Powder metallurgy preparation method

(1) Powder production. The powder production process includes steps such as powder preparation and powder blending. Plasticizers such as engine oil, rubber, or paraffin are usually added to improve the formability and plasticity of the powder.
(2) Compaction molding. The powder is pressed into the desired shape under a pressure of 15-600MPa experimental conditions.
(3) Sintering. This is carried out in a high-temperature furnace with a protective atmosphere or a vacuum furnace. Sintering is different from metal melting; at least one element remains in a solid state during sintering. During the sintering process, powder particles undergo a series of physical and chemical processes such as diffusion, recrystallization, fusion welding, compound formation, and dissolution, becoming a metallurgical product with a certain porosity.
(4) Post-processing. Generally, sintered parts can be used directly. However, for certain parts requiring high dimensional accuracy, high hardness, and wear resistance, post-sintering treatment is necessary. Post-processing includes coining, rolling, extrusion, quenching, surface quenching, oil impregnation, and infiltration.
Methods for powder preparation
Powder preparation is the first step in powder metallurgy. As powder metallurgy materials and products continuously increase and their quality improves, the demand for more types of powders grows. For instance, in terms of material range, not only metal powders but also alloy powders and metal compound powders are used; regarding powder shape, various shapes of powders are required, such as spherical powder for filter production; in terms of particle size, powders of various sizes are needed, including coarse powders with sizes from 500-1000 micrometers and ultrafine powders with sizes less than 0.5 micrometers, etc.
To meet the various requirements for powders, diverse methods for producing powders are necessary. These methods essentially involve transforming metals, alloys, or metal compounds from solid, liquid, or gaseous states into powder form. Various methods for powder preparation and the powders produced by these methods will be introduced.
Methods for transforming metals, alloys, or metal compounds into powder from a solid state include:
(1) Mechanical comminution and electrochemical corrosion methods for producing metal and alloy powders from solid metals and alloys;
(2) Reduction method for producing metal and alloy powders from solid metal oxides and salts; Reduction-synthesis method for producing metal compound powders from metal and alloy powders, metal oxides, and non-metal powders
Methods for transforming metals, alloys, or metal compounds into powder from a liquid state include:
(1) Atomization method for producing metal and alloy powders from liquid metals and alloys
(2) Displacement method and solution hydrogen reduction method for producing metal alloys and coated powders from metal salt solutions by displacement and reduction; Fused salt precipitation method for producing metal powders from molten metal salts; Metal bath method for precipitating and producing metal compound powders from auxiliary metal baths.
(3) Aqueous solution electrolysis method for producing metal and alloy powders from metal salt solutions by electrolysis; Molten salt electrolysis method for producing metal and metal compound powders from molten metal salts by electrolysis.
Methods for transforming metals or metal compounds into powder from a gaseous state:
(1) Vapor condensation method for producing metal powders from metal vapor condensation;
(2) Thermal decomposition method of carbonyls for producing metals, alloys, and coated powders from gaseous metal carbonyls
(3) Vapor phase hydrogen reduction method for producing metal and alloy powders, and metal and alloy coatings from gaseous metal halides by vapor phase reduction; Chemical vapor deposition method for producing metal compound powders and coatings from gaseous metal halide deposition.
However, in terms of the essence of the process, existing powder preparation methods can be broadly categorized into two major types: mechanical methods and physicochemical methods. Mechanical methods involve the mechanical crushing of raw materials, a process where the chemical composition remains essentially unchanged; physicochemical methods are processes that obtain powder by changing the chemical composition or aggregation state of the raw materials through chemical or physical actions. Many powder production methods exist, but on an industrial scale, the most widely used are the Hans reduction method, atomization method, and electrolysis method. Some methods, such as vapor deposition and liquid phase deposition, are also very important for special applications. [1]
The basic processes of powder metallurgy are:
1. Preparation of raw material powders. Existing powder preparation methods can be broadly divided into two categories: mechanical methods and physicochemical methods. Mechanical methods can be further divided into: mechanical comminution and atomization; Physicochemical methods are further divided into: electrochemical corrosion, reduction, synthesis, reduction-synthesis, vapor deposition, liquid phase deposition, and electrolysis. Among these, the most widely used are the reduction method, atomization method, and electrolysis method.
2. Compaction of powders into billets of desired shape. The purpose of compaction is to produce green compacts of a certain shape and size, with specific density and strength. Compaction methods are basically divided into pressure compaction and non-pressure compaction. Die pressing is the most commonly used method in pressure compaction. Additionally, 3D printing technology can also be used for billet production.
3. Sintering of billets. Sintering is a critical process in powder metallurgy. The compacted green parts acquire their desired final physical and mechanical properties through sintering. Sintering is further divided into single-component system sintering and multi-component system sintering. For single-component and multi-component solid-phase sintering, the sintering temperature is lower than the melting point of the metals and alloys used; for multi-component liquid-phase sintering, the sintering temperature is generally lower than the melting point of the refractory component but higher than the melting point of the easily fusible component. Besides conventional sintering, there are also special sintering processes such as loose sintering, infiltration, and hot pressing.
4. Post-sintering treatment of products. Post-sintering treatment can adopt various methods depending on product requirements, such as finishing, oil impregnation, machining, heat treatment, and electroplating. Furthermore, in recent years, new processes such as rolling and forging have also been applied to the post-sintering processing of powder metallurgy materials, achieving more desirable results.
Powder properties (property of powder)
A general term for all properties of powder. It includes: geometric properties of powder (particle size, specific surface area, pore size, shape, etc.); chemical properties of powder (chemical composition, purity, oxygen content, acid insolubles, etc.); mechanical properties of powder (apparent density, flowability, formability, compressibility, angle of repose, shear angle, etc.); physical properties and surface characteristics of powder (true density, luster, wave absorption, surface activity, zeta potential, magnetism, etc.). Powder properties often largely determine the performance of powder metallurgy products.
The most fundamental geometric properties are powder particle size and shape.
(1) Particle size. It affects the processing and forming of powder, shrinkage during sintering, and the final performance of the product. The performance of certain powder metallurgy products is almost directly related to particle size. For example, the filtration accuracy of filter materials can empirically be obtained by dividing the average particle size of the original powder particles by 10; the performance of cemented carbide products is greatly related to the grain size of the WC phase. To obtain cemented carbide with finer grain size, it is only possible to use WC raw materials with finer particle sizes. The powders used in production practice range from hundreds of nanometers to hundreds of micrometers in particle size. The smaller the particle size, the greater the activity, and the easier the surface is to oxidize and absorb water. When the size is as small as hundreds of nanometers, the storage and transportation of powder become very difficult, and when it reaches a certain small size, quantum effects begin to play a role, and its physical properties will undergo significant changes, such as ferromagnetic powder becoming superparamagnetic powder, and the melting point also decreasing with the reduction of particle size.
(2) Powder particle shape. It depends on the powder production method, such as powder produced by electrolysis, which has a dendritic shape; iron powder particles produced by the reduction method appear sponge-like flakes; powder produced by gas atomization is basically spherical. In addition, some powders are egg-shaped, disc-shaped, needle-shaped, onion-head shaped, etc. The shape of powder particles affects the powder's flowability and apparent density. Due to mechanical interlocking between particles, irregular powder has higher green strength, especially dendritic powder, which has the highest pressed billet strength. However, for porous materials, spherical powder is best.
Mechanical properties The mechanical properties of powder, also known as its process properties, are important process parameters in powder metallurgy forming. The apparent density of powder serves as the basis for volumetric weighing during pressing; the flowability of powder determines the filling speed of the powder into the die and the production capacity of the press; the compressibility of powder determines the difficulty of the pressing process and the level of applied pressure; and the formability of powder determines the strength of the green compact.
Chemical properties primarily depend on the chemical purity of raw materials and the powder production method. Higher oxygen content can reduce pressing performance, green strength, and the mechanical properties of sintered products. Therefore, most technical specifications in powder metallurgy have certain regulations regarding this. For example, the permissible oxygen content of powder is 0.2% to 1.5%, which corresponds to an oxide content of 1% to 10%.