Spherical Tungsten Powder Research Articles

Preparation of spherical tungsten powder with uniform distribution of lanthania by plasma spheroidization

The high ductile-brittle transition temperature of tungsten (W) makes it prone to crack during additive manufacturing. Incorporating lanthania (La2O3) to tungsten had been proven to be an efficient way to solve the cracking problem. However, the current incorporation methods are often accompanied by problems such as uneven dispersion of lanthania. In this study, W-La2O3 spherical powder with uniform distribution of lanthania was successfully prepared by spray granulation, heat treatment, and plasma spheroidization. The morphology, particle size distribution, flowability, apparent density, and element distribution of W-La2O3 powder at different stages were studied. The results show that the final obtained W-La2O3 powder has smooth surface and narrow particle size distribution, which also has excellent flowability (8.2 s/50 g) and apparent density (10.7 g/cm3). Moreover, it was observed that the plasma spheroidization resulted in the decrease of lanthania content in the powder, and the law of variation of lanthania content was obtained by the linear fitting method. The spheroidization mechanism of W-La2O3 composite powders was also proposed by studying the microstructure of incompletely spheroidized powders and the change of lanthania content.

Research on Spheroidization of Tungsten Powder from Three Different Raw Materials

In this work, three kinds of tungsten powders with different particle sizes were spheroidized by radio-frequency (RF) inductively coupled plasma spheroidization. The spheroidization behavior of these tungsten powders was investigated and compared. The spheroidization effects of irregular tungsten powder improves with the decrease in degree of agglomeration and increases with primary particle size. Spherical tungsten powder from irregular powder with a primary particle size of 19.9 μm and an agglomeration coefficient of 1.59 had the best spheroidization effect; its apparent density, hall flow time, and spheroidization ratio are 9.36 g/cm3, 6.28 s/50 g, and 98%, respectively. The results show that irregular feedstock tungsten powder with a smaller primary particle size and higher agglomeration degree has a poor spheroidization effect because it is easily affected by the gas flow and deviates from the high temperature zone. On the contrary, irregular feedstock tungsten powder with larger primary particle sizes and lower agglomeration degrees has better spheroidization effects.

Investigation on spheroidization of refractory tungsten powders by laminar DC plasma torch

The spherical tungsten powders were prepared by a low-power DC laminar plasma torch. At a discharge current of 50 A, the tungsten powder fed downstream cannot be spheroidized by using Ar discharge, while a small number of spherical particles can be observed with upstream feeding. When N2 is added to the working gas, the input power is increased to maintain the same discharge current, which leads to the increase of plasma jet length, thereby expanding the effective high temperature region. Optical emission spectroscopy measurements show that the electron excitation temperature of N2-Ar mixed discharge is higher than that of argon discharge. In addition, compared with argon plasma, N2-Ar mixed gas plasma has larger thermal conductivity due to the molecular dissociation. Therefore, the heat is more efficiently transferred from the plasma to the tungsten particles. The improvement of plasma jet and heat transfer characteristics by introducing nitrogen into the discharge contributes to the increase of spheroidization rate. EDS analysis shows that the oxygen content in the tungsten particles decreases after spheroidization, which is caused by the evaporation of tungsten oxide during plasma treatment.

A Tungsten Heavy Alloy with Enhanced Performance Prepared by Spark Plasma Sintering from Fine Spherical Tungsten Powders

Traditional tungsten heavy alloys (WHAs), with a coarse liquid‐phase sintered microstructure, have moderate strength and excellent ductility. Grain refinement is one of the effective methods to improve the strength of these WHAs. The routine spark plasma sintering (SPS) method can refine W grains; however, the obtained typical solid‐state sintered microstructure is detrimental to the performance. Adapted SPS methods such as cyclic SPS and transient liquid‐phase SPS, proposed in the previous study, are capable of modifying W grain morphology to further enhance the performance of parts. They are, however, complex processes, especially when it comes to mass production. In this research, 93W–5.6Ni–1.4Fe heavy alloys (93WHAs) with similar liquid‐phase sintered microstructure are prepared via SPS at solid‐state sintering temperature using fine spherical W powders as raw materials. The influences of the W powder shape and powder mixing condition on the microstructural and mechanical properties of the prepared 93WHAs are investigated. An improved microstructure is obtained in the 93WHA prepared using spherical W powders blended with nano Ni and Fe powders as raw materials, as compared to using the irregular W powders. The alloy thus shows elevated mechanical properties, due to the improved matrix phase distribution and the W grain morphology.

Electrochemical Dissolution Process of Tungsten Carbide in Low Temperature Molten Salt System

Tungsten was extracted from LiCl-KCl-Li2WO4 molten salt with tungsten carbide as soluble anode, and its electrochemical dissolution was studied. Although the fused salt electrochemical method has the advantages of short process and easy equipment operation, the current electrolysis process faces problems such as higher electrolysis temperature, high energy consumption, and complex composition of the products. Therefore, in this study, in order to reduce the electrolysis temperature and energy consumption, tungsten was extracted by LiCl-KCl-Li2WO4 molten salt system in the temperature range 400 °C–600 °C. In addition, compared to the blank salt electrolysis, the addition of Li2WO4 as the active material facilitated the reaction and improved the dissolution efficiency, and the current efficiency. Through a series of electrochemical tests, it is proved that the adding of Li2WO4 decreases the charge transfer resistance and speeds up the reaction. Moreover, the oxidation–reduction process of tungsten ion in tungstate was investigated, indicating that the redox process is a reversible process controlled by diffusion. Clusters of spherical tungsten powders were prepared at 500 °C by optimizing the experimental parameters.

Estimation of plasma parameters in the process of micro-scale powder plastic and characteristics of its products

The temperature and density of plasma jets were estimated with a Boltzmann plot and Stark broadening of Ar I (696.54 nm) lines by optical emission spectroscopy (OES) in the process of plasma plastic, and the morphology and microstructure of tungsten (W) powders were investigated by scanning electron microscope (SEM) and x-ray Diffraction (XRD), respectively. The results show that the assumption of local thermodynamic equilibrium (LTE) was invalid at the end of the plasma jets, and earlier than this after the injection of tungsten powder. The temperature and electron density of the plasma jets were up to about T = 6797 K with Qc = 50 slpm and ne = 1.05 × 1016 cm−3 with Qs = 115 slpm at Z = 60 mm, respectively, and both dropped rapidly with the injected tungsten powders of 20 μm. After the plasma plastic process, the spherical tungsten powders were prepared and there were some satellite particles on the surface of the spherical products. The tungsten powders were both composed of a single equilibrium α-W phase with a body centered cubic (bbc) crystal structure before and after plasma treatment.

Spheroidisation of tungsten powder by radio frequency plasma for selective laser melting

ABSTRACTThe growing interest in additive manufacturing of refractory metals is accompanied by the demand on spherical refractory meal powders with good flowability. In this work, the particle size distribution, particle morphology, phase and extent of oxidation of the tungsten powders spheroidised by radio frequency (RF) plasma with different feeding rates were investigated using scanning electron microscopy, laser particle size analysis and X-ray diffraction. It was demonstrated that spherical tungsten powders with extremely fine flowability and high apparent density were obtained by the RF plasma spheroidisation technology. High-density tungsten samples with negligible defects were prepared with the spheroidised powders by selective laser melting. The results would help pave the way for solving the problems in customising refractory metal parts by additive manufacturing.

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.

Fabrication of fine-grained spherical tungsten powder by radio frequency (RF) inductively coupled plasma spheroidization combined with jet milling

Fine-grained spherical tungsten powder with particle size from 6μm to 11μm has been fabricated by a well-designed method with two simple steps. A pretreatment of tungsten powder was carried out in the first step. Complete dispersion, effective classification and favorable shape modification have been achieved by jet milling process. Accordingly, monodisperse tungsten powders with narrow particle size distribution were obtained. On the basis of the existing calculation and simulation results, plasma spheroidization parameters have been optimized. Thus, fine-grained spherical tungsten powder with narrow particle size distribution has been prepared by subsequent radio frequency (RF) inductively coupled plasma spheroidization. Furthermore, the presented method can be useful in fabricating spherical tungsten powder of different particle size with narrow particle size distribution as well as for process control in large-scale continuous production.

Properties evolution of additive manufacture used tungsten powders prepared by radio frequency induction plasma

In this study, radio frequency induction plasma was adopted to improve tungsten powder properties to meet the requirements of additive manufacturing process. After plasma spheroidization process operated, tungsten particles changed from irregular shape to uniform spherical shape. The average size and oxygen content of raw tungsten powders were both decreased, meanwhile, the laser absorption coefficient was increased by plasma spheroidization process. Furthermore, a series of tests were carried out for raw and spherical tungsten powders to investigate the influence of plasma spheroidization process on powder flowability evolution. The results displayed that, plasma spheroidized powders had a better comprehensive flowability, such as basic flow property, aerated flow property, compressibility, permeability and shear property, comparing to raw tungsten powders. In conclusion, plasma spheroidized tungsten powders met demands of additive manufacturing process very well.

Fabrication of homogeneous tungsten porous matrix using spherical tungsten powders prepared by thermal plasma spheroidization process

In this work, spherical and dense tungsten particles with average size of 13μm were synthesized by thermal plasma spheroidization process, and were further used to fabricate porous tungsten matrix with homogeneous pore distribution and open pore channel. The influences of sintering temperatures, dwelling time and additive on the microstructure and microhardness evolution of porous products were investigated, and the experimental results show that spherical and dense particles could keep their initial shape and favor the reservation of packed pores with narrow pore size distribution, which exhibits superiority in fabrication of tungsten matrix with uniform pore distribution compared with irregular tungsten powders. Specially, the porosity of porous tungsten matrix could be finely tuned from 25% to 30%, which has obvious effect on microhardness of obtained porous skeleton. The sintering kinetic analysis indicates that grain boundary diffusion is the primary mass transport mechanism during the fabricating porous tungsten matrix process. Furthermore, WCu composites fabricated by spherical powders exhibit higher thermal conductivity than that of irregular powders, which reveals the superiority of spherical tungsten powder.

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 particle 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 6.41 μm is obtained from polyhedral tungsten powder with an average diameter of 7.50 μm. 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 processing 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.

High Purity Tungsten Spherical Particle Preparation From WC-Co Spent Hard Scrap

Abstract Tungsten carbide-cobalt hard metal scrap was recycled to obtain high purity spherical tungsten powder by a combined hydrometallurgy and physical metallurgy pathway. Selective leaching of tungsten element from hard metal scrap occurs at solid / liquid interface and therefore enlargement of effective surface area is advantageous. Linear oxidation behavior of Tungsten carbide-cobalt and the oxidized scrap is friable to be pulverized by milling process. In this regard, isothermally oxidized Tungsten carbide-cobalt hard metal scrap was mechanically broken into particles and then tungsten trioxide particle was recovered by hydrometallurgical method. Recovered tungsten trioxide was reduced to tungsten particle in a hydrogen environment. After that, tungsten particle was melted and solidified to make a spherical one by RF (Ratio Frequency) thermal plasma process. Well spherical tungsten micro-particle was successfully obtained from spent scrap. In addition to the morphological change, thermal plasma process showed an advantage for the purification of feedstock particle.

Spherical modification of tungsten oxide powder and its mechanism analysis

Owing to its high production costs, complexity of equipment, and difficulty in controlling parameters, spherical or subglobose tungsten powder preparation method cannot meet the demand of industrial production. Tungstic oxide powder was treated by particle composite system and its effects were studied. Morphologies of particles were investigated by scanning electron microscopy (SEM). Particle size analysis was carried out and the related mechanism was discussed. The results show that the processing effect is best when the rotational speed is set at 4,000 r·min−1 for 15 min: the powder particles become nearly spherical and their sharp edge angles are rounded off and reshaped. When the processing time is 60 min, the powders smash to pieces because of too much energy inputting. So the test results, such as grain size distribution, can be explained well. Nearly spherical tungsten powder is obtained after reduction at 780 °C for 2 h and its flow ability is significantly improved.

Preparation of spherical tungsten and titanium powders by RF induction plasma processing

Spherical tungsten and titanium powders were prepared by radio frequency inductively coupled plasma torch with irregular shaped tungsten and titanium hydride (TiH2) powders, respectively. The effect of the feed rate on spheroidization efficiency was investigated. The phase composition, oxygen content, morphology, and particle size distribution of the powders before and after spheroidization were characterized by X-ray diffraction (XRD), nitrogen/oxygen analyzer, scanning electron microscopy (SEM), and laser micron sizer (LMS). The results show that both kinds of plasma-processed powders have good dispersity, smooth surface, and single phase. A maximum of 100 % of spheroidization efficiency can be achieved at a lower feed rate. The spherical titanium powder obtained by the plasma treatment consists of particles with mean diameter of 33.34 μm, while the mean diameter of original TiH2 powder is 136.56 μm. The apparent density and flowability of the spherical tungsten powder are 6.3 g·cm−3 and 0.16 s·g−1, respectively. The apparent density and flowability of the spherical titanium powder are 2.8 g·cm−3 and 0.52 s·g−1, respectively. With the increase of the feed rate, the spheroidization efficiency of raw powders drops gradually.

Fine Spherical Tungsten Powder Prepared by Precipitation Reaction

The spherical tungsten powder was prepared by chemical reaction with ammonium tungstate and strong acids under ultrasonic and mechanical agitation. After precipitation reaction, the precipitate was dried and grinded, and then reduced into tungsten powder with hydrogen. The effects of acid kinds and dispersant on the fine tungsten powder were studied in this paper. The result shows that the acid kinds and its addition amount have great effect on the shape of tungsten particles. The tungsten powder with uniform particle size and spherical could be prepared by adding 17ml sulfric acid into 100ml ammonium tungstate. The tungsten particles can be finer and more dispersive, and have a spherical with addition of dispersant SDS (Sodium dodecyl sulfate). The particle size is about 1.5 micrometer.

Study on Spheroidization of Tungsten Powders by RF Plasma Processing

Spherical tungsten powders were synthesized by (RF) argon plasma with irregular tungsten powders in this paper. The microstructure and crystallographic phases of the synthesized powders were characterized using scanning electron microscopy (SEM) and X-ray diffractometry (XRD). The results showed that fine spherical powder composed of pure tungsten phase with favorable dispersity and its surface becomes smooth. The spheroidization efficiency of synthesized powders was almost 100 %, and the particle size distribution became narrower after spheroidization treatment. The spheroidization efficiency of the sample decreases gradually as increasing of the feeding rate. The RF plasma spheroidization of tungsten powder is an ideal route synthesis of well- spherical metallic tungsten powders which is widely used in thermal spraying and metal injection moulding process.

Hydrostatic pressing of tungsten powders

1. It is shown that in the whole pressure range investigated (9–180 kg/mm2) coarse-grained spherical tungsten powders surpass standard fine-grained powders in compressibility. 2. In a wide pressure range (from 9 to 100–150 kg/mm2) the compressibility of the powders investigated is satisfactorily described by M. Yu. Bal'shin's second equation. The limiting pressure below which this equation is applicable falls with increase in the relative density of powders. 3. For spherical powders the character of density, microstress, and electrical resistivity vs compaction pressure curves changes at pressures above 100 kg/mm2, which is apparently indicative of a change in the mechanism of their densification.

High-performance cesium ionizers made from sized spherical tungsten powder

Tungsten microspheres used to fabricate cesium ionizers having improved uniformity, pore character and ionization performance