Basic process of powder metallurgy process
(Summary description)
1. Preparation of raw material powder. The existing milling methods can be roughly divided into two categories: mechanical methods and physical chemical methods. The mechanical method can be divided into: mechanical crushing and atomization method; physical chemical method is further divided into: electrochemical corrosion method, reduction method, chemical method, reduction-chemical method, vapor deposition method, liquid deposition method and electrolysis method. Among them, the most widely used are reduction method, atomization method and electrolysis method.
1. Preparation of raw material powder. The existing milling methods can be roughly divided into two categories: mechanical methods and physical chemical methods. The mechanical method can be divided into: mechanical crushing and atomization method; physical chemical method is further divided into: electrochemical corrosion method, reduction method, chemical method, reduction-chemical method, vapor deposition method, liquid deposition method and electrolysis method. Among them, the most widely used are reduction method, atomization method and electrolysis method.
2. The powder is formed into a compact of the required shape. The purpose of forming is to produce a compact of a certain shape and size, and make it have a certain density and strength. The molding method is basically divided into pressure molding and pressureless molding. Compression molding is most widely used in compression molding. In addition, 3D printing technology can also be used for the production of billets.
3. Sintering of the compact. Sintering is a key process in the powder metallurgy process. The formed compact is sintered to obtain the required final physical and mechanical properties. Sintering is divided into unit system sintering and multi-system sintering. For the solid phase sintering of the unit system and the multi-component system, the sintering temperature is lower than the melting point of the metal and alloy used; for the liquid-phase sintering of the multi-component system, the sintering temperature is generally lower than the melting point of the refractory component and higher than that of the fusible component. Melting point. In addition to ordinary sintering, there are also special sintering processes such as loose sintering, immersion method, and hot pressing method.
4. The subsequent processing of the product. The treatment after sintering can be done in various ways according to different product requirements. Such as finishing, oil immersion, machining, heat treatment and electroplating. In addition, in recent years, some new processes such as rolling and forging have also been applied to the processing of powder metallurgy materials after sintering, and have achieved ideal results.
Property of powder
The general term for all the properties of the 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.); the mechanical properties of the powder (loose density, fluidity, etc.) , Formability, compressibility, stacking angle and shear angle, etc.); physical properties and surface characteristics of the powder (true density, gloss, wave absorption, surface activity, ze%26mdash;ta(%26ccedil;) potential and magnetism, etc. ). Powder properties often determine the properties of powder metallurgy products to a large extent.
The most basic geometric properties are the particle size and shape of the powder.
(1) Granularity. It affects the powder processing, 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 obtained by dividing the average particle size of the original powder particles by 10; The big problem is that to obtain finer grain size cemented carbide, it is only possible to use finer grain size wc raw materials. The powder used in production practice has a particle size ranging from a few hundred nanometers to a few hundred micrometers. The smaller the particle size, the greater the activity, and the easier it is for the surface 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 too small to a certain extent, the quantum effect starts to work, and its physical properties will change dramatically. For example, ferromagnetic powder will become superparamagnetic. Powder, the melting point also decreases as the particle size decreases.
(2) The particle shape of the powder. It depends on the powdering method, such as the powder obtained by the electrolysis method, the particles are dendritic; the iron powder particles obtained by the reduction method are sponge flakes; the gas atomization method is basically spherical powder. In addition, some powders are egg-shaped, disc-shaped, needle-shaped, onion-shaped, etc. The shape of powder particles will affect the fluidity and bulk density of the powder. Due to the mechanical engagement between particles, the compact strength of irregular powder is also high, especially dendritic powder has the highest compact strength. But for porous materials, spherical powder is best.
Mechanical properties The mechanical properties of the powder are the technological properties of the powder, and it is an important process parameter in the powder metallurgy forming process. The bulk density of the powder is the basis for weighing by volumetric method during compression; 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 High and low; and the formability of the powder determines the strength of the blank.
The chemical performance mainly depends on the chemical purity of the raw materials and the method of making powder. Higher oxygen content will reduce compaction performance, green compact strength and mechanical properties of sintered products, so there are certain regulations in most technical conditions of powder metallurgy. For example, the allowable oxygen content of the powder is 0.2% to 1.5%, which is equivalent to an oxide content of 1% to 10%.
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