Research Progress on Gas Atomization for Preparation of Metal Powders Used in 3D Printing
3D printing technology, also known as additive manufacturing, emerged in the 1930s. It is a technique that builds parts layer by layer (e.g., using metal powders) based on a computer-generated three-dimensional model . Compared with traditional subtractive manufacturing, 3D printing offers advantages such as high design freedom, high material utilization, and the ability to fabricate complex structures, showing broad application prospects in aerospace, medical devices, precision manufacturing, and other fields. The development of metal 3D printing relies heavily on high-performance metal powders, whose characteristics directly determine the quality and performance of printed parts. Ideal metal powders for 3D printing must possess high sphericity, small particle size, narrow particle size distribution, high purity, and good flowability .
Gas atomization (GA), as the primary method for producing metal powders, originated in the 1920s. It uses a high-speed gas jet to impinge on a stream of molten metal, breaking it into tiny droplets that solidify into spherical powders upon cooling . Powders produced by this method exhibit uniform composition, high sphericity, and low impurity content, and currently account for 30%–50% of the total production of metal powders for 3D printing . However, gas atomization is a complex process involving the coupling of gas–liquid two-phase flow; subtle changes in process parameters can lead to significant differences in powder characteristics, thereby affecting the performance of printed parts. Therefore, a systematic review of research progress in gas atomization technology is of great importance for promoting the development of the 3D printing industry.
Performance requirements of metal powders for 3D printing
In the 3D printing process, metal powders serve as the raw material, and their performance directly affects the uniformity of powder spreading, sintering quality, and final mechanical properties of the parts. Key performance indicators include powder purity, particle size, oxygen content, and recyclability.
2.1 Powder purity
The presence of impurity elements can react with the base powder, reducing the thermodynamic stability and mechanical properties of the material. For example, Ti-6Al-4V alloy powder readily adsorbs N, O, H, and other elements when heated, leading to performance degradation . However, in some cases, impurities may have positive effects, such as impurities in Al2O3 ceramic powder that can lower the sintering temperature and facilitate the process . Therefore, powder purity must be adjusted according to the specific application.
2.2 Particle size
Particle size directly affects the layer thickness of powder spreading (typically 50–100 μm) and the sintering driving force. Laser forming processes commonly use powder particle sizes of 30–50 μm, while electron beam forming uses 50–90 μm. Particles that are too fine tend to cause uneven spreading and "balling" phenomena, while excessively coarse particles increase the divergence angle and reduce utilization efficiency . Ideally, a mixture of fine and coarse powders in an appropriate ratio is needed to balance flowability and sintering performance.
2.3 Oxygen content
Oxygen content mainly originates from the atomization gas and equipment. High oxygen contamination can form prior particle boundaries (PPB), deteriorating alloy performance. Studies have shown that the oxygen content of powders is related to particle size; as particle size decreases, oxygen content increases . Vacuum heating or reduction treatment can effectively reduce oxygen content.
2.4 Powder recyclability
The cost of 3D printing powders is high, and recycling can significantly save raw materials. For example, after 21 cycles of use, Ti-6Al-4V powder showed an increase in oxygen content from only 0.08% to 0.19%, and unmelted particles could be reused . Increasing the number of recycling cycles may improve tap density and flowability without affecting the microstructure or mechanical properties.
Basic principles of powder production by gas atomization
Gas atomization uses a high-speed gas jet to break a stream of molten metal into droplets, which then solidify into spherical powders upon cooling. The process consists of three stages: metal melting, atomization impingement, and cooling/solidification. Inert gases (e.g., nitrogen, argon) are commonly used as the atomization medium to avoid oxidation. The gas atomization process can be divided into four zones:
Zone I (negative pressure turbulence zone): The atomization gas creates a negative pressure at the nozzle exit, which draws the molten metal stream and disperses it into liquid strips.
Zone II (primary droplet formation zone): The liquid strips are initially broken into droplets under the actions of surface tension and gas flow.
Zone III (effective atomization zone): The high-speed gas jet further breaks the droplets into finer particles.
Zone IV (cooling and solidification zone): The droplets fall freely and solidify into spherical powders upon cooling .