Metal powder for 3D printing is the most important part of the 3D printing industry chain for metal parts, and also holds the greatest value. At the “2013 World 3D Printing Technology Industry Conference”, authoritative experts in the global 3D printing industry defined metal powder for 3D printing as a group of metal particles with a size less than 1mm. This includes single metal powders, alloy powders, and certain refractory compound powders with metallic properties. Currently, 3D printing metal powder materials include cobalt-chromium alloys, stainless steel, industrial steel, bronze alloys, titanium alloys, and nickel-aluminum alloys. However, in addition to good plasticity, 3D printing metal powders must also meet requirements such as fine particle size, narrow particle size distribution, high sphericity, good flowability, and high bulk density.

Due to the different applications and subsequent forming process requirements, the preparation methods for metal powders also vary. The main preparation processes include physical-chemical methods and mechanical methods. In the powder metallurgy industry, electrolysis, reduction, and atomization are widely used preparation methods. However, it should be noted that both electrolysis and reduction have certain limitations and are not suitable for the preparation of alloy powders. Currently, the metal powders used in additive manufacturing mainly focus on titanium alloys, high-temperature alloys, cobalt-chromium alloys, high-strength steels, and tool steels. To meet the requirements of additive manufacturing equipment and processes, metal powders must have low oxygen and nitrogen content, good sphericity, narrow particle size distribution, and high bulk density. The Plasma Rotating Electrode Process (PREP), Plasma Atomization (PA), Gas Atomization (GA), and Plasma Spheroidization (PS) are the main preparation methods for metal powders used in additive manufacturing, all of which can produce spherical or near-spherical metal powders.

Summary of Metal 3D Printing Powder Forming Techniques

Methods for Preparing Metal Powders

1. Plasma Rotating Electrode Process (PREP)

The Plasma Rotating Electrode Process (PREP) is a spherical powder preparation process developed in Russia. Its principle is illustrated in the diagram, where metal or alloy is processed into rod material and the end of the rod is heated using plasma while the rod rotates at high speed. The centrifugal force helps to refine the molten droplets, which solidify in an inert gas environment and form powder under surface tension; different particle sizes are classified by screening, and the final powder product is obtained after electrostatic removal of inclusions (only for high-temperature alloys).

The PREP method is suitable for preparing alloy powders such as titanium alloys and high-temperature alloys. The metal powders produced by this method have high sphericity and good flowability, but the particle size is relatively coarse, and the yield of fine particle size (0-45μm) for SLM processes is low, with high costs for fine powders. The size of the powder, or droplet size, primarily depends on increasing the rod’s rotation speed or increasing the rod’s diameter, which raises requirements for equipment sealing and vibration.

At present, the most advanced equipment and core technology for PREP are still held by Russia. Domestic units mainly rely on direct introduction or on absorbing, digesting, and improving the technology after introduction. The Iron and Steel Research Institute, Beijing Aeronautical Materials Research Institute, and Northwest Nonferrous Metal Research Institute introduced Russian PREP equipment earlier, but the current equipment process technology level has a significant gap compared to international advanced levels. Domestic universities such as Xi’an Jiaotong University and Central South University are conducting basic research on PREP technology. The Iron and Steel Research Institute and Zhengzhou Mechanical Research Institute jointly developed the first large PREP equipment in China for the development of alloy powder materials, but the yield of fine titanium alloy powder is still not ideal. In recent years, Xi’an Ouzhong Company has introduced two sets of PREP equipment from Russia, and AVIC MATE and Hunan Dingli have also successively developed complete sets of PREP equipment. The yield of fine titanium alloy powder (≤45μm) is less than 20%. Overall, there is still a performance gap between the early introduced and currently self-developed PREP equipment in China compared to Russia. Advantages: Clean surface, high sphericity, fewer by-products, no hollow/satellite powders, good flowability, high purity, low oxygen content, narrow particle size distribution. Disadvantages: Coarse powder size, low yield of fine powders, high cost of fine powders.

2. Plasma Atomization (PA)

Plasma Atomization (PA) is a unique metal powder preparation technology developed by the Canadian company AP&C. It uses a plasma torch symmetrically mounted at the top of the melting chamber to create a high-temperature plasma focal point, with temperatures reaching up to 10,000 K. A specialized feeding device sends metal wire into the plasma focal point, where the raw material is rapidly melted or vaporized, and then dispersed and atomized into ultra-fine droplets or gas mist by high-speed plasma impact. During the flight and deposition process in the atomization tower, it exchanges heat with the cooling argon gas introduced into the tower, cooling and solidifying into ultra-fine powder.

The metal powders produced by the PA method are nearly spherical, and the overall particle size is relatively fine. AP&C has collaborated with Sweden’s Arcam company to expand and enhance production capacity in response to the rapid development of the current additive manufacturing market. Due to the high temperature of the plasma torch, theoretically, the PA method can produce all existing high melting point metal alloy powders, but the technology is limited in preparing powders for many difficult-to-deform alloy materials, such as titanium-aluminum intermetallic compounds. Additionally, the pre-preparation of raw material wires increases production costs, and the production efficiency needs improvement to ensure powder particle size and quality control.

Advantages: Extremely high yield for powders below 45μm, almost no hollow spherical gas entrapment, superior to gas atomization. The TC4 alloy used in Arcam’s electron beam forming is prepared using this method. Disadvantages: Slightly lower sphericity, presence of satellite powders, and high cost of wire materials.

3. Gas Atomization (GA)

Currently, commonly used gas atomization preparation techniques for metal powders for additive manufacturing include Vacuum Induction-melting Gas Atomization (VIGA) and Electrode Induction-melting Inert Gas Atomization (EIGA). The VIGA method uses crucible melting of alloy materials, where the alloy liquid flows through a conduit at the bottom of the intermediate tank to the atomization nozzle, where it is impacted and broken by supersonic gas, atomizing into micron-scale molten droplets that coalesce and solidify into powder. This method is mainly suitable for the production of powders made from iron-based alloys, nickel-based alloys, cobalt-based alloys, aluminum-based alloys, and copper-based alloys.

The EIGA method combines gas atomization technology with electrode induction melting technology, eliminating components that come into contact with the molten metal, such as crucibles. The slowly rotating pre-alloy rod metal electrode is lowered into an annular induction coil for electrode melting, and the electrode droplets fall into the gas atomization nozzle system, utilizing inert gas for atomization, effectively reducing the introduction of impurities during the melting process, achieving safe and clean melting of reactive metals. This method is mainly applied to the preparation of powders for reactive metals and their alloys, intermetallic compounds, and refractory metals, such as titanium and titanium alloys, and titanium-aluminum intermetallic compounds.

In recent years, powder manufacturers and powder-making equipment manufacturers have developed technologies such as ultrasonic gas atomization, tightly coupled gas atomization, laminar flow gas atomization, and thermal gas atomization through improvements to gas atomization technology. They have also made modifications targeting the characteristics of additive manufacturing technology, and now can produce powders that meet the requirements for use in laser selective melting (SLM), laser coaxial feeding, and other additive manufacturing processes. Advantages: High yield of fine powders, suitable for laser selective melting below 45μm, and lower costs. Disadvantages: Slightly lower sphericity, more satellite powders, high hollow powder rate for 45-406μm powders, and the presence of air entrapment, making it unsuitable for electron beam selective melting forming, direct hot isostatic pressing, and other powder metallurgy fields.

4. Plasma Spheroidization (PS)

Radio frequency plasma has high energy density, strong heating intensity, and a large volume of the plasma arc. Since there are no electrodes, there is no contamination of the product due to electrode evaporation. The principle of radio frequency plasma powder spheroidization technology is that under the action of a high-frequency power supply, inert gas (such as argon) is ionized to form a stable high-temperature inert gas plasma; irregularly shaped raw powder is injected into the plasma torch using a carrier gas (nitrogen). The powder particles absorb a large amount of heat in the high-temperature plasma, rapidly melting on the surface; they then enter the reactor at very high speeds, rapidly cooling in an inert atmosphere, and solidifying into spherical powders under the action of surface tension, before being collected in the collection chamber.

Advantages: High sphericity and good surface smoothness. Can prepare refractory metals with high melting temperatures, such as tantalum, tungsten, niobium, and molybdenum. Disadvantages: Long heating cycle, volatile elements may evaporate, irregular powders have large surface areas, and high oxygen content.

5. Comparison of Methods

The powder produced by the PREP method has a narrow particle size distribution and is not easy to obtain fine powders, with low yield of fine powders. Due to the high cost of fine powders, this significantly limits its application in SLM processes. The coarse powders produced by this technology are used in laser rapid forming (LSF) processes. The PA method has been used for the batch production of conventional titanium and titanium alloy powders, with the powders containing satellite powders, flaky powders, and nanoparticles, which have good flowability after treatment. However, the need for wire material as a raw material has created bottlenecks in preparing difficult-to-deform metal materials, limiting the range of materials applicable. In the production of non-reactive metal powders such as nickel-based alloys and iron-based alloys, the production costs are relatively high. The VIGA method is widely adopted by global additive manufacturing powder suppliers due to its high efficiency, wide range of alloy adaptability, low cost, and controllable powder particle size. The EIGA method has advantages over the PREP method in producing reactive metal powders, such as material savings, flexible production, and high output of fine powders, making it suitable for the production of titanium alloy powders for SLM processes. The PS method uses high-energy plasma to produce highly spherical and dense metal powders. Its raw materials are non-spherical powders with high oxygen and hydrogen content, making it challenging to control the oxygen content of the spherical powders, and the yield of fine powders also depends on the particle size of the original powders. Repeatedly used metal powders for additive manufacturing can be used as raw materials for the PS method for re-powdering.

Comparison of Several Metal Powder Preparation Methods

The global additive manufacturing technology industry is currently in a period of rapid development, with more comprehensive and systematic advancements in materials, equipment, processes, and applications in Europe and the United States. The development of additive manufacturing technology in China is mainly concentrated on the printing and forming processes, with insufficient attention to alloy material research, and significant under-investment in the research of powder material manufacturing devices and process technologies. The development of new alloy powder materials and low-cost multi-process composite powder-making technologies has not yet been widely carried out. The lack of foundational equipment construction and process technology research has largely affected the construction and development of China’s independent additive manufacturing material technology system.

Source: Micro-Nano Additive

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