Page 27 - MSAM-1-1

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

vaporization of C and N during spheroidization increased 3.4. Flowability
the Cr /Ni ratio, promoting FA solidification mode Table 5 provides the results of the Revolution Powder
eq
eq
(Figure 11). Since the cooling rate of particles during plasma Analyzer flowability test. The avalanche angle and break
spheroidization is large, primary ferrite cannot undergo a energy measurements were repeated 150 times, and the
solid-state transformation to austenite. The competition means and standard deviations were calculated. Due to the
between austenite and delta ferrite phases within steel larger avalanche angle and break energy of the as-received
depends on the material chemistry and the cooling rate.
powder over the spheroidized powder, it is concluded that
Moreover, the cooling rate increases with decreasing the spheroidized powder has higher flowability than the
particle size, providing variation in the microstructure as-received powder. This data suggests that the as-received
in polydisperse powders. Kelly et al. [40] studied the size- material is more cohesive due to its larger asperity (as shown
dependent microstructure of 303 stainless steel droplets in Figure 7).
with sizes ranging from 20 to 120 µm. They concluded
that the smallest particles were predominantly BCC, 3.5. Characterization of fabricated parts
whereas the largest particles contained an FCC Figure 12 presents the results of tensile tests, and Table 6
microstructure. The increase in BCC, or delta ferrite, shows their mean and standard deviations. Performing
with decreasing particle size, was a consequence an ANOVA with a significance level of 0.05 showed
of enhanced supercooling. Moreover, since small that the YS, UTS, and strain at break of as-received and
particles have the lowest probability of containing reconditioned powders are significantly different (P <
potent nucleants, large undercooling can be reached 0.0001). The parts fabricated with the spheroidized powder
before solidification begins. Since the enthalpy of BCC had lower YS and UTS than those of the as-received
crystallization is less than FCC, ferrite is more prone powder by approximately 70 and 30 MPa, respectively,
to solidification before austenite in smaller particles. while their strain at break was higher by about 0.2. The
The conclusions provided by Kelly et al. coupled lower strength and higher ductility follow the chemistry
with the bulk chemistry of the powder suggest that
plasma spheroidization increases the chrome-nickel change in the powder, i.e., reduction in N, C, Cr, and O,
equivalency of the input material, which solidifies as a and these results agree with the studies reported in the
combination of ferrite and austenite. literature. By investigating the effects of N content on
stainless steel powders during the plasma spheroidization
process, Razumov et al. found that the reduction in
[41]
A B N content (which is an austenite stabilizer) decreased
the tensile properties of fabricated parts. Wang et al.
[42]
assessed the effects of chemical composition on tensile
properties of austenitic stainless steels. They found that

Figure 10. EBSD phase patterns of (A) as-received and (B) spheroidized
304L powders. The blue phase corresponds to austenite and the red phase Figure 11. Weld crack length of austenitic stainless steel vs. chrome-
to delta ferrite. The spheroidized powder is found to contain a larger nickel equivalency (taken from ). The spheroidized powder has a higher
[39]
amount of delta ferrite. chrome-nickel equivalency than the as-received powder.

Volume 1 Issue 1 (2022) 7 http://doi.org/10.18063/msam.v1i1.1