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Materials Science in Additive Manufacturing Laser absorption and printability of 90W-Ni-Fe

morphologies [10,11,25-27] . According to the specific impact The calculated laser absorptivity and their standard
energy equation : deviations are presented in Figure 7. As shown from
[28]
Figure 7A, the laser absorptivity of the 90W-Ni-Fe powder
1
i
E = n j 1 2 M s Mv 2 (7) bed model tended to decrease with the increase of milling
bj
energy. The laser absorptivity decreased significantly
where, E is the specific impact energy in ball milling, when the matrix particles were broken or deformed. The
i
Ms is the mass of powder, M is the total mass of grinding standard deviation of calculated laser absorptivity tended
b
balls, v is the relative impact velocity between two to decrease and then increased with the milling energy
j
grinding balls and/or a ball grinding against the grinding increasing (Figure 7B). The standard deviation was lowest
bowel wall, and n is the number of collision of a ball when the reinforced particles were uniformly distributed
against other balls and/or the grinding bowl wall within and the matrix particles were unbroken or undeformed,
a second. As can be seen from the above equation, the indicating that the laser energy conversion factor was high
higher the milling speed or ball-to-powder weight ratio and stable. This was attributed to the improved homogeneity
is applied, the higher the impact energy is obtained. The of the powder bed [25,26,29] . When the nanoparticles were
nanoparticle-coated 90W-Ni-Fe powder morphology agglomerated, the decreased ratio of spot size to irradiated
evolved with the change of milling energy (Figure 5A). particles promoted the multiple reflections of the laser,
[8]
Reinforced particles were gradually dispersed uniformly improving the laser absorptivity . However, the powder
with increased specific impact energy in ball milling, but bed was uneven in this case, so the laser energy conversion
particle deformation and breakage were more likely to was instability, which tended to produce highly unstable
occur (Figure 5B). When the ball-to-powder weight ratio molten pools, affecting the printing quality of LPBF [8,14,22] .
was 1:2, the milling speed was 250 rpm with a milling When the matrix particles were broken or deformed, the
time of 6 h, the Ni and Fe nanoparticles were uniformly powder bed had low packing density and high porosity,
dispersed around W particles, and the sufficiently high which weakened the multiple reflections and reduced the
sphericity of the W matrix particles was maintained. laser energy conversion factor, affecting the wetting and
Combining the above, 3D microscopic GO-RT models spreading of melt during LPBF [8,30,31] . These may cause
with different powder morphologies were established balling effects and reduce the printing quality of fabricated
(Figure 6). specimens [8,24,31] .

A

B

Figure 5. Schematic of the evolution mechanism of nanoparticle-coated powder during ball milling (A) and SEM images showing the different nanoparticle-
coated 90W-Ni-Fe powder (B).

Volume 1 Issue 2 (2022) 5 http://doi.org/10.18063/msam.v1i2.11