Study on behaviors of tungsten powders in radio frequency thermal plasma

This paper describes an investigational study of the spheroidization of refractory metal tungsten powders by radio frequency thermal plasma, with emphasis on the melting, solidification and growth behavior of the tungsten powder particles during the spheroidization process. The flight time and melting time of tungsten powder particles in the plasma were estimated, and the growth behavior of the tungsten powder particles was analyzed in detail by investigating the change in the average particle size before and after plasma spheroidization. The morphology and spheroidization rate were analyzed using field emission scanning electron microscopy. The flight time and melting time for tungsten powder particles with radius of 7.8 μm were calculated to be 9.3 ms and 2.9 ms, respectively. The change in powder particle size during the process showed that the growth of tungsten powder particles was mainly caused by the coalescence of droplets in the thermal plasma system. The experimental results demonstrated that the spheroidization rate can reach up to 95% under the operating conditions used in this work.     

Forming and sintering behaviors of commercial α-Al2O3 powders with different particle size distribution and agglomeration

The Al₂O₃ structure ceramics have been investigated extensively in previous studies. In order to compare micron- with submicron-scale powder on forming and sintering behaviors, three commercial α-Al₂O₃ powders were studied: 0.15 μm (denoted S as small in the paper) (granulating), 0.43 μm (denoted M as middle) (granulating), and 1.8 μm (denoted L as large) (granulate-free) at d₅₀ (median size). Although the (M) powder contains hard agglomerates, it forms more easily than the (S) powder. This is principally because the (M)'s soft agglomeration strength (0.03 MPa) is weaker than (S) (7 MPa). The (L) bulk formed easily with lower pressure 10 MPa because of wider starting-particle size distribution, 0.2–15 μm. The (S) primary particles rearranged before sintering, so it postponed its sintering onset temperature to about 1200 °C. Additionally, its shrinkage rate becomes maximal and concentrated at the 2nd stage of sintering from 1300 to 1400 °C. (M) bulk revealed the longest shrinkage range from 1000 to 1500 °C because the sintering occurred with its hard agglomerates at first. Although (L) powder formed rather easily, its sintering was impeded by a much wider particle size distribution.     

Sintering of tungsten powder with and without tungsten carbide additive by field assisted sintering technology

Tungsten powder (0.6–0.9 μm) was sintered by field assisted sintering technology (FAST) at various processing conditions. The sample sintered with in-situ hydrogen reduction pretreatment and pulsed electric current during heating showed the lowest amount of oxygen. The maximum relative density achieved was 98.5%, which is from the sample sintered at 2000 °C, 85 MPa for 30 min. However, the corresponding sintered grain size was 22.2 μm. To minimize grain growth, nano tungsten carbide powder (0.1–0.2 μm) was used as sintering additive. By mixing 5 and 10 vol.% WC with W powder, densification was enhanced and finer grain size was obtained. Relative density above 99% with grain size around 3 μm was achieved in W–10 vol.% WC sintered at 1700 °C, 85 MPa, for 5 min.     

The effect of submicron-sized initial tungsten powders on microstructure and properties of infiltrated W-25 wt.% Cu alloys

In the present work, several W-25 wt% Cu alloys have been prepared through combined processes of high-energy ball-milling, liquid-phase sintering and infiltration, using the precursors of industrial copper powders with an average particle size of 50 μm and tungsten powders with alternative average particle size of 8 μm, 800 nm, 600 nm or 400 nm. Microstructure characteristics, relative density, hardness and electrical conductivity of the WCu alloys were investigated to elucidate the effect of initial particle size of tungsten powders. EBSD was further utilized to reveal the orientation and grain size distribution in the WCu alloys prepared by 8 μm and 400 nm-sized tungsten powders. The results showed that the WCu alloy made by 400 nm-sized tungsten powders exhibited excellent homogeneity for both sintered tungsten powders and grains, together with the highest relative density of 98.9%, the highest hardness of 230 HB, and good electrical conductivity of 48.7% IACS. Moreover, it also showed highly improved arc erosion and mechanical wear resistances.     

Study on preparation and property of porous tungsten via tape-casting

A novel method of preparing porous tungsten via tape-casting is developed in this study. Micron-sized bulk porous tungsten with an open, biporous structure with large pores of 3–6 μm and with small pores of ~ 1 μm was successfully fabricated. The morphology of large pore depends on NaCl space-holder, and the uniform porous structure can be attributed to dispersant and binder added in tape-casting slurry which keeps tungsten powders decentralized, make the slurry stable and ordered. Compared with conventional process, the sintering temperature is reduced by at least 300 °C with the help of exothermic carburization of tungsten where carbon is introduced in the process of removing organics. W₂C phase was in situ generated on the surface of W particle and became the boundary between W grains. Furthermore, tape-casting samples show typical compressive properties of brittle porous material with higher compressive strength, which is attributed to the hard phase (W₂C) and uniform porous structure.     

The effects of ball-milling treatment on the densification behavior of ultra-fine tungsten powder

Ultra-fine tungsten powder with a BET particle size of 210 nm was synthesized by sol spray drying, calcination and subsequent hydrogen reduction process. Then this powder was treated by ball-milling, the characteristic changes of this powder before and after milling were investigated. Then the sintering densification behavior of these powders with different ball-milling time (0 h, 5 h, 10 h) were also studied. The results show that ball-milling treatment greatly activates the sintering process of ultra-fine tungsten powder. The relative density of the powder ball-milled for 10 h could reach 97.3% of theoretical density (TD) when sintered at 1900 °C for 2 h, which is 600 °C lower than the required temperature of the traditional micro-scaled powder sintered for the same density. At the same time, ball-milling treatment could substantially reduce the onset temperature of sintering as well as recrystallization, and bulk tungsten materials with more uniform and finer microstructure and much better mechanical properties (hardness) could be obtained.     

Fabrication and characterization of pure tungsten using the hot-shock consolidation

Crack-free pure W bulks have been fabricated by SHS assisted hot-shock consolidation (HSC). The tungsten powders were preheated by the heat released through a SHS (self-propagating high-temperature synthesis) reaction before shock wave loading. The duration of preheating was less than 3 min and the preheating temperature was controlled in the range of 700 ~ 1300 °C by adjusting the mass of the SHS mixture. The highest relative density of compacted samples can reach 96.7% T.D. (theoretical density) at 1300 °C under the shock pressure of 3.14GPa. The grain sizes of all compacted samples are nearly the same as the initial powder size of 2 μm. The hardness and modulus of the consolidated pure W bulks were measured using nanoindentation test; and the microstructure was investigated using light microscopy (LM) and scanning electron microscopy (SEM). It is found that the shock pressure plays a more important role than preheating temperature, after the pressure exceeding the crush strength of tungsten powder during the sintering process. At the preheating temperature of 1300 °C, the increase in shock pressure leads to obvious surface melting. For HSC of pure tungsten, the void collapse and surface melting are the main sintering mechanisms. The former one contributes to the densification behavior of powders, and the later one is responsible for the inter-particle bonding; and both of which are dominated by the shock pressure. The advantage of preheating for eliminating the cracks is also demonstrated by the experimental results.     

The aqueous electrophoretic deposition (EPD) of diamond–diamond laminates

The aqueous electrophoretic deposition (EPD) of a diamond/diamond laminate with two alternating grades of diamond was investigated. Diamond particles of average particle size 0.5 μm and 2 μm were deposited in an alternating manner onto tungsten carbide substrate. The layered diamond deposit was sintered with the carbide substrate in a high-pressure, high-temperature press. The sintered deposit was examined for evidence of alternating residual stresses. Differences of cobalt content in the 0.5 and 2 μm layers were observed by image analysis. The sintered diamond laminate demonstrated only minimal crack deflection during three-point bending.     

Effect of initial particulate and sintering temperature on mechanical properties and microstructure of WC–ZrO2–VC ceramic composites

Owing to improving the mechanical properties of cemented carbides in high speed machining fields, a new composite tool material WC–ZrO₂–VC (WZV) is prepared from a mixture of yttria stabilized zirconia (YSZ) and micrometer VC particles by hot-press-sintering in nitrogenous atmosphere. Commercial WC, of which the initial particle sizes are 0.2 μm, 0.4 μm, 0.6 μm and 0.8 μm, is mixed with zirconia and VC powder in aqueous medium by following a ball mill process. The sintering behavior is investigated by isostatic pressing under different sintering temperature. The relative density and bending strength are measured by Archimedes methods and three-point bending mode, respectively. Hardness and fracture toughness are performed by Vickers indentation method. Microstructure of the composite is characterized by scanning electron microscopy (SEM). The correlations between initial particles, densification mechanism, sintering temperature, microstructure and mechanical properties are studied. Experimental results show that maximum densification 99.5% is achieved at 1650 °C and the initial particle size is 0.8 μm. When temperature is 1550 °C and particle size is 0.4 μm, the optimized bending strength (943 MPa) is obtained. The best hardness record is 19.2 GPa when sintering temperature is 1650 and particle size is 0.8 μm. The indention cracks propagate around the grain boundaries and the WC particles fracture, which is associated with particle and microcrack toughening mechanism.     

Explosive compaction of tungsten powder using a converging underwater shock wave

Tungsten and tungsten-based alloys have wide applications in industries. Powder metallurgy is one of the major processes for production of tungsten parts, but tungsten parts with high density cannot be produced by this method. Two explosive compaction processes using converging underwater and no-water shock wave, were applied to compact tungsten powder in the present investigation. C4 as an explosive material with a detonation velocity of 8.2 km/s applied to consolidate amorphous powder with a mean grain size of 5 μm. The density and hardness of consolidated tungsten parts were determined and by scanning electron microscope (SEM) analyzed their fragment surfaces. In addition to explosion experiments, a numerical simulation of compaction processes conducted by use of LS-DYNA program. Finally, the experimental results of two processes and numerical simulation results of the same processes compared. The results indicated that the tungsten parts without cracks and with a hardness equal to 570 Vickers and a density equal to 18.5 g/cm³ can be obtained by underwater shock wave compaction method.     

Fabrication of bronze components by metal injection moulding using powders with different particle characteristics

The aim of this work is to fabricate bronze components by metal injection moulding (MIM) studying the possibility of changing partially or totally the gas atomised powder by water atomised ones that are cheaper than the former. In order to carry out this study, a bronze 90/10 gas atomised spherical powder (usual MIM powder <22 μm) and two water atomised irregular powders (particle size <35 μm and <140 μm) were mixed in different proportions. As received powders and their mixtures were used to fabricate feedstocks and processed by MIM to evaluate the influence of powder particle size and morphology on debinding and sintering stages. Finally, both mechanical properties hardness and maximum flexural stress were determined to characterize the sintered materials. The addition of irregular fine and coarse powders was found to affect the moulding process, although densities and mechanical properties close to values of gas atomised one were obtained after sintering. Therefore, the use of water atomised bronze powders could be a promising way to diminish production costs in this technology.     

Preparation of ultra fine copper–nickel bimetallic powders for conductive thick film

In this paper, ultrafine nickel-rich Cu–Ni bimetallic powders were synthesized with hydrothermal-reduction method. When polyethyleneglycol (PEG) was employed as protective agent, flake bimetallic powder particles, which have an excellent dispersibility and uniform size of 1.8–2.0 μm, can be prepared. Polyhedral powder particles, which have a uniform particle size in the range from 0.5 to 0.8 μm, were successfully synthesized using gelatin as protective agent. By thermal analysis, it was found that the oxidation-resistance of Cu–Ni powder particles was strong. Above-mentioned flake/polyhedral bimetallic powders were mixed with inorganic binder and vehicle to make conductive thick film. The low resistivity and high adhesion strength of thick film were attributed to high densification and rough interface from interfacial reaction, respectively.     

Thermal properties of W–Cu composites manufactured by copper infiltration into tungsten fiber matrix

W–Cu composites were produced by the technique of copper infiltration into tungsten fiber preforms (CITFP) under vacuum circumstance. Fibrous structure preforms with various volume fraction of tungsten fiber were fabricated by the process of mold pressing and sintering. The molten copper was infiltrated into the open pores of the preforms under vacuum at 1473 K to 1573 K for 1 h to produce W–Cu composites with compositions of 10–30 wt.% copper balanced with tungsten. The microstructure, relative densities, and thermal properties of the composites were investigated and measured. The relative as-sintered density was enhanced with the increase of the sintering temperature. The thermal conductivity of the W–Cu30 composite with 28.2 wt.% Cu was 241 W/(m · K) at 298 K, 10% higher than that of the W–Cu alloy with similar copper content produced by conventional powder metallurgy process. The thermal expansion of the composites was decreased with the increase of tungsten content, keeping the same tendency as the prediction by the rule of weighted average of volume ratio of compositions.     

Effect of starch filler content and sintering temperature on the processing of porous 3Y–ZrO2 ceramics

A direct casting process was used to produce porous 3Y–ZrO₂ ceramics using starch as a fugitive filler and binder. The compositions with low additions of starch had higher porosity than the volume fraction of starch initially in the green body (X_st), whereas, the compositions with high amounts of starch produced lower porosity than the predicted value. The well ordered structure consisted of spherical pores of 8–10 μm diameter, retained from the original starch particles, connected by channels. The interconnection between pores was dependent on the volume fraction of starch incorporated, as well as on the sintering temperature. Pore interconnection was observed for all the compositions sintered at 1000–1300 °C. Increasing the sintering temperature to 1400–1500 °C produced a marked dependence of the open to total porosity ratio on X_st. For a high porosity material, a bimodal channel size distribution was found at 1400 and 1500 °C. The primary pore channel diameter was 0.7 μm and the secondary one was close to 4 μm. As the sintering temperature increased, the volume of the connecting channels decreased; at 1500 °C only a minor volume of the larger channels was found.     

Microstructure evolution of grey cast iron powder by high pressure gas atomization

The grey cast iron powders were prepared by high pressure gas atomization. Calculation results show that the cooling rates of droplets of grey cast iron reach to 10⁴ to 10⁶ K s^?1 in the experiments. Microstructures of atomized grey cast iron powders with different diameters were characterized by X-ray diffraction, optical microscopy and scanning electron microscopy. Microstructures of powders under 38 μm in diameter are mainly γ-Fe and a little α-Fe. With the increase of powder diameter, content of γ-Fe phase gradually decreases, while content of α-Fe phase increases. When the diameter is over 150 μm, powders are completely composed of α-Fe phase. By measuring the spatial variation in microstructural scales within powders, the results show the lamellar spacing increases with the increase of powder diameter. As the powder size is larger than 106 μm, the increase trend of the lamellar spacing becomes smaller.     

Effect of particle size in aggregated and agglomerated ceramic powders

This work describes the compaction of agglomerated and aggregated ceramic powders with special emphasis on the role of primary particle size. Discrete element simulations are used to model weakly bonded agglomerates as well as strongly bonded aggregates. Crushing tests are carried out to obtain the characteristic strength of single agglomerate and aggregate. Microstructure evolution and stress–strain curves indicate that aggregates undergo a brittle to plastic-like transition as particle size decreases below 50 nm. It is shown that agglomerates made of nanoparticles exhibit much greater strength than those made of micron-sized particles, with an approximately inverse linear relationship with primary particle size. Simulation of the uniaxial compaction of a representative volume element of powder demonstrates that adhesive effects are responsible for the difficulty to compact nanopowders and for the heterogeneity of microstructure prior to sintering.     

Near-nano and coarse-grain WC powders obtained by the self-propagating high-temperature synthesis and cemented carbides on their basis. Part I: Structure,composition and properties of WC powders

Near-nano WC powders with mean grain sizes of about 200 nm were prepared by the SHS method including the reduction of WO₃ by Mg in the presence of carbon and regulating additives. The chemical leaching and refinement of the SHS reaction products allowed one to obtain stoichiometric WC containing only traces of oxygen and magnesium. The thermal reduction of WO₃ and V₂O₅ by magnesium in the presence of carbon resulted in obtaining two carbide phases of WC and complex carbide (W,V)C with the fcc crystal lattice having a grain size of less than 300 nm. It was established that the tungsten oxide reduction by magnesium in the presence of carbon cannot be used to synthesize coarse-grain WC powders. Coarse-grained WC powders were obtained using the W + C mixture heated to high temperatures by a simultaneous exothermic reaction of interaction between magnesium perchlorate Mg(ClO₄) and magnesium. The coarse-grain WC powder synthesized in such a way is nearly stoichiometric and consists of sintered round-shaped agglomerates with the average grain size of up to 16 μm and containing only traces of magnesium and oxygen. The agglomerates comprise WC single-crystals of roughly 1 μm to 8 μm in size.     

Warm die compaction and sintering of titanium and titanium alloy powders

A study has been made of the effect of non-lubricated warm die (200 °C) compaction on the densification of hydride–dehydride (HDH) Ti powder, pre-alloyed (PA) Ti-6Al-4V and Ti-10V-2Fe-3Al powders, and HDH Ti and V-Fe-Al master alloy powder blends, compared to cold die compaction. Depending on the compaction pressure, which was varied from 200 to 1000 MPa, non-lubricated warm die (200 °C) compaction was very effective for −100 mesh HDH Ti powder, increasing the green density by 5.0–9.4% theoretical density (TD). Die wall lubrication with stearic acid showed no influence on the green density when compacted at 800 MPa. With warm die (200 °C) compaction, achieving a green density of greater than 90%TD was straightforward for HDH Ti powder when compacted at ≥750 MPa. Accordingly, near pore-free (≥99.5%TD) Ti microstructures were obtained after sintering at 1300 °C for 120 min in vacuum when compacted at 1000 MPa. The resulting increment in the sintered density was between 2.0%TD and 4.4%TD. Warm die (200 °C) compaction showed no effect on PA Ti-10V-2Fe-3Al powder and only a small effect on PA Ti-6Al-4V powder when compacted at 1000 MPa. However, it was still virtually effective for Ti-10V-2Fe-3Al powder blends made of HDH Ti powder and V-Fe-Al master alloy powder. The observations were compared with literature data and discussed in accordance with the yield strength of Ti, Ti-6Al-4V, Ti-10V-2Fe-3Al and Al₃V as a function of temperature.     

Effects of processing parameters on density and electric properties of electric ceramic compacted by low-voltage electromagnetic compaction

In this research, low-voltage electromagnetic compaction (EMC) was applied to compact TiO₂ and PZT powders in the indirect way. After selecting the appropriate processing parameters, TiO₂ and PZT ceramics of higher density and better electrical properties were produced compared with traditional static compaction. The microstructures of two ceramics produced by two above-mentioned methods respectively show that, the average grain size of TiO₂ and PZT compacted by low-voltage EMC are about 8 μm and 4 μm which are smaller than that by static compaction respectively (15 μm and 7 μm) under the same sintered condition. Discharge voltage and charge capacitance are important factors to the green density and sintered part's density of each ceramics. Meanwhile, TiO₂ and PZT have their own discharge voltage range (700–1100 V for TiO₂ and 600–1000 V for PZT), during which each ceramic powder could be pressed effectively. With the same condition of charge capacitance, as the discharge voltage increases toward a peak value, the green density and sintered part's density increase, then tend to decrease after that peak value. The green density and sintered part's density of each ceramic increase and the above peak discharge voltage decrease slightly, as charge capacitance enlarges in the range investigated. In addition, effects of pancake coil turns and field shaper structure on the ceramic density were investigated. In most of cases investigated, the higher the ceramic part's density, the better the dielectric constants of TiO₂ parts and the piezoelectric constants of PZT parts.     

The effect of additive and sintering mechanism on the microstructural characteristics of W–40Cu composites

In this research, effect of cobalt and nickel additives on the W–40wt.% Cu composites prepared by solid phase sintering and infiltration (SPS + I) as well as liquid phase sintering (LPS) processes has been investigated. For this purpose, three types of powder consist of pure tungsten, mixture of tungsten–1wt.%Co and mixture of tungsten-1wt.%Ni were separately prepared and compacted by cold isostatic pressing (CIP). In the SPS + I process, compacted specimens were sintered at 1100 °C for 1 h and subsequently infiltrated by liquid copper at 1250 °C for 1 h. In the LPS process, compacted samples were directly infiltrated without initial sintering. Density of samples was measured by Archimedes method. Microstructure (i.e. contiguity, porosity, grain size) and chemical composition were studied by SEM and EDS, respectively. It is found that microstructural characteristics of the W–40wt.%Cu composites depend on sintering mechanism as well as additive type. Density of samples prepared by LPS process was higher in compared with ones obtained via SPS + I process. This behavior was related to W–W contiguity as well as tungsten particles wettability.