Production of Spheroidized Micropowders of W-Ni-Fe Pseudo-Alloy Using Plasma Technology

Abstract

The process of obtaining powders from the 5–50 μm fraction of a W-Ni-Fe system consisting of particles with predominantly spherical shapes was investigated. Experimental studies on the plasma–chemical synthesis of a nanopowder composed of WNiFe-90 were carried out in a plasma reactor with a confined jet flow. A mixture of tungsten trioxide, nickel oxide, and iron oxide powders interacted with a flow of hydrogen-containing plasma generated in an electric-arc plasma torch. The parameters of the spray-drying process and the composition of a suspension consisting of WNiFe-90 nanoparticles were determined, which provided mechanically strong nanopowder microgranules with a rounded shape and a homogeneous internal structure that contained no cavities. The yield of the granule fraction under 50 μm was 60%. The influence of the process parameters of the plasma treatment of the nanopowder microgranules in the thermal plasma flow on the degree of spheroidization and the microstructure of the obtained particles, seen as their bulk density and fluidity, was established. It was shown that the plasma spheroidization of the microgranules of the W-Ni-Fe system promoted the formation of a submicron internal structure in the obtained spherical particles, which were characterized by an average tungsten grain size of 0.7 μm.

1. Introduction

Tungsten and its pseudo-alloys and composites are widely used nowadays as materials for the creation of equipment assemblies operating in extreme conditions. In particular, they are currently considered the main materials for fusion reactor assemblies that have direct contact with high-temperature plasma and are exposed to powerful heat and radiation flows [1,2,3,4]. The industrial production of tungsten-based products uses powder metallurgy methods because tungsten’s high melting point and insufficient plasticity under normal conditions complicate the use of casting and machining technologies. Modern additive technologies (ATs) significantly expand the possibilities of powder metallurgy, facilitating layer-by-layer fabrication of parts with complex shapes, which has attracted the attention of designers of fusion reactor design elements [5,6,7,8]. According to the data given in [9], the number of publications related to ATs of tungsten and its alloys has been increasing exponentially since 2008.

To realize ATs, micropowders of metals and alloys consisting of spherical particles of a given particle size distribution in the size range of 10–50 μm are used [10,11]. To obtain such powders, including tungsten-based powders, irregularly shaped microparticles are spheroidized by melting them in the thermal plasma flows of electric discharges [12,13,14,15,16].

Currently, fabrication of compacts from W-Ni-Fe pseudo-alloys, including using AT methods, is based on liquid-phase sintering of a mixture of powders of individual metals. Traditional production of tungsten powders reduces tungsten trioxide powder with hydrogen in a stationary or moving bed in electric furnaces [17,18]. A significant intensification of this reduction process is achieved when it is carried out in a flow of hydrogen thermal plasma from electric discharges [19,20,21,22,23,24,25]. Under these conditions, the vaporization of tungsten trioxide particles is ensured and tungsten nanoparticles form as a result of chemical condensation from the gas phase.

Plasma technology also makes it possible to produce nanocomposite powders of a W-Ni-Fe pseudo-alloy by reducing a mixture of oxides of individual metals in a flow of hydrogen-containing thermal plasma; the resulting nanoparticles have a core (W)–shell (Ni-Fe-W) structure. Nanopowders of tungsten and its pseudo-alloys are of interest for use in various applications, particularly in the creation of nanostructured materials [26], but nanopowders are not suitable for direct use in ATs.

High-intensity plasma technologies that provide target products in the form of metallic nanopowders can be used as an initial stage for the production of spherical micropowders for ATs. Nanopowders undergo granulation to obtain porous microgranules, followed by densification and spheroidization in a thermal plasma flow. Different methods can be used for the granulation of nanopowders. In particular, in our previous work [27], for the fabrication of W-Ni-Fe microgranules, the primary nanopowder was subjected to “dry” granulation via pressing, followed by milling and sieve classification, to obtain the target fraction of the microgranules, which was then densified and spheroidized in the thermal plasma flow of an electric-arc plasma torch. However, this granulation method is labor-intensive and inefficient compared to the granulation of nanopowders via spray drying, which is widely used in both production and research practices [28,29,30].

The purpose of this work was to experimentally investigate the production of micropowders of a W-Ni-Fe pseudo-alloy consisting of dense spherical particles with a submicron structure in a process including the following stages:

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The plasma–chemical synthesis of a W-Ni-Fe nanopowder;

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The granulation of the nanopowder via spray drying;

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The spheroidization and compaction of microgranules in a flow of electric-arc thermal plasma.

In addition to additive technologies, the use of spheroidized metal powders of tungsten pseudo-alloys that are obtained from nanopowders and possess submicron structures will be able to improve the characteristics of compacts produced using other modern powder metallurgy methods (hot isostatic pressing, plasma spraying, electric-pulse plasma sintering, etc.) [31,32].

2. Materials and Methods

A nanopowder with the composition W-7Ni-3Fe (WNiFe-90) was obtained by processing a mixture of tungsten trioxide powders (89.6 wt. % WO₃ (TU 6-09-17-250-88)), nickel oxide powders (7.0 wt. % NiO (TU 6-09-3642-74)), and iron oxide powders (3.4 wt. % Fe₂O₃ (TU 6-09-1404-76)) of corresponding compositions in a thermal plasma jet composed of nitrogen (purity: 99.99%) with hydrogen added up to 20 vol. % (purity: 99.99%). The plasma jet was generated in an electric-arc plasma torch with a nominal power of 30 kW. A powder feedstock with a particle size of less than 50 μm was fed into the plasma stream as a system of gas-dispersed jets using nitrogen as a carrier gas. A detailed description of the plasma–chemical unit used to produce nanopowders of metals and their inorganic compounds is presented in [33].

Studies on the granulation of the nanopowder of the W-Ni-Fe system were carried out using a Buchi Mini Spray Dryer B-290 (Flavil, Switzerland) laboratory unit that was equipped with an ultrasonic nozzle and an inert Buchi B-295 circuit for circulating working gas. Technical nitrogen from cylinders (purity: 99.6%) was used as a working gas in the process of nanopowder granulation.

A nanopowder suspension was prepared in distilled water using a Bandelin Sonopuls HD 3100 (Berlin, Germany) ultrasonic dispersant. To maintain the homogeneity of the suspension fed into the drying chamber of the dispersing unit and to prevent possible sedimentation of particles, the suspension was continuously stirred with an electromechanical stirrer.

From the nanopowder microgranules obtained using a Retsch AS 300 Control (Dusseldorf, Germany) sieve machine, the fraction under 50 μm was separated for further processing (densification and spheroidization of particles) in a jet of thermal plasma composed of argon (purity: 99.993%) with hydrogen added up to 7.5 vol. % (purity: 99.99%). A description of a plasma spheroidization unit for powder materials based on an electric-arc plasmatron is given in [34].

The powders obtained in the experiments were subjected to a comprehensive analysis of their physical and chemical properties. The morphologies and structural characteristics of the powder materials were studied using an Osiris (Hillsboro, OR, USA) transmission electron microscope (TEM) (“FEI”) and a Scios (Hillsboro, OR, USA) scanning electron microscope (SEM) (“FEI”) with elemental energy-dispersive microanalysis via an EDAX attachment and an FIB column for gallium ion etching. The specific surface areas of the powders were determined using the Brunauer–Emmett–Teller (BET) adsorption method on a Micromeritics TriStar 3000 (Norcross, GA, USA) specific surface area analyzer. The contents of oxygen and nitrogen were determined on a Leco TC-600 (St. Joseph, MI, USA) analyzer via reductive melting in an inert gas current (helium) in a graphite crucible in a pulsed resistance furnace. Oxygen was detected based on the amount of gaseous CO₂ released using the infrared absorption method. Nitrogen was detected based on thermal conductivity. Sample preparation involved packing the powder in an inert glovebox environment in a tin capsule. The hydrogen content was determined on a Leco RHEN-602 (St. Joseph, MI, USA) analyzer via reductive melting in an inert gas (argon) current in a graphite crucible in a pulsed resistance furnace. Hydrogen was determined based on thermal conductivity. The carbon content was determined on a Leco CS-600 (St. Joseph, MI, USA) analyzer via oxidative melting in a ceramic crucible in an induction furnace. Carbon detection was carried out via infrared absorption based on the amount of gaseous CO₂ released.

Diffraction studies using X-rays were carried out on an Rigaku Ultima-4 X-ray (Tokyo, Japan) diffractometer. The particle size distributions of the powder materials were determined via laser diffraction on a Malvern Mastersizer 2000M (Worcestershire, UK) analyzer. The roundness coefficient of the microgranules (the average value of the ratio between the perimeter of a microgranule and the perimeter of a circle with the same area), the degree of spheroidization of the microparticles (the ratio between the number of spherical particles and the total number of analyzed particles), and the internal structure of the latter (the number of cavities) were investigated via statistical image processing using an Olympus CX31 (Tokyo, Japan) optical microscope (OM) and ImageScope Color software (version M of 19.01.2010).

The separation of nanoparticles from plasma-treated micropowders was carried out using fractional separation in a liquid via sedimentation of a suspension after treatment on a Bandelin Sonopuls HD 3100 (Berlin, Germany) ultrasonic disperser. The flowability of the powder materials was determined for 50 g samples using a calibrated funnel (Hall flow meter) and a stopwatch according to GOST 20899-98 [35]. The bulk density of the powder materials was determined via the weight method using a funnel according to GOST 19440-94 [36].

Thermodynamic analysis was carried out using the TERRA software package [37] for the modeling phase and the chemical equilibria in multicomponent systems. The equilibrium compositions of the W-Ni-Fe-O-H system were calculated for isobaric and isothermal conditions at temperatures from 500 to 5000 K with a total pressure of 0.1 MPa.

3. Results and Discussion

3.1. Synthesis of W-Ni-Fe Nanopowder

The plasma–chemical synthesis of W-Ni-Fe nanopowder was carried out by feeding an initial mixture of oxide powders via a transport gas into a high-temperature plasma jet propagating in a cylindrical reactor with water-cooled walls. As a result of the heating and vaporization of particles of the dispersed raw materials, there was a reduction in the obtained oxide vapors. Metal vapors formed and subsequently condensed during cooling in the volume of the reactor. The formed nanoparticles were deposited on the walls and bottom of the reactor and were partially carried to the filtration apparatus with the exhaust gasses, where they were subsequently collected.

An argon–hydrogen mixture with flow rates from 0.5 to 2.5 m³(STP)/h was used as a plasma gas. The input power of the plasmatron used in the experiments ranged from 21 to 30 kW. The values of the average mass-specific enthalpy of the plasma jet were in a range from 3.5 to 8.5 kWh/m³(STP). The flow rates of the conveying gas varied from 0.5 to 1.5 m³(STP)/h. The initial powder mixture was fed at flow rates from 2 to 8 g/min.

The specific surface area of the nanopowder of the W-Ni-Fe system was 3.6 m²/g when it was synthesized using the optimal regime. This corresponded to an average particle size of 70 nm. The nanopowder was polydispersed and consisted of individual particles with a predominantly rounded shape (Figure 1a) and a core–shell structure (Figure 1b), where nickel, iron, and tungsten formed an alloy in the near-surface layers of the nanoparticles and the cores consisted of tungsten.

The presence of this particle structure was due to the large difference in the condensation temperatures of tungsten and iron-group metals. In our previous work [38], the thermodynamic calculations of the equilibrium compositions and characteristics of multicomponent W–Ni–Fe–O–H systems were performed. Based on the thermodynamic model of the process, at the equilibrium cooling of the W-Ni-Fe-O-H system (at different levels of excess hydrogen) and the temperature corresponding to the beginning of the condensation of nickel and iron vapors, all tungsten was present in the condensed state (Figure 2). Condensation of nickel and iron vapor with the formation of molten nanoparticles occurred in the presence of crystalline tungsten nanoparticles. Therefore, during collisions between nanoparticles, the complete fusion and alignment of concentrations over the volume of the particle did not occur. There was only partial dissolution of tungsten in the film of the iron and nickel alloy formed on the surface of the tungsten nanoparticles.

The sizes of the obtained nanoparticles were in a range from 20 to 400 nm, and it was found that the nanopowder contained up to 0.5 wt. % of particles in the micron-size range (Figure 3) that formed as a result of incomplete processing of the initial raw material, as well as atomization of the melt film from the surface of the garnish that formed in the near-surface area of the arc plasma torch.

The contents of the impurities in the obtained W-Ni-Fe nanopowder were as follows: oxygen—1.7 wt. %, nitrogen—0.02 wt. %, hydrogen—0.034 wt. %, and carbon—0.02 wt. %. The phase composition of the obtained nanopowder of the W-Ni-Fe system was mainly represented by a stable cubic alpha phase of tungsten (W) and a gamma phase of W-Ni-Fe, as well as traces of a metastable beta phase of tungsten (W) (Figure 4).

The nanopowder was characterized by a homogeneous distribution of the metals Ni and Fe in its constituent nanoparticles (Figure 5).

3.2. Granulation of Nanopowder

The process of W-Ni-Fe nanopowder granulation was carried out in order to obtain homogeneous nanopowder microgranules with spherical shapes and maximum yields of fractions smaller than 50 μm that had mechanical strength sufficient for transportation without destruction in the gas flow in the pipeline from the powder feeder to the plasma jet.

The results of our previous studies [39] were used for nanopowder granulation. To obtain nanopowder microgranules, aqueous suspensions containing from 50 to 60 wt. % of nanopowder were prepared. Sucrose was used as an organic binder to ensure the strength of the granules, and its content was 2 wt. % of the dry nanopowder. To increase the stability of the suspension, 0.2 wt. % of polyacrylic acid was present in the composition.

The W-Ni-Fe nanopowder microgranules of the fraction under 50 μm obtained in the experiments had a predominantly rounded shape (Figure 6). The microgranules ranged in size from 10 to 47 μm. According to the results of a statistical analysis of the granule morphology in the optical microscope images, it was determined that the roundness coefficient of the granules was maximized and was 1.0. The dispersion characteristics were D₁₀ = 15 μm; D₅₀ = 24 μm; and D₉₀ = 37 μm.

The internal structure of the microgranules was mainly characterized by homogeneity and the absence of internal cavities (Figure 7a). A uniform distribution of Ni and Fe was found over the volume of the microgranules at the submicron level (Figure 7b–d). According to the results of an EDS microanalysis of the surfaces of the microgranules and their volumes, the elemental compositions corresponded to the contents of the metals W/Ni/Fe as follows: 89.3/7.4/3.3 and 89.0/7.3/3.7 wt. %, respectively.

It was experimentally established that the maximum yield of the target fraction of W-Ni-Fe nanopowder microgranules under 50 μm was 60% when spray drying a nanosuspension with a nanoparticle concentration of 55 wt. %. The bulk density of the obtained microgranules was 2.8 g/cm³, and the fluidity was 42 s/50 g.

The contents of the oxygen, nitrogen, hydrogen, and carbon impurities in the microgranules were 2.6 wt. %, 0.4 wt. %, 0.034 wt. %, and 1.0 wt. %. The increases in the contents of the impurities in the obtained microgranules compared to the initial nanopowder were due to the presence of the organic binder sucrose (C₁₂H₂₂O₁₁).

To evaluate the possibility of reusing the nanoparticles that make up the microgranules of the fraction over 50 μm in spray-drying processes, experiments were performed to remove the organic binder from the microgranules of the above fraction. The microgranules were ultrasonically treated in water at 70 °C for 10, 20, or 50 min in order to dissolve the sucrose used as an organic binder and release the individual nanoparticles. A microscopic analysis of the particle morphology after treatment and washing showed that the microgranules were not destroyed. This may have been because in the process of spray drying at a temperature of 110–200 °C, the caramelization of sucrose occurred, which would have sharply worsened its dissolution in water. The fraction of microgranules over 50 microns could be subjected to milling and subsequent classification with separation of the target fraction of microgranules under 50 microns, which would provide a significant increase in its yield at the granulation stage.

To evaluate the mechanical strength of the obtained microgranules, abrasion tests were carried out on the fraction of granules between 20 and 60 μm with sucrose contents that ranged from 0.5 to 5 wt. % when mixing the samples (m = 10 g) in a “Turbula” C 2.0 mixer using different modes (Figure 8).

Based on the results of the particle size analysis, it was found that granules with a sucrose concentration of 0.5 wt. % did not collapse and tended to increase in size based on a small increase in particle diameter throughout the size range. When considering the low organic binder content in this sample, this may have been due to the aggregation of the particles during processing due to their plasticity (Figure 8a). The size of the microgranules containing sucrose at 2 wt. % remained unchanged during the mechanical actions (Figure 8b). A slight decrease in the size of the microgranules with a sucrose concentration of 5 wt. % can be explained by the possible destruction of the surface layers of the particles during abrasion due to a decrease in plasticity with the increased content of the organic binder (Figure 8c).

Taking into account the absence of mechanical loads on the microgranules during their transportation along the path to the plasma jet, it can be concluded that microgranules with sucrose concentrations in the vicinity of 2 wt. % provide sufficient fracture strength.

3.3. Spheroidization of Microgranules

The process of densification and spheroidization of the porous nanopowder microgranules in the plasma unit of IMET RAS was based on intensive heating of initial particles fed into a plasma flow by a transporting gas, the particles melting, and the melt droplets acquiring a spherical shape due to surface tension forces. The spherical shape of the microparticles was preserved during their crystallization in the cooling gas-dispersed flow.

In the experiments, the plasma gas flow rate varied from 2.0 to 2.5 m³/h, and the input power of the plasma torch ranged from 9 to 30 kW. The average mass-specific enthalpy of the plasma jet ranged from 1.0 to 2.7 kWh/m³. The flow rate of the transport gas was 0.5 m³/h, and the feedstock was fed at flow rates from 1 to 3 kg/h.

As a result of the experiments, micropowders of a W-Ni-Fe pseudo-alloy were obtained, consisting mainly of spherical particles. In addition to the latter, the powders contained nanoparticles of the W-Ni-Fe system that were enriched in Ni and Fe. They formed as a result of the partial vaporization of metals during thermal interactions between microgranules and the high-temperature gas flow. Due to the large difference in the boiling points of W and Ni-Fe, the chemical composition of the nanoparticles was dominated by iron-group metals. During the experimental studies, the mass contents of the nanoparticles ranged from 1 to 10 wt. % and were removed by washing the suspensions in distilled water during ultrasonic treatment. A photograph of a characteristic sample of plasma-treated micropowder after removal of nanoparticles is presented in Figure 9.

The phase composition of the obtained micropowders was mainly represented by the stable cubic alpha phase of tungsten (W) and the gamma phase of W-Ni-Fe (Figure 10).

The particle size in the characteristic sample of spheroidized W-Ni-Fe micropowder ranged from 8 to 43 μm (D₁₀ = 13 μm; D₅₀ = 19 μm; and D₉₀ = 28 μm) (Figure 11).

During the processing of the nanopowder microgranules in the plasma flow in the volume of each granule, processes occurred that led to the recrystallization of tungsten particles as a result of the sintering of nanoparticles, the partial dissolution of tungsten in the Ni-Fe melt, and its subsequent crystallization from the melt during cooling. This led to a radical change in the microstructure of the particles processed in the plasma compared to the original nanopowder microgranules. Differences in the heating conditions of the microgranules in the plasma flow, which was characterized by the presence of significant temperature and velocity gradients, led to the formation of two main types of microstructures in the obtained particles: a dense fine-grained microstructure and a fine-grained microstructure with cavities (Figure 12a). The first type of particle was predominant, and the countable contents of particles of the second type ranged from 3 to 30%. According to the results of a statistical analysis of the grain sizes in the internal structures of spheroidized micropowder in images obtained using a scanning electron microscope, it was determined that the tungsten grains in the microparticles had a rounded shape, and their sizes ranged from 0.2 to 1.5 μm (Dav = 0.7 μm) (Figure 12b).

According to the results of an EDS microanalysis of the surfaces and cross-sections of spheroidized microparticles, the elemental compositions corresponded to metal contents of W/Ni/Fe = 94.4/3.9/1.7 and 94.6/3.5/1.9 wt. %, respectively. Uniform distributions of nickel (Ni) and iron (Fe) along the microparticle cross-sections were established (Figure 13).

It was experimentally demonstrated that the presence of hydrogen markedly increased the heat transfer coefficient of the gas and intensified the heating of the particles, since the heating of the metallic particles in the thermal plasma was limited by the heat transfer from the gas to the particle surface (

Table 1. Characteristics of spheroidized W-Ni-Fe micropowders with different hydrogen contents obtained in a plasma jet.

	Enthalpy Value, kW·h/m3	Hydrogen Content, vol. %
		0	3.7	7.5
Fluidity, s/50 g	2.7	7.3	7.0	6.0
Bulk density, g/cm3		9.0	9.4	9.4
Degree of spheroidization, %		94.0	97	97

).

The highest level of microparticle spheroidization achieved in these experiments was 97%, and this value was obtained using argon–hydrogen plasma with hydrogen contents up to 7.5 vol. % and plasma jet enthalpy values of 2.0 kWh/m³ and higher (

Table 2. Characteristics of spheroidized W-Ni-Fe micropowders obtained using different values of plasma jet enthalpy.

	Hydrogen Content, vol. %	Enthalpy Value, kW·h/m3
		1.0	2.0	2.7
Fluidity, s/50 g	7.5	8.9	7.7	6.0
Bulk density, g/cm3		8.1	9.2	9.4
Degree of spheroidization, %		90	97	97

). The nanoparticle contents in the plasma-treated micropowders ranged from 6.0 to 10.0 wt. % under these conditions. Treating the nanopowder microgranules in plasma using the optimal regime yielded dense spheroidized W-Ni-Fe micropowders with a fluidity of 6.0 s/50 g and a bulk density of 9.4 g/cm³.

Testing the LPBF process using the spheroidized W-Ni-Fe micropowders showed the principal possibility of obtaining dense compact samples. Further studies in this direction will allow us to determine the optimal conditions and parameters for the process stages to obtain LPBF samples with the best possible physical and mechanical characteristics.

4. Conclusions

Experimental studies were carried out on the process of obtaining micropowders of a pseudo-alloy composed of WNiFe-95. The micropowders consisted of spherical microparticles with a submicron internal structure characterized by an average tungsten grain size of 0.7 μm.

The plasma processes were carried out in apparatuses based on electric-arc plasma torches. As a result of plasma–chemical synthesis, a nanopowder composed of WNiFe-90 was obtained. It consisted of individual particles with a predominantly round shape and a core–shell structure, where nickel, iron, and tungsten formed an alloy in the near-surface layers of the nanoparticles and the cores consisted of tungsten. Using a spray-drying method and a suspension consisting of nanoparticles, mechanically strong porous nanopowder microgranules with a rounded shape and a homogeneous internal structure containing no cavities were obtained. The yield of the microgranule fraction under 50 μm was 60%. The bulk density of the obtained microgranules was 2.8 g/cm³, and the fluidity was 42 s/50 g.

It was established that the parameters of the microgranule plasma treatment process significantly influenced the degree of spheroidization and the microstructure of the obtained dense particles, as well as their bulk density and fluidity. The possibility of significantly refining treated microgranules in the process of plasma spheroidization in terms of gas impurities (O, N, and H) and carbon was shown. The use of the optimal mode allowed a micropowder consisting of dense spherical particles to be obtained from the 5–50 micron fraction, with a fluidity of 6 s/50 g and a bulk density of 9.4 g/cm³.

The technical solutions applied to carry out all stages of this process provide the possibility of scaling to implement this process at an industrial level with a capacity on the order of tens of kilograms per hour.