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简体中文 Spherical tungsten carbide powder is an essential surface functional material, widely applied in aerospace, electronic information, electric power, energy, petroleum, chemical engineering, metallurgy and machinery industries. In traditional production, tungsten carbide powder, tungsten powder and recycled materials are mixed with a specific carbon ratio, placed into graphite boats, and smelted in tilting carbon tube furnaces at nearly 3000°C. Tungsten carbide powder produced by conventional methods is mostly flaky, strip-shaped or polygonal, with a low content of acicular eutectic structures.
Based on abundant preliminary studies on refractory metal powder preparation, this research independently develops a set of ultra-high temperature atomization equipment. The integrated ultra-high temperature melting and inert gas atomization device can heat the melt above 3000°C and realize continuous atomization. With tungsten powder, carbon black, tungsten carbide precursors and polygonal tungsten carbide as raw materials, high-quality spherical tungsten carbide is prepared through ultra-high temperature atomization. This paper explores the influence rules of ultra-high temperature atomization parameters on the preparation of spherical tungsten carbide, and analyzes the performance characteristics and formation mechanism of finished powder.
Core Reasons for Adopting Ultra-high Temperature Atomization Equipment
Solve Core Technical Difficulties
Traditional smelting technology cannot realize particle spheroidization, resulting in irregular particle morphology and insufficient eutectic structures, which weaken practical application performance. The excellent comprehensive properties of spherical powder depend on regular morphology and uniform microstructure. Ultra-high temperature atomization disperses high-temperature melt into fine droplets, which shrink into spherical shapes under surface tension and form uniform WC and W₂C eutectic structures during rapid cooling. Meanwhile, tungsten carbide and its precursors feature ultra-high melting points, which cannot be fully melted and reacted by ordinary equipment. This customized device provides a stable ultra-high temperature environment over 3000°C, ensuring thorough melting, sufficient chemical reactions and stable component uniformity of raw materials.
Break Through Industrialization Limitations
Current mainstream technologies such as plasma processing and water-cooled crucible method are restricted by low output, complex processes, high cost and difficult large-scale promotion. The ultra-high temperature atomization equipment integrates melting and continuous atomization, realizing integrated continuous production, effectively improving production efficiency and reducing processing costs to support industrial mass production. In addition, the fully sealed structure and inert gas isolation system greatly reduce carbon burning loss and oxidation contamination, stably maintaining the chemical composition and performance consistency of powder products to meet the application demands of high-end industrial fields.
Experiment
Experimental Equipment and Core Characteristics
The experiment was carried out on the ultra-high temperature atomization equipment independently developed by Hunan Meiteyou Cemented Carbide Co., Ltd., as shown in Figure 1. The whole system consists of nine major modules, including feeding system, heating and melting system, melt conveying and atomization system, powder collection system, classification treatment system, inert gas protection system, automatic control system, power supply system and gas recovery system. It innovatively combines ultra-high temperature melting with gas atomization, breaking through the technical bottleneck of gas atomization for ultra-high melting point materials. During operation, the furnace tube is heated to about 3000°C, and materials are continuously and evenly supplied through granulators. Under the injection of atomizing nozzles, the one-time powder forming rate reaches more than 95%, and the one-time spheroidization rate is over 90%. The fully sealed heating and atomization design minimizes material burning loss. The compact and simplified process greatly cuts the production cost of cast spherical WC, and supports continuous and efficient industrial production.
Experimental Process and Testing Methods
Raw materials with specific particle sizes, including tungsten powder, carbon black and tungsten carbide precursors, were proportionally mixed according to carbon content requirements. After briquetting treatment, the mixtures were added into the high-temperature melting atomization furnace at a constant speed. Raw materials were melted into liquid at nearly 3000°C, and the melt entered the high-temperature atomization area. Under the high-pressure impact of 0.8~1.5 MPa high-purity argon, the melt was crushed into tiny droplets. Droplets entered the cooling zone and rapidly condensed into spherical cast spherical tungsten carbide powder relying on surface tension.
After sample preparation, an XJP-100 metallographic microscope and LEO1450 scanning electron microscope were used to observe and analyze the microstructure. An M21X X-ray diffractometer was adopted for phase composition detection, and a Hall flow meter was used to test powder fluidity for comprehensive performance characterization.
Results and Discussion
Powder Morphology
Reaction Mechanism and Influencing Factors in Atomization
The closed ultra-high temperature melting and atomization device realizes continuous melting and atomization without interruption. Raw materials are continuously sent into the reactor, fully heated and melted during the falling process, and then transported to the atomizer through the beam flow hole. The residence time of materials in the reactor is only about 1 second. Within this short period, raw materials complete heat absorption, solid-solid reaction and eutectic formation of WC and W₂C. The core chemical reaction is shown below:
W + WC -> W2C
In this solid-solid reaction, tungsten particles capture carbon elements from WC and trigger surface reactions, which gradually penetrate into particle interiors to generate W₂C. The particle size and activity of tungsten powder directly control the reaction progress. Unreacted tungsten cannot be melted under conventional atomization temperatures due to its melting point of 3410°C, remaining as impurity grains in finished powder. Similarly, the particle size and grain structure of WC also affect reaction efficiency, melting effect and eutectic quality, making particle size distribution a critical process control index.
Optimization of Process Parameters
To guarantee complete melting and sufficient interfacial reaction of all raw materials, the particle size of precursors was controlled within 80~200 μm. The melting point of cast spherical tungsten carbide is 2525°C, and extra superheat is required for qualified spheroidization. Five temperature gradients of 2600°C, 2700°C, 2800°C, 2900°C and 3000°C were set to explore the temperature influence on product performance.
Influence of Atomization Temperature on Powder Performance
As listed in Table 1, temperature dominates the spheroidization effect. Powders prepared at 2600°C are hard to be spheroidized. Samples obtained at 2700°C present a low spheroidization rate with irregular shapes and rough surfaces. Complete spheroidization can only be achieved when the temperature rises above 2800°C. In the range of 2700°C to 3000°C, the increase of atomization temperature effectively improves spheroidization rate, surface smoothness and overall particle morphology.
Atomization temperature shows a positive correlation with spheroidization rate and powder fluidity, as recorded in Table 2. The spheroidization rate is relatively low at 2700°C, reaches 90% at 2800°C, and achieves 100% full spheroidization at 2900°C. Higher temperature also brings better powder fluidity. In terms of particle size distribution, fine powder content rises while coarse powder proportion decreases with the increase of temperature from 2700°C to 3000°C. Higher superheat reduces the viscosity of spherical tungsten carbide melt and lowers the energy required for droplet crushing, which promotes the formation of fine powder under the same atomization conditions.
Microstructure, Phase Structure and Properties
Figure 3 shows the macro morphology and microstructure of spherical tungsten carbide powder. The samples present complete spherical shapes and smooth surfaces, with fine feather-like eutectic structures inside. It proves that precursors fully absorbed heat to complete the phase transformation from W to W₂C and WC in the smelting process, and finally formed stable WC-W₂C eutectic structures after cooling.
Figure 4 displays the X-ray diffraction phase analysis results of spherical tungsten carbide prepared by ultra-high temperature atomization. Only WC and W₂C dual phases exist in all samples. With reasonable temperature and precursor particle size, raw materials are fully melted and reacted to form pure spherical tungsten carbide eutectic organizations.
Analysis of Atomization Mechanism
Temperature Dependence of Droplet Spheroidization
After the formation of metal droplets in atomization, surface tension drives irregular droplets to shrink into regular spheres. Under high superheat, droplets own sufficient cooling time to complete spheroidization before solidification, forming complete spherical or subspherical powder. At low superheat, droplets solidify rapidly in primary or secondary crushing. The insufficient spheroidization time leads to coarse particles, dumbbell-shaped structures and other defective morphologies.
Influence of Melt Physical Properties on Atomization
Melt density, surface tension and viscosity jointly determine the flow behavior and atomization characteristics. Limited by compressibility, the density variation of liquid melt is negligible, exerting little impact on the overall atomization process. Surface tension maintains droplet stability and controls powder morphology, while viscosity plays a key role in adjusting particle size. The minimum energy required for atomization is positively correlated with surface tension and liquid surface area variation. For spherical tungsten carbide melt, surface tension and viscosity decrease with the increase of superheat, providing theoretical support for temperature regulation.
Correlation Between Spheroidization Index and Superheat
Spheroidization index is adopted to quantitatively evaluate spheroidization degree, defined as the length ratio of X and Y axes in powder projection. When the index equals 1, the powder is an ideal sphere; when the index is greater than 1, particles are irregular. Effective spheroidization must meet the critical superheat condition. In the range of 2700°C to 2850°C, rising temperature reduces melt surface tension and viscosity, significantly increasing spheroidization rate. Particles can automatically shrink into regular spheres under adequate superheat conditions.
Conclusions
- With tungsten powder, carbon black, tungsten carbide precursors and polygonal tungsten carbide as raw materials, high-fluidity, wear-resistant and high-performance spherical tungsten carbide can be mass produced via ultra-high temperature melting and inert gas atomization technology.
- Within the temperature range of 2800~3000°C, higher atomization temperature increases fine powder content and synchronously optimizes particle sphericity and spheroidization rate.
- The prepared spherical tungsten carbide features regular spherical appearance and excellent fluidity. Its interior is composed of fine acicular eutectic structures with pure WC-W₂C eutectic phase, with a microhardness of 3200HV and a Hall flow rate of 6.5s/50g.
- Based on the influence law of melt superheat on spheroidization, the spheroidization index is introduced for quantitative characterization, and the correlation formula between spheroidization index and melt superheat of tungsten carbide powder is established.
