Tungsten Powder Research Articles

SYNTHESIS OF TUNGSTEN CARBIDE-FILLED MULTIWALLED CARBON NANOTUBES BY DC ARC DISCHARGE PLASMA IN AMBIENT AIR

In this paper, the possibility of synthesizing multiwalled carbon nanotubes (CNTs) by DC arc discharge plasma in ambient air is shown. The synthesis takes place in a graphite cathode with a crucible shape using tungsten powder as a catalyst. In the non-vacuum synthesis method the CO atmosphere is generated in the reaction zone during the arcing, preventing the products of the synthesis from oxidation. According to the X-ray diffraction analysis (XRD), the synthesized product includes more than 85 vol % of carbon crystalline phases with a graphite-like structure, a cubic tungsten phase, and two phases of tungsten carbide WC and W2C. The transmission electron microscopy data shows that the product of the synthesis contains CNTs with a relatively large number of walls (up to ~50 pcs) (MWCNTs) with a diameter of up to ~100 nm and a length of up to ~1.5 μm. It was found that crystalline objects corresponding in structure to WC tungsten carbide are identified inside CNTs.

Magnesium thermal nano-sized powders of the chromium subgroup metals

A method for magnesium vapor reduction of the double oxides of chromium subgroup metals at low static pressure and dynamical vacuum was used. The study results of the production and investigation of tungsten, molybdenum, chromium, W-Cr mix and Mo-W alloy powders are presented. The specific surface area of the resulting powders was in the range from 14 to 41 m2·g−1. The average crystalline grain size estimated from the value of the range of coherent X-ray scattering was 8-40 nm.

The preparation of TiC dispersion strengthened tungsten alloy via freeze-drying method

First time, TiC dispersion strengthened tungsten powder was prepared by freeze-drying method. The aqueous solution containing dispersant (Polyvinyl Pyrrolidone (PVP)+ Methacrylic Acid (MA)), ammonium metatungstate (AMT) and TiC particles was sprayed into liquid nitrogen. After vacuum drying, the amorphous precursor powder was obtained. Rapid cooling and vacuum drying made precursors have unique fishbone like porous structure. After calcining and reduction, the W-TiC powder was obtained. The W-TiC powder made by freeze-drying method had small size, near-spherical appearance and well chemical activity. After being sintered by spark plasma sintering (SPS), small size sintered W-0.2/1.0(wt%) TiC alloys with significantly improved mechanical properties were obtained. The microstructure and fracture surface of the W-TiC alloys showed that TiC particles were evenly distributed in the grain interiors and boundaries. With the TiC content increasing to 1%, no obvious particle agglomeration was detected. Uniformity of TiC dispersion was attributed to the combination of dispersant (PVP + MA) and rapid cooling. Based on the successful preparation of small sintered samples, 1.0 kg W-0.2TiC powder was prepared to verify the possibility of mass production by the same method. After medium frequency induction sintering and hot rolling, W-0.2TiC plate with 60% deformation was successfully prepared. Obtained W-0.2TiC plate had strength of more than 1700 MPa and a ductile brittle transition temperature (DBTT) of about 200 °C.

Gamma-ray shielding characteristics of flexible silicone tungsten composites

Silicone being a hybrid elastomer is well known for its excellent thermal and mechanical properties, chemical resistance, compatibility with organic and inorganic fillers, nontoxicity, and flexibility. As the reported literature on silicone tungsten composite is rare, thus, a complete possible spectrum of silicone tungsten composites series with tungsten loading of 0–88.1 %wt has been fabricated by RTV method and studied as a flexible gamma shielding material. Flexible silicone/tungsten composite formulations containing different weight percentages of tungsten powder (0, 30.1, 47.8, 59.8, 68.1, and 88.1 wt %) were developed by the room-temperature vulcanization route. Two lead collimators with diameters of 0.6 cm were used to make a narrow beam geometry for gamma rays emitted from a137Cs (gamma-ray energy of 662 keV) point source. Uncollided flux was measured with a NaI(Tl) scintillation detector enclosed in lead shielding to reduce the background radiation level. The measured mass attenuation coefficient for our composites with 88.1 wt % tungsten was 0.1035 cm2/g, which is nearly 3.5% higher than that of commercially available silicone/tungsten composites named T-Flex (nearly 0.095 cm2/g) containing the same tungsten loading. Similarly, superior half value layers (HVL) of our composites with 88.1 % wt loading of tungsten i.e 1.01 cm versus 1.27 cm for the reported T-Flex counterpart with additional advantage of insitu fabrication on complex geometries. It was also found that the effectiveness of gamma-ray shielding increases with increase in density of the composites, which is due to the increase in the weight percent of tungsten powder. Our material will have applications as shielding material for both mobile and stationary radiation sources and it can also be used as fabrication material for gloves, safety shoes, coats, etc. to protect workers in a radiation environment.

Crystallite size induced bandgap tuning in WO3 derived from nanocrystalline tungsten

In this work, nanocrystalline tungsten oxide (WO3) powder was obtained by high energy ball milling (HEBM) followed by heat treatment of tungsten (W) powder. HEBM significantly influenced the oxidation behavior by reducing the crystallite size and inducing lattice strain in W powder. The resultant nanocrystalline WO3 shows variation in the optical bandgap with respect to the bulk counterparts. The alteration in the bandgap of WO3 is ascribed to the changes in crystallite size and the unit cell volume. In the end, a thorough correlation between fundamental interactions of these changes was succinctly highlighted.

Synthesis of W-AG Nanoalloys and Computer Aided Thermal Analysis of W-AG Nanoalloys Based Heat Sink

W-Ag finds applications in many frontier areas such as heat sinks and microwave absorbers for microelectronics devices, arcing tips, resistance welding electrodes, electrical contacts and heavy duty electronic contacts. These nanoalloys prepared by mechanical milling process result in highly dense product. Chemical approaches have significant advantages in terms of better stoichiometric control, purity and controllable particle size. The variation in chemical composition of these composites allows the match of thermal expansion coefficient with ceramic substrates used in semiconductor devices. In the present study, Ag nanoalloys with average sizes below 200 nm were prepared using tungsten powder and silver nitrate as precursors. Compositional characterization of W-Ag nanoalloys was carried out using portable XRF.The developed material i.e. W-Ag is used as heat sink application in which the heat sink is modeled and thermally analyzed for evaluation of heat flux. The heat flux obtained was compared with aluminum heat sink. It was perceived that W-Ag heat sink exhibited superiority in withstanding higher temperature. Whereas, heat sink designed from aluminum was limited with their parameter which is thermal conductivity.

Tungsten Matrix Composite Reinforced with CoCrFeMnNi High-Entropy Alloy: Impact of Processing Routes on Microstructure and Mechanical Properties

Tungsten heavy alloy composite was developed by using novel CoCrFeMnNi high-entropy alloy as the binder/reinforcement phase. Elemental tungsten (W) powder and mechanically alloyed CoCrFeMnNi high-entropy alloy were mixed gently in high energy ball mill and consolidated using different sintering process with varying heating rate (in trend of conventional sintering < microwave sintering < spark plasma sintering). Mechanically alloyed CoCrFeMnNi high-entropy alloy have shown a predominant face-centered cubic (fcc) phase with minor Cr-rich σ-phase. Consolidated tungsten heavy high-entropy alloys (WHHEA) composites reveal the presence of Cr–Mn-rich oxide phase in addition to W-grains and high-entropy alloys (HEA) phase. An increase in heating rate restricts the tungsten grain growth with reduces the volume fraction of the Cr–Mn-rich phase. Finally, spark plasma sintering with a higher heating rate and shorter sintering time has revealed higher compressive strength (~2041 MPa) than the other two competitors (microwave sintering: ~1962 MPa and conventional sintering: ~1758 MPa), which may be attributed to finer W-grains and reduced fraction of Cr–Mn rich oxide phase.

Effect of Y(NO3)3 additive on morphologies and size of metallic W particles produced by hydrogen reduction

Tungsten, one of the refractory metals, has lots of excellent properties, making tungsten powders become a choice of raw material for many applications. This work reported a simple method (the salt-assisted hydrogen reduction of WO3 powders) for preparing tungsten powders with desired morphologies and size. By adjusting the amount of additive (Y(NO3)3) and reduction temperature, metallic tungsten particles with different morphologies and size were obtained. It was found that both temperature and additive had apparent effects on the morphology and size of metallic W particles, and the large parallelepiped-shaped powders disappeared and large irregular blocky-shaped powders were degraded under the influence of additive. With the increase of temperature and amount of salt, particles became more dispersed and the size increased at higher temperatures, and the shape of particles was changed from spherical to polyhedral as well. On the contrary, the size of sub-particles became smaller as increasing amount of Y(NO3)3 at lower temperatures. Based on the results, the reaction and refining mechanisms for preparing tungsten were proposed as well.

The influence of the local effect of electric current on densification of tungsten powder during spark plasma sintering

The local overheating effect induced by the uneven distribution of current density is a crucial physical phenomenon during the spark plasma sintering (SPS) of metallic powder, which can promote the densification. In order to investigate the local effects of current on densification kinetics behaviors of tungsten powder during SPS, an electromigration-creep model was constructed by introducing the local current density and was used to describe the densification process. The numerical calculation based on the model indicates that the local effect of electric current can induce a certain impact on densification during SPS at the intermediate sintering stage, in which an extremely high local current density is generated. Additionally, with the decrease of porosity, the local effect of electric current on densification is weakened.

Preparation of Ultrafine W Powder via Carbothermic Prereduction of Tungsten Oxide Followed by Deep Reduction with Hydrogen

A two-step reduction method to synthesize tungsten powder with controllable size is proposed. Firstly, precursors composed of W and WO2 were synthesized via carbothermic prereduction of tungsten trioxide with different C/WO3 ratios at 1323 K, then the precursors were completely reduced by hydrogen to remove residual oxygen at 1023 K and 1223 K, respectively. It was observed that increasing the C/WO3 molar ratio in the prereduction stage was beneficial to decrease the particle size of the W powder. In the subsequent hydrogen reduction stage, the presence of the gaseous intermediate phase W-O-H had a great influence on the morphology of the final product. Tungsten powders with particle size ranging from micron to nanometer range could be prepared by controlling the C/WO3 ratio and hydrogen reduction temperature.

Investigation of densification behavior of tungsten powders during hot isostatic pressing with a 3D multi-particle FEM approach

In this paper, hot isostatic pressing (HIPing) process of pure tungsten powders was numerically reproduced using multi-particle finite element method (MPFEM) at a particulate scale level. The effects of pressure, temperature and particle size ratio on the densification behavior of the tungsten powders during HIPing were systematically investigated. Various macro- and microscopic properties including relative density, overall/local stress distributions, deformation status, pore filling behavior and densification mechanisms were characterized and analyzed. The results show that the HIPing process of tungsten components can be numerically realized from particulate scale by properly controlling the HIPing conditions such as pressure and temperature. The pressure and temperature during HIPing play a dominant role in determining both the final properties of the compacts and the microscopic properties of individual particles. Also, different particle size ratios of the tungsten powder mass can affect both the macro- and microscopic properties of the compacts during and after HIPing. It is indicated that MPFEM modelling used in this paper can effectively conquer the deficiencies in physical experiments and traditional FEM modelling on the HIPing of refractory tungsten metals, which can provide an effective way to reproduce the HIPing process and identify the densification dynamics and mechanisms more accurately and realistically.

Study on the properties of PPS composite modified with Tungsten powder

The tungsten powder (W) and polyphenylene sulfide (PPS) as the main raw material, through mixing, extrusion molding and extrusion molding method contrast, determines the extrusion method for the preparation of tungsten powder/PPS (W/PPS) composites, was studied by DSC, XRD and SEM after modification, the W powder for W/PPS material microstructure and the influence of thermal performance. W was mixed evenly in PPS, and the thermal decomposition temperature of PPS was increased.

Diamond Thin Film Coating on WC Substrate by HFCVD Method using Different Seeding Powders and Its Characterization

The formation of diamond thin solid film is studied by Hot Filament Chemical Vapor Deposition (HFCVD) by using different seeding powders on WC substrate. Here, we have deposited microcrystalline diamond (MCD) on SPUN cemented carbide (WC-Co) cutting tool insert using conventional HFCVD technique. The substrates were ultrasonically seeded with titanium, tungsten, molybdenum, diamond, combined diamond and tungsten powder at the same operation parameters to observe the difference in the growth of diamond by different seeding. Nucleation of diamond coating was studied after 40 minutes of coating with seeded substrate. The physical characteristics were studied by X-ray Diffraction (XRD).The Scanning Electron Microscope (SEM) was used to study the surface morphology. The diamond purity was studied with the help of Raman spectroscopy (RS). Reduction of cobalt percentage before and after pretreatment was studied by Energy Dispersive Spectroscopy (EDS). A very dense and uniform coating was deposited on the diamond powder seeded substrate. The nucleation density was found to be highest in diamond powder when compared with molybdenum and tungsten powder individually, because in initial stage the deposited material contains more sp2 content as compared to sp3 content. By using diamond and tungsten mixed powder the coating was uniform rather in case of only tungsten powder coating the discrete crystals were formed.

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.

Mechanical alloying assisted spark plasma sintering of Tungsten diboride ceramics

Herein, we demonstrate the fabrication of tungsten boride from high-purity tungsten (W) and boron (B) powders through mechanical alloying (MA), followed by spark plasma sintering (SPS) for the first time. First, WB2 powders are synthesized by mechanical alloying, which is achieved by high energy ball milling for 50 h. The phase composition and microstructure of the as-milled powders are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The as-milled WB2 powders possess an AlB2-type structure, with a crystallite size of 5–10 nm. The as-synthesized powders are sintered by SPS at 1600 °C, under the pressure of 30 MPa, for 10 min and 5 wt % of MgO is added as a sintering aid. Furthermore, the as-sintered WB2 and MgO-added WB2 ceramics are characterized by XRD, SEM and microindentation. The results reveal that the metastable AlB2-type WB2 phase has been transformed into a stable WB2-type WB2 phase after sintering. Moreover, a slight increase in the Vickers hardness is observed due to the addition of MgO, which can be ascribed to the grain refinement effect in the presence of sintering aid.

Investigations on surface morphologies, microstructure and thermal properties of tungsten-based composites

The purpose of this research is to determine the surface morphologies, microstructural and thermal properties of tungsten-based composites that consist of 93 wt.% tungsten (W), 6 wt.% vanadium carbide (VC) and 1 wt.% graphite (C) powders. W-6 wt.% VC-1 wt.% C powders were mechanically alloyed (MA’d) for 6 hrs using a [Formula: see text] with a rate rpm using tungsten carbide vial and balls and sintered at [Formula: see text] for under [Formula: see text], [Formula: see text] gas flow conditions. The phase composition and microstructural characterization of the tungsten composites were carried out using X-ray diffractometer (XRD), Scanning Electron Microscopy (SEM) and Raman Spectroscopy. SEM images showed the distribution of the tungsten (W), vanadium carbide (VC) and graphite (C) powders and porosity in the tungsten matrix. The Raman spectra exhibited two major peaks, which are recorded at 1331 (vs) [Formula: see text] and 1583 (vs) [Formula: see text] in the Raman spectra. These bands represented carbon phases such as disordered graphite (D) and graphite (G). Thermogravimetric analysis (TGA) measurements were performed to obtain the weight loss and thermal stability of samples in the temperature range [Formula: see text]–[Formula: see text] under argon gas atmosphere. The TG curve revealed a total loss of 3.3% of weight at this temperature range. It is considered that the cause of mass loss is due to the oxidation and gas desorption of materials.

Three-dimensional MPFEM modelling on isostatic pressing and solid phase sintering of tungsten powders

Particulate scale three-dimensional CIP and solid phase sintering of tungsten powder in powder metallurgy process was numerically reproduced using MPFEM method. The effects of initial packing structures on the densification behavior of the tungsten powders during CIP and sintering were systematically investigated. Various macroscopic and microscopic properties including relative density, overall/local stress distributions, deformation status, pore filling behavior as well as densification mechanisms were characterized and analyzed. The results indicate that the initial packing structure before CIP plays an important role in determining the properties of both green compact and sintered part after compaction and sintering. Detailed analyses of individual particles revealed that the particles with larger deformation tend to generate greater stress concentration during CIP and sintering; meanwhile, the stress distribution in different location of each particle also demonstrated that a close relationship exists between the stress concentration/release process of individual particles and their initial packing structure before CIP.

Influence of Particle Size on Laser Absorption and Scanning Track Formation Mechanisms of Pure Tungsten Powder During Selective Laser Melting

Abstract A three-dimensional laser absorption model based on ray tracing was established to describe the coupled interaction of a laser beam with particles in the powder layers of pure tungsten (W) material processed by selective laser melting (SLM). The influence of particle size on the powder-to-laser absorptivity and underlying absorption behavior was investigated. An intrinsic relationship between the absorption, distribution of absorbed irradiance within the powder layers, and surface morphology and geometric characteristics (e.g., contact angle, width and height of tracks, and remelted depth) of the laser scanning tracks is presented here. Simulation conclusions indicate that the absorptivity of the powder layers considerably exceeds the single powder particle value or the dense solid material value. With an increase in particle size, the powder layer absorbs less laser energy. The maximum absorptivity of the W powder layers reached 0.6030 at the particle size of 5 μm. The distribution of laser irradiance on the particle surface was sensitive to particle size, azimuthal angle, and the position of the powder particles on the substrate. The maximum irradiance in the powder layers decreased from 1.117 × 10–3 to 0.85 × 10–3 W·μm−2 and the contour of the irradiance distribution in the center of the irradiated area gradually contracted when the particle size increased from 5 to 45 μm. An experimental study on the surface morphologies and cross-sectional geometric characteristics of SLM-fabricated W material was performed, and the experimental results validated the mechanisms of the powder-to-laser-absorption behavior that were obtained in simulations. This work provides a scientific basis for the application of the ray-tracing model to predict the wetting and spreading ability of melted tracks during SLM additive manufacturing in order to yield a sound laser processability.

Effect of Nanosize Tungsten Powder on the Microstructure and Mechanical Properties of Silumins

The microstructure and mechanical properties of alloys of the Al – Si system modified with nanosize tungsten powder in an amount of 0.01 – 0.5 wt.% during casting are studied. The phase composition, the sizes, and the localization of phases in the microstructure of the initial and modified Al – Si alloys are investigated by the methods of optical, scanning, and transmission electron microscopy. The ultimate strength, the elongation, and the impact toughness of the Al – Si alloys are determined before and after modification with nanosize tungsten powder.

Study of the operating in conditions of abrasive wear deposited layer obtained with the use of new flux-cored wires

The effect of introduction of chromium with increased concentration as a reducing agent was studied at the manufacture of flux cored wire of the Fe – C – Si – Мn – Сr – Ni – Mo system. Metal welding deposition was carried out on the plates of St3 steel with the help of AN-26С flux with preliminary heating of the base metal to 250 – 300 °С. Flux cored wire of5 mmin diameter, made on a laboratory machine, was deposited on ASAW-1250 welding tractor in the following modes: current 420 –520 A, voltage 28 – 32 V, welding speed 7.2 – 9.0 m/h. After it the metal was cooled at room temperature. For manufacture of the samples, the corresponding powdery materials were used as fillers (charge): iron powder PZhV1 according to GOST (State Standards) 9849 – 86, FS 75 ferrosilicon powder according to GOST 1415 – 93, FKh900A high carbon ferrochrome powder according to GOST 4757 – 91, FMn 78(A) carbonaceous ferromanganese powder according to GOST 4755 – 91, PNK-1L5 nickel powder according to GOST 9722 – 97, FMo60 ferromolybdenum powder according to GOST 4759 – 91, FV50U 0.6 ferrovanadium powder according to GOST 27130 – 94, PC-1U cobalt powder according to GOST 9721 – 79 and tungsten powder PVN TU 48-19-72-92. It was determined that carbon, manganese, chromium, molybdenum, nickel and, to a small extent, vanadium, within the limits studied, simultaneously increase hardness of the deposited layer and decrease wear rate of the samples. It is shown that the low viscosity of the matrix does not allow tungsten carbides to be kept on the surface, as a result of which the wear is carried out not according to the uniform abrasion of the surface, but according to the scheme of spalling high-strength carbides particles from the matrix. As a result, new cracks are formed in the matrix, contributing to its additional wear. According to the results of the multifactor correlation analysis, dependences of hardness and wear resistance of the deposited layer on mass fraction of the elements included in the flux-cored wires of the Fe – C – Si – Мn – Сr – Ni – Mo system were determined. The obtained dependences can be used to predict hardness and wear resistance of the deposited layer with a change in the chemical composition of the weld metal.