Principle of Metal Powder Preparation by Gas Atomization and Factors Affecting Powder Performance

As a key raw material for additive manufacturing, the quality of metal powder largely determines the final quality of the product. With the rapid development of additive manufacturing technology and its process specificity, the quality requirements for metal powders are getting higher and higher, such as high sphericity, good fluidity, low gas and impurity content, etc. At the same time, with the continuous expansion of the application field of additive manufacturing, more and more types of metal powders are needed. There are many methods for preparing metal powders, among which gas atomization has become the main method for preparing high-performance metal and alloy powders.

Compared with other powder making methods, gas atomization has the following advantages: wide range of applications, can produce a variety of metal powders and pre-alloyed powders that cannot be produced by other methods; high cooling rate (10^4 ~10^6 ℃/s) and high supercooling; the prepared powder has high sphericity and controllable powder particle size, etc.

The principle of gas atomization powder making The basic principle of gas atomization is to use high-speed airflow to impact the molten metal, convert the kinetic energy of the gas into the surface energy of the molten metal through collision, so that the molten metal flow is broken into fine droplets, and then quickly cooled and solidified in the airflow atmosphere to form powder.

The master alloy raw material undergoes three main processes in the process of gas atomization powder making: melting, atomization and solidification. At present, the mainstream atomization process is carried out in a vacuum or inert gas environment to reduce the oxygen content and impurity content in the powder and improve the purity of the powder. Studies have shown that the oxygen in the powder is basically brought in during the smelting process. Therefore, whether in the preparation of the master alloy or the atomization process, a vacuum or inert gas environment must be maintained. After the master alloy is melted, it is broken and dispersed into small droplets by a high-pressure and high-speed airflow (inert gas). The small droplets lose heat rapidly during the falling process and quickly solidify into spherical powder under the action of surface tension.

Factors affecting powder morphology

The morphology of aerosolized powder can be divided into regular spheres and irregular shapes, which is related to the relative size of the spheroidization time and solidification time of the droplets formed after the metal melt is broken. When the spheroidization time of the metal droplets is shorter than the solidification time, the droplets can be fully spheroidized before solidification, and the powder particles formed after solidification are more regular in shape and smoother in surface; if the spheroidization time of the atomized droplets is longer than the solidification time, the droplets cannot be fully spheroidized before solidification, and irregular powder particles are formed after solidification. The spheroidization time of the droplets mainly depends on the viscosity of the liquid metal, the surface tension and the size of the droplets; the solidification time mainly depends on the specific heat of the droplets, the thermal conductivity of the droplets and the superheat of the metal.

1. The influence of the superheat of the melt. Some studies have shown that as the superheat increases, the powder changes from an irregular dumbbell shape and a rod shape to a spherical shape. When Zhu Jianyong et al. used gas to atomize solder powder, they found that the superheat of the metal liquid flow had a certain influence on the powder morphology. Increasing the atomization temperature could increase the powder balling rate, and pointed out the reason. Increasing the atomization temperature can not only prolong the solidification time, but also reduce the viscosity of the metal liquid flow, thereby shortening the balling time.

2. Influence of atomization medium Xu Tianhao et al. used air, argon, nitrogen and helium as four different atomization media to atomize SnAgCu powder, and found that the sphericity of the produced powder became better in turn. The sphericity of the powder produced with air as the atomization medium was the worst, and the morphology was extremely irregular. In addition, the purity of the atomization medium directly affects the powder balling rate, because the low purity of the atomization medium will cause an oxide film to form on the surface of the molten droplet, which will increase the viscosity of the molten droplet. Pu Youfu et al. also came to a similar conclusion by comparing the particle size distribution of atomized solder powder in air, helium, nitrogen, argon, nitrogen-helium mixture, and nitrogen-argon mixture atmospheres. Nichiporenko atomized lead by changing the atomizing medium and found that the powders obtained by atomizing with air were all non-spherical; while when the atomizing medium was changed to argon, 85% of the powders were spherical.

3. The influence of atomization pressure The atomization pressure also has a certain influence on the powder morphology. When the atomization pressure is high, the particle size of the metal powder is finer, and many fine powders adhere to each other. This is because when the atomizing gas pressure is high, the energy of the atomizing gas and the metal droplets is greater, and the gas provides more energy to the metal liquid flow to break it into fine droplets, making the powder finer; at the same time, the heat exchange between the atomizing gas and the metal liquid flow is more, resulting in faster solidification of the metal powder, causing adhesion and agglomeration between the fine powders.

Hollow powder formation mechanism and control method

Hollow powder is a common type of defect in aerosolized powder. Holes generally exist in powders in two forms: one is the closed pores formed by the atomized gas being encapsulated inside the powder, and its size is generally 10% to 90% of the powder, which is generally most common in powders with coarser particle sizes (>70 μm); the other is the pores formed by the solidification and shrinkage between dendrites, whose size is generally less than 5% of the powder size, and are distributed inside and on the surface of the powder. Generally speaking, with the increase of powder particle size, the number, size and gas content of the pores in the powder will increase accordingly. The formation of hollow powder is related to the droplet breakup mechanism during atomization. During the aerosolization process, depending on the different energy of the interaction between the atomizing gas and the molten metal, there are many different types of droplet breakup mechanisms that occur simultaneously. When bag breakage, one of the mechanisms with the highest energy, occurs, large droplets will form bag-like flakes under the action of the airflow and diffuse in a direction perpendicular to the gas flow. When the viscosity of the liquid is low, the outer side of the liquid film breaks to form fine droplets; but during the aerosolization process, the droplets cool very quickly, and as the droplet temperature drops rapidly, the viscosity increases sharply. When the droplet viscosity is high enough, the breakage of the bagged film is suppressed, and the ports on both sides of the liquid film combine to form a hollow droplet wrapped in atomized gas, as shown in the figure below. Therefore, in order to suppress the generation of hollow powder, the energy of the crushing process must be reduced to avoid the occurrence of bag-type crushing, but this is difficult to do without precise control of the atomization process.

Mechanism and control method of satellite powder formation Satellite powder refers to small-sized powder adhering to the surface of large-sized powder to form a satellite-shaped powder structure, as shown in the figure below. Satellite powder reduces the sphericity, fluidity and bulk density of the powder, and is another common defect in aerosol powder making. There are currently two different theories to explain the appearance of satellite powder. A classic theory attributes the appearance of satellite powder to the collision and adhesion of fine powder and coarse powder during the downward flight of the atomization chamber. Studies have shown that during the atomization process, fine droplets cool and solidify before larger droplets solidify, accelerate in high-speed airflow, and eventually collide and weld to larger molten droplets, forming satellite powder. Ozbilen found that when the particle size distribution of the atomized powder is wide and the surface of the large particle powder is rough, the probability of satellite powder appears increases.

Anderson et al. observed in the atomization experiment that vertical upward fine powder flow can be seen along the wall of the atomization chamber, and the airflow sends these fine powders into the flow field below the nozzle. Therefore, another theory was proposed: it is believed that the solidified fine powder is sucked into the injection area below the nozzle by the swirling airflow and collides with the droplets that have not yet completely solidified, eventually forming satellite powder. As a result, a 30 cm diameter atomization chamber was developed, and experiments proved that the probability of satellite powder appearance was reduced. However, this method will cause the droplets to collide with the inner wall of the atomization chamber prematurely, reducing the powder recovery rate.

Factors affecting powder particle size

1. Influence of melt superheat Lv Haibo studied the effect of superheat temperature on powder particle size and found that as the superheat increases, the powder particle size becomes finer. Ouyang Hongwu also found that as the superheat increases, the fine particles tend to increase and the average particle size of the powder decreases. When the metal melt reaches a certain temperature, the effect of changing the temperature of the metal melt to increase the particle size of the alloy powder is not obvious. The main reason may be that the metal melt is inevitably in contact with the air during atomization. As the temperature increases, the oxidation trend of the metal is also strengthened. The generated oxides are mixed in the melt, which reduces the fluidity of the metal and increases the viscosity. When its effect is offset by the effect of reducing the viscosity of the metal melt due to the increase in temperature, the particle size of the atomized powder with increased temperature will not increase significantly.

2. Influence of atomization pressure Studies have found that as the atomization pressure increases, the yield of fine powder increases. However, when the atomization pressure increases to a certain extent, the increase in the amount of fine powder is not very large even if the atomization pressure is increased, indicating that the atomization pressure has an optimal value. Others have found that when the atomization pressure exceeds the optimal value, the fine powder yield decreases with the increase of atomization pressure. This is because the atomization pressure is too high, the suction force of the jet gas is enhanced, and the flow rate of the metal liquid flow increases. Mates et al. found that with the increase of atomization pressure, the standard deviation of the particle size distribution of the obtained powder decreases, indicating that the higher the atomization pressure, the narrower the particle size distribution.

3. Influence of gas-liquid ratio (GMR) Wolf found that the mass average particle size decreases continuously with the increase of gas-liquid mass flow rate ratio (GMR), but when the ratio exceeds a fixed value, the powder particle size will not be further reduced by increasing the atomization gas pressure or reducing the flow rate of the metal liquid flow. This value is the atomization limit of the atomization equipment used, and it is also the most economical value for atomizing ultrafine powders.

4. Influence of atomization gas flow characteristics The relative velocity of the metal liquid flow and the gas jet plays an important role in the entire atomization process. The greater the relative velocity, the smaller the average particle size of the atomized powder. Ünal studied the effect of changing the length of the supersonic zone of the atomizing airflow on the particle size of the powder. It is believed that the longer the supersonic zone in the atomizing airflow, the smaller the particle size of the obtained powder. This is because fine powder is mainly obtained through secondary crushing during the gas atomization process of the metal liquid flow. In the area where secondary crushing occurs, the greater the speed of the atomizing airflow, the smaller the critical size of the metal droplets, that is, the smaller the size of the mother liquid droplets after crushing, and therefore the smaller the average particle size of the obtained atomized powder.