process at 7 kA, 600 ms and 100 MPa, in an alumina die, followed by a treatment of residual stress relief (800 °C, 2 h in high vacuum), allows to obtain WC-Co pellets with the best balance between an homogeneous microstructure and mechanical behaviour.     

Fatigue testing and properties of hardmetals in the gigacycle range

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 10¹⁰ 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.     

The mathematical modeling of phase transformation of steel during quenching

In the heat treatment of steel, uneven cooling in variably introduces residual stresses in the workpiece. These residual stresses can combine with the thermomechanical stresses encountered in operation to cause premature fatigue failure of the material. A prediction of the residual and thermoelastoplastic stresses developed during heat treatment would be beneficial for component design. In this article a numerical model is developed to predict the thermoelastoplastic and residual stresses during rapid cooling of a long solid cylinder. The total strains developed during cooling of the cylinder comprise elastic, thermal, and plastic strains and strains due to phase transformation. For plastic deformation an extension of Jiang’s constitutive equations developed by Jahanian is adopted. The properties of the material are assumed to be temperature dependent and characterized by nonlinear strain hardening. For phase transformation two parts are considered: nucleation according to Scheil’s method and phase growth according to Johnson and Mehl’s law. For martensitic transformation, a law established by Koisteinin and Marburger is used. Non-additivity of pearlitic and bainitic nucleation suggested by Manning and Lorig is taken into account by means of a correction factor to Scheil’s summation of the transition from pearlitic to bainitic. The effect of phase transformation and temperature dependence of material properties is investigated. It is shown that by neglecting the temperature dependency and phase transformation in numerical calculations, the results are underestimated. The numerical results are compared with the available experimental data in the literature, and good agreement is observed.     

7075铝厚板内淬火残余应力的形成机理分析与控制策略

残余应力在材料去除后的释放与再分布是引起飞机结构件加工变形的重要因素。而淬火过程中,铝合金厚板内部产生高温度梯度场,由其造成的不均匀塑性变形是形成淬火残余应力的关键所在。为此,通过作为表示工件与介质之间换热能力的对流换热系数,建立了淬火工艺的有限元分析模型,与实验结果的比较分析表明,无论是残余应力幅值还是分布趋势,仿真值均有很高的吻合度。在此基础上,利用有限元方法进行了淬火残余应力的形成机理,认为芯层在终滑点塑性变形结束时最终的残余应力已经形成,之后继续降温过程残余应力基本保持不变。最后以芯层终滑点为依据提出了热换系数的残余应力控性控形方法。从分析结果可知,控形区对最终残余应力的影响很大,控性区段基本不影响最终残余应力。因此,可以通过合理降低控形区邻域和增加控性区邻域内对流热换系数的方法,实现冷却速度的增加和残余应力的减少。     

Modeling of a substrate thermomechanical behavior during plasma spraying

Plasma coating performances and lifetimes may be ruined during service conditions because of uncontrolled residual stress development within the coating. This study presents the results of a CAST3M^® thermomechanical numerical model which purpose is to simulate the different residual stresses development within the duplex coating–substrate during the coating built-up and its comparison with the experimental results. To achieve the thermal spray process understanding all the thermal fluxes transferred to a metallic beam and surrounding temperatures were measured so as to provide the CAST3M^® model with precise boundary conditions, corresponding to a specific geometry. The residual stresses were experimentally determined by the in situ curvature measurement and, afterwards, by the hole drilling method. The plasma torch stand-off distance, the relative torch/substrate velocity and the substrate material were considered as the parameters of this study. The main results concern the substrate temperature and deflection during the preheating stage, the thermal energy transferred by the molten splats to the substrate together with the quenching stress and the development of thermal stress during the final cooling.     

Effect of Residual Stresses and Prediction of Possible Failure Mechanisms on Thermal Barrier Coating System by Finite Element Method

This work is focused on the effect of the residual stresses resulting from the coating process and thermal cycling on the failure mechanisms within the thermal barrier coating (TBC) system. To reach this objective, we studied the effect of the substrate preheating and cooling rate on the coating process conditions. A new thermomechanical finite element model (FEM) considering a nonhomogeneous temperature distribution has been developed. In the results, we observed a critical stress corresponding to a low substrate temperature and high cooling rate during spraying of the top-coat material. Moreover, the analysis of the stress distribution after service shows that more critical stresses are obtained in the case where residual stresses are taken into account.