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Materials Science in Additive Manufacturing Spheroidization of 304L SS powder for LPBF process

2.2. Plasma spheroidization the powder type: as-received versus spheroidized. The

An inductively coupled plasma (ICP) system, TekSphero-15 response variables were mechanical properties in terms
(Tekna Plasma System Inc., Canada), was utilized with of yield strength (YS), ultimate tensile strength (UTS),
a maximum power capacity of 15 kW. The machine was and strain at break. As shown in Figure 2, to evaluate the
equipped with Tekna’s PN-35M induction plasma torch, repeatability of tensile properties, 15 cubic specimens with
operating at the 2 – 5 MHz frequency range. 10 mm edge dimension were built per powder (as-received
and spheroidized) using the Renishaw AM250 machine,
Figure 1 shows an illustration of the plasma where X-axis is the powder spreading direction and Z-axis
spheroidization process where powder with irregular is the part building direction. All the 15 cubic specimens
particle geometries is injected into a high-temperature per powder type were fabricated in one build. The LPBF
plasma (3000 – 10,000 K) under an inert atmosphere process parameters were chosen based on the reported
(argon) to get melted . The surface tension effects on results of building fully dense parts with the least energy
[26]
the molten liquid droplets reshape the particles to spheres input , which are listed in Table 2.
[32]
while the particles fall under gravity inside the water-
cooled processing chamber . The molten particles would 2.4. Characterization techniques
[31]
be solidified at high cooling rates (~10 K/s) and deposited Analyzing powder particles using image analysis of
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in the main collection bin. To successfully spheroidize scanning electron microscope (SEM) micrographs
powder, several parameters (including the sheath, central, is a popular method among researchers because of
carrier, and hydrogen gas flow rates), the power delivered to these images’ reliability. SEM images can be used to
the plasma, and the powder mass flow rate can be adjusted. determine particle size distributions and analyze particle
The parameters used in this study are listed in Table 1. geometries [33] . The morphology of powder was studied

2.3. Part fabrication through analysis of micrographs taken by ASPEX
SEM (Aspex Corp., USA). The change in the shape of
The used LPBF machine was a Renishaw AM250 powder particles was quantified using the aspect ratio,
(Renishaw plc., UK), capable of fabricating parts with defined as the ratio of the major axis to the minor axis
complex geometries from a range of metal powders. This of the bounding ellipse for each particle [34] . Through this
machine is equipped with a high-precision fiber laser definition, the particle sphericity increases as the aspect
(70 µm focal diameter) in 200 W power and a build ratio approaches unity.
volume of 250 × 250 × 300 mm . Before feeding the
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powder into the LPBF machine, the Renishaw powder The method of ICP optical emission spectroscopy
recovery system (sieving to particle size <63 µm) was used (ICP-OES) was deployed to study powder chemistry. The
to eliminate the larger particles that may result from the microstructures of powder particles were analyzed by the
agglomeration of particles during powder handling. For electron backscattered diffraction (EBSD) probe provided
the experiment of part fabrication, the variable factor was
Table 1. Process parameters for spheroidizing 304L SS
powder in a TekSphero‑15 system

Power Gas flow rates
Carrier Central Sheath Hydrogen
12 kW 4 lpm 10 lpm 40 lpm 2 lpm

Table 2. Process parameters used for part fabrication in
Renishaw AM250

Parameter Value
Laser power (W) 200
Spot diameter (µm) 70
Point distance (µm) 70
Exposure time (µs) 88
Scan speed (m/s) 0.8
Hatch spacing (µm) 85
Figure 1. Schematic of the TekSphero-15 showing the injection of the
powder into the plasma. Layer thickness (µm) 50

Volume 1 Issue 1 (2022) 3 http://doi.org/10.18063/msam.v1i1.1