Preparation of spherical metal powder for 3D printing

Spherical powders have been widely used in many fields because of their good fluidity and high solid density. In the field of thermal spraying, spherical powder makes the coating more uniform and compact because of its good fluidity, so the coating has better wear resistance; In the field of powder metallurgy, the shaped parts prepared by spherical powder have high density and uniform shrinkage in the sintering process, so the products obtained have high precision and good performance, and have obvious advantages in the application of advanced powder metallurgy forming technology such as injection molding, gel injection molding and additive manufacturing (such as 3D printing technology).

Current status of metal powder preparation for 3D printing

At present, the main methods of preparing metal powder for 3D printing include atomization, rotating electrode, spheroidization, etc.

1. Atomization method

The powder obtained by atomization method has accounted for more than 80% of the metal 3D printing powder in the world today. The principle of the powder production method is that the metal or alloy liquid flow is broken into small droplets by impact of fast moving fluid (atomization medium) or in other ways, and then condensed into solid powder. The principle structure diagram is shown in Figure 1. Atomization is mainly divided into water atomization and gas atomization.

1.1 Water atomization

Water atomization is to prepare metal powder with water as atomizing medium, which has low production cost and high atomization efficiency. It is commonly used to produce steel powder, pre-alloy powder for oil-bearing, nickel-based magnetic material powder and so on. Compared with aerosolization, the specific heat capacity of water is relatively large. The metal droplets broken in the atomization process solidified rapidly and became irregular, resulting in difficult to control the shape of powder and difficult to meet the requirements of metal 3D printing on the sphericity of powder. In addition, the active metal and its alloy will react after contact with atomizing medium water at high temperature, increasing the oxygen content of powder. These problems limit the preparation of metal powders with high sphericity and low oxygen content by water atomization.

1.2 Aerosolization

Aerosolization is a process by which a flow of liquid metal is crushed into small droplets by a high-speed air stream and quickly condensed into a powder. Aerosol preparation of metal powder has the advantages of fine particle size, high sphericity and high purity. It is the main method of producing metal powder for 3D printing at present, and the 3D-printed metal powder prepared by aerosol method accounts for about 40% of the powder prepared by atomization method. However, the atomization technology also has some shortcomings. In the process of breaking metal liquid by air flow, 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, German Nanoval Company improved the nozzle structure and proposed laminar atomization technology. 

The structure of laminar atomization nozzle is shown in Figure 2. The technology makes the air flow and the metal liquid flow present laminar flow distribution in the laminar flow atomizing nozzle. The shear force and extrusion pressure generated by the air flow on the metal surface shear the metal liquid flow into droplets with decreasing diameter. The cooling rate reaches 106 ~ 107 K/s and the particle size distribution of the prepared powder is narrow. The average particle size of the atomized metal powder can reach 10μm. With low gas consumption and low production cost, it is suitable for the production of most metal powders used in 3D printing. However, this preparation technology is not stable in the atomization process, so it is difficult to effectively control the atomization process and has low production efficiency, which limits the production capacity and is difficult to be applied to large-scale metal powder production for 3D printing.

On the basis of the tight coupling atomization technology, British PSI company optimized and improved the structure of the tight coupling ring seam nozzle structure, so that the air outlet speed exceeds the speed of sound, and the high speed air flow can be obtained under the small atomization pressure. Under the pressure of 2.5MPa, the gas rate can reach 540m/s. In addition, ultrasonic tightly coupled atomization technology can improve the cooling speed of powder, high efficiency, low cost, and wide application range. It is one of the important development directions of aerosol technology, and has industrial practical significance, and has important significance for promoting the industrial production and preparation of metal powder used in 3D printing.

HJE Company and PSI Company have developed a new technology of hot gas atomization preparation. By heating the atomizing medium, it can further improve the fine powder yield and reduce gas consumption. The practical application effect is good, and it is a promising technology. Under the condition of atomization pressure of 1.72MPa, when the gas is heated to 200~400℃, the average particle size and standard deviation of the atomized powder decrease with the increase of temperature. However, because the hot gas atomization technology is limited by the gas heating system and nozzles, only a few research institutions have conducted the research.

By improving atomizing nozzle, 316L stainless steel metal powder was prepared by nitrogen atomization technology under the condition of temperature 1800℃ and atomizing pressure 2.0MPa.

2. Rotating electrode method

The rotating electrode method uses metal or alloy as the consumable electrode, whose end face is heated by the arc and fused into a liquid. Under the action of the centrifugal force of the high-speed rotation of the electrode, the liquid is thrown out and crushed into small droplets. Generally speaking, the cooling rate of the rotating electrode is about 10 3 ~10 4 K/s, and the rotating speed of the electrode is 10000~30 00r/min. The particle size of the prepared powder decreases with the increase of the rotating speed and diameter of the electrode, and the range is usually between 50~350μm.

Compared with the aerosol process, the spherical powder prepared by rotating electrode method does not have the common associated phase in the aerosol spherical powder, and the sphericity and smoothness are higher, the particle size distribution range is narrow, there is no agglomeration phenomenon, and the fluidity is good. In the metal 3D printing process, the powder spreading uniformity is good, and the printed product has high density and high surface finish. In addition, the whole process generally adopts inert gas protection, and does not need crucible melting, to avoid the metal or alloy and slag making and refractory contact, reduce the pollution source of metal powder, can produce high purity metal powder.

3. Spheroidization

The spheroidization method is mainly used for spheroidization of irregular powder produced by crushing method and physical and chemical method. It is considered to be the most effective process to obtain highly dense spherical powder. Its principle is to use heat source (plasma) with high temperature and high energy density to heat powder particles rapidly and melt them, and to condense into spherical droplets under the action of surface tension. After entering the cooling chamber, the spherical powder is cooled quickly. At present, the preparation process of spheroidization is mainly divided into radio frequency ion spheroidization and laser spheroidization. Due to the agglomeration phenomenon of the initial powder, it will be fused in the spheroidization process, resulting in the particle size of the prepared spherical metal powder increases.

At present, the RF plasma powder treatment system developed by TEKNA (Canada) is in a leading position in the world, which can realize the production of Ti, Ti-6Al-4V, W, Mo, Ta, Ni and other metals and their alloy powders. Domestic University of Science and Technology Beijing has conducted a lot of research in RF plasma spheroidization, which can test the spheroidization treatment of W, Mo, Ti and other metal powders. Meanwhile, the University of Science and Technology Beijing uses titanium hydride as raw material, combines RF plasma spheroidization treatment with "hydrogen explosion", realizes dehydrogenation in the spheroidization process, and prepares ultra-fine spherical titanium powder. The particle size range can reach 20~50μm.

With the rapid development of metal 3D printing industry, the spherical 3D printing metal powder preparation technology will be further improved and industrialized. In view of the strict requirements of 3D printing on the performance of metal powder, there is a certain production capacity in China at present, aerosol method and rotating electrode method can achieve a certain scale production, spheroidization method is still in the laboratory stage, there is a certain distance to achieve scale, but there are process stability problems, metal-based powder for high-end 3D printing basically depends on imports, therefore, Our country should increase technology investment, draw lessons from mature research experience, and develop new technology and new process independently to promote the development and progress of metal powder preparation technology for 3D printing.