Properties of Tungsten Metal Powder

The performance of tungsten powder has a significant impact on the processing performance and quality of subsequent products. Therefore, both the hard alloy field and the tungsten material processing field have put forward corresponding requirements for the chemical purity and physical properties of raw tungsten powder, especially the requirements for physical properties are getting higher and higher. 

Chemical purity

When manufacturing cemented carbide and tungsten products, the chemical purity of tungsten powder is required to be relatively high. The residual impurity elements in tungsten powder have an impact on the processing performance and service performance of the products. The influence is very complex, with some being harmful and some beneficial. Current research suggests that Ca, Mg, P, As, Si, S, Fe, Ni, Cu, Al, and Mo can reduce the strength of the alloy, while K and Na promote the growth of WC grains. V and Cr, on the other hand, inhibit the growth of grains. If the Mo content in WO exceeds 0.5%, it will cause a decrease in the flexural strength of the alloy. In most tungsten powder varieties currently produced, the content of residual metallic impurities (excluding those added as additives) is in the range of a few parts per ten thousand to a few parts per hundred thousand. 

Oxygen in tungsten powder can react with carbides, absorbing carbon from carbides and causing decarburization of cemented carbide. When the alloy is severely decarburized, the γ phase appears, making the alloy brittle. The gas released from the reaction increases the porosity of the alloy and reduces its strength. Depending on different reduction processes and equipment, the oxygen content in tungsten powder is generally between 0.05% and 0.5%, and it increases with the decrease in tungsten powder particle size and the increase in specific surface area. Therefore, the requirement for oxygen content in fine-grained tungsten powder has to be appropriately relaxed. The chemical purity requirements for tungsten powder are shown in Table 4-1, and the oxygen content requirements are shown in Table 4-2.

The impurity elements in tungsten powder may come from the raw materials or be introduced during the production process. Therefore, preventing the contamination of materials during the process is of great significance. For example, in the production of tungsten powder using APT as the raw material, the materials come into direct contact with the calcining furnace, reduction furnace tubes, and crucibles, resulting in an increase in the content of impurities such as Fe, Ni, Cr, and Si, and a decrease in chemical purity. When their content reaches a certain level or they aggregate to a sufficient size, they may become sources of defects for subsequent processing or use. Therefore, to ensure the purity of tungsten powder, in addition to strictly controlling the quality of the raw material APT, it is also very important to prevent contamination during the process. 

2. Physical Properties

The physical properties of metallic tungsten powder mainly include average particle size, particle size distribution, particle aggregation degree, particle morphology, specific surface area, bulk density, compacted density, and Hall flow rate, etc. 

(Average particle size and particle size distribution)

Whether it is cemented carbide or tungsten products, there are strict requirements for the average particle size and particle size distribution of tungsten powder. In the field of cemented carbide, the particle size and particle size distribution of W powder directly affect the particle size and particle size distribution of the produced WC powder. The particle size of WC powder further influences the performance of the cemented carbide products. 

The research has found that the properties of WC powder are restricted by those of W powder. After W powder is carbonized to form WC, the particle size undergoes a slight change. To produce WC powder of coarse, medium and fine particle sizes, coarse, medium and fine particle W powder needs to be used. Uneven W powder carbonization results in uneven WC powder. The changes in powder particle size after the carbonization of coarse, medium and fine particle W powder are shown in Table 4-3.

The requirements for tungsten powder particle size vary among different users. For the hard alloy field, for various types of hard alloys used for different purposes, due to the different particle sizes of WC powder used, there are different requirements for the average particle size and particle size composition of the raw material W powder. All cutting tools require the W powder and WC powder to have a fine particle size and a narrow particle size distribution. Impact tools require the W powder and WC powder to be coarse, with a wider particle size distribution. The average particle size used for preparing coarse-grained WC is 25.8 μm. The representative particle size distribution of the W powder is shown in Figure 4-1.

For tungsten material processing, the average particle size and particle size distribution of tungsten powder have an impact on the pressing performance of the subsequent products, the density of the green body (also known as the pressed body), and the sintering performance. Smaller powder particle size and more complex shapes will result in greater friction between particles, leading to a decrease in the density of the green body. The narrower the particle size distribution, the more loosely the particles are arranged. A wider particle size distribution, or even mixing powders of different average particle sizes, can achieve better particle arrangement and obtain higher green body strength. In the tungsten material processing field, the average particle size of tungsten powder is generally required to be within the range of 2 to 6 μm. 

There are many methods for determining powder particle size and particle size distribution. The Fischers' apparatus and the laser particle size analyzer are widely used in tungsten powder. However, due to the different principles of these two measurement methods, the measured values obtained from the same powder may vary. Therefore, the particle size of tungsten powder should generally be stated as the Fischers' average particle size or the laser average particle size. Additionally, it should be noted that "supplied state" tungsten powder usually has varying degrees of agglomeration, which is related to the production conditions. The average particle size of tungsten powder measured using such samples may differ from the actual particle size of the powder. For example, the particle size of some dull fine tungsten powder in the "supplied state" is 1-2 μm, and after depolymerization and dispersion, the value drops to 0.4-0.5 μm. For tungsten powder with particle sizes in the range of 1-10 μm, in most cases, measuring the "supplied state" particle size can meet the production requirements. For submicron tungsten powder and coarser tungsten powder, in order to more accurately characterize the size of the particles, "grinding state" samples must be used for average particle size and particle size distribution tests. 

The particle size distribution of tungsten powder is related to its particle size. Generally, the larger the average particle size of tungsten powder, the wider the particle size distribution. For a given particle size, in production, methods such as using wet hydrogen or adding alkali metal compounds to the tungsten oxide can make the particle size larger and control the particle size distribution range more narrowly. The determination of particle size distribution is often carried out using "ground state" samples. 

The average particle size of tungsten powder is generally expressed by its diameter (in micrometers). However, in production practice, some semi-quantitative concepts are often used. The common classifications include: 

Very coarse particles: Average particle size > 30 μm; 

Coarse particles: Average particle size 10 to 30 μm; 

Medium-sized particles: Average particle size 3 to 10 μm; 

Fine particles: Average particle size 0.5 - 3 μm; 

Ultrafine particles: average particle size < 0.5 μm. 

(2) Aggregation degree

The aggregation degree of powders is usually characterized by the difference in particle size between "supplied state" powders and "ground state" powders. The aggregation degree of fine tungsten powder is generally higher than that of coarse tungsten powder. For tungsten material production, the aggregation degree directly affects the strength of the green piece. In the WC production process, the aggregation degree of w powder has an impact on the uniformity of carbon distribution. 

(3) Particle morphology

The particle morphology of tungsten powder has an impact on its pressing performance and the strength of the green body. Irregular particle morphology leads to interlocking between particles, thereby enhancing the strength of the green body. Spherical tungsten powder has good fluidity and is particularly suitable for spraying materials. Similarly, when preparing WC, the morphology of tungsten powder also affects the morphology of WC powder. 

(4) Specific Surface Area

The total surface area possessed by a unit mass of tungsten powder is referred to as the specific surface area of the tungsten powder, which is usually expressed in units of m2·g-1. The specific surface area of tungsten powder typically ranges from 0.01 to 12 m2·g-1. It indirectly reflects the particle size and morphology of the tungsten powder and is an important indicator for evaluating the sintering activity, dissolution characteristics, and reaction ability with gaseous and solid substances during the carbonization process of the tungsten powder. 

(5) Loose density and compacted density

The loose density and compacted density of tungsten powder increase with the increase of the average particle size of the powder. The relationship between the loose density of tungsten powder produced by a certain factory and its Fischers' average particle size is shown in Table 4-4. The narrower the particle size distribution of the powder, the more complex the particle morphology, and the more severe the aggregation, the smaller the loose density. Generally, the process parameters of the reduction process can be adjusted to control it.

(6) Fluidity

The fluidity of tungsten powder is influenced by particle size, particle size distribution, and particle morphology. The coarser the powder particles, the rounder the particles, and the smoother the surface, the better the fluidity. The fluidity of tungsten powder is usually measured by Hall flow rate, which is expressed as the time it takes for 50g of tungsten powder to flow through a specified small hole in a Hall flow meter. The fluidity of the powder directly affects the volumetric loading during the pressing process and the uniformity of the die-cast density. 

(7) Compressibility

Compressibility refers to the ability of tungsten powder to be compressed under specified pressing conditions. It is usually measured in standard molds under specified lubrication conditions, and is expressed by the powder's density of the pressed product under the specified pressure. It can also be represented by a curve graph showing the change of pressed product density with pressing pressure. 

(8) Formability

This refers to the ability of the tungsten powder compact to maintain its predetermined shape. It can be expressed by the minimum pressure required for the powder to be formed, or by the strength of the compact under a certain forming pressure.