The wear of WC-Co drill bits during rotary-percussive drilling of reinforced concrete

The wear mechanisms of WC-Co hardmetal drill bits during rotary-percussive drilling of reinforced concrete depend a number of factors such as the loading spectrum (thermo-mechanical stresses); workpiece material (type of concrete and reinforcement steel); hardmetal grade (WC-grainsize and Co-content); and drill bit geometry (design and location on the drill bit).Differences in wear mechanisms and wear intensity are evident, being highly dependent on the factors listed above. Additionally, the extreme conditions acting on the cutting edges during drilling of reinforcement steel (rebar) within the concrete induce significant microstructural and metallurgical changes within the near-surface regions of the drill bit.In this work, experimental studies have documented, analysed and quantified the active wear mechanisms in this complex and challenging application. A wide range of experimental variables have been investigated covering the large proportion of real-world cases encountered in the application of these drill bits on the construction site.     

Sintering behavior and microstructural evolution of NbC-Ni cemented carbides with Mo2C additions

The effect of Mo₂C additions to a NbC-12vol%Ni-0.5vol%WC material on sintering at 1450 °C and on the resulting microstructure was investigated by replacing different amounts of NbC with Mo₂C (0%, 3%, 6% or 9% in volume) and by performing interrupted tests at key temperatures (1080 °C, 1180 °C and 1320 °C). Chemical reactions and phase transformations were followed by Thermogravimetric Analysis (TGA) and sintering behavior by dilatometry. The microstructure was studied by Scanning and Transmission Electron Microscopy (SEM and TEM), Energy Dispersive Spectrometry (EDS) and X-ray diffraction (XRD). It is observed that Mo₂C and WC particles, present at the onset of solid state sintering, delay the shrinkage of the material. An increase of hardness with Mo₂C addition is measured, mainly due to grain refinement.     

Influence of nitridation on surface microstructure and properties of graded cemented carbides with Co and Ni binders

The influence of a nitridation treatment on the microstructure and the properties of (W,Ti)C-based cemented carbides with Co and Ni binders is investigated. Nitridation results in the formation of a very fine-grained (Ti,Ta,Nb)(C,N) phase (γN-phase) with inclusions of binder and WC clusters in the near-surface region of the cemented carbides. Rietveld refinements of X-ray diffractograms reveal differences in the domain size and the microstrains of the γN-phase for cemented carbides with Co-binder and Ni-binder. Nitridation reduces the domain size and increases microstrains in the γ-phase. The chemical composition, the morphology and defects of the binder were investigated by TEM/EDX before and after the nitridation treatment. The results reveal that a high nitrogen activity leads to the preferred dissolution of the core-rim structure of the mixed cubic carbide grains during the formation of the γN-phase. The nitridation heat treatment improves the wear and corrosion resistance of cemented carbides with both Co-binder and Ni-binder.     

Kinetics of formation of graded layers on cemented carbides: Experimental investigations and DICTRA simulations

Kinetics of formation of fcc-free layers on Co-W-Ti-Ta-Nb-C-N cemented carbides was investigated by experimental methods and DICTRA simulations. The layer formation obeys a parabolic law, indicating a diffusion-controlled process. For DICTRA simulations, the influence of the mobilities for all diffusing elements in the liquid binder phase at the sintering temperature was investigated. The best agreement between experimental and simulations was obtained considering that the mobility of all metallic elements is two times slower compared with the mobility of the non-metallic elements.     

The manufacture and characterization of WC-(Al)CoCrCuFeNi cemented carbides with nominally high entropy alloy binders

Recently, there has been increasing interest in cemented tungsten carbide hardmetals and titanium carbonitride cermets with binders of multi-component alloys (≥4 elements) or high entropy alloys (≥5 principal elements in equimolar ratios). Property improvements have been reported, such as increased ambient and elevated temperature hardness, as well as greater oxidation resistance.This study has thoroughly investigated model cemented carbides manufactured using coarse WC with a binder content of 20 wt% (32–37 vol%) from three different (Al)CoCrCuFeNi high entropy alloys (HEAs) and at different carbon levels (low, medium and high).Binder alloys were manufactured by both planetary ball milling of elemental powder mixtures and gas atomizing. Sintering was performed in vacuum for 2 h at different temperatures between 1200 °C and 1500 °C. Post-HIP treatments were also applied in some cases as all systems were difficult to densify without residual porosity.Detailed analyses were performed on the as-manufactured binder alloys, sintered binder alloys (without WC) and the actual sintered cemented carbides (WC + HEA). Various analysis methods were used to examine the materials. These included thermogravimetry analysis (TGA) and differential scanning calorimetry (DSC) to determine the melting behaviour; X-Ray Diffraction (XRD), Electron Backscatter Diffraction (EBSD) and Energy-Dispersive X-Ray Spectroscopy (EDS) to identify the type, crystal structure and exact composition of the phases present; and light optical microscopy (LOM) and scanning electron microscopy (SEM) for microstructural characterization. Additionally, the hardness and Palmqvist indentation toughness of each composition were also measured.2-Phase WC-HEA microstructures could not be obtained using the investigated high entropy alloys. Several solid solution binder alloys and numerous carbide phases were present after sintering, formed by segregation and reaction. The type and quantity of the phases depend on the carbon balance. For the compositions containing aluminium, it was found that aluminium forms oxides and intermetallic phases during sintering. The paper presents these findings in detail.     

梯度硬质合金梯度层形成的计算机模拟及验证

结合相图热力学计算,使用DICTRA软件计算模拟Co-W-Ti-C-N、Co-W-Ti-Nb-C-N和Co-W-Ti-Ta-C-N体系梯度硬质合金梯度层形成过程,对比计算模拟和实测的梯度硬质合金中Co含量的距离变化曲线。通过分析各相体积分数及组元成分随距离的分布研究烧结时间、烧结温度、Co含量和Ti含量对梯度层厚度的影响。结果表明:计算模拟与实验数据吻合较好。延长烧结时间、升高烧结温度和增加Co含量均会促进梯度层厚度的增加,而增加Ti含量则会抑制梯度层厚度的增加。     

Damaging of cemented carbide end mill with different grain sizes: experimental and simulation

The main purpose of the current study was to investigate the effects of the size of WC grains on the damage evolution of WC-Co junk mills.The finite element met...     

Synthesis and processing of nanocrystalline tungsten carbide: Towards cemented carbides with optimal mechanical properties

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.     

Wear damage of cemented carbides with different combinations of WC mean grain size and Co content. Part I: ASTM wear tests

In the present work we made and examined cemented carbides characterized by very different WC grain sizes varying from near-nano with a WC mean grain size of about 200 nm to coarse-grain with a WC mean grain size of about 4.5 μm and Co contents varying from 3 to 24 wt.%. The major objective of the present work was to examine the wear damage, wear behavior and wear mechanisms of cemented carbides having nearly the same hardness but greatly varying with respect to their WC grain size and Co content in the high-load ASTM B611 test and low-load G65 test.Both the hardness and resistance to fracture and micro-fatigue of cemented carbides play an important role in the wear damage by use of the high-stress ASTM B611 test when the carbide surface is subjected to alumina particles at high loads. In this case, the wear-resistance increases with increasing the WC mean grain size and decreasing the Co content at nearly the same hardness of the different cemented carbides. The submicron and near-nano cemented carbides are characterized by lower wear-resistance in comparison with the coarse-grain grade due to their reduced fracture toughness, fracture resistance and resistance to micro-fatigue.The Co mean free path in the carbide microstructure plays an important role with respect to wear-resistance in the low-stress ASTM G65 test when the carbide surface is subjected to gentle scratching by abrasive silica particles. The predominant wear of the thick Co interlayers leaving unsupported WC grains plays the decisive role in the wear behavior of the coarse-grain grade resulting in its low wear-resistance. In contrast to the ASTM B611 test the wear rate decreases with decreasing the WC mean grain size and increasing the Co content due to the corresponding reduction of Co mean free path in the carbide microstructure. As a result, the wear-resistance of the near-nano grade in the ASTM G65 test is the best of all in spite of its reduced fracture toughness.Phenomena of micro-fatigue, micro-fracturing and micro-chipping are found to play a decisive role in the wear damage of cemented carbides if they are subjected to abrasion wear, high loads and severe fatigue.     

Study of cemented carbonitrides with nickel as binder: Experimental investigations and computer calculations

Cobalt is the most common binder in cemented carbides industry. However, there are some interests in use of alternatives. The similarity in properties has made nickel the first choice. In the present work, the effect of initial composition on modern hardmetals containing transition metal carbides/carbonitrides that are called “cemented carbonitrides” with nickel as binder was investigated. Change in quantity of additive carbides and tungsten to carbon (C/W) weight ratio through applying metallic tungsten powder in primary powder mixture had some effects on final hardness, transverse rupture strength, and microstructure of studied alloys. Addition of vanadium carbide not more than 0.2 wt.%, increased the final hardness. Application of (Ta,Nb)C solid solution carbide cancelled the grain refinement effect of VC. Formation of eta (η) phase was observed in SEM micrographs as a result of increase in W/C weight ratio. Surface modified layers with thicknesses ranging from 55 to 65 μm called Cubic Free Layer (CFL) was observed in vacuum sintered specimens. DICTRA® module from Thermo-Calc® software package was applied for simulation of CFL formation process in studied alloys. Formation of eta phase and consumption of metallic binder was predicted using calculations of Thermo-Calc® ver. P software. A state of the art technique was developed to prove linear relationship between Labyrinth factor (λ) and binder phase volume fraction (f).     

不同粒度的金刚石套料钻钻削碳纤维复合材料时对轴向力及出入口质量的影响

碳纤维增强复合材料在加工过程中,容易产生分层、毛刺、撕裂等缺陷,还会加剧刀具的磨损。针对以上问题,研制了不同粒度的金刚石套料钻,进行了碳纤维复合材料制孔加工实验,分析了磨粒粒度对轴向力、扭矩和出口质量的影响。得到以下结论:采用有细粒度金刚石的钎焊套料钻可以减小钻孔时的轴向力和扭矩。不同粒度套料钻所加工的孔入口处均未出现任何缺陷,出口处的缺陷主要以撕裂缺陷为主。金刚石粒度80/100的钎焊套料钻在进给速度为100 mm/min,主轴转速15 000 r/min的条件下出入口质量优于其他粒度。     

Sintering behavior and mechanical properties of Cr3C2 doped ultra-fine WC-Co cemented carbides: Experment guided with thermodynamic calculations

The influence of Cr₃C₂ doping on the sintering process and mechanical properties of WC-Co cemented carbides was studied. Using differential thermal analysis of green powders and thermodynamic calculations, the disappearing temperature of solid-state binder phase in the ultra-fine WC-Co cemented carbides with different amounts of Cr₃C₂ dopant was first investigated and then verified to descend with the increase of Cr content. Based on these investigations, the sintering temperatures of three alloys with 0.3, 0.5 and 0.65 wt% Cr were selected to high by 50 °C than the phase disappearing temperature of solid-state binder. Compared with the commercial sample with the sintering temperature at 1410 °C for Cr₃C₂ doped WC-Co cemented carbides, the optimized sintering temperatures lead to finer microstructures and better mechanical properties, such as transverse rupture strength and hardness. In addition, the reliability for the performance of WC-Co cemented carbides prepared with the optimized sintering schedule is significantly improved in comparison with the commercial sample. The strategy from the present work can be used to design sintering process parameters during the manufacture of ultrafine WC-Co cemented carbides.     

WC-base cemented carbides with partial and total substitution of Co as binder: Evaluation of mechanical response by means of uniaxial compression of micropillars

The influence of the chemical nature of the metallic binder on the plastic deformation of cemented carbides was studied. Three different cemented carbide grades - WC-Co, WC-CoNi and WC-NiMo - with similar microstructural characteristics (binder content and carbide grain size) were investigated. Mechanical response was evaluated by means of uniaxial compression of micropillars, and tests were carried out in-situ in a FESEM with a nanoindenter equipped with a flat-diamond punch. After uniaxial compression, inspection of deformation phenomena was done at both surface and bulk of micropillars through scanning and transmission electron microscopy, respectively. It is found that yielding phenomena and strain hardening increase as Co is totally substituted by a NiMo alloy, while contrary effect results from partial replacement of Co with Ni. Relative differences are directly linked to intrinsic ductility of the metallic phase and operative plastic deformation mechanisms. Moreover, for the three materials studied, stress-strain responses show pronounced yielding events related to glide at WC/WC interfaces. Although they are discerned at different stress levels, estimated values of sliding resistance of WC/WC boundaries are found to be alike for the three grades studied.     

Threshold tool-radius condition maximizing the formability in SPIF considering a variety of materials: Experimental and FE investigations

In the current study, a new level of understanding on the influence of using small tool radii on the formability (θ_max) is identified for single point incremental forming (SPIF). The relative value of tool radius and blank thickness (i.e., R/T_B, where R is the tool radius and T_B is the blank thickness) was varied over a range (from 1.1 to 3.9), and a formability diagram in the R/T_B–θ_max space was obtained. The formability was observed to show an inverse V-type pattern which revealed that there is a critical radius of tool (R_c) that maximizes the formability in SPIF. Further, this radius which was found to be independent of the material type (or property) is a function of blank thickness related as, R_c≈2.2T_B. This radius was termed as threshold radius. The formability, in agreement with general opinion in the literature, was noticed to increase with the decrease in the tool radius above the threshold value. However, contrarily it reduced with the decrease in the tool radius below the threshold value. In fact, undue surface cutting and metal squeezing was detected when the tests were performed with pointed tools, i.e., below threshold radius. This unstable deformation, which according to the FE analyses was found to be an outgrowth of in-plane compression under the tool center, increasingly weakened the material by inducing corresponding increase in damage (quantified by stress triaxiality) with the decrease in the tool radius. On the other hand, the damage was also observed to increase due to decrease in compression with the increase in the tool radius above the threshold value. This revealed high compression with low damage constitutes the most conducive condition that maximizes the formability in SPIF, which is realized when R≈2.2T_B.     

Experimental evaluation and FE simulation of thermal conditions at tool surface during cooling and deformation phases in hot forging operations

In hot and warm forging operations, surface layers of tools at the tool-workpiece interface are not only exposed to high mechanical stresses but also to severe temperature cycles, which often lead to loss of strength and hardness and thermal fatigue failure as well.This paper offers an approach for determining heat transfer conditions at the surface of punches and dies during both the deformation and the cooling-lubrication phases of forging cycles. The approach is based on temperature readings inside the tool, FE simulation and inverse analysis. An application case is illustrated where operating conditions reproduce hot forging of turbine airfoil sections.     

Mechanical and microstructural single-crystal Bauschinger effects: Observation of reversible plasticity in copper during bending

We study the Bauschinger effect on a bent and straightened micro-sized single-crystal copper beam (width: 8.64 μm; thickness: 7.05 μm) over three consecutive cycles. The reverse yield strengths (straightening step) are much smaller than those in forward loading (bending step). An upper bound estimate shows a load drop of 73% (1st cycle), 76% (2nd cycle) and 83% (3rd cycle) relative to the forward yield stress. Electron backscatter diffraction reveals a dramatic reduction in the bending-induced misorientation gradients upon load reversal (straightening), documenting an unexpected form of microstructure reversibility. The observed Bauschinger softening is interpreted in terms of two effects. The first consists of internal backstresses that support load reversal. They are created by polarized dislocation arrays that are accumulated during forward bending. The second effect is the reduced requirement to activate dislocation sources during reverse loading as the dislocations that were stored during bending did not participate much in cross-hardening and, hence, serve as mobile dislocations upon reverse loading. After straightening the misorientation gradients are largely removed but the non-polarized dislocations remain. We therefore introduce a revised terminology, namely the “mechanical Bauschinger effect” and the “microstructural Bauschinger effect”. The former term describes a yield stress drop and the latter one the degree of microstructure reversibility upon load path changes.     

Splat morphology of plasma sprayed aluminum oxide reinforced with carbon nanotubes: A comparison between experiments and simulation

This study elucidates the effect of carbon nanotube (CNT) addition on the splat formation in plasma sprayed aluminum oxide (Al₂O₃) composite coating using experimental and computational methods. CNT content was varied as 0, 4 and 8 wt.% in Al₂O₃ matrix. With an increasing CNT content, splat morphology became more circular and disk-shaped. The average diameter of disk-shaped splats increased from 28.6 ± 1.4 μm for Al₂O₃ to 43.2 ± 1.3 μm for Al₂O₃–8 wt.% CNT. The population density of splats with fingers, fragments, and voids was the lowest for the highest (8 wt.%) CNT content. The addition of CNTs resulted in two simultaneously competing phenomena viz. increased thermal capacity and increased viscosity of the melt. Increased thermal capacity delayed the localized solidification resulting in higher splat diameter while agglomeration of CNTs at the periphery of the splat results in higher viscosity of the melt which suppresses the splat fragmentation that leads to increased population density of disk shaped splats. Splat morphology of three compositions was also simulated using SIMDROP software, which showed a good agreement with the experimentally collected splats.     

Polarization resistance measurements of bars embedded in concrete with different chloride concentrations: EIS and DC comparison

The experimental conditions for corrosion rate measurements of reinforcement by means of the polarization resistance technique, R_p, in order to obtain reliable values (in accordance with the gravimetric losses simultaneously measured) must consider some factors as are: the need to subtract ohmic drop generated by the high resistivity of the concrete, the need to wait a certain time after application of the current, the use of a certain sweep rate in order to obtain a quasi‐steady‐state value of the conjugated variable (potential/current), the need to avoid the overlapping with other processes, and the need to use a correct B value in Stern formula. These experimental requirements should be optimized for the individual kinetic of each system. From time to time several papers are published wondering whether R_p measurements in concrete are reliable. Present paper extends the study of the experimental conditions influence in the R_p determination in order to look more in detail to the accuracy of R_p technique. Several sweep rates in potentiodynamic tests, different waiting times with galvanostatic and potentiostatic pulses and different frequencies in electrochemical impedance spectroscopy, EIS, were tested in order to compare the most suitable parameters that provide correct values of the corrosion rate, I_corr.     

Influence of the depth and morphology of real cracks on diffuse ultrasound in concrete: A simulation study

The aim of the present paper is to simulate the propagation of diffuse ultrasonic energy in concrete in the presence of a real crack. The numerical model is presented and validated by the comparison with experimental data from the literature. Unlike most of the studies which consider a crack as a notch, a realistic crack morphology exhibits partial contacts along its lips. These contacts are modeled in order to study their influence on the diffusion parameters. The feasibility of determining the contact density of the crack is shown, revealing practice implications for non-destructive crack sizing and imaging in concrete.