堆焊工艺制备金属间化合物复合材料的组织和性能

以WC，NiAl，NiB和Ni粉末等混合球磨、烧结制备复合材料焊条，在球磨过程中，WC颗粒被破碎，NiAl，NiB和Ni反应生成金属间化合物Ni3A1。用氩弧焊将这种复合材料焊条堆焊在1Cr25Ni20Si2不锈钢的表面，形成5mm厚的金属间化合物耐磨复合材料。堆焊过程中，部分WC溶解，析出新碳化物W2C，Ni3Al转变成新金属间化合物Ni3(A1Ti)C。这种复合材料的耐磨性可达45钢的3倍以上。     

氧化钨的形貌结构对纳米WC粉末均匀性的影响

研究了在传统氢还原工艺制备纳米碳化钨粉末过程中不同氧化钨的形貌结构对纳米W/WC粉末均匀性的影响,并对粉末及其WC-Co烧结体的性能进行了表征。结果表明,用具有疏松、多孔形貌结构的细小氧化钨颗粒更容易制备出结构较疏松、分散性较好的纳米W粉和WC粉。晶粒聚集和异常粗大颗粒的产生,主要与碳化过程中团聚纳米钨粉颗粒因烧结合并增粗有关。     

Processing of WC/W composites for extreme environments by colloidal dispersion of powders and SPS sintering

Tungsten and tungsten carbide are materials with high thermomechanical response that are used or have been proposed for extreme environment applications such as first plasma face, or cutting tools. The high melting temperature and strong bonding energy of both materials force the use of powder metallurgical processes and non-conventional sintering routes to achieve dense parts. Consequently, a high dispersion and close contacts of the starting powders are required. In this paper tungsten and tungsten carbide powders are colloidally processed and mixed to achieve composite powders that are sintered later by Spark Plasma Sintering. Starting micrometric tungsten carbide and nanosized tungsten powders are dispersed in water at pH 3. By using a cationic dispersant, the surface charge of the nanosized W suspended in water reverses to positive, ensuring its attachment to the carbide surfaces and the good dispersion of the two phases when both slurries are mixed.Composite powders with volumetric rations of 50WC/50W, 80WC/20W and 90WC/10W as well as pure WC and W are sintered by SPS following the dimensional change of the specimens during the process. It has been proved that complete coverage of the micronic WC by the nanosized W powders, achieved with this colloidal approach, makes the tungsten govern the initial sintering stages. The derivative of the sintering curves is used to detect the solid state reactive sintering temperature of W₂C. After sintering, XRD and SEM observations indicate that all the mixture compositions yield to ceramic materials with different W₂C/WC ratios, depending on the initial compositions. Dispersion of the two phases is high and no remaining W is detected. Flexure tests at room temperature show that composite materials present a slightly lower fracture strength than pure WC.     

细WC配比对非均匀YG20C合金性能的影响

研究了碳化钨粗颗细粒搭配对YG20C合金的力学性能与组织结构的影响,即在WC颗粒平均粒径为25μm的合金中,加入不同比例的平均粒径为2.0μm的WC颗粒,构成非均匀晶粒硬质合金。结果表明,非均匀YG20C合金的硬度和抗弯强度随着细晶粒碳化钨含量的增加都是先升高再降低。当细颗粒碳化钨比例为20%(文中含量均为质量分数)时,YG20C合金的综合力学性能最好,硬度和抗弯强度都达到"双高",其值分别为HRA83.5和2 800 MPa,比均匀YG20C合金的硬度和抗弯强度分别提高HRA1.2和400 MPa。并且非均匀YG20C合金的冲压使用效果最佳。     

WC strengthened W–Cu nanocomposite powder synthesized by in-situ reactions

A novel strategy to prepare W–Cu nanocomposite powder with WC nanoparticles for strengthening was proposed. The feasibility and mechanisms of the present in-situ reactions method for synthesizing multicomponent powders were demonstrated by both thermodynamic calculations and experiments. The W-Cu-WC nanopowder can be prepared at a low temperature with pure phase constitution and homogeneous microstructure with a mean particle size of 60 nm. Owing to in-situ reactions, the orientation relationship of WC and W is formed, which facilitates strong bonding between the strengthening particles and matrix. The new method is applicable to synthesize a variety of composite powders with tailorable phase and particle size.     

Solid solution extent of WC and TaC in Ti(C,N) as revealed by lattice parameter increase

Materials development of cermets for cutting tools has come to a new stage where molybdenum is no longer indispensable. The key is the addition of a solid-solution treatment of hard phase powder mixtures before sintering with metallic binders. In this paper, ceramic powders of Ti(CN), TaC and WC are mixed, compacted and heat treated to form (Ti, W, Ta)(C,N) solid solutions at elevated temperatures. The influence of temperature and duration of the treatment on the solid solution extent is examined through measurement of the X-ray diffraction (XRD) spectra and the lattice constant. After treating at 1700°C for 1 h, the XRD spectra revealed the disappearance of the WC peaks. The lattice parameter of titanium carbonitride increased with increasing reaction time and temperature and stabilized after treating at 1700°C for 1·5 h. Cold isostatic pressing (CIP) at 350 MPa following uniaxial pressing at 180 MPa did not seem to have too much added influence on the solid solution extent, but CIP ed samples did give less scattered data.     

Optimized production route of WC powder pre-alloyed with Ta for fine grained cemented carbides

Powders of hexagonal (W,Ta)C were produced following a two-step carburization process with (W,Ta)₂C powder as an intermediate product. XRD measurements indicate that higher temperatures during the first carburization step increase the fraction of Ta in the (W,Ta)₂C structure where lower temperatures during the second carburization step seems to increase the fraction of Ta in the (W,Ta)C structure. Lower temperatures during the carburization steps increase the fraction of Σ2 WC/WC grain boundaries and cause the formation of what is interpreted as Σ4 boundaries. The (W,Ta)C powders can be successfully used to produce fine grained WC-Co based cemented carbides. The pre-alloying with Ta appears to have a softening effect on the material.     

WC/W_2C_P在NiCrBSi合金中的分解机理及其表面渗碳改性

为了抑制铸造碳化钨颗粒(WC/W2C P)在复合材料制备过程中的过度分解,利用SEM、EDS和XRD等测试手段对WC/W2C P在Ni Cr BSi合金中的分解机理及其表面改性渗碳处理进行了研究。结果表明,铸造碳化钨由WC和W2C两相组成,在Ni Cr BSi合金中WC相比W2C相具有更好的化学稳定性。在熔烧法制备WC/W2C P增强Ni Cr BSi基复合材料涂层的过程中,基体熔液和WC/W2C P发生元素互扩散;WC/W2C P中的化学稳定性差的W2C相与从基体熔液中扩散过来的Ni、Cr等元素反应生成了富W、Ni的碳化物,而化学稳定性好的WC相几乎完整的保留下来;基体中的Ni、Cr元素与从WC/W2C P中扩散过来的W、C元素形成了富W、Ni、Cr的碳化物在凝固过程中析出。经渗碳表面改性后,WC/W2C P表面形成了化学稳定性好的WC壳层,该壳层能有效抑制WC/W2C P在基体中的扩散分解,减少基体中碳化物的析出。     

Effect of pre-alloyed raw materials on the microstructure of a (Ti,W)(C,N)–Co cermet

Four alloys manufactured from different combinations of powders (TiC + TiN + WC; Ti(C,N) + WC; (Ti,W)C + TiN and (Ti,W)(C,N)) were studied using X-ray diffractometry, optical microscopy (OM), scanning electron microscopy (SEM) and analytical electron microscopy (AEM). The alloy manufactured from binary powders had a smaller grain size and a more inhomogeneous microstructure than the other alloys. The alloys manufactured with WC contained an inner rim around Ti(C,N) cores, as well as W-rich cores. Thermodynamic calculations suggest that these are formed during solid-state sintering at 900°C in a low nitrogen activity. The outer rim had a composition that is in good agreement with calculations of the equilibrium during liquid phase sintering at 1450°C.     

Fabrication of nanocrystalline WC and nanocomposite WC–MgO refractory materials at room temperature

Reactant material powders of pure WO₃, Mg and graphite have been milled at room temperature using a high-energy ball mill. After a few kiloseconds of milling (11 ks), numerous fresh surfaces of the reactant materials are created as a result of the repeated impact and shear forces generated by the balls. After 86 ks of milling, a mechanical solid state reduction is successfully achieved between the fresh Mg and WO₃ particles to form a product of nanocrystalline mixture of MgO and W. A typical mechanical solid state reaction takes place between the W particles and graphite powders to obtain fine grains of nanocrystalline WC. Towards the end-stage of ball-milling (173 ks), the nanocrystalline MgO grains (10 nm) are embedded into the fine matrix of WC to form fine nanocomposite powders (1 μm in diameter) of WC–18% MgO material with spherical-like morphology. This composite powder was then consolidated under vacuum at 1963 K, with a pressure ranging from 19.6 to 38.2 MPa for 0.3 ks, using a plasma activated sintering method. In addition, pure nanocrystalline WC powders (7 nm in diameter) obtained by removing the MgO from the milled powders, using a simple leaching technique have been also consolidated by the same consolidation technique. The consolidation step does not lead to a dramatic grain growth and the compacted samples that are fully dense still maintain their unique nanocrystalline characteristics. The elastic properties and the hardness of both consolidated samples have been investigated. A model for fabrication of refractory nanocrystalline WC and nanocomposite WC–18% MgO materials at room temperature is proposed.     

Microstructure and properties of ultrafine WC-10Co composites with chemically doped VC

Vanadium carbide is the most effective grain growth inhibitor for ultrafine WC-Co composites due to its high solubility and mobility in the cobalt phase at relatively low temperatures;however,there are still some debates over the best way to introduce it into the WC-Co formulation.In this paper,the differences between admixed and chemically doped grain growth inhibitors on the microstructural development and properties of an ultrafine WC-10Co composite are discussed.The densification rate of chemically doped samples is slower in the early stage of sintering and the WC grain sizes of the sintered alloys are finer than those of admixed samples,leading to the increase of hardness and transverse rupture strength of the sintered alloys.The effectiveness of the chemically doped inhibitor is attributed to the formation of vanadium rich layers on the surfaces of tungsten carbide powders during reduction and carbonization,which alters the surface and interface energies of WC grains,impedes the contact with each other of WC grains and contributes to the resistance to W diffusion across the layer during sintering,resulting in the inhibition of nanosized particle coalescence.     

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.     

A study on the fine-finish die-sinking micro-EDM of tungsten carbide using different electrode materials

In recent years, tungsten carbide (WC) and its composites (WC–Co) are widely used in the die and mold industries due to their unique combination of hardness, strength and wear resistance. Micro-EDM is one of the most effective methods for machining these extremely difficult-to-cut materials. However, numerous applications of WC often involve intense mechanical demands at the surface. Therefore, fine-finish micro-EDM of WC is becoming an imminent and important issue. In this study, investigations have been conducted with view of obtaining fine surface finish in the micro-EDM of WC using tungsten (W), copper tungsten (CuW) and silver tungsten (AgW) electrodes. It was found that the surface characteristics are dependent mostly on the discharge energy during machining. The fine-finish micro-EDM requires minimization of the pulse energy supplied into the gap. In addition, the surface finish was found to be influenced greatly by the electrical and thermal properties of the electrode material. The performance of the electrodes for the finishing micro-EDM was evaluated based on the achieved surface roughness and surface characteristics with respect to material removal rate (MRR) and electrode wear ratio (EWR). It was found that AgW electrode produces smoother and defect-free nanosurface with the lowest R_a and R_max among the three electrodes. Besides, a minimum amount of material migrates from the AgW electrode to the WC workpiece during the finishing micro-EDM. On the other hand, CuW electrodes achieved the highest MRR followed by AgW. In the case of electrode wear, the W electrode has the lowest wear followed by CuW and AgW. Finally, considering all the performance parameters, AgW appears to be the best choice for finish die-sinking micro-EDM of WC.     

Ni+Al元素粉末法制备WC-Ni3Al-B复合材料(英文)

为发展WC-Ni3Al-B复合材料的液相烧结制备技术,研究由羰基Ni粉、分析纯Al粉和粗WC粉的混合粉末反应合成制备的WC+Ni3Al预合金粉末。采用DSC和XRD分别研究3Ni+Al和70%WC+(3Ni+Al)混合粉末在550～1200°C和25～1400°C温度范围的相变过程。结果表明:Ni3Al相的形成取决于反应温度。在200～660°C热处理温度范围内,除了WC相外,还存在Ni2Al3、NiAl和Ni3Al相;而在660～1100°C温度范围内,仅存在NiAl和Ni3Al相;在1100～1200°C温度范围可以获得均匀的WC+Ni3Al预合金粉末混合物。采用该预合金粉末制备的WC-30%(Ni3Al-B)复合材料具有很高的致密度,且WC晶粒呈圆形。与普通商用YGR45(WC-30%(Co-Ni-Cr))相比,WC-30%(Ni3Al-B)复合材料具有更高的硬度(9.7GPa),低的抗弯强度(1800MPa)和相近的断裂韧性(18MPa.m1/2)。     

High energy milling on tungsten powders

Nanocrystalline tungsten powders were produced by high energy mechanical milling, using both tungsten carbide (WC) and tungsten (W) balls as grinding media. X-ray diffraction study indicated that the lattice parameter of tungsten decreased (from 3.162 to 3.149 Å) with increasing milling time from 0 to 15 h. Considerable decrease in particle size was observed in both W and WC grinding media after 15 h of milling duration. Rietveld analysis of the X-ray data along the Williamson-Hall plots revealed that the crystallite size also decreased with increasing milling time. Chemical analyses showed that the total amount of cobalt and carbon in the milled samples were higher in WC grinding media, as compared to W grinding media. The sintered density increased from 80% to 98% from as received to milled tungsten powders, when sintered at 1790 °C. The mechanical properties of as sintered alloys were evaluated and were found to be strongly influenced by the milling time and grinding media.     

Influence of Mo on the microstructure and mechanical properties of TiC-based cermets

The microstructures and mechanics properties of TiC-based cermets composed of TiC, WC, Ni, Co, Mo, and Cr₃C₂ were investigated. The results show that Mo has a great effect on the sintering densification, microstructures, and mechanical properties. The microstructures and distribution of Mo and Ti in the TiC-based cermets were analyzed. It was indicated that a new phase with Ti, Mo, W, and C was formed on the rim of (Ti,W)C grains by means of an addition of Mo into the TiC-based cermets. The new phase with a surrounding structure was of great aid to improve the wettability of the liquid phase on the solid phase surface of TiC, decrease the porosity and refine the grains of the hard phase, which gave rise to the increase in strength and hardness. The properties of the TiC-based cermets could be further improved to some extent by adding WC, Cr₂C₃, and Co.     

纳米钨合金粉末的制备技术

钨合金包括W－Ni-Fe,W-Ni-Cu,W-Cu,WC-Co等钨基合金材料。钨合金材料将是21世纪出现的一种多功能高性能的多胜任的新型材料。有杉纳米粉末制备的亚微或微米钨合金块体材料具有非常优越的潜在物理力学性能,用作高性能结构件和高性能电子、微电子等功能材料方面都将具有很大的潜在优势,可以更好地满足高性能新型材料的要求。本文综合近几年来国内外纳米钨合金的研究状况,详细地介绍了有关纳米钨合金粉末的制备技术,预测了今后钨合金材料的研究方向。     

机械活化-反应热处理制备纳米晶WC-Co复合粉末的研究

以W、C、Co为原料粉末,经机械活化-反应热处理工艺制备纳米晶WC-Co复合粉末。实验发现活化粉末的固相反应具有以下特征:反应温度低,反应速度快。在800℃热处理时已有大量的WC生成。在850℃保温25minW2C就完成了向WC的转化。经900℃保温35min制备了晶粒尺寸为30.5nm的WC-Co复合粉末。     

Effect of direct current patterns on densification and mechanical properties of binderless tungsten carbides fabricated by the spark plasma sintering system

Binderless tungsten carbide materials (bWCs) were fabricated by the spark plasma sintering (SPS) system. Ultrafine WC powders with adjusted oxygen contents and C/W atomic ratios were used as raw materials. Constant and pulsed direct current patterns (constant DC and pulsed DC) were chosen as the power supplies. The results indicate that for WC starting powders with either low (0.31%) or high (0.95%) oxygen contents, a relative density larger than 99.0% can be reached by pulsed DC at 1820 °C. Nevertheless, the severely oxidized WC powders cannot be well-densified by constant DC. A high degree of densification of bWCs facilitates the collaborative improvement of the toughness and hardness. The existence of W₂C facilitates the improvement of the hardness at the high expense of the toughness. The existence of graphite phase is substantially detriment to the toughness. The grain coarsening facilitates the improvement of the toughness with sacrificed hardness. The related mechanism is discussed.