What is metal powder
What is metal powder?
Metal powder refers to a group of metal particles with a size of less than 1 mm. Including single metal powder, alloy powder and some refractory compound powder with metallic properties, it is the main raw material of powder metallurgy.
Metal element is generally silver-white, when the metal is under certain conditions, it is black powder. Most metal powders are black.
Method for preparing metal powder
At present, many methods have been developed for the preparation of metal powders, which are mainly divided into physical and chemical methods and mechanical methods according to production principles. The most important ones in the mechanical method are the atomization method and the mechanical crushing method. The most important physical and chemical methods are reduction method, electrolysis method and hydroxyl method.
Physical Chemistry
Reduction method
The reduction method of metal oxides and salts is the most widely used powder preparation method. Iron powder and tungsten powder can be reduced by solid carbon, tungsten, molybdenum, iron, copper, cobalt, nickel and other powders can be produced by hydrogen or ammonia; iron powder can be produced by converting natural gas and coal gas, and sodium, calcium, and magnesium can be produced Metals such as tantalum, niobium, titanium, zirconium, thorium, uranium and other rare metal powders can be prepared as reducing agents. The basic principle of the reduction method of metal oxides and salts is that the reducing agent used has a greater affinity for oxygen than the corresponding metal in the oxide and the salt used, so it can deprive the oxygen in the metal oxide or salt. And the metal is reduced. Since different metal elements have different effects on oxygen, the stability of the generated oxides is also different. The ΔG during the oxidation reaction can be used to characterize the stability of the oxide. For example, the smaller the ΔG value during the reaction, the higher the stability of the oxide, that is, the greater its affinity for oxygen.
Its advantages are simple operation, easy control of process parameters, high production efficiency, low cost, and suitable for industrial production; the disadvantage is that it is only suitable for metal materials that are easy to react with hydrogen and become brittle and breakable after hydrogen absorption.
Metal thermal reduction and reduction law
Metal thermal reduction means that the reduced raw material can be solid, gaseous, or molten salt. The latter two correspondingly have the characteristics of gas phase reduction and liquid phase precipitation. The metal thermal reducing agent method is more commonly used in industry: reducing TiO2, ThO2, UO2, etc. with calcium; reducing TiCl4, ZrCl4, TaCl5, etc. with magnesium; reducing TiCl4, ZrCl4, K2ZrF6, K2TaF7, etc., with sodium; reducing calcium hydride (CaH2) ) Co-reducing chromium oxide and nickel oxide to prepare nickel-chromium stainless steel powder.
Reduction method refers to the use of carbon, boron carbide, silicon, nitrogen and refractory metal oxides to obtain carbides and borides. Nitride method.
Electrolysis
Electrolysis is a method in which metal powder is deposited on the cathode by electrolyzing molten salt or an aqueous solution of salt. Almost all metal powders can be prepared by electrolysis, and copper powder, silver powder, and tin powder are particularly suitable. Electrolysis powder can be divided into aqueous solution electrolysis, organic electrolyte electrolysis, molten salt electrolysis and liquid metal cathode electrolysis.
The advantage is that the purity of the prepared metal powder is relatively high, and the purity of general elemental powder can reach more than 99.7%; in addition, the electrolysis method can well control the particle size of the powder and can produce ultra-fine powder. However, the electrolysis method consumes a lot of electricity and the cost of powder production is high. The electrolytic aqueous solution can produce Cu, Ni, Fe, Ag, Sn, Fe-Ni and other metal (alloy) powders, and the electrolytic molten salt can produce Zr, Ta, Ti, Nb and other metal powders.
Hydroxy method
Some metals (iron, nickel, etc.) and carbon monoxide are synthesized into metal carbonyl compounds, and then thermally decomposed into metal powder and carbon monoxide. The powder prepared in this way is very fine and has high purity, but the cost is high. Industrially, it is mainly used to produce fine and ultra-fine powders of nickel and iron, as well as alloy powders such as Fe-Ni, Fe-Co, and Ni-Co.
Chemical replacement method
The chemical substitution method is based on the activeness of the metal. The active metal is used to replace the less active metal from the metal salt solution, and the metal (metal powder) obtained by the replacement is further processed by other methods. Refinement. This method is mainly used in the preparation of inactive metal powders such as Cu, Ag and Au.
Mechanical Law
Atomization method
The atomization method belongs to the mechanical powdering method, which is a method of directly crushing liquid metal or alloy and worthy of powder. It is widely used and is second only to the reduction method in scale. The atomization method is also called the spray method, which can produce metal powders such as lead, tin, aluminum, copper, nickel, and iron, and can also be used to produce alloy powders such as bronze, brass, carbon steel, and alloy steel.
The atomization method generally uses high-pressure gas, high-pressure liquid or high-speed rotating blades to break the metal or alloy melted at high temperature and high pressure into fine droplets, which are then condensed in a collector to obtain ultrafine metal powder. A chemical change occurred. The atomization method is one of the main methods for producing metal and alloy powders. There are many atomization methods, such as dual-flow atomization, centrifugal atomization, multi-stage atomization, ultrasonic atomization technology, tight-coupling atomization technology, high-pressure gas atomization, laminar flow atomization, ultrasonic tight-coupling atomization and hot gas atomization化 etc.
Atomized powder has the advantages of high sphericity, controllable powder particle size, low oxygen content, low production cost and adaptability to the production of various metal powders. It has become the main development direction of high-performance and special alloy powder preparation technology, but the atomization method It has the disadvantages of low production efficiency, low yield of ultra-fine powder, and relatively large energy consumption.
Mechanical crushing method
The mechanical pulverization of solid metals is an independent powdering method. The development of its mechanism is closely related to the final state of solid strain and the formation and propagation of cracks during pulverization. At the same time, it serves as an indispensable supplementary process for some powder milling methods. For example, grinding hard and brittle cathode precipitates produced by electrolysis, sponge-like metal blocks produced by grinding reduction, etc. Therefore, mechanical pulverization occupies an important position in powder production.
The crushing method is different due to the nature of the material and the required crushing fineness. According to different ways of applying external force, material crushing is generally carried out by squeezing, impacting, grinding and splitting concentrated methods, and the working principles of various crushing equipment are mostly based on these principles.
Among them, the ball milling method is mainly divided into rolling ball method and vibration ball milling method. This method utilizes the mechanism that metal particles are broken and refined due to deformation under different strain rates. Its advantages are that the selectivity of materials is not strong, it can be operated continuously, and the production efficiency is high. It is suitable for dry and wet grinding, and can be used for powder preparation of various metals and alloys. The disadvantage is that it is difficult to classify in the powder preparation process.
Schematic diagram of metal powder prepared by high-energy ball milling
Grinding method
The grinding method is to spray compressed gas into the grinding area after passing through a special nozzle, so as to drive the materials in the grinding area to collide with each other and rub into powder; after the airflow expands, it rises with the material into the classification area, and is sorted by the turbo classifier to reach the particle size. The remaining coarse powder returns to the grinding zone to continue grinding until it reaches the required particle size and is separated. Because the grinding method adopts dry production, the process of dehydration and drying of the material is omitted; the product has high purity, high activity, good dispersibility, fine particle size and narrow distribution, and smooth particle surface. It is widely used in non Metal, chemical raw materials, pigments, abrasives, health care drugs and other industries in ultra-fine grinding. However, the grinding method also has high equipment manufacturing costs. In the production process of metal powder, a continuous inert gas or nitrogen must be used as a compressed gas source, which consumes a large amount of gas and is only suitable for the crushing and powdering of brittle metals and alloys.
Performance index of metal powder
Metal powder is a loose substance, and its performance comprehensively reflects the nature of the metal itself, the properties of individual particles and the characteristics of particle groups. Generally, the properties of metal powders are divided into chemical properties, physical properties and process properties. Chemical properties refer to metal content and impurity content. The physical properties include the average particle size and particle size distribution of the powder, the specific surface and true density of the powder, the shape, surface morphology and internal microstructure of the particles. Process performance is a comprehensive performance, including powder fluidity, loose packing density, tap density, compressibility, formability and sintering size change. In addition, for some special applications, powders are required to have other chemical and physical properties, such as catalytic properties, electrochemical activity, corrosion resistance, electromagnetic properties, internal friction coefficient, etc. The performance of the metal powder depends to a large extent on the production method of the powder and its preparation process. The basic properties of the powder can be determined by specific standard testing methods. There are many methods for measuring the particle size and distribution of powders, generally sieve analysis (>44μm), sedimentation analysis (0.5-100μm), gas permeation method, microscope method, etc. Ultrafine powder (<0.5μm) was measured by electron microscope and X-ray small angle scattering method. Metal powder is conventionally divided into five grades: coarse powder, medium powder, fine powder, fine powder and superfine powder.
Chemical elements of metal powder
The purity of metal powder for 3D printing is very demanding. In addition to determining the main elements and impurity elements, there are also requirements for the oxygen, nitrogen, and hydrogen content of the raw materials.
Measurement method: Due to the numerous measurement methods, this article uses titanium alloy as an example to illustrate: the spectrum analyzer measures Fe, Al, V and other elements in titanium alloy; the oxygen, nitrogen and hydrogen analyzer based on inert gas melting heat conduction/infrared principle measures the content of materials Oxygen, nitrogen, hydrogen content; carbon/sulfur analyzer determines the content of carbon element in raw materials, the above measurement methods can be used in a comprehensive way. In addition, an energy spectrometer and X-ray diffractometer can also be used to determine the element composition qualitatively or semi-quantitatively.
Particle shape of metal powder
Particle shape refers to the geometric shape of powder particles. It can be broadly divided into regular shapes and irregular shapes. The shape of the particles has a great influence on the fluidity of the powder, the packing density and the sintering and melting process. Under normal circumstances, the metal powder bed melting process requires the higher the sphericity of the powder, the better. The surface shape factor, volume shape factor and proportional shape factor are commonly used when determining particle morphology.
Under normal circumstances, the surface and internal structure of non-spherical powders are loose, leading to certain pore defects inside the printed parts, and spherical powders can be better improved in this respect.
Measurement method: The particle surface area observation equipment can be a scanning electron microscope.
Particle size and particle size distribution of metal powder
Usually the diameter is used to express the size of the particles and is called the particle size. Since the countless particles that make up the powder do not belong to the same particle size, it is necessary to use the percentage of particles with different particle sizes to account for the total powder to characterize the distribution of powder particles. Represents the volume percentage of different sizes in a certain size interval.
Bulk density of metal powder
The bulk density is the mass of powder per unit volume when the powder sample naturally fills the specified container. In general, the coarser the powder size, the greater the bulk density. Powders with fine and coarse combinations can achieve higher delivery density.
Measuring method: The measuring method of bulk density is: funnel method, Scott volume meter method or vibration funnel method. The funnel method uses a constant volume and standard funnel, and the powder freely passes through the funnel aperture into the measuring cup until the measuring cup is completely filled and the powder overflows from the measuring cup. Use the formula: ρ=m/v, where m is the mass of the powder sample; V is the volume of the measuring cup.
Fluidity of metal powder
Fluidity refers to the time required for 50g of powder to flow out of a standard flow funnel, in s/50g. The reciprocal is the mass of powder flowing out per unit time, called flow rate. Fluidity is a comprehensive parameter related to morphology, particle size distribution and delivery density. Generally speaking, the larger the powder particles, the more regular the particle shape, the smaller the proportion of extremely fine powder in the particle size composition, and the relatively better fluidity. Adsorption of water and gas on the surface of the particles will reduce the fluidity of the powder.
Three test points related to fluidity: angle of repose, outflow speed and degree of compression. The angle of repose is the maximum angle formed by the free slope of the powder accumulation layer and the horizontal plane, and is when the particles slide on the free slope of the powder accumulation layer It is measured when the force of gravity and the friction between the particles reaches a balance and is in a static state. The outflow rate is described by adding the material to the funnel by measuring the time required for all the materials to flow out. The degree of compression reflects the cohesiveness and softness of the powder, and is an important indicator of powder fluidity.
Measurement method: Two flow meters are used to determine the fluidity of powder: Hall flowmeter funnel and Carney funnel.
Related standards for metal powder
| Standard number | Standard name |
| GB/T 1480-2012 | metal powder Determination of particle size by dry sieving |
| GB/T 19077-2016 | Particle size analysis Laser diffraction |
| GB/T 15445.6-2014 | Expression of particle size analysis results Section six part : Qualitative and quantitative expression of particle shape and morphology |
| GB/T 1479.1-2011 | metal powder Determination of bulk density Section one Part: Funnel method |
| GB/T 1479.2-2011 | metal powder Determination of bulk density The second part: Scott volumeter method |
| GB/T 5162-2006 | metal powder . Determination of tap density |
| ASTM B923-2016 | Standard Test Method for bone mineral density of metal powders by helium or nitrogen Hydrometry |
| GB/T 1482-2010 | metal powder Determination of liquidity Standard funnel method (Hall current meter) |
| ASTM B964- 2016 | use Carney Standard Test Method for flow rate of metal powder by funnel |
| Q /CAM T006-2018 | Computer tomography of hollow powder ratio of metal powder for additive manufacturing（ CT ）Law |
| Q /CAM T011-2020 | Foreign material inclusion in metal powder for additive manufacturing test method |
| ASTM B215-2010 | Standard Practice for sampling metallic powders |
| ASTM B215-2008 | Standard Practice for sampling metallic powders |
| ASTM B312-2009 | Standard Test Method for unburned strength of metal powder compact specimens |
| ASTM B330-2007 | Standard Test Method for determination of Fischer number of metal powders and related compounds |
| ASTM B331-2010 | Standard Test Method for compressibility of metallic powders in uniaxial compression |
Application scenarios of metal powder
Metal powder is often produced into structural and functional devices through powder metallurgy, injection molding and other processes. It is also often directly applied to various industries.
Development of metal powder preparation technology
Metal powder is the main raw material of powder metallurgy, and it continues to develop with the growth of powder metallurgy technology and its downstream demand. Powder metallurgy technology has extremely wide application potential in high-end equipment manufacturing fields such as robots, high-end CNC machine tools, aerospace equipment, high-speed rail magnetic levitation and other advanced rail transit, new energy vehicles, medical imaging equipment, etc. With continuous transformation and upgrading and continuous development to high-end manufacturing, its market space is gradually expanding, and 3D laser printing technology continues to mature. In the future, the demand for metal powder is likely to rise at a high speed.
At the same time, the industry has a good policy environment, such as “Made in China 2025”, “Special Plan for Technological Innovation in the “Thirteenth Five-Year” Materials Field”, “Thirteenth Five-Year Development Plan for the New Material Industry”, “New Material Industry Development Guide”, etc. Other plans and related industrial policies have a significant effect on the formation of a good market environment for the metal powder industry and the development of enterprises.
Source: 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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