Page 55 - MSAM-1-4

Materials Science in Additive Manufacturing LPBF of Mg and its bio-applications

spherical morphology to form a homogeneous layer of A B
continuously deposited powder. This feature is vital to
improve the flowability and packing density. At present,
the methods for preparing Mg alloys powder mainly
include water, gas, and plasma atomization, plasma
rotating electrode (PRE) atomization, and evaporation-
condensation. Among them, aerosolization is widely used
for the preparation of Mg powder for LPBF processing Figure 2. The SEM image of (A) water-atomized powders and (B) gas-
[35]
with a particle size distribution between 20 and 70 μm. atomized powders .
Moreover, the sphericity of the powder can reach 98%.
Usually, high sphericity means superior flowability, which focused argon plasma jet generated by a plasma torch to
is conducive to powder spreading with scraper and as-built melt the wire as initial material, which then results in the
parts with high quality. formation of tiny metal droplets. The plasma torch produces
an extremely high-temperature argon plasma, which can
2.1.1. Water atomization inhibit the rapid solidification of molten and superheated
[36]
Water atomization is an atomization method that uses metal droplets into irregular shapes . Therefore, the
water as the atomization medium to break the molten droplets spend sufficient time in the superheated state and
metal. The advantages of water atomization include simple adopt an equilibrium shape (ideal spheres) driven by the
equipment structure, high efficiency, and low atomization surface tension during the falling process. The atomization
cost. However, the disadvantage of water atomization and condensation process of the metal powder is carried
is that the prepared powder has high impurity and poor out in an inert atmosphere.
sphericity. It was due to the fact that the reaction between 2.1.4. Plasma rotating electrode atomization
active metal and atomization medium at high temperature
increases oxygen content. In addition, the metal droplets Plasma rotating electrode (PRE) atomization is a
formed by water atomization breaking in the rapid centrifugal atomization process. It uses the plasma arc
solidification stage are irregular due to the large specific generated by a plasma gun as a high-temperature heat
heat capacity of water. source to melt the raw material, which is in the form of
an electrode rod and rotate at approximately 15,000 rpm.
2.1.2. Gas atomization Meanwhile, centrifugal force separates the molten material
The aerosolization method uses high-speed airflow to from the rod, which then results in the formation of tiny
crush the liquid metal stream, forming tiny droplets, and metal droplets. The metal droplets solidify into spherical
then quickly condensing to obtain the formed powder. powders before they hit the chamber walls. This method
Compared with water atomization, gas atomization uses was invented to reduce the volume of gas consumed to
inert gas (i.e., argon and nitrogen) as atomization medium. prepare each unit weight of atomized powder, as well as to
The metal prepared by this method has the advantages address the generally low yield of fine powders prepared by
of small powder (particle size <150 μm), uniform conventional gas atomization processes. Notably, the yield
composition, and high sphericity. Gas atomization is for the process was closely related to this centrifugal force,
suitable for producing most metals and alloys, which is the that is, the rotation speed. In addition, the process can
primary production method for spherical metal powders in eliminate the risk of ceramic contamination due to melting
AM processing. At present, most of the powder is produced of reactive metals in ceramic crucibles, thus obtaining high-
[37]
and prepared by the gas atomization method, accounting purity spherical metal powder. Zheng et al. reported that
for 30%–50% of the powder produced in the world . The Mg-15Gd-0.4Zr (wt%) alloy powders with the size ranging
[29]
density and microstructure of 17-4PH stainless steel parts from 75 to 250 μm were prepared by PRE atomization.
prepared from gas-atomized and water-atomized powders As compared with aluminum (Al) powder prepared by
were comprehensively investigated by Irrinki et al. . The gas atomization, it exhibited relatively high sphericity,
[34]
morphology of water-atomized powder and gas-atomized as shown in Figure 3. The Mg alloy powder was adopted
powder is shown in Figure 2. as the base material, and corresponding samples were
successfully prepared by laser-directed energy deposition.
2.1.3. Plasma atomization
Based on the above description, the powder preparation
Plasma atomization was reported by Entezarian et al. in methods are capable of producing spherical powders
1996 for the production of highly spherical powders with tailored for AM process. Recently, the rapid development
an average particle size of 40 μm . This approach uses the of AM significantly increased the demand for spherical
[36]

Volume 1 Issue 4 (2022) 4 https://doi.org/10.18063/msam.v1i4.24