Page 25 - MSAM-1-1

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

were altered to almost complete spheres after the revealing that the spheroidization process drastically
spheroidization process. reduced the maximum asperity of 90% of the particles from
The particle size distributions obtained from 1200 approximately 1.8 to 1.2.
particles of the as-received and spheroidized powders 3.2. Powder chemistry
shown in Figure 6 indicate that the as-received powder
was primarily reshaped by spheroidization, rather than The possible impact of the plasma spheroidization process
reduced in size by vaporization or enlarged through the on the material’s bulk chemistry is another crucial aspect
agglomeration of molten particles during solidification. that should also be considered. Table 4 shows the bulk
This invariation in size is further evidenced by the lack chemistries of as-received and spheroidized powders,
of change in the 10 , 50 , and 90 cumulative percentiles obtained through a combination of ICP-OES and inert gas
th
th
th
of the particle size distributions in Figure 6 and their fusion to determine the heavy and light alloying elements,
values in Table 3. This nearly identical particle size for the respectively. Comparing the bulk chemistries before and
as-received and spheroidized 304L powders is desirable as after spheroidization reveals that the wt% of each Mn, C, and
it indicates the advantage of the spheroidization process N, which are volatile elements within AISI 304L stainless
in improving the particle geometries while keeping the steel powder, reduced during the plasma spheroidization
particle size constant. process due to vaporization. However, it should be noted
that despite these changes in chemistry, the spheroidized
Also, the plasma spheroidization improved the powder still lies within the AISI specifications for 304L
uniformity of the as-received powder for particles below stainless steel.
30 µm, which exhibited higher sphericity. Since a particle
size of 30 µm is close to the D of the spheroidized powder, The chrome-nickel equivalency was determined using
90
[38]
the results in Figure 7 indicate that at least 90% of the the WRC-1992 model for insight into differences in the
particles by number distribution showed enhancements in solidification behavior, and the obtained data are included
the particle shape. To further demonstrate the improvement in Table 4. An increase in the chrome-nickel equivalency
in sphericity, the cumulative aspect ratio distributions of (Cr /Ni ) from 1.50 to 1.74 due to plasma spheroidization
eq
eq
[39]
the as-received and spheroidized powders were compared, was found. Based on the work done by Korinko et al.
on the solidification behavior of stainless steels under
welding conditions, a shift from austenite-to-ferrite (AF)
Table 3. Cumulative percentiles of particle size distributions solidification mode to ferrite-to-austenite (FA) solidification
in Figure 6 comparing the as‑received and spheroidized mode is therefore expected due to the chemistry change. As
powders such, parts produced using as-received powder will solidify
Sample D (µm) D (µm) D (µm) as primary austenite and secondary delta ferrite. In contrast,
10 50 90 the spheroidized powder will produce a microstructure
As-received 13.41 ± 0.03 19.9 ± 0.01 30.4 ± 0.04
that exhibits primary ferrite with secondary austenite
Spheroidized 13.6 ± 0.02 20.4 ± 0.02 31.3 ± 0.06 after solidification. The ramifications of such change in

Figure 6. Particle size distributions of the as-received and spheroidized Figure 7. Particle shapes of as-received and spheroidized powders,
AISI 304L stainless steel powder. quantified using the aspect ratio.

Volume 1 Issue 1 (2022) 5 http://doi.org/10.18063/msam.v1i1.1