What are the characteristics of powder properties in powder metallurgy
The general term for all properties of a powder. It includes: the geometric properties of the powder (particle size, specific surface, pore size and shape, etc.); the chemical properties of the powder (chemical composition, purity, oxygen content and acid insoluble matter, etc.); mechanical properties of the powder (bulk density, fluidity, etc.) , formability, compressibility, stacking angle and shear angle, etc.); physical properties and surface properties of powder (true density, gloss, wave absorption, surface activity, ze%26mdash;ta(%26ccedil;) potential and magnetic properties, etc. ). Powder properties often determine the performance of powder metallurgy products to a large extent.
The most fundamental geometric properties are the particle size and shape of the powder.
(1) Granularity. It affects the shaping of the powder, 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 obtained by dividing the average particle size of the original powder particles by 10; In order to obtain a finer grained cemented carbide, it is only possible to use a finer grained wc raw material. The powders used in production practice have particle sizes ranging from a few hundred nanometers to a few hundred micrometers. The smaller the particle size, the greater the activity, and the easier the surface is to oxidize and absorb water. When it is as small as a few hundred nanometers, the storage and transportation of the powder is not easy, and when it is small to a certain extent, quantum effects start to work, and its physical properties will change dramatically, such as ferromagnetic powder will become superparamagnetic For powders, the melting point also decreases with decreasing particle size.
(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 reduction method are sponge flakes; the powder obtained by gas atomization is basically spherical powder. In addition, some powders are oval, disc, needle, onion head, etc. The shape of the powder particles will affect the fluidity and bulk density of the powder. Due to the mechanical meshing between the particles, the compact strength of the irregular powder is also large, especially the dendritic powder has the highest compact strength. But for porous materials, it is best to use spherical powder.
Mechanical properties The mechanical properties of the powder are the process properties of the powder, which are important process parameters in the powder metallurgy forming process. The bulk density of the powder is the basis for weighing by the volumetric method during pressing; the fluidity of the powder determines the filling speed of the powder to the die and the production capacity of the press; the compressibility of the powder determines the difficulty of the pressing process and the pressure applied. 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, green strength and mechanical properties of sintered products, so most of the technical conditions of powder metallurgy have certain provisions for this. For example, the allowable oxygen content of the powder is 0.2% to 1.5%, which corresponds to an oxide content of 1% to 10%.