Page 90 - IJB-8-1

Laser Additive Manufacturing of Zinc
Table 1. The intrinsic characteristic features of Zn, Fe, and Mg powders used by LPBF
Metal Melting Boiling Specific Surface Laser Density
powders point point heat tension absorptivity
(0C) (0C) (J/kg·K) (mN/m) (%) (g/cm )
3
Zn 420 907 382 782 70 7.14
Fe 1538 2862 444 1835 75 7.87
Mg 650 1091 1360 559 \ 1.74
LPBF: Laser powder bed fusion

are easily melted and increase the thermal penetration general, the densification rate and surface quality are
depth, thus improving the densification rate . It should the two important indexes to estimate the formation
[41]
be mentioned that the presence of surface-active elements quality of LPBF-processed parts . Excellent formation
[49]
in powder, such as oxygen and sulfur, is able to increase quality is also the key to preventing fatigue damage [50,51] .
the thermal capillary force within the molten pool, which Unfortunately, serious evaporation of Zn easily occurs
yields a huge surface tension gradient, and then triggers during LPBF even at low laser energy input, which exerts
a relatively strong fluid circulation and instability in the a significantly negative influence on the formation quality.
molten pool . Due to the recoil force of evaporation, the molten pool
[42]
At present, the suitable powders designed for LPBF moves violently and results in the ejection of massive
are quite limited, which becomes one of the obstacles Zn liquid from the molten pool, which will disturb the
for LPBF of Zn. The powder requires proper rheological adjacent powders and then push the powders away from
properties to form a thin, dense, and uniform powder the molten pool . The melted Zn solidifies into spherical
[52]
layer. At present, the processing technologies of powders particles and adheres to the track surface under the action
for LPBF technique include plasma, gas, and water of surface tension, which eventually leads to the surface
atomization [43-45] . Each processing technology can produce roughness deformation and density reduction of Zn-based
specific powder characteristics including morphology, materials.
particle size, and porosity, which significantly affects The related process parameters mainly include
the rheological behavior of Zn powder. Demir et al. laser power, scanning rate, hatching space, and
[6]
fabricated Zn powders by water atomization and sieved layer thickness . In general, laser power affects the
[53]
them into coarse powder (15 μm) and fine powder (9 thermodynamics and temperature distribution of molten
μm), as displayed in Figure 1A and B. Compared with pool. As shown in Figure 2A, a limited portion of powder
coarse powder, fine powder is relatively susceptible to the particles near the laser irradiation center area is obviously
variation of laser energy input. Ruvalcaba et al. utilized melted at relatively low laser power. Meanwhile, powder
[46]
water-atomized Zn powder, as shown in Figure 1C, to particles far from the irradiation region actually maintain
fabricate testing samples with a density of only 95% under their original spherical shape and point contact with
the action of the optimized parameters. It is revealed that each another. Apparently, the shell at the border of the
the water-atomized Zn powder is difficult to fabricate powder particles is melted by laser beam. However, the
high-density samples, which is attributed to the forming core of the powder particles is not significantly affected
of high oxygen content under the impact of water jet. and retains the initial solid state, which causes relatively
The air-atomized Zn powders with elongated flake, stick high viscosity of liquid pool, thereby resulting in low
shape, and minor portion of teardrop shape are exhibited melt flow capacity. As a result, the formed molten metal
in Figure 1D and E . The oxidation on the surface cannot diffuse completely, leaving a valley between two
[47]
of the droplet can form oxide film, which prevents the adjacent particles and a rough surface. As the laser power
spheroidization of the melt droplet during solidification. increases, the enhanced width generated in the molten
Wen et al. fabricated Zn parts using nitrogen atomized pool significantly promotes the melt spreading ratio,
[48]
powder with spherical shape. Their results reveal that the which produces effective wetting ability and a desired
spherical powder improves the powder fluidity and favors metallurgical bonding with adjacent tracks. With the
for the deposition of uniform powder layer, thereby further increase of laser power, massive powder particles
obtaining Zn parts with high densification. completely melt. Then, adjacent particles adhere to the
molten pool, which causes a lack of powders in the
2.2. Process parameters
adjacent area, thus resulting in the formation of pores and
Processing parameter, which directly determines the balling droplets that aggravate the surface roughness .
[54]
laser energy input, is the other important factor that Therefore, with the increase of laser power, the combined
affects the formation quality of LPBF-processed Zn. In width of scanning trace and temperature within the molten

76 International Journal of Bioprinting (2022)–Volume 8, Issue 1