Preparation of spherical metal powder for 3D printing
Spherical metal powder is the core material of 3D printing, and is the most important link in the 3D printing industry chain, and is closely related to the development of 3D printing technology. In the 2013 World 3D printing technology industry conference, authoritative experts gave a clear definition of the performance requirements of 3D printing metal powder, that is, metal powder with size less than 1 mm. In addition, it also requires that metals meet the requirements of high purity, good spherical degree, narrow particle size distribution, low oxygen content and good fluidity. ASTM f3049-14, issued in June 2014, specifies the scope and characterization of 3D printed metal powder properties. At present, the metal powder materials for 3D printing mainly focus on the metal and its alloys such as iron, titanium, cobalt, copper, nickel, etc.
With the rapid development of 3D printing technology, the market of spherical metal powder will maintain a high growth trend. The market size of 3D printing metal powder in 2016 is about $250million, according to IDTechEx, the market size of 3D printing metal powder will reach US $5billion by 2025. But at present, spherical metal powder for 3D printing is mainly monopolized by foreign manufacturers. The spherical powder produced in China has some problems such as unstable performance, high cost and low yield. Therefore, it is very important to study the preparation of 3D printing metal powder. This paper expounds the basic principle of the main preparation technology of metal powder for 3D printing, and analyzes its advantages and disadvantages, aiming at further improving the preparation technology level of 3D printing metal powder and promoting the development and application of 3D printing technology.
The present situation of metal powder preparation for 3D printing
Table of Contents
At present, the preparation methods of metal powder for 3D printing mainly include atomization, rotary electrode and spheroidization.
The powder obtained by atomization method has accounted for more than 80% of the 3D printing powder in the world. Its principle is to break the liquid flow of metal or alloy into small droplets by the impact of fast moving fluid (atomizing medium) or other ways, and then condense into solid powder. The principle structure diagram is shown in Fig. 1. According to the different atomization medium, the principle structure of the powder is as shown in Fig. 1, The atomization method is mainly divided into water atomization and air atomization.
Fig. 1 Schematic diagram of atomization
Water atomization is a kind of metal powder which is made by water as atomizing medium. It has low production cost and high atomization efficiency. It is often used to produce steel powder, pre alloy powder for oil bearing, nickel based magnetic material powder, etc. Compared with atomization, the specific heat capacity of water is relatively large. The metal droplets which are broken in the atomization process rapidly solidify into irregular shape, which makes the shape of powder difficult to control and can not meet the requirements of 3D printing of metal on the spherical degree of powder. In addition, the reactive metal and its alloy will react with the atomizing medium water at high temperature, increasing the oxygen content of powder, These problems limit the preparation of metal powders with high spherical degree and low oxygen content by water atomization.
The principle of atomization is the process of crushing liquid metal flow into droplets and condensing into powder quickly by high-speed air flow. The atomization of metal powder has the advantages of fine particle size, high spherical degree and high purity. It is the main method to produce metal powder for 3D printing. The metal of 3D printing powder accounts for about 40% of the powder prepared by atomization. However, there are some shortcomings in the atomization technology. In the process of air breaking metal liquid, the energy of air flow is low and the atomization efficiency is low, which increases the preparation cost of metal powder.
Based on the atomization technology, the structure of nozzle is improved by nanoval company in Germany, and the laminar atomization technology is proposed. The structure of laminar atomization nozzle is shown in Fig. 2. The technology makes the flow and metal flow distributed in laminar flow atomization nozzle. The shear force and extrusion pressure produced by the airflow on the metal surface cut the metal flow into droplets with the diameter decreasing continuously. The cooling speed is 106-107k / s, and the particle size distribution of the prepared powder is narrow. Under the atomization pressure of 2.0MPa, the average particle size of the metal powder prepared by atomization can reach 10 μ m。 The technology is unstable in the atomization process, and it is difficult to control the atomization process effectively, and the production efficiency is low, which limits the production capacity and is difficult to be applied to the production of metal powder for large-scale 3D printing.
Fig. 2 structure of laminar atomization nozzle
Based on the close coupling atomization technology, British psi company has optimized and improved the structure of the tight coupling annular slot nozzle structure, which makes the outlet velocity of the air flow exceed the sound speed, and can obtain high-speed air flow under the lower atomization pressure. At the pressure of 2.5MPa, the gas rate can reach 540 m / s. In addition, the ultrasonic tight coupling atomization technology can improve the cooling speed of powder, high efficiency and low cost, It is one of the important development directions of aerosol technology and has practical significance in industry. It is of great significance to promote the industrial production and preparation of metal powder for 3D printing.
Hje company and PSI company have developed a new technology of hot gas atomization. Heating the atomizing medium can further improve the fine powder yield and reduce the gas consumption. The practical application effect is good, which is a technology with application prospect. Under the condition of atomization pressure of 1.72 MPa, the average particle size and standard deviation of the powder obtained by atomization are reduced with the increase of temperature. However, due to the limitation of the heating system and nozzle, only a few research institutions have carried out research.
Through the improvement of atomizing nozzle, Kunming Metallurgical Research Institute has prepared 316 L stainless steel metal powder by nitrogen atomization technology under the condition of 1800 ℃ and atomization pressure of 2.0MPa. The results are compared with the morphology of powders of EOS company in Germany. See Fig. 3 for the microstructure.
Fig. 3 microstructure of 316 L stainless steel
Rotating electrode method
The rotating electrode method is to use metal or alloy as self-consumption electrode, and its end face is heated by arc and melted into liquid. Under the action of centrifugal force of high speed rotation of electrode, the liquid is thrown out and crushed into small droplets. Its principle structure is shown in Fig. 4. Generally speaking, the cooling rate of rotating electrode is about 103-104k / s, and the rotating speed of electrode is 10000-3000 R / min. the particle size of the prepared powder decreases with the increase of electrode rotation speed and electrode diameter, and the range is usually 50-350 μ M.
Fig. 4 process schematic diagram of rotating electrode
Fig. 5 spherical titanium powder prepared by air atomization and rotary electrode process
Figure 5 is a spherical titanium powder prepared by the atomization process and the rotary electrode process. Compared with the atomization process, the spherical powder prepared by rotary electrode method has no common associated phase in the atomized spherical powder, and the spherical degree and smooth finish are high, the particle size distribution is narrow, there is no agglomeration, and the fluidity is good. The powder uniformity is good in the 3D printing process of metal, and the printing products have high density and high surface finish. In addition, the whole process is generally protected by inert gas, and it does not need to be melted by crucible. It avoids the contact between metal or alloy and slag making and refractory, reduces the source of metal powder pollution, and can produce high purity metal powder.
Spheroidization is mainly to spheroidize irregular powders produced by crushing and physical and chemical methods. It is considered to be the most effective process to obtain high density spherical powder. The principle is to use the heat source (plasma) with high temperature and high energy density to quickly heat and melt the powder particles, and condense into spherical droplets under the action of surface tension, After entering the cooling chamber, the spherical powder is obtained by rapid cooling. At present, the preparation technology of spheroidization is mainly divided into two kinds: radiofrequency ion spheroidization and laser spheroidization. Because of the agglomeration of the initial powder, the whole melting will be made during the spheroidization, which results in the increase of the particle size of the spherical metal powder.
Fig. 6 microstructure of titanium hydride powder before and after frequency plasma spheroidization
Currently, the RF plasma powder processing system developed by TEKNA in Canada is in the leading position in the world, and the production of Ti, Ti-6Al-4V, W, Mo, Ta, Ni and other metals and alloy powders can be realized. In China, Beijing University of science and technology has carried out a lot of research on the spheroidization of RF plasma. It can test the spheroidization of metal powders such as W, Mo, Ti. At the same time, the University of science and technology of Beijing takes titanium hydride powder as raw material to combine the radiofrequency plasma spheroidization with hydrogen explosion to realize dehydrogenation and prepare ultrafine spherical titanium powder. The particle size range can reach 20-50 μ m. Fig. 6 is a picture of the powder morphology of titanium hydride powder before and after radiofrequency plasma spheroidization.
With the rapid development of 3D printing industry, the technology of spherical 3D printing metal powder will be further improved and industrialized. In view of the strict requirements of 3D printing on the performance of metal powder, at present, domestic production capacity is certain. The atomization method and rotary electrode method can realize a certain scale production. The spheroidization method is still in the laboratory stage, and there is a certain distance to realize the scale. However, there are problems in process stability. The metal based powder for high-end 3D printing is basically imported. Therefore, the high-end 3D printing metal based powder is basically imported, China should increase the technical investment, learn from mature R & D experience, independently develop new technology and new technology and technology, and promote the development and progress of metal powder preparation technology for 3D printing.
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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