3D printing with spherical metal powder preparation process

Spherical metal powder is the core material of metal 3D printing, is the most important link in the 3D printing industry chain, and is closely related to the development of 3D printing technology. At the 2013 World 3D Printing Technology Industry Congress, authoritative experts gave a clear definition of the performance requirements of 3D printed metal powders, i.e. metal powders smaller than 1 mm in size.
addition, metals are required to meet the requirements of high purity, good sphericality, narrow particle size distribution, low oxygen content, good liquidity and so on
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. The ASTM F3049-14 standard, issued in June 2014, defines the range and conditions for the performance of 3D-printed metal powders. At present, 3D printing metal powder materials are mainly concentrated in iron, titanium, cobalt, copper, nickel and other metals and their alloys.
with the rapid development of metal 3D printing technology, the market of spherical metal powder will maintain a high growth trend. The market size of 3D-printed metal powders was approximately $250 million in 2016, and according to IDTechEx, the market size of 3D-printed metal powders will reach $5 billion by 2025.
at present, 3D printing spherical metal powder is mainly monopolized by foreign manufacturers, domestic production of spherical powder has unstable performance, high cost, low yield and other issues. Therefore, it is particularly important to study the preparation of 3D printing metal powder, this paper expounds the basic principles of the main preparation process of metal powder for 3D printing, and analyzes its advantages and disadvantages, with the aim of further improving the level of preparation technology of metal powder for 3D printing, and promoting the development and application of 3D printing technology.
the current situation of metal powder preparation process for 3D printing
Currently, the preparation methods for metal powder for 3D printing mainly include atomization, rotary electrode, spun and so on.
1.1 Atomization method
atomization legal system of powder has accounted for more than 80% of the world's metal 3D printing powder, the principle is to fast-moving fluid (atomization medium) impact or other ways to break the metal or alloy flow into small droplets, followed by condensation into solid powder powder powder method, the principle of the structure map as shown in Figure 1, according to the atomization medium, fogging method is mainly divided into atomization and atomization.
Figure 1 Atomization Powder Schematic
1.1.1 Water Atomization
Water atomization is the preparation of metal powders with water as atomized medium, which has low production costs and high atomization efficiency, and is commonly used to produce steel powder, pre-alloyed powders for oil bearings, nickel-based magnetic material powders, etc. Relatively aerosolization, the water is longer than the thermal capacity, in the atomization process broken metal melt droplets quickly solidified into irregular, resulting in powder shape difficult to control, and difficult to meet the requirements of metal 3D printing on powder sphericality, in addition, because the active metal and its alloys at high temperature contact with the atomized medium water will react, increase the powder oxygen content, these problems limit the water atomization legal system of high spherical, low oxygen content of the metal powder.
1.1.2 Atomization
The principle of atomization is the process of crushing liquid metal flows into small droplets and quickly condensing them into powders through high-speed airflow. Aerosol preparation metal powder has the advantages of fine particle size, high sphericality and high purity, and is the main method of producing metal powder for 3D printing, and the 3D printing powder metal accounts for about 40% of the atomized legal preparation powder. However, aerosolization technology also has some shortcomings, in the process of air flow broken metal liquid, air flow energy is low, atomization efficiency is low, increasing the cost of metal powder preparation.
, on the basis of aerosolization technology, the German company Nianoval improved the nozzle structure and proposed stratified atomization technology. The structure of the stratified atomized nozzle is shown in Figure 2. The technology makes the air flow and the metal liquid flow in the layer flow atomization nozzle layer flow distribution, the air flow in the metal surface of the shear force and extrusion pressure, the metal flow cut into a shrinking diameter droplet, its cooling speed of 106 to 107K/s, the preparation of the powder particle size distribution narrow, in 2.0MPa atomization pressure, the average particle size of the metal prepared metal can reach 10 m.
and low gas consumption, low production costs, suitable for most 3D printing metal powder production, but this preparation technology in the process of atomization is unstable, difficult to effectively control the atomization process, low production efficiency, limited its production capacity, difficult to apply to large-scale 3D printing metal powder production.
2 layer flow atomization nozzle structure diagramPSI UK has optimized and improved the structure of the tightly coupled ring seam nozzle on the basis of tightly coupled atomization technology. So that the outlet speed of air flow exceeds the speed of sound, can obtain high-speed air flow under small atomization pressure, at 2.5 MPa pressure, gas rate can reach 540 m/s, in addition, ultrasonic tightly coupled atomization technology can improve the cooling speed of powder, high efficiency, low cost, and wide application range, is one of the important development directions of aerosolization technology, and has industrial significance, for promoting the industrial production of 3D printing metal powder has important significance.
HJE and PSI have developed a new technology for the preparation of thermal gas atomization, heating the atomized medium, which can further improve the yield of fine powder, reduce gas consumption, and have good practical application results, and is a technology with application prospects. Under the atomization pressure of 1.72 MPa, the gas is heated to 200 to 400 degrees C, and the average particle size and standard deviation of the powder obtained from atomization decrease with the temperature, but only a few research institutions study it because the thermal gas atomization technology is limited by the gas heating system and nozzle.
Kunming Metallurgical Research Institute through the improvement of atomized nozzles, at a temperature of 1,800 degrees C, atomization pressure 2.0MPa conditions, the use of nitrogen atomization technology to prepare 316 L stainless steel metal powder, and with the German company EOS powder shape comparison, microstructure situation in Figure 3.
3 316 L stainless steel microstructure1.2 Rotating electrode method
rotary electrode method is based on metal or alloy as a self-consuming electrode, its end surface is heated by an arc and melted into a liquid, and under the influence of the centrifugal force of the electrode high-speed rotation, the liquid is thrown and crushed into tiny droplets, the principle structure shown in Figure 4. In general, the cooling rate of the rotating electrode is about 103 to 104K/s, the rotation speed of the electrode is 10,000 to 30,000 r/min, and the prepared powder particle size decreases with the speed of electrode rotation and the diameter of the electrode, usually between 50 and 350 m.
Figure 4 Rotating Electrode Process Schematic
Figure 5 Spherical Titanium White Powder prepared by aerosol powdering process and rotating electrode process
Figure 5 is spherical titanium powder prepared using aerosolization process and rotary electrode process. Compared with the aerosolization process, the spherical powder of rotating electrode legal system does not have the accompaniment common in aerosol spherical powder, and the spherical and smoothness is higher, the particle size distribution range is narrow, no reunion phenomenon, the mobility is good, the powder uniformity in the metal 3D printing process is good, the printing product density is high, the surface finish is high. In addition, the entire process, generally using inert gas protection, and do not require melting, avoid metal or alloy and slag and contact with refractable materials, reduce the source of metal powder pollution, can produce high purity metal powder.
1.3 spherical method
spherical method is mainly the spherical treatment of the irregular powder produced by crushing method and physical and chemical method, is considered to be the most effective process to obtain high-density spherical powder, the principle is to use high temperature, energy density heat source (plasma), the powder particles are rapidly heated and melted, and under the effect of its surface pressure to shrink into spherical droplets, into the cooling chamber to quickly cool the spherical powder. At present, the splocky legal preparation process is mainly divided into radio frequency ion splorization and laser splorization. Because the initial powder will produce a certain reunion phenomenon, in the process of sphericalization will make its overall melting, resulting in the preparation of spherical metal powder particle size increased.
6 Microstructures before and after the methicilonic plasma icing of hydrogenated titanium At present, TEKNA Canada has developed an RF plasma powder processing system that is a worldwide leader in the production of metals such as Ti, Ti-6Al-4V, W, Mo, Ta, Ni and their alloy powders. Beijing University of Science and Technology has done a lot of research on radio frequency plasma ballification. Can test W, Mo, Ti and other metal powder spherical treatment, at the same time, Beijing University of Science and Technology to hydrogenated titanium powder as raw material, radio frequency plasma spherical treatment and "hydrogen explosion" combined, the spherical process to achieve dehydrogenation, preparation of ultra-fine spherical titanium powder, particle size range can reach 20 to 50 m, Figure 6 is hydrogenated titanium powder by radio frequency plasma ionization before and after the powder morphological map.
2 Conclusion
With the rapid development of the metal 3D printing industry, spherical 3D printing metal powder preparation technology will be further improved and industrialization. In view of the strictness of the performance requirements of 3D printing on metal powder, the domestic production capacity, aerosolization method and rotating electrode method can achieve a certain scale of production, spun method is still in the laboratory stage, the realization of scale still has a certain distance, but there are process stability problems, high-end 3D printing metal-based powder basically rely on imports, for this reason, China should increase technology investment, learn from mature research and development experience, independent research and development of new technology, and promote the development of metal powder preparation technology for 3D printing.