Powder metallurgy is a metallurgical process technology in which metal powders (or mixtures of metal powders and non-metal powders) are used as raw materials to form metal products or materials by forming and sintering. Powder metallurgy can be used according to the use of parts, and the material composition can be adjusted at will; it is close to the final shape of the workpiece, the subsequent processing allowance is small, the production process is free from noise pollution, no treatment/no emission of toxic substances; and it can be molded without any cutting process. Complex shapes.
The powder metallurgy process is divided into five steps: milling, mixing, compression molding, sintering, and steam treatment.
The powder preparation method can be divided into three types: mechanical method, without changing the chemical composition of the raw material, preparing the powder by cutting/grinding the metal; physical method, preparing the powder by cooling and atomizing the liquid metal; chemical method, reducing and dissociating, etc. Based on the chemical reaction, the chemical composition and the state of aggregation are changed to prepare a powder.
Mixing powder refers to mixing in a desired ratio for preparing a mixture having a certain chemical composition and a certain particle size, and suitable other physicochemical properties. The properties of the metal powder have a major impact on its forming and sintering processes as well as the quality of the product. The properties of metal powders can be characterized by chemical composition, physical properties, and process properties.
Press forming is to apply a certain pressure (15-600 MPa) to the loose powder to become a compact having a certain size, shape and a certain density and strength. The pressing method is divided into one-way pressing and two-way pressing. Since the powder body flows in various directions after being pressed in the stamper, it causes a side pressure perpendicular to the wall of the stamper. The side pressure causes friction, which causes a significant pressure drop in the height direction of the compact. Therefore, the density is different for different pressing methods.
Sintering is to heat and keep the compact at a temperature below the melting point of the base metal, and atomic diffusion, solid solution, compounding and welding between the powder particles causes the compact to shrink and strengthen. Sintering differs from metal melting in that at least one element remains in the solid state during sintering. During the sintering process, a series of physical and chemical processes, such as diffusion, recrystallization, fusion welding, compounding and dissolution, become metallurgical products with a certain porosity.
The steam treatment is to form an oxide film by the sintered body under the action of high temperature and water vapor for a certain period of time, thereby improving the anti-rust ability of the powder metallurgical part and sealing the pores. The factors affecting the formation of ST film are temperature, steam quantity, water-gas ratio, and must be strictly controlled in the process.