Preparation of Ultrafine Tungsten Powder by Sol-Gel Method

Sol-gel method was employed for the preparation of nanoscale tungsten powder. The effects of different preparation conditions on particle size were discussed and the optimum preparation condition was found. The products were characterized by X-ray diffraction, scan electron microscopy and so on. The results show that the intermediate is monoclinic WO3, its particle shape is approximately spherical, and the particle size distribution is narrow. The average particle size is about 60 nm. After deoxidization, WO3 turns into cubic tungsten powder with small particle size （average particle size about 120 nm） and narrow size distribution.

STUDY OF PREPARATION PROCESS OF TUNGSTEN POWDER BY SHS WITH A MAGNESIUM THERMIT STAGE

Tungsten powder was fabricated from the system CaWO4-Mg by self-propagating high-temperature synthesis (SHS) with a magnesium thermit stage. The physic-chemical change during heating and the effects of pressure of sample and diluents (W powder) on product have been studied. The experimental results show that the porosity of combustion product and the particle size of final tungsten powder decrease with increasing pressure of sample. Addition of diluents could increase the particle size of final tungsten powder. The purity of tungsten is improved by leaching in NaOH solution. The results of spectral analysis and particle size distribution of final tungsten powder show that the final Tungsten powder has a median diameter of 0.87μm,specific surface area of 1.09m2/g and purity of above 99.0%.

Synthesis of nanosized tungsten powder

Nanosized tungsten powder was synthesized by means of different methods and under different conditions with nanosized WO3 powder. The powder and the intermediate products were characterized using XRD, SEM, TEM, BET （Brunauer Emmett Teller Procedure） and SAXS （X-ray diffracto-spectrometer/Kratky small angle scattering goniometer）. The results show that nanosized WO3 can be completely reduced to WO2 at 600℃ after 40 min, and WO2 can be reduced to W at 700℃ after 90 min, moreover, the mean size of W particles is less than 40 nm. Furthermore, the process of WO3→WO2→W excelled that of WO3→W in getting stable nanosized tungsten powder with less grain size.

Characterization of nanometer tungsten powders

Three types of tungsten powders were prepared by hydrogen reduction of three precursor powders at low temperature, which were used as samples, and were then characterized by Brunauer-Emmer-Teller （BET） method, scanning electron microscopy （SEM）, transmission electronic microscopy （TEM）, small angle X-ray scattering （SAXS）, and field-emission scanning election microscopy （FESEM） respectively. The results showed that although BET and SEM could not characterize the particle size of nanometer powders, they were important means of assistance to exclude non-nanometer powders. TEM and FESEM could directly measure the particle size of nanometer powders, but this needs a lot of time, to count the average particle size and particle size distribution. SAXS could not describe the state of agglomeration. By the combination of FESEM and SAXS, the particle size, particle size distribution, and particle shape of nanometer powders could be precisely characterized.

Influence of phase components of tungsten oxide on homogeneity of ultrafine tungsten powder

The influence of phase components of tungsten oxide on homogeneity of ultrafine tungsten powder by conventional hydrogen reduction techniques was studied. Results show that phase components of tungsten oxide play a crucial role on homogeneity of metal tungsten powder; ultrafine and homogeneous tungsten powder can be produced from oxides which consist of only one phase. Due to the different reduction rates (or different reduction paths) of oxide which comprises different phases, the multi phase components tungsten oxide leads to a fine but in homogeneous metal tungsten powder.

Microstructure and properties of liquid-phase sintered tungsten heavy alloys by using ultra-fine tungsten powders

The microstructure and properties of liquid-phase sintered 93W-4.9Ni-2.1Fe tungsten heavy alloys using ultra-fine tungsten powders (medium particle size of 700 nm) and original tungsten powders (medium particle size of 3 μm) were investigated respectively. Commercial tungsten powders (original tungsten powders) were mechanically milled in a high-energy attritor mill for 35 h. Ultra-fine tungsten powders and commercial Ni, Fe powders were consolidated into green compacts by using CIP method and liquid-phase sintering at 1 465 ℃ for 30 min in the dissociated ammonia atmosphere. Liquid-phase sintered tungsten heavy alloys using ultra-fine tungsten powders exhibit full densification (above 99% in relative density) and higher strength and elongation compared with conventional liquid-phase sintered alloys using original tungsten powders due to lower sintering temperature at 1 465 ℃ and short sintering time. The mechanical properties of sintered tungsten heavy alloy are found to be mainly dependent on the particles size of raw tungsten powders and liquid-phase sintering temperature.

Effects of Cobalt on the Sintering Behavior of Mechanically Activated Tungsten Powder

Tungsten alloys were prepared with mechanically activated powder added microelement cobalt in order to improve the process and properties of alloys. Properties of alloys such as density, hardness and bending strength were measured. The results show that through mechanical activation, cobalt can accelerate the sintering process of these alloys By the combination of mechanical activation and adding microelement cobalt, tungsten alloys with higher density and better properties can be obtained.

PREPARATION OF NANOSTRUCTURED TUNGSTEN POWDERS

The preparation of nanostructured tungsten powders has been studied by using the characteristics of high-er temperature, higher chemical reactivity and quenching technology of hydrogen plasma,in which WO3 solid particles served as raw materials. The reduction mechanism of WO3 at high temperature has been also discussed. The composition . particle size distribution and morphology of nanostructured tungsten powders have been measured by X?ray diffraction small angle X-ray scatter and TEM. Tungsten powders of mean particle size of 40 nanometer,and specific surface area of 3 X10m2/kg have been prepared. The shape of nanostruc-tutted tungsten powders is sphere.

Effect of Y_(2)O_(3)doping on preparation ultrafine/nano-tungsten powder and refinement mechanism

In this study,ultrafine/nano W-Y_(2)O_(3)composite powders were synthesized by spray drying,roasting and two-step hydrogen reduction using ammonium metatungstate and yttrium nitrate as raw materials.The mechanism of the influence of Y_(2)O_(3)on the growth of WO_(2.72)and the particle refinement of tungsten powder is discussed.The effect of Y_(2)O_(3)particles on the reduction behavior of tungsten powder was investigated using scanning electron microscopy to study the near surface morphology and X-ray diffraction for phase ID(composition)and crystal structural changes for the reduced powders at each step.The results show that the doping of 0.3 wt.%Y_(2)O_(3)can significantly increase the aspect ratio of WO_(2.72)in the first step of hydrogen reduction.Moreover,Y_(2)O_(3)can effectively inhibit the growth of tungsten particles in the hydrogen reduction process.Therefore,The Y_(2)O_(3)-doped tungsten powders have finer particles and a narrower particle size distribution range than the undoped powders.The average particle diameter of 0.3 wt.%Y_(2)O_(3)doping tungsten powder was in the range of 90-120 nm.

A Study of Scandia Doped Tungsten Nano-Powders

Scandia and rhenium doped tungsten powders were prepared by solid-liquid doping combined with two-step reduction method. The particle size of doped tungsten and distribution of scandia and rhenium were studied by SEM, EDS, XRD and granularity analysis. Experimental results showed that scandia distributed evenly on the surface of tungsten particles. Addition of scandia and rhenium decreased the particle size of doped tungsten, and the more the content of scandia and rhenium, the smaller the doped tungsten particles. Tungsten powders doped with 3 % Sc2O3 and 3 % Re （mass fraction） had an average size of about 80 nm in diameter. The mechanism of the decrease in the tungsten particle size was discussed.

Effect of rare earth element cerium on preparation of tungsten powders

Tungsten powders and Ce doped powders were prepared by hydrogen reduction combined with the liquid-solid doping method. The phase composition, particle size and powder morphology of Ce doped tungsten powders were analyzed by X-ray diffrac-tion, scanning electron microscopy and transmission electron microscopy, respectively. The results indicated that 10000 ppm Ce doped tungsten oxide powders were consisted of WO3 phase and Ce4W9O33 phase. The hydrogen reduction of Ce doped tungsten powders was basically accomplished at 800 oC for 3 h. The size of Ce doped W powders was remarkably decreased compared to the undoped W powders. The phase of Ce4W9O33 was reduced to Ce2 （WO4）3 phase and Ce2W2O9 phase during the process of hydrogen reduction. Moreover, Ce2 （WO4）3 phase and Ce2W2O9 phase were observed form their morphologies, where the doping content of Ce was more than 100 ppm. The ternary phase embedding into W particles was assigned to Ce2 （WO4）3, while the ternary phase distrib-uting among W particles corresponded to Ce2W2O9. The phase of Ce2 （WO4）3 might be the nucleus of W particles and increase the number of the nucleus. And the particles of Ce2W2O9 covered WO2 particles and might inhibit the growth of W particles. These two reasons resulted in the decrease of the size of Ce doped W particles. Uniform fine W powders were fabricated with the doping content of Ce more than 100 ppm.

Spherical modification of tungsten powder by particle composite system

Owing to contradiction between increasing demand of spherical tungsten powder and limitation of traditional manufacturing technology,a novel preparation method was developed to sphericize the polygonal tungsten powder by means of modification of particle composite system.Tungsten powder particles were modified by par-ticle composite system,and detailed characterization by scanning electron microscopy(SEM)was studied.Particle size distribution and function mechanism were analyzed,and the internal relationship between average diameter and processing time was discussed.The results show that the spherical tungsten powder with an average diameter of 6.41μm is obtained from polyhedral tungsten powder with an average diameter of 7.50μm.The spherical effect could be achieved(sharp edge angles of particles are rounded off and reshaped)when the processing time is over 30 min.The relationship between average diameter(d)and pro-cessing time can be described by the exponential decay model,which provides a good interpretation for the process of modification.The relationship between them can be expressed by the equation d=1.87406exp(-x/8.92718)+6.4182.The proposed method could readily enable large-scale production of spherical tungsten powder.

Hot-pressed sintering of W/Cu functionally graded materials prepared from copper-coated tungsten powders

The three-layered(W-60 vol%Cu/W-40 vol%Cu/W-20 vol%Cu)W/Cu functionally graded material(FGM)containing a Cu network structure was fabricated at different temperatures by hot-pressed sintering produced from copper-coated tungsten powders.The effects of various sintering temperatures on relative density,microstructure,thermal conductivity,hardness and flexural strength were investigated.Scanning electron microscopy(SEM)and X-ray diffraction(XRD)analysis show that a Cu network extends throughout the W/Cu FGM specimens sintered at 1065℃and the graded structure can be retained perfectly,and W particles are distributed homogeneously.The low-temperature sintering densification of W/Cu FGM arises because the sintering mode of the copper-coated tungsten particles includes just sintering Cu to Cu,rather than Cu to W,Cu to Cu and W to W,as required for conventional powder particles.The relative density of W/Cu FGM sintered at 1065℃for 3 h under a load of25 MPa is 96.1%.The thermal conductivity is up to204 W·m^-1·K^-1 at normal temperature and 150 W·m^-1·K^-1at 800℃.And the Vickers hardness varies with the gradient of different layers from 3.34 to 4.05 GPa.

INDUCTION PLASMA REACTIVE DEPOSITION OF TUNGSTEN CARBIDE FROM TUNGSTEN METAL POWDER

Experimental results on the primary carburization reaction between the tungsten powder and methane in the induction plasma, and the secondary carburization of the deposit on substrate at high temperature are reported. Optical microscopy and scanning electron microscopy were used to examine the microstructures of starting tungsten powder, carburized powder, and deposit. X-ray diffraction analysis, thermal gravimetric analysis and microhardness measurement were used to characterize the structures and properties of the powder and the deposit. It is found that the primary carburization reaction in the induction plasma starts from the surface of tungsten particles when the particles are melted. Tungsten particles are partially carburized inside the reactive plasma. Complete carburization is achieved through the secondary carburization reaction of the deposit on substrate at high temperature.

A novel binder and binder extraction method for powder injection molding of tungsten cemented carbide

An improved wax based multi component binder and a new debinding method termed high pressure condensed solvent extraction were developed for powder injection molding of tungsten cemented carbide. The results indicate that a critical powder loading of 65% (volume fraction) and an ideal rheological properties were obtained by the feedstock based on the binder. High debinding rate and specimens with high strength were obtained by the debinding method. Moreover, by making high temperature holding time adjustable, it makes the subsequent thermal degradation process more flexible to debinding atmosphere and carbon content of the as debinded specimens controllable. The transverse rupture strength, hardness and density of the as sintered specimens made by an optimized PIM process are 2.48 GPa, HRA90 and 14.72 g/cm 3, respectively. Good shape retention and about 0.02% dimension deviation were achieved.

Numerical simulation of tungsten alloy in powder injection molding process

The flow behavior of feedstock for the tungsten alloy powder in the mold cavity was approximately described using Hele-Shaw flow model. The math model consisting of momentum equation, consecutive equation and thermo-conduction equation for describing the injection process was established. The equations are solved by the finite element/finite difference hybrid method that means dispersing the feedstock model with finite element method, resolving the model along the depth with finite difference method, and tracking the movable boundary with control volume method, then the pressure equation and energy equation can be resolved in turn. The numerical simulation of the injection process and the identification of the process parameters were realized by the Moldflow software. The results indicate that there is low temperature gradient in the cavity while the pressure and shear rate gradient are high at high flow rate. The selection of the flow rate is affected by the structure of the gate. The shear rate and the pressure near the gate can be decreased by properly widening the dimension of the gate. There is a good agreement between the process parameters obtained by the numerical simulation and the actual ones.

钨粉制备及其对钨合金性能影响的研究进展

金属钨因具有高熔点、高硬度、耐腐蚀、耐磨和热膨胀系数小等优点而被广泛应用于制备各种合金材料。本研究综述了钨粉的制备方法,如熔盐电解法、溶胶凝胶法、高能球磨法和氢气还原法。针对钨粉均匀性问题,重点阐述了目前使用最广泛的氢气还原法的研究现状,其中,调控氢气中水蒸气分压有利于提高钨粉均匀性;气流磨和球化等钨粉的处理工艺有助于提高钨粉均匀性和分散性。另外,介绍了钨粉粒度和分散性等对钨合金性能的影响,均匀分散的钨粉对制备组织均匀的钨合金优势巨大。针对钨粉和钨合金中钨晶粒之间的相关性介绍了晶粒细化的相关研究。简要介绍了钨粉性能对增材制造钨合金性能的影响。

超细晶/纳米晶钨材料制备技术的研究进展

晶粒细化可显著提高超细晶/纳米晶钨材料的性能,介绍了超细晶/纳米晶钨材料制备技术的最新研究进展,包括粉末冶金法和深度塑性变形法,粉末冶金法的烧结工艺主要包括热等静压烧结、超高压通电烧结、放电等离子体烧结、微波烧结等;深度塑性变形法包括高压扭转、等通道挤压、表面机械研磨处理和累积轧制等。由于超细晶/纳米晶钨材料存在大量的晶界可有效提高材料的力学性能和抗辐照性能,因此有望解决纯钨材料作为核聚变堆面向等离子体材料存在的问题,对超细晶/纳米晶钨材料的发展提出了建议。

Hf含量对湿化学法制备超细W-Y_(2)O_(3)复合材料显微组织与性能的影响

本文在现有W-Y_(2)O_(3)材料基础上,引入微量Hf4+掺杂入Y_(2)O_(3),调节Y_(2)O_(3)与W晶粒之间的界面关系,从而改善W基材料的综合性能。通过改变Y与Hf元素的掺杂比例,获得纳米级W基复合粉体,在氢气气氛下常规烧结制备W-Y_(2)(Hf)O_(3)复合材料。采用SEM、TEM等表征手段对W-Y_(2)(Hf)O_(3)复合材料的性能进行表征分析,研究Y与Hf元素在材料中的作用规律。结果表明:掺杂Hf元素有利于后续氢气还原,在第二相掺杂量不变条件下,当Hf含量增加时,所获得的粉体粒径减小,W-3Y-7Hf的粒径约为100 nm,明显小于传统制备的W-Y_(2)O_(3)粉体。烧结后的块体晶粒尺寸细化,显微硬度和相对密度随之增大,成分为W-3Y-7Hf烧结块体显微硬度最高,为513.7HV_(0.2),致密度为97.6%。在钨基材料中同时添加Y与Hf元素会在钨晶粒的晶界与晶内处形成复合第二相氧化物Y_(2)Hf_(2)O_(7)颗粒,尺寸更小,弥散强化作用更强;其中,W-3Y-7Hf中第二相氧化物颗粒尺寸仅为200 nm左右,与钨晶界产生良好的界面结合关系,形成半共格界面。

Study on the RF inductively coupled plasma spheroidization of refractory W and W-Ta alloy powders

Spherical powders with good flowability and high stacking density are mandatory for powder bed additive manufacturing. Nevertheless, the preparation of spherical refractory tungsten and tungsten alloy powders is a formidable task. In this paper, spherical refractory metal powders processed by high-energy stir ball milling and RF inductively coupled plasma were investigated. By utilizing the technical route, pure spherical tungsten powders were prepared successfully, the flowability increased from 10.7 s/50 g to 5.5 s/50 g and apparent density increased from 6.916 g cm-3 to 11.041 g cm-3. Alloying element tantalum can reduce the tendency to micro- crack during tungsten laser melting and rapid solidification process. Spherical W-6Ta （%wt） powders were prepared in this way, homogeneous dispersion of tantalum in a tungsten matrix occurred but a small amount of flake-like shape particles appeared after high-energy stir ball milling. The flake-like shape particles can hardly be spheroidized in subsequent RF inductively coupled plasma process, might result from the unique suspended state of flaky particles under complex electric and magnetic fields as well as plasma-particle heat exchange was different under various turbulence models. As a result, the flake-like shape particles cannot pass through the high-temperature area of thermal plasma torch and cannot be spheroidized properly.