What is powder metallurgy
What is powder metallurgy?
Powder metallurgy is the process technology of making metal powder or using metal powder (or the mixture of metal powder and non-metallic powder) as raw material, forming and sintering, and manufacturing metal materials, composites and various types of products. Powder metallurgy is similar to the production of ceramics. It belongs to powder sintering technology. Therefore, a series of new powder metallurgy technologies can also be used for the preparation of ceramic materials. Due to the advantages of powder metallurgy technology, it has become the key to solve the problem of new materials and plays an important role in the development of new materials.
Powder metallurgy includes powder making and products. Among them, pulverization is mainly a metallurgical process, which is consistent with the literal meaning. Powder metallurgy products often go far beyond the scope of materials and metallurgy, and are often interdisciplinary technologies (materials and metallurgy, machinery and mechanics, etc.). In particular, modern metal powder 3D printing integrates mechanical engineering, CAD, reverse engineering technology, layered manufacturing technology, numerical control technology, material science and laser technology, making powder metallurgy product technology a modern comprehensive technology across more disciplines.
Characteristics of powder metallurgy
Powder metallurgy has unique chemical composition and mechanical and physical properties, which can not be obtained by traditional melting and casting methods. The use of powder metallurgy technology can directly make porous, semi dense or fully dense materials and products, such as oil bearing, gear, cam, guide rod, cutting tool and so on.
- (1) Powder metallurgy technology can minimize alloy composition segregation and eliminate coarse and uneven casting structure. It plays an important role in the preparation of high-performance rare earth permanent magnet materials, rare earth hydrogen storage materials, rare earth luminescent materials, rare earth catalysts, high-temperature superconducting materials, new metal materials (such as Al Li alloy, heat-resistant Al alloy, superalloy, powder corrosion-resistant stainless steel, powder high-speed steel, intermetallic compound high-temperature structural materials, etc.).
- (2) A series of high-performance nonequilibrium materials such as amorphous, microcrystalline, quasicrystal, nanocrystalline and supersaturated solid solution can be prepared. These materials have excellent electrical, magnetic, optical and mechanical properties.
- (3) It can easily realize various types of composites and give full play to the respective characteristics of each component material. It is a process technology for producing high-performance metal matrix and ceramic composites at low cost.
- (4) It can produce materials and products with special structure and properties that cannot be produced by ordinary smelting method, such as new porous biomaterials, porous separation membrane materials, high-performance structural ceramic abrasives and functional ceramic materials.
- (5) It can realize near net formation and automatic batch production, so as to effectively reduce the resource and energy consumption of production.
- (6) It can make full use of ore, tailings, steelmaking sludge, steel rolling scale and recycled waste metal as raw materials. It is a new technology that can effectively regenerate and comprehensively utilize materials.
Many of our common machining tools and hardware abrasives are made by powder metallurgy technology.
Technological process of powder metallurgy
(1) Production of powder.
The production process of powder includes powder preparation, powder mixing and other steps. In order to improve the formability and plasticity of powder, plasticizers such as engine oil, rubber or paraffin are usually added.
(2) Press molding.
The powder is pressed into the required shape under the pressure of 15-600Mpa.
It is carried out in a high temperature furnace or vacuum furnace with protective atmosphere. Sintering is different from metal melting. At least one element is still in solid state during sintering. In the sintering process, powder particles become metallurgical products with certain porosity through a series of physical and chemical processes such as diffusion, recrystallization, fusion welding, combination and dissolution.
(4) Post processing.
Generally, sintered parts can be used directly. However, for some parts with high dimensional accuracy, high hardness and wear resistance, post sintering treatment should be carried out. Post treatment includes fine pressing, rolling, extrusion, quenching, surface quenching, oil immersion, and infiltration.
Preparation method of powder
Preparation of powder is the first step of powder metallurgy. With the continuous increase of powder metallurgy materials and products and the continuous improvement of their quality, more and more kinds of powder are required. For example, in terms of material range, not only metal powder, but also alloy powder and metal compound powder are used; In terms of powder shape, it is required to use various shapes of powder. For example, when generating a filter, it is required to form powder; From the point of view of powder particle size, various particle sizes are required. The coarse powder particle size is 500 ~ 1000 microns, and the ultra-fine powder particle size is less than 0.5 microns.
In order to meet the various requirements for powder, there must be various methods for producing powder. These methods are nothing more than changing metal, alloy or metal compound into powder state in solid, liquid or gaseous state. Various methods for preparing powder and powder prepared by various methods.
A method for converting metals and alloys or metal compounds into powder in a solid state includes:
- (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 oxide and salt reduction chemical method for preparing metal compound powder from metal and alloy powder, metal oxide and non-metallic powder.
A method for converting metals and alloys or metal compounds into powder in liquid state includes:
- (1) Atomization method for preparing alloy powder from liquid metal and alloy
- (2) There are replacement method and solution hydrogen reduction method for preparing metal alloy and coating powder from metal salt solution replacement and reduction; A molten salt aging method for preparing metal powder by precipitation from molten metal salt; Metal bath method for preparing metal compound powder by precipitation from auxiliary metal bath.
- (3) Aqueous solution 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 molten salt electrolysis of metal.
Method for converting metal or metal compound into powder in gaseous state:
- (1) Steam condensation method for preparing metal powder from metal steam condensation;
- (2) Preparation of metals, alloys and coated powders from gaseous metal carbon based dissociation
- (3) Vapor phase hydrogen reduction method for preparing metal and alloy powder and metal and alloy coating from gaseous metal halide gas phase reduction; Chemical vapor deposition method for preparing metal compound powder and coating from gaseous metal halide deposition.
However, from the essence of the process, the existing pulverizing methods can be divided into two categories: mechanical method and physicochemical method. Mechanical method is a technological process in which raw materials are mechanically crushed without changing their chemical composition; Physicochemical method is a process to obtain powder by changing the chemical composition or aggregation state of raw materials with the help of chemical or physical effects. There are many production methods of powder. From the industrial scale, Hans reduction method, atomization method and electrolysis method are the most widely used. Some methods such as vapor deposition method and liquid deposition method are also very important in special applications.
The basic processes of powder metallurgy process are:
- 1. Preparation of raw material powder. The existing pulverizing methods can be divided into two categories: mechanical method and physicochemical method. The mechanical method can be divided into: mechanical crushing and atomization; Physicochemical methods are divided into electrochemical corrosion method, reduction method, chemical method, reduction chemical method, vapor deposition method, liquid deposition method and electrolysis method. Among them, reduction method, atomization method and electrolysis method are the most widely used.
- 2. The powder is formed into a blank of the desired shape. The purpose of forming is to make a blank with a certain shape and size, and make it have a certain density and strength. The forming methods are basically divided into pressure forming and non pressure forming. Compression molding is the most widely used in compression molding. In addition, 3D printing technology can be used to make embryo blocks.
- 3. Sintering of green blocks. Sintering is a key process in powder metallurgy. The formed compact is sintered to obtain the required final physical and mechanical properties. Sintering can be divided into unit system sintering and multi-element system sintering. For the solid-state sintering of unit system and multicomponent system, the sintering temperature is lower than the melting point of the metals and alloys used; For liquid phase sintering of multicomponent systems, the sintering temperature is generally lower than the melting point of refractory components and higher than that of fusible components. In addition to ordinary sintering, there are also special sintering processes such as loose sintering, melt leaching and hot pressing.
- 4. Post processing of products. The treatment after sintering can take a variety of 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 achieved ideal results.
Properties of powder
Powder is the general term of all properties. It includes: geometric properties of powder (particle size, specific surface, pore size and shape, etc.); Chemical properties of the powder (chemical composition, purity, oxygen content, acid insoluble matter, etc.); Mechanical properties of powder (loose density, fluidity, formability, compressibility, stacking angle and shear angle, etc.); Physical properties and surface properties of powder (true density, gloss, microwave absorption, surface activity, ze%26mdash; ta(%26ccedil;) Potential and magnetism, etc.). Powder properties often determine the properties of powder metallurgy products to a great extent.
The most basic geometric properties are the particle size and shape of the powder.
- (1) Granularity. It affects the processing and forming of powder, shrinkage during sintering and the final properties of products. The performance of some powder metallurgy products is almost directly related to particle size. For example, the filtration accuracy of filter materials can be empirically obtained by dividing the average particle size of original powder particles by 10; The properties of cemented carbide products are closely related to the grains of WC phase. In order to obtain cemented carbide with finer grains, it is only possible to use WC raw materials with finer grains. The powder used in production practice has a particle size ranging from hundreds of nanometers to hundreds of microns. The smaller the particle size, the greater the activity, and the easier the surface is to oxidize and absorb water. When it is as small as hundreds of nanometers, the storage and transportation of powder is not easy, and when it is as small as a certain extent, the quantum effect begins to work, and its physical properties will change greatly. For example, ferromagnetic powder will become superparamagnetic powder, and the melting point will decrease with the decrease of particle size.
- (2) Particle shape of powder. It depends on the powder making method, such as the powder obtained by electrolytic method, and the particles are dendritic; The iron powder particles obtained by reduction method are sponge flakes; The powder obtained by gas atomization is basically spherical powder. In addition, some powders are egg, disc, needle, onion, etc. The shape of powder particles will affect the fluidity and loose density of powder. Due to the mechanical meshing between particles, the compact strength of irregular powder is also large, especially the dendritic powder has the greatest compact strength. But for porous materials, spherical powder is the best.
Mechanical properties the mechanical properties of powder, that is, the process properties of powder, are important process parameters in powder metallurgy forming process. The bulk density of powder is the basis for weighing by volumetric method during pressing; The fluidity of powder determines the filling speed of powder to the die and the production capacity of the press; The compressibility of powder determines the difficulty of pressing process and the level of applied pressure; The formability of powder determines the strength of billet.
The chemical properties mainly depend on the chemical purity of raw materials and milling method. The mechanical properties of most pressed and sintered products will be reduced due to the specified oxygen content. For example, the allowable oxygen content of the powder is 0.2% ~ 1.5%, which is equivalent to the oxide content of 1% ~ 10%.
Application fields of powder metallurgy
Powder metallurgy related enterprises are mainly applicable to the production and research of spare parts in automobile industry, equipment manufacturing industry, metal industry, aerospace, military industry, instruments and meters, hardware tools, electronic appliances and other fields, the production of relevant raw materials and auxiliary materials, and the manufacturing of various powder preparation equipment and sintering equipment. Products include bearings, gears, cemented carbide cutting tools, molds, friction products and so on. In military enterprises, heavy weapons and equipment such as armor piercing bullets, torpedoes, aircraft tanks and other brake pairs need to be produced by powder metallurgy technology. In recent years, powder metallurgy auto parts have become the largest market in China’s powder metallurgy industry, and about 50% of auto parts are powder metallurgy parts.
- (1) Application: (automobile, motorcycle, textile machinery, industrial sewing machine, electric tools, hardware tools. Electrical appliances, construction machinery, etc.) various powder metallurgy (iron copper based) parts.
- (2) Classification: powder metallurgy porous materials, powder metallurgy antifriction materials, powder metallurgy friction materials, powder metallurgy structural parts, powder metallurgy tool and die materials, powder metallurgy electromagnetic materials and powder metallurgy high temperature materials, etc.
Powder metallurgy parts
Source: China Flanges Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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