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(2) The particle shape of the powder. It depends on the milling method, such as the powder obtained by electrolysis, the particles are dendritic; the iron powder particles obtained by the reduction method are in the form of sponge sheets; the gas atomization method is basically a spherical powder. In addition, some powders are ovoid, disc-shaped, needle-shaped, onion-shaped, and the like. The shape of the powder particles affects the fluidity and bulk density of the powder. Due to the mechanical meshing between the particles, the strength of the green compact of the irregular powder is also large, and in particular, the dendritic powder has the highest strength of the compact. When it is as small as a few hundred nanometers, the storage and transport of the powder is not easy, and when it is small to a certain extent, the quantum effect begins to function, and its physical properties will change greatly, such as ferromagnetic powder will become superparamagnetic Powder, the melting point also decreases as the particle size decreases. However, for porous materials, it is best to use spherical powder.
Mechanical properties The mechanical properties of the powder, ie the process properties of the powder, are important process parameters in the powder metallurgy forming process. The bulk density of the powder is the basis for volumetric weighing during pressing; the fluidity of the powder determines the filling speed of the powder and the productivity of the press; the compressibility of the powder determines the ease of pressing and the application of pressure. High and low; and the formability of the powder determines the strength of the blank.
The chemical properties mainly depend on the chemical purity of the raw materials and the milling method. Higher oxygen content will reduce the pressing properties, the strength of the compact and the mechanical properties of the sintered product, so there are certain provisions in most technical conditions of powder metallurgy. For example, the powder may have an oxygen content of 0.2% to 1.5%, which corresponds to an oxide content of 1% to 10%.
(1) Particle size. It affects the processing of the powder, the shrinkage during sintering, and the final properties of the product. The performance of some powder metallurgy products is almost directly related to the particle size. For example, the filtration accuracy of the filter material can be empirically determined by dividing the average particle size of the original powder particles by 10; the properties of the cemented carbide product are very similar to those of the wc phase. In the big relationship, it is only possible to obtain a fine-grained cemented carbide with a finer-grained wc raw material. Powders used in production practice range in size from a few hundred nanometers to hundreds of microns. The smaller the particle size, the greater the activity and the easier the surface will oxidize and absorb water.