(1) Production of powder. The powder production process includes steps such as powder preparation and powder mixing. In order to improve the moldability and plasticity of the powder, plasticizers such as engine oil, rubber or paraffin are usually added.
(2) Compression molding. The powder is pressed into the required shape under the pressure of 15-600MPa.
(3) Sintering. It is carried out in a high-temperature furnace or vacuum furnace with a protective atmosphere. Sintering is different from metal melting, at least one element is still in the solid state during sintering. During the sintering process, powder particles undergo a series of physical and chemical processes such as diffusion, recrystallization, fusion welding, compounding, and dissolution, and become metallurgical products with a certain degree of porosity.
(4) Post-processing. In general, the sintered parts can be used directly. But for some parts that require high precision, high hardness and wear resistance, post-sintering treatment is required. Post-processing includes precision pressing, rolling, extrusion, quenching, surface quenching, oil immersion, and infiltration, etc.
Preparation method of powder
Making powder is the first step in powder metallurgy. Powder metallurgy materials and products continue to increase, their quality continues to improve, and more and more types of powder are required. For example, from the perspective of material range, not only metal powder, but also alloy powder, metal compound powder, etc. are used; from the perspective of powder shape, various shapes of powder are required, such as when a filter is produced, it is required to form a powder; from powder In terms of particle size, powders of various particle sizes are required. The coarse powder has a particle size of 500-1000 microns, ultra-fine powder with a particle size of less than 0.5 microns, and so on.
In order to meet the various requirements for powders, there are also various methods for producing powders. These methods are nothing more than turning metals, alloys or metal compounds into a powder state in a solid, liquid or gaseous state. Various methods of preparing powders and powders prepared by various methods.
The methods of turning metals and alloys or metal compounds into powders in a solid state include:
(1) There are mechanical crushing method and electrochemical corrosion method for preparing metal and alloy powder from solid metal and alloy:
(2) Reduction method for preparing metal and alloy powder from solid metal oxides and salts Methods of converting metal compounds into powders include:
(1) Atomizing method for preparing alloy powder from liquid metals and alloys
(2) Replacement and reduction of metal salt solutions to prepare metal alloys and coated powders include replacement method and solution hydrogen reduction method; molten salt aging method to prepare metal powder from metal molten salt precipitation; from auxiliary metal bath The metal bath method is used to prepare metal compound powder by precipitation.
(3) Aqueous electrolysis method for preparing metal and alloy powder from metal salt solution electrolysis; molten salt electrolysis method for preparing metal and metal compound powder from metal molten salt electrolysis.
The method of turning metal or metal compound into powder in gaseous state:
(1) Steam condensation method for preparing metal powder from metal vapor condensation;
(2) Carbon-based thermal dissociation method for preparing metals, alloys and coated powders from gaseous metal carbon-based dissociation
(3) The vapor-phase hydrogen reduction method is used to prepare metals, alloy powders, and metal and alloy coatings from gaseous metal halide vapor-phase reduction; the chemical vapor deposition method is used to prepare metal compound powders and coatings from gaseous metal halide deposition.
However, from the perspective of the essence of the process, the existing milling methods can be roughly classified into two categories, namely mechanical methods and physical chemical methods. Mechanical method is a process in which raw materials are mechanically crushed, and the chemical composition is basically unchanged; physical chemical method is a process in which powder is obtained by changing the chemical composition or aggregation state of raw materials by means of chemical or physical action. There are many production methods. From the industrial scale, the most widely used Hans reduction method, atomization method and electrolysis method. Some methods such as vapor deposition and liquid deposition are also important in special applications.