Page 26 - MSAM-1-2

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

maintained good sphericity (Figure 10B and E), which laser energy, reducing the temperature of the droplet. This
can be attributed to the high and relatively stable laser increased the viscosity of the droplet and they solidified
absorption of the powder layer [24-26] . The fully melted before spreading sufficiently, resulting in the balling effects.
particles and stable molten pool improved the wetting and This phenomenon is detrimental to interlayer bonding and
spreading of liquid metal, enhancing the surface quality tends to affect the densification behavior during LPBF [31,32] .
of the LPBF-processed parts [14,25] . Comparatively, with Before LPBF fabrication, a computer-aided design
enhanced particles agglomerated (Figure 10A and D), model of the part is processed by software to plan the laser
the LPBF 90W-Ni-Fe single track had a fluctuated path line-by-line [4,33] . Therefore, the quality of the scanning
boundary. This can be attributed to the high and uneven track is of great importance to metallurgical bonding
laser absorption behavior which caused the nonuniform and the surface quality of fabricated components [6,14,24] .
spreading [11,24,26] . When the matrix particles were broken or To further investigate the effect of nanoparticle-coated
deformed, the LPBF scanning track morphology changed 90W-Ni-Fe powder morphology on LPBF printing quality,
to irregular shapes (Figure 10F), and the balling effects SEM images of the upper surfaces, and morphology of the
were observed (Figure 10C), which can be attributed to the side surfaces of different fabricated 90W-Ni-Fe alloys are
increase in viscosity. When the temperature of the laser- presented in Figure 11. It can be seen that the evolution
irradiated zone is higher than the melting point of W, the of the powder morphology does affect the surface quality
viscosity of the W droplet can be defined as : of LPBF-processed specimens due to the changes in
[30]
dynamic viscosity. LPBF processing involves the flow

1.28 10 5
()T =0.108exp( RT ) (8) and solidification of liquid metal in the molten pool; the
d
dynamic viscosity n of the molten pool is defined as :
[29]
where n is the viscosity of the W droplet, R is the gas f 16 m
d
constant (equal to 8.31 J·mol ·K ), and the temperature = 15 kT (9)
−1
−1
f
T ranges of 3350–3700 K. From the above equation, the
higher the molten pool temperature, the lower the W where m is the atomic mass, k is the Boltzmann
droplet viscosity. The previous studies indicated that there constant, T is the temperature of the molten pool, and
is a competing mechanism for the wetting, spreading, and σ is the surface tension. The previous studies indicated
solidification of W droplets during LPBF . Due to the that the surface tension is higher when the molten pool
[31]
low laser absorptivity of the powder layer with deformed temperature is lower . Therefore, the higher the molten
[21]
matrix particles, the molten pool cannot absorb enough pool temperature, the lower the dynamic viscosity. The laser
A B C

D E F

Figure 10. Simulated single-track morphologies (A-C) and SEM images showing as-fabricated 90W-Ni-Fe scanning tracks (D-F) with powders of different
morphologies: severely agglomeration (A and D), uniformly dispersion (B and E), and deformation (C and F).

Volume 1 Issue 2 (2022) 8 http://doi.org/10.18063/msam.v1i2.11