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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.
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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.

Preparation method 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.
  • (3) Sintering. 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.

Property 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 the powder (true density, gloss, microwave absorption, surface activity, 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. Higher oxygen content will reduce the pressing performance, blank strength and mechanical properties of sintered products, so it is specified in most technical conditions of powder metallurgy. For example, the allowable oxygen content of the powder is 0.2% – 1.5%, which is equivalent to the oxide content of 1% – 10%.

Heat treatment process of powder metallurgy

Powder metallurgy materials are more and more widely used in modern industry, especially in automobile industry, daily necessities, mechanical equipment and so on. They have obvious advantages in replacing cast iron materials with low density, low hardness and strength, and are gradually popularized in the application of precision and complex parts with high hardness, high precision and strength, which is due to the rapid development of powder metallurgy technology. The heat treatment process of fully dense steel has been successful, but there are still some defects in the heat treatment of powder metallurgy materials due to the differences in physical properties and heat treatment processes of powder metallurgy materials. In the technical research of powder metallurgy materials, various casting and smelting enterprises have achieved certain results in the improvement of physical and mechanical property defects of powder metallurgy materials, such as hot forging, powder injection molding, hot isostatic pressing, liquid phase sintering and combined sintering, and improved the strength and wear resistance of powder metallurgy materials, It will greatly expand the application range of powder metallurgy.

Heat treatment process of powder metallurgy materials

The heat treatment of powder metallurgy materials should be determined according to its chemical composition and grain size, in which the existence of pores is an important factor. In the process of pressing and sintering, the pores formed by powder metallurgy materials run through the whole part, and the existence of pores affects the mode and effect of heat treatment.

The heat treatment of powder metallurgy materials includes quenching, chemical heat treatment, steam treatment and special heat treatment:

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Quenching heat treatment process

Due to the existence of pores, the heat transfer rate of powder metallurgy materials is lower than that of dense materials, so the hardenability is relatively poor during quenching. In addition, during quenching, the sintering density of the powder material is directly proportional to the thermal conductivity of the material; Due to the difference between sintering process and dense material, the internal structure uniformity of powder metallurgy material is better than that of dense material, but there is a small heterogeneity in micro area. Therefore, the complete austenitizing time is 50% longer than that of corresponding forgings. When alloy elements are added, the complete austenitizing temperature will be higher and the time will be longer.
In the heat treatment of powder metallurgy materials, in order to improve the hardenability, some alloy elements such as nickel, molybdenum, manganese, chromium and vanadium are usually added. Their action mechanism is the same as that in dense materials. They can obviously refine the grains. When they are dissolved in austenite, they will increase the stability of supercooled austenite and ensure the austenite transformation during quenching, The surface hardness and hardening depth of the quenched material increase. In addition, P / M materials should be tempered after quenching. The temperature control of tempering treatment has a great impact on the properties of P / M materials. Therefore, the tempering temperature should be determined according to the characteristics of different materials to reduce the impact of tempering brittleness. General materials can be tempered in air or oil at 175-250 ℃ for 0.5-1.0h.

Chemical heat treatment process

Chemical heat treatment generally includes three basic processes: decomposition, absorption and diffusion. For example, the reaction of carburizing heat treatment is as follows:

  • 2CO ≈ [C] + CO2 (exothermic reaction)
  • CH4 ≈ [C] + 2h2 (endothermic reaction)

After decomposition, carbon is absorbed by the metal surface and gradually diffused to the interior. Quenching and tempering treatment after obtaining sufficient carbon concentration on the surface of the material will improve the surface hardness and hardening depth of powder metallurgy materials. Due to the existence of pores in powder metallurgy materials, activated carbon atoms penetrate into the interior from the surface to complete the process of chemical heat treatment. However, the higher the material density, the weaker the pore effect and the less obvious the effect of chemical heat treatment. Therefore, the reducing atmosphere with high carbon potential should be used for protection. According to the pore characteristics of powder metallurgy materials, the heating and cooling rate is lower than that of dense materials, so the holding time should be prolonged and the heating temperature should be increased.
The chemical heat treatment of powder metallurgy materials includes carburizing, nitriding, sulfurizing and multicomponent co infiltration. In the chemical heat treatment, the hardening depth is mainly related to the density of the material. Therefore, corresponding measures can be taken in the heat treatment process, such as prolonging the time when the material density is greater than 7g / cm3 during carburization. The wear resistance of materials can be improved by chemical heat treatment. The uneven austenitic carburizing process of powder metallurgy materials can make the carbon content of the treated material carburized layer surface reach more than 2%, and the carbides are evenly distributed on the carburized layer surface, which can improve the hardness and wear resistance.

Steam treatment

Steam treatment is to oxidize the surface of materials by heating steam and form oxide film on the surface of materials, so as to improve the properties of powder metallurgy materials. Especially for the surface anti-corrosion of powder metallurgy materials, its validity period is more obvious than that of bluing treatment, and the hardness and wear resistance of the treated materials are significantly increased.

Special heat treatment process

Special heat treatment process is the product of scientific and Technological Development in recent years, including induction heating quenching, laser surface hardening and so on. Induction heating quenching is under the influence of high-frequency electromagnetic induction eddy current. The heating temperature increases rapidly, which has a significant effect on the increase of surface hardness, but it is prone to soft spots. Generally, intermittent heating method can be used to prolong the austenitizing time; Laser surface hardening process uses laser as heat source to rapidly heat up and cool the metal surface, so that the substructure in austenite grain has no time to recover and recrystallize to obtain ultra-fine structure.

Analysis of influencing factors on heat treatment of powder metallurgy materials

The pores generated in the sintering process of powder metallurgy materials are their inherent characteristics, which also have a great impact on the heat treatment, especially the relationship between the change of porosity and heat treatment. In order to improve the compactness and grain size, the added alloy elements also have a certain impact on the heat treatment:

Effect of porosity on heat treatment process

During heat treatment of powder metallurgy materials, rapid cooling inhibits the diffusion and transformation of austenite into other structures, so as to obtain martensite, and the existence of pores has a great influence on the heat dissipation of the materials. Through the thermal conductivity formula:

  • Thermal conductivity = theoretical thermal conductivity of metal × (1-2 × Porosity) / 100

It can be seen that the hardenability decreases with the increase of porosity. On the other hand, porosity also affects the density of the material, which is related to the effect of surface hardness and hardening depth after heat treatment, which reduces the surface hardness of the material. Moreover, due to the existence of pores, brine cannot be used as the medium during quenching to avoid corrosion due to salt residue. Therefore, the general heat treatment is carried out in vacuum or gas medium.

Effect of porosity on surface hardening depth during heat treatment

The heat treatment effect of powder metallurgy material is related to the density, permeability (hardenability), thermal conductivity and electrical resistance of the material. Porosity is the biggest reason for these factors. When the porosity exceeds 8%, the gas will penetrate rapidly through the gap. When carburizing and hardening, increasing the carburizing depth will reduce the effect of surface hardening. Moreover, if the penetration rate of carburizing gas is too fast, soft spots will be produced in quenching, which will reduce the surface hardness and make the material brittle and deformed.

Effect of alloy content and type on powder metallurgy heat treatment

Copper and nickel are common alloy elements. Their content and type will affect the effect of heat treatment. The hardening depth of heat treatment increases with the increase of copper content and carbon content, and decreases when it reaches a certain content; The stiffness of nickel alloy is greater than that of copper alloy, but the non-uniformity of nickel content will lead to the non-uniformity of austenite structure.

Effect of high temperature sintering

Although high temperature sintering can obtain the best alloying effect and promote densification, the difference of sintering temperature, especially when the temperature is low, will lead to the decrease of heat treatment sensitivity (the decrease of alloy in solid solution) and mechanical properties. Therefore, good heat treatment effect can be obtained by high temperature sintering and sufficient reducing atmosphere.
The heat treatment process of powder metallurgy materials is a complex process, which is related to porosity, alloy type, alloy element content and sintering temperature. Compared with dense materials, the internal uniformity is poor. In order to obtain higher hardenability, it is necessary to improve the complete austenitizing temperature and prolong the time, Heterogeneous austenite carburizing can obtain high carbon concentration which is not limited by austenite saturated carbon concentration. In addition, the addition of alloying elements can also improve the hardenability. Steam treatment can significantly improve its corrosion resistance and surface hardness.

Application 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, engineering machinery, etc.) all kinds of 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.

Source: Network Arrangement – China Metal Flanges Manufacturer – Yaang Pipe Industry (www.epowermetals.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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