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

by the FEI Helios NanoLab 600 (Thermo Fisher Scientific, The tensile properties of parts fabricated with as-received
USA); samples of as-received and spheroidized powders and spheroidized powders were measured using an Instron
were set in conductive bakelite, mechanically polished 5969 Dual Column Universal Testing System (Instron,
down to 0.05 µm, and vibratorily polished in 0.02 µm USA) with 50 kN force capacity at a strain rate 0.015/min.
colloidal silica for 4 h. The phase identification of powder To evaluate the repeatability of tensile properties per each
was performed using X-ray diffraction (XRD). fabricated part, 5 mini-tensile specimens (with thickness
The dynamic powder flow properties were measured 1 ± 0.02 mm and dimensions shown in Figure 4) were
and analyzed using a Revolution Powder Analyzer cut from each cube and tested using a universal Instron
(Mercury Scientific Inc., USA). As demonstrated in machine at room temperature with a crosshead speed
[36]
Figure 3, this analyzer rotates a drum composed of a of 0.015/min . The analysis of variance (ANOVA) was
powder sample. The drum has glasses on both sides, conducted on the obtained results of tensile tests.
which let the coaxial camera take images of the powder 3. Results and discussion
movement to quantify flowability when the powder is
illuminated with a backlight on the opposite side. While 3.1. Powder morphology
rotating, the powder will occasionally avalanche as it The SEM micrographs provided in Figure 5 indicate
succumbs to the force of gravity, where it eventually comes the morphological differences between as-received
to rest. At the onset of slumping, the angle is known as the and plasma spheroidized powders, where particles
avalanche angle and is calculated at the cycle’s peak. The with irregular geometries in the as-received powder
break energy can be used to evaluate the amount of energy
needed to begin each avalanche and is the subtraction of
the maximum powder’s energy level before an avalanche
begins from the starting powder’s energy level before
starting the rotation process. A standard rotational
speed recommended by the equipment’s manufacturer is
0.3 rpm, which was used to observe powder flow for the
as-received and spheroidized 304L powders.

Figure 4. Dimensions of mini-tensile test specimens with thickness 1 ±
Figure 2. Schematic of specimens built with the Renishaw AM250. 0.02 mm (taken from ).
[37]
A B A B

Figure 3. Illustration of powder flowability test measured by the
Revolution Powder Analyzer. (A) The rotating drum watched by a digital
camera. (B) The side-view of rotating drum with a powder sample in the Figure 5. SEM micrographs of AISI 304L stainless steel powder in
beginning of an avalanche cycle . (A) as-received and (B) spheroidized conditions.
[35]

Volume 1 Issue 1 (2022) 4 http://doi.org/10.18063/msam.v1i1.1