Powder metallurgy preparation method

(1) Powder production. The powder production process includes powder preparation and powder mixing steps. To improve the formability and plasticity of the powder, plasticizers such as machine oil, rubber, or paraffin wax are usually added.
(2) Pressing and molding. The powder is pressed into the desired shape under a pressure of 15-600 MPa.
(3) Sintering. This is carried out in a high-temperature furnace or vacuum furnace under a protective atmosphere. Sintering is different from metal melting; during sintering, at least one element remains in the solid state. During the sintering process, powder particles undergo a series of physicochemical processes such as diffusion, recrystallization, welding, compounding, and dissolution, becoming metallurgical products with a certain porosity.
(4) Post-processing. In general, sintered parts can be used directly. However, for some parts with high Size requirements and high hardness and wear resistance, post-sintering treatment is required. Post-processing includes fine pressing, rolling, extrusion, quenching, surface quenching, oil impregnation, and melt infiltration.
Powder preparation methods
Powder preparation is the first step in powder metallurgy. With the increasing number and improving quality of powder metallurgy materials and products, the demand for a wider variety of powders is growing. For example, in terms of material range, not only metal powders but also alloy powders and metal compound powders are used; in terms of powder shape, various shapes of powders are required, such as when producing filters, the formation of powders is required; in terms of powder Size, various Sizes of powders are required, with coarse powder Sizes ranging from 500 to 1000 micrometers and ultrafine powder Sizes less than 0.5 micrometers.
To meet the various requirements for powders, various methods for producing powders are needed. These methods generally involve converting metals, alloys, or metal compounds from solid, liquid, or gaseous states into powder states. Various methods for preparing powders and the powders produced by various methods.
Methods for converting metals and alloys or metal compounds into powder in the solid state include:
(1) Mechanical pulverization and electrochemical corrosion methods for preparing metal and alloy powders from solid metals and alloys:
(2) Reduction methods for preparing metal and alloy powders from solid metal oxides and salts; reduction-chemical combination methods for preparing metal compound powders from metal and alloy powders, metal oxides, and non-metal powders
Methods for converting metals and alloys or metal compounds into powder in the liquid state include:
(1) Atomization methods for preparing alloy powders from liquid metals and alloys
(2) Displacement methods, solution hydrogen reduction methods for preparing metal alloys and coated powders by displacement and reduction from metal salt solutions; molten salt deposition methods for preparing metal powders by precipitation from metal molten salts; metal bath methods for preparing metal compound powders by precipitation from auxiliary metal baths.
(3) Aqueous solution electrolysis methods for preparing metal and alloy powders by electrolysis of metal salt solutions; molten salt electrolysis methods for preparing metal and metal compound powders by electrolysis of metal molten salts.
Methods for converting metals or metal compounds into powder in the gaseous state:
(1) Vapor condensation methods for preparing metal powders from metal vapors;
(2) Carbon-based thermal decomposition methods for preparing metals, alloys, and coated powders from gaseous metal carbon-based materials
(3) Gas-phase hydrogen reduction methods for preparing metal and alloy powders and metal and alloy coatings from gaseous metal halides; chemical vapor deposition methods for preparing metal compound powders and coatings from gaseous metal halide deposition.
However, from the essence of the process, existing powder preparation methods can be broadly classified into two categories: mechanical methods and physicochemical methods. Mechanical methods are processes in which raw materials are mechanically pulverized, and the chemical composition essentially does not change; physicochemical methods are processes in which powders are obtained by changing the chemical composition or aggregation state of raw materials using chemical or physical actions. There are many powder production methods. In terms of industrial scale, the most widely used are the Hans reduction method, atomization method, and electrolysis method. Some methods, such as gas-phase deposition and liquid-phase deposition, are also important in special applications. [1]
The basic processes of powder metallurgy are:
1. Preparation of raw material powder. Existing powder preparation methods can be broadly divided into two categories: mechanical methods and physicochemical methods. Mechanical methods can be further divided into: mechanical pulverization and atomization methods; physicochemical methods are further divided into: electrochemical corrosion methods, reduction methods, chemical combination methods, reduction-chemical combination methods, gas-phase deposition methods, liquid-phase deposition methods, and electrolysis methods. The most widely used are reduction methods, atomization methods, and electrolysis methods.
2. Forming the powder into the desired shape of the blank. The purpose of forming is to produce a compact of a certain shape and Size and give it a certain density and strength. Forming methods are basically divided into pressure forming and pressureless forming. The most widely used in pressure forming is die pressing. In addition, 3D printing technology can also be used to produce blanks.
3. Sintering of the blanks. Sintering is a key process in powder metallurgy. The formed compact is sintered to obtain the required final physical and mechanical properties. Sintering is divided into single-system sintering and multi-system sintering. For single-system and multi-system solid-phase sintering, the sintering temperature is lower than the melting point of the metals and alloys used; for multi-system liquid-phase sintering, the sintering temperature is generally lower than the melting point of the refractory component and higher than the melting point of the easily melting component. In addition to ordinary sintering, there are also special sintering processes such as loose sintering, melt infiltration, and hot pressing.
4. Subsequent processing of the product. Post-sintering treatment can take various forms depending on the product requirements. Such as finishing, oil impregnation, machining, heat treatment, and electroplating. In addition, in recent years, some new processes such as rolling and forging have also been applied to the post-sintering processing of powder metallurgy materials, achieving satisfactory results.
Powder properties
A general term for all the properties of powder. It includes: the geometric properties of the powder (particle size, specific surface area, pore size and shape, etc.); the chemical properties of the powder (chemical composition, purity, oxygen content and acid insolubles, etc.); the mechanical properties of the powder (bulk density, fluidity, formability, compressibility, angle of repose and shear angle, etc.); and the physical properties and surface properties of the powder (true density, gloss, wave absorption, surface activity, zeta potential and magnetism, etc.). The properties of the powder often largely determine the properties of powder metallurgy products.
The most basic geometric properties are the particle size and shape of the powder.
(1) Particle size. It affects the processing and forming of the powder, shrinkage during sintering, and the final performance of the product. The performance of some powder metallurgy products is almost directly related to the particle size. For example, the filtration accuracy of filter materials can be empirically obtained by dividing the average particle size of the original powder particles by 10; the performance of cemented carbide products is largely related to the grain size of the WC phase. To obtain cemented carbide with finer grain size, it is only possible to use finer WC raw materials. The particle size of the powder used in production practice ranges 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 are very difficult, and when it is small to a certain extent, quantum effects begin to take effect, and its physical properties will change dramatically, such as ferromagnetic powder will become superparamagnetic powder, and the melting point will also decrease with decreasing particle size.
(2) Powder particle shape. It depends on the powder making method. For example, the powder obtained by electrolysis is dendritic; the iron powder obtained by reduction is sponge-like; and the powder obtained by gas atomization is basically spherical. In addition, some powders are oval, disc-shaped, needle-shaped, onion-shaped, etc. The shape of the powder particles will affect the fluidity and bulk density of the powder. Due to the mechanical interlock between the particles, the green strength of irregular powders is also high, especially the dendritic powder has the highest green strength. However, for porous materials, spherical powder is best.
Mechanical properties The mechanical properties of powder are the process properties of powder, which are important process parameters in powder metallurgy forming process. The bulk density of the powder is the basis for weighing by 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 level of applied pressure; and the formability of the powder determines the strength of the compact.
Chemical properties mainly depend on the chemical purity of raw materials and powder making methods. Higher oxygen content will reduce pressing performance, green strength and mechanical properties of sintered products, so most of the technical conditions in powder metallurgy have certain provisions for this. For example, the allowable oxygen content of the powder is 0.2%～1.5%, which is equivalent to an oxide content of 1%～10%.