Electrochemical dissolution of cemented carbide scrap and electrochemical preparation of tungsten and cobalt metals

In this study, WC and WC-6 wt% Co scraps were utilized in molten salt state as two consumable anodes to circulate tungsten and cobalt during electrolysis. Because of poor dissolution of cemented carbide scrap in NaCl-KCl salt, tungsten trioxide was added to NaCl-KCl salt as active substance to improve the dissolution of the anode. Therefore, dissolutions of WC and WC-6 wt% Co anodes were studied at different tungsten trioxide contents. It was shown that addition of tungsten trioxide significantly increased dissolution of the anode. The addition of tungsten trioxide drastically reduced charge transfer impedance and accelerated the dissolution. The electrolyte resistance also reduced slightly. The electrolysis of WC anode was investigated at optimal tungsten trioxide content. Tungsten with single phase was obtained at current density of 0.1 A·cm⁻² for 8 h. The electrolysis of WC-6 wt% Co anode was also explored using same tungsten trioxide content. The data suggested that electrolysis time and current density greatly influenced the composition of cathode product. The preliminary conditions were determined for separation of tungsten and cobalt. Overall, the electrochemical preparations of cobalt and tungsten metal were achieved by two-step electrolysis processes.     

Microstructures and mechanical properties of FeCoCrNi high entropy alloy/WC reinforcing particles composite coatings prepared by laser cladding and plasma cladding

FeCoCrNi HEA coatings with 20% mass fraction of WC reinforcing particles were prepared by two different cladding methods, laser cladding (LC) and plasma cladding (PC). The microstructure of HEA matrix and WC particles of LC and PC coatings were discussed respectively. For HEA matrix, dendritic morphology was observed in both coatings. For WC particles, a few granular (Cr,W)₂C carbides around WC particles in LC coatings, and a large number of crystal and fishbone Fe₃W₃C carbides around WC particles in PC coatings. Mechanical properties as hardness and wear resistance of the two kinds of coatings were also investigated. The interstitial solution strengthening effect of C element is stronger in PC coating, and the hardness of HEA matrix in LC coatings is twice that of in PC coating, which shows a strong retention force on WC particles. The friction coefficient of LC coating is lower and stable, with the volume wear rate of 0.7 × 10⁻⁵ mm⁻³/N·m, showing high wear resistance. PC coatings have poor wear resistance due to decarbonization and oxidation of WC particles and reduction of retention force of HEA matrix, with the volume wear rate of 8.29 × 10⁻⁵ mm⁻³/N·m. The wear mechanism of both coatings were also discussed.     

Synthesis of WC-Co composite powders with two-step carbonization and sintering performance study

In this study, WC-Co composite powder was synthesized by two-step carbonization method using W, Co and C as raw materials. X-ray diffraction (XRD) showed that the η phase (Co₆W₆C) was kept at 1100 °C for 1 h under vacuum, and it could be completely carbonized into WC-Co composite powders. The surface morphology of WC-Co composite powders was analyzed by scanning electron microscope (SEM). The effects of η phase and second phase (W phase) on WC morphology and Co phase distribution were investigated. Electron backscattered diffraction (EBSD) was used to analyze WC-10 wt% Co cemented carbide particle distribution. Comparison of transverse rupture strength, hardness and fracture toughness of two kinds of WC-10 wt% Co cemented carbides synthesized by WC-Co composite powders + WC and WC + Co respectively, the cemented carbide of composite powders + WC increases the fracture toughness from 11.4 ± 0.3 MPa·m^1/2 to 12.4 ± 0.3 MPa·m^1/2.     

WC-Co reinforced NiCoCrAlYTa composite coating: Effect of the proportion on microstructure and tribological properties

The properties of the working surfaces are linked to the safety and lifespan of the modern machines so that variety of coatings are used to protect the parts from breakdown. The NiCoCrAlYTa coating, which has an excellent oxidation resistance, usually undergoes more serious friction and wear due to its lower micro-hardness in contrast to the ceramic coatings. Therefore, the composite coatings reinforced by WC-Co are prepared by HVOF sprayed technology and are also characterized by scanning electron microscope, Raman spectrometer and X-ray diffraction. At the same time, the friction and wear behaviors as well as the mechanisms of different friction pairs are also discussed, in detail. The composite coatings, which mainly consist of γ-(Ni, Co), β-NiAl, γ′-Ni₃Al, WC and W₂C, are dense and uniform. With the increase of WC-17Co, the microhardness of NiCoCrAlYTa/WC-Co composite coating has enhanced from 641.4 HV_300g to 859.7 HV_300g. The wear rates of the composite coatings (10⁻⁵–10⁻⁶ mm³·N⁻¹ m⁻¹) are far lower than those of the as-sprayed NiCoCrAlYTa coating (10⁻⁴ mm³·N⁻¹ m⁻¹). Overall, the mechanical properties and tribological behaviors of the coatings are greatly improved with the addition of WC-Co.     

High-temperature sliding wear,elastic modulus and transverse rupture strength of Ni bonded NbC and WC cermets

The effects of rapid pulse electric current sintering (PECS), substitution of WC by NbC and Co by Ni, and carbide additives (TiC and Mo₂C) on the microstructure, elastic modulus, B3B transverse rupture strength (TRS) and high temperature sliding wear on WC-Co, WC-Ni, NbC-Co and NbC-Ni cermets were studied. Additions of x% Mo₂C and y% TiC (where x and y were <10 wt%), coupled with PECS, significantly refined the NbC-Ni cermet's carbide grain size from ~5.0 μm to <0.8 μm, giving mechanical properties comparable to WC-Co and WC-Ni cermets: >14 GPa hardness and ~10 MPa.m^1/2 fracture toughness (K_IC) and ball-on-three-balls (B3B) TRS > 1600 MPa. The sintering techniques had negligible effect on the samples' elastic and shear modulus, and all WC-based samples had higher elastic modulus than all NbC-based samples (by ~120 GPa). High temperature sliding wear tests were carried out using a ball-on-disk tribometer, with a 10 N force, at a sliding speed of 1.34 m/s for 0.8 km (10 min) and 2.4 km (30 min), using 100Cr6 (AISI 52100) steel balls at 400 °C and 0% humidity. For the 2.4 km sliding distance, all the WC cermets had lower wear volumes than NbC cermets, with LPS WC-0.5Cr₃C₂-10Co having the lowest wear volume. Additions of TiC and Mo₂C to NbC-12Ni improved the sliding wear resistance, with TiC having the greater effect, reducing the sample wear rate by over 30% from 15.1 × 10⁻⁶ mm³/N·m to 9.4 × 10⁻⁶ mm³/N·m after sliding distance of 2.4 km. Generally, the LPS samples had lower wear volumes than the corresponding SPS samples, due to higher K_1c and TRS.     

亚微米WC添加对超音速火焰喷涂WC-Co涂层磨损性能影响

采用超音速火焰喷涂方法(HVOF)在304不锈钢基体表面制备WC和WC-12Co的复合涂层WC-Co,研究亚微米WC的添加对涂层相组成、显微硬度、耐磨性能和表面形貌的影响。利用X射线衍射、压痕法、往复式摩擦磨损实验和扫描电子显微镜(SEM)分别对涂层的相组成、显微硬度、磨损性能和表面形貌进行分析测试,并分析涂层的磨损过程和机制。结果表明,添加质量分数5%的亚微米WC颗粒显著提高了涂层的显微硬度(16.3%);增强了涂层的耐磨性,磨损率从6.09×10-7 mm3/Nm减小到5.15×10-7 mm3/Nm(减小13.8%);亚微米WC颗粒喷涂后在涂层中保持了WC相,并主要存在于WC-Co扁平粒子界面和孔隙。基于涂层中扁平粒子的结合特性与磨损失效特征,建立强化模型,分析亚微米WC颗粒对涂层扁平粒子界面的强化机制。     

Microstructure and properties of WC-12Co composite coatings prepared by laser cladding

A comprehensive study of the phase composition, microstructure evolution, microhardness and wear performance of WC-12Co composite coatings fabricated by laser cladding using coaxial powder-feed mode was presented. It was shown that a combination of high scan speed and high laser energy density made WC on the edge of WC-12Co composite powders partially melt in liquid Co and 304 stainless steel matrix, and then new carbides consisting of lamellar WC and herringbone M₃W₃C (M=Fe, Co) were formed. Meanwhile, WC-12Co composite coatings with no porosity, cracks and drawbacks like decarburization were obtained, showing high densification and good metallurgical bonding with the substrate. Furthermore, a considerably high microhardness of HV_0.3 1500-1600, low coefficient of friction of 0.55 and wear rate of (2.15±0.31)×10^-7 mm³/(N·m) were achieved owing to the synergistic effect of excellent metallurgical bonding and fine microstructures of composite coating under laser power of 1500 W.     

High temperature performance of arc-sprayed aluminum bronze coatings for steel

1 Introduction The aluminum bronze coatings on steel substrate which are sprayed by electric arc process have been developed as possible new candidates for the use to high temperature applications, while their corrosion properties at high temperature in …     

Effect of melting rate on microsegregation and primary MC carbides in M2 high-speed steel during electroslag remelting

Large-size primary MC carbides can significantly reduce the performance of M2 high-speed steel. To better control the morphology and size of primary MC carbides, the effect of melting rate on microsegregation and primary MC carbides of M2 steel during electroslag remelting was investigated. When the melting rate is decreased from 2 kg·min⁻¹ to 0.8 kg·min⁻¹, the columnar dendrites are gradually coarsened, and the extent of segregation of Mo and V is alleviated, while the segregation of Cr becomes severe. At 2 kg·min⁻¹, the number of primary MC carbides per unit area with the sizes in the range of 2 µm to 6 µm accounts for about 75% of all MC carbides, while the carbides are mainly concentrated on the size larger than 8 µm at 0.8 kg·min⁻¹. Thermodynamic calculations based on the Clyne-Kurz (simplified to C-K) model shows that MC carbide can be precipitated in the final solidification stage and a smaller secondary dendrite arm spacing caused by higher melting rate (2 kg·min⁻¹ in this experiment) facilitates the refinement of primary MC carbides.

     

Effect of Ti content in Ag–Cu–Ti activated filler metal on dissimilar joint formation of sialon and WC–Co alloy by laser brazing

Laser brazing was carried out for dissimilar joining of sialon and a WC–Co alloy. Eutectic type Ag–Cu alloys as filler metals with different Ti content ranging from 0 to 2·8 mass-% were used to investigate the effects of Ti on the interface structure and strength of the joint. The filler metal sheet was sandwiched between a sialon block and a WC–Co alloy plate, and a laser beam was irradiated selectively on the WC–Co alloy plate. The brazed joint was obtained using the filler metal containing >0·3 mass-%Ti. TiN, Ti₅Si₃, and Cu₄Ti layers were formed at the interface of sialon and brazed metal as compound layers. The shear strength of the brazed joint increased with increasing Ti content in the filler metal in the range 0·3–1·7 mass-%, reaching a maximum value of 106 MPa. However, the strength decreased when the Ti content became higher than 1·7 mass-%.     

Sub-micron binderless tungsten carbide sintering behavior under high pressure and high temperature

Pure tungsten carbide (WC) compacts of about 200 nm grain size were prepared by high pressure and high temperature (HPHT) method. The best property sample with high relative density (99.2%), high Vickers hardness (2925 kg·mm^− 2) and high fracture toughness (8.9 MPa·m^1/2) was obtained in the condition of 1500 °C temperature and 5 GPa pressure. By means of scanning electron microscopy (SEM) and transmission electron microscope (TEM) observations, a large number of twins and stacking faults appeared in sintered samples, and the grain size of sintered samples maintained in the initial range. The XRD patterns of bulk samples reveal that there is a phase transition from WC to W₂C with the increasing of temperature. Moreover, the effect of HPHT condition for sintering kinetics, microstructure evolutions, and mechanical properties of the sintered samples were also discussed.     

燃料类型及喷涂参数对HVOF喷涂WC10Co4Cr涂层的组织及力学性能的影响

超音速火焰喷涂（HVOF）制备的WC基金属陶瓷涂层广泛应用于金属构件的磨损、腐蚀及空蚀防护。分别采用氢气燃料及煤油液体燃料HVOF喷涂设备分别在9种不同的工艺条件下制备了WC10Co4Cr涂层,研究了燃料类型对涂层的组织、残余应力及力学性能的影响规律。在两种燃料HVOF工艺各自优化的喷涂参数条件下,通过对基体曲率的原位监测对比测试了涂层中的平均残余应力;利用显微维氏硬度、压痕法（断裂韧性）和球盘摩擦磨损对比研究了涂层的力学性能。结果表明：液体燃料（LF）HVOF焰流中粒子的温度更低,速度更高。LF-HVOF喷涂的WC10Co4Cr涂层内的残余压应力更高且涂层致密度更高,而气体燃料（GF）HVOF喷涂的WC10Co4Cr涂层内为残余拉应力。LF-HVOF涂层（1280 HV_0.3, 7.3 MPa·m^0.5）比GF-HVOF涂层（1032 HV_0.3, 4.5 MPa·m^0.5）具有更高的硬度和断裂韧性,LF-HVOF涂层的耐磨性约为GF-HVOF涂层的1.7倍。     

Enhancement of photocatalytic activity of TiO2 film electrode by in situ photoelectro-generating active chlorine

The photoelectrocatalytic activity of TiO2 film electrodes in the degradation of nitrite ion was greatly enhanced in the presence of chlorine ion. The influences of NaCI concentration and initial pH value on the degradation rate of NO2^- and active chlorine production were studied. The results show that the decay rate of NO2^- and the accumulation rate of active chlorine increase with increasing NaCl concentration. At pH〈8, both the decay of increasing NaCl concentration, while at pH〉 10, they are suppressed NO2^- and active chlorine formation rates are enhanced with In addition, contrast to conventionally accepted view, in which an advantage of anatase over the rutile modification of TiO2 is in terms of photoactivity, it is found that a thermal oxidation futile TiO2 electrode is more suitable for both photogenerating active chlorine and degradingNO2^- in the presence of Cl^-. The correlative mechanism was also discussed in detail. Specific adsorption of Cl^- on the electrode causes its energy band edges to move towards positive value and also lower the photocurrent, thus less OH· radicals are produced. However, more active species of Cl that have longer lifetime are available to take part in the oxidation of NO2^- thus improving its degradation rate.     

Electrodeposition of high performance multilayer coatings of Zn–Co using triangular current pulses

Abstract

Compositionally modulated alloy (CMA) coatings of Zn–Co were electrodeposited on to mild steel from an acid chloride bath containing thiamine hydrochloride, as an additive. Electroplating was carried out galvanostatically from a single bath containing Zn²⁺ and Co²⁺ ions. Gradual change in composition in each layer was effected by triangular current pulses, cycling between two cathode current densities. Compositionally modulated alloy coatings were developed under different conditions of cyclic cathode current density and number of layers, and their corrosion resistances were evaluated by potentiodynamic polarisation and electrochemical impedance spectroscopy. The formation of multilayer and corrosion mechanism was analysed using scanning electron microscopy. The corrosion resistances of CMA and monolithic alloy coatings were compared with that of the base metal. Compositionally modulated alloy coating at optimal configuration, represented as (Zn–Co)_{2·0/4·0/300}, was found to exhibit ～80 times better corrosion resistance compared with monolithic (Zn–Co)_3·0 alloy, deposited for the same length of time from the same bath. Improved corrosion resistance was attributed to the formation of n-type semiconductor film at the interface, supported by Mott–Schottky plots. Decrease in corrosion resistance at high degree of layering was found, and is due to lower relaxation time for redistribution of solutes in the diffusion double layer, during plating.     

Friction and wear behaviours of in situ reduced graphene oxide reinforced zirconia ceramic

The tribological behaviour of zirconia composites reinforced by in situ reduced graphene oxide (IrGO) was investigated by a rotating ball-on-plate configuration at room temperature, and was followed by a comparison with composites reinforced by pre-reduced graphene oxide(rGO). The results indicate that both the friction and wear resistance of ceramics increase with the incorporation of graphene oxide (GO). IrGO is better for enhancing the tribological performance of zirconia than rGO. The wear rate decreases by up to one order of magnitude (from 2.33 × 10⁻⁵ mm³·N⁻¹·m⁻¹ to 4.66 × 10⁻⁶ mm³·N⁻¹·m⁻¹) with 0.5 wt% GO. A protective tribofilm containing reduced graphene oxide forms at wear surfaces of ceramics, and the protruding-out of rGO is more pronounced. The main wear mechanism changes from severe delamination to plastic deformation and micro-cracking with increasing GO content. The analysis of wear tracks for ceramics with GO additives by Raman mapping reveals a decrease in the tetragonal-monoclinic phase transformation after sliding wear, which is regarded as the intrinsic reason for the improved wear resistance.     

Enhanced growth of intermetallic phases in the Ni–Ti system by current effects

The effect of direct current upon interfacial reactions in the Ni–Ti system was investigated. Isothermal diffusion couple experiments were conducted under varying current densities to de-couple Joule heating from intrinsic effects of the current flux. Current densities of up to 2546 A cm⁻² were used in the temperature range of 625–850 °C. All of the intermetallic compounds (NiTi, Ni₃Ti and NiTi₂) present in the equilibrium phase diagram were identified in the product layer. In addition, β-Ti solid solutions formed in samples annealed above the α→β temperature, 765 °C. The growth of all product layers was found to be parabolic and the applied current was found to significantly increase the growth rate of the intermetallic layers. Using Wagner’s analysis the present results were compared to published results on current-free diffusion couples. The intrinsic growth rate constant of the NiTi₂ intermetallic was found to be 43 times higher under the influence of 2546 A cm⁻² than that obtained without a current at 650 °C. The effective activation energy for the formation of all phases was found to decrease with increasing current density. The effect was strong for all phases but the decrease was most marked for Ni₃Ti. In this case, the activation energy decreased from 292 kJ mol⁻¹ under the influence of a current density of 1527 A cm⁻² to 86 kJ mol⁻¹ when the current density was 2036 A cm⁻². The results are explained in terms of current induced changes in the growth mechanism arising from changes in the concentration of point defects or their mobility.     

Microstructure and wear resistance of AlCrFeNiMo0.5Six high-entropy alloy coatings prepared by laser cladding

In recent years, the coating prepared by laser cladding has attracted much attention in the field of wear research. In this work, AlCrFeNiMo_0.5Si_x (x=0, 0.5, 1.0, 1.5, 2.0) high-entropy alloy coatings were designed and prepared on Q235 steel by laser cladding. The effect of Si content on microstructure, microhardness and wear resistance of the coatings was studied in detail. The results indicate that the AlCrFeNiMo_0.5Si_x high-entropy alloy coatings show an excellent bonding between substrate and the cladding layer. The AlCrFeNiMo_0.5Si_x coatings are composed of nano-precipitated phase with BCC structure and matrix with ordered B2 structure. With the addition of Si, the white phase (Cr, Mo)₃Si with cubic structure appears in the interdendritic, and the morphology of the coating (x=2.0) transforms into lamellar eutectic-like structures. The addition of Si enhances the microhardness and significantly improves the wear resistance of the coatings. As x increases from 0 to 2.0, the average hardness of the cladding zone increases from 632 HV to 835 HV, and the wear rate decreases from 1.64×10⁻⁵ mm³·(N·m)⁻¹ to 5.13×10⁻⁶ mm³·(N·m)⁻¹. When x≥1.5, the decreasing trend of the wear rate gradually slows down. The wear rates of Si1.5 and Si2.0 coatings are 5.85×10⁻⁶ mm³·(N·m)⁻¹ and 5.13×10⁻⁶ mm³·(N·m)⁻¹, respectively, which is an order of magnitude lower than that of Q235 steel.

     

HVAP 及 HVOF 工艺制备 WC10Co4Cr 复合涂层及其性能研究

目的对比研究超音速等离子喷涂(HVAP)技术与超音速火焰喷涂(HVOF)技术制备WC10Co4Cr涂层,并根据涂层组织形貌与电化学特性判断两种工艺的优劣。方法采用SEM及XRD分析WC10Co4Cr复合涂层的微观形貌和物相,在3.5%(质量分数)Na Cl溶液中对涂层进行电化学分析。结果 WC10Co4Cr涂层由较大的WC颗粒及粘结相组成,在喷涂过程中WC颗粒不断累积形成层片状结构,涂层有较小程度的失碳,形成了具有脆性的W2C。电化学极化测试表明,超音速等离子喷涂技术制备的涂层表现出优异的抗电化学腐蚀性能。结论超音速等离子喷涂技术制备的WC10Co4Cr涂层显微硬度为1197HV,孔隙率为0.50%,腐蚀电位为-0.3947 V,腐蚀电流密度为9.19×10-7A/cm2,腐蚀速率为1.01×10-2g/(m2·h),腐蚀深度为1.09×10-2mm/a,具有与超音速火焰喷涂涂层相似的耐腐蚀性能。     

In-situ synthesis of silicide coatings on molybdenum substrates by electrodeposition in chloride-fluoride molten salts

Silicide coatings including MoSi₂ and Si-MoSi₂ were prepared on molybdenum substrates by electrodeposition in NaCl-KCl-NaF-K₂SiF₆ molten salt. The experimental was conducted under different cathodic current densities at the temperature of 1073 K. The effect of the current density on the microstructure, phase composition, cross-section morphologies and elemental distribution of the as-prepared coatings were investigated by means of SEM, XRD and EDX. The results revealed that the type of the silicide coatings was strongly dependent on the current density and the relevant deposition mechanism was discussed. Besides, the electrochemical behavior of silicon ion in the chloride-fluoride molten salts was also studied using cyclic voltammetry and chronopotentiometry to reveal the electroreduction mechanism. The reduction of Si(IV) to Si was proved to be a quasi-reversible or irreversible diffusion-controlled single-step reaction and the diffusion coefficient of Si(IV) calculated from chronopotentiograms was (1.28 ± 0.25) × 10⁻⁵ cm²·s⁻¹.     

Effect of VC and NbC additions on microstructure and properties of ultrafine WC-10Co cemented carbides

The nanocomposite WC-Co powders were prepared through planetary ball milling method. Effects of grain growth inhibitor addition and the vacuum sintering parameters on the microstructure and properties of ultrafine WC-10Co cemented carbides were investigated using X-ray diffractometer, scanning electron microscope and mechanical property tester. The results show that VC and NbC additions can refine the WC grains, decrease the volume fraction of Co₃W₃C phase in ultrafine WC-10Co cemented carbides, and increase the hardness and fracture toughness of the base alloys. After sintering for 60 min at 1400 °C, the average grain size and hardness of ultrafine-grained WC-10Co-1VC cemented carbide are 470 nm and HRA 91.5, respectively. The fracture toughness of cemented carbide WC-10Co-1NbC alloy is over 7 MN·m^?3/2.