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Zhou, et al.
99.5%. According to different regions of the processing optimized gas circulation system to eliminate the negative
window, the Zn-Al parts are also fabricated, as exhibited effect of evaporation by numerical analysis. Results show
in Figure 3B. It is found that reasonable laser energy that a stable LPBF process was achieved to prepared
density can reduce the Zn evaporation degree and improve high-density porous Zn scaffolds under suitable blow-off
the formation quality. Notably, under optimized process and suction. Jauer et al. first adopted a specialized gas
[78]
parameters, rare earth elements (REs) like neodymium circulation system to remove the evaporation particles
(Nd) can release a large amount of heat, which reduces within the processing chamber during LPBF, thereby
the liquid temperature and creates a narrow crystallization reducing the negative impact of the metal evaporation
interval during LPBF . Meanwhile, REs as surface fume on formation quality and providing a stable
[66]
active elements are also able to deoxidize and purify the processing environment. Based on this foundation, a
molten pool . As a consequence, evaporation of Zn is gas circulation system was specially designed to prepare
[67]
reduced and the fluidity is raised, thereby improving the pure Zn during LPBF, and the influence of gas flow
formation quality of laser metal Zn parts. system design on the elimination of evaporated fume
Apart from the process parameters, the scanning was numerically simulated. The results show that partial
strategy also has a significant effect on the formality of evaporated fume was effectively sucked out. Specially,
LPBF-processed parts. Usually, the rapid movement of the flowing inert gas generated directional flow in the
laser beam and high-energy input at the local zone results in cavity, which absorbed the evaporation products near the
sharp changes in temperature distribution and temperature laser melting zone to prevent the contamination of the
gradient during LPBF . As a result, high residual stress transmission mirror. As a result, the side effects of falling
[68]
and uneven deformation may be produced. At present, spatters on the next scanning track were decreased. It is
several teams of researchers reported that one alternative ascribed to the fact that the quick and efficient removal
linear hatching scanning strategy was used in LPBF of of the evaporation products can void the attenuation of
Zn [69-71] . It is believed that such scanning strategy is able to laser energy, thereby maintaining the stability of laser
reduce the residual stress and is easy to be generated from melting Zn-based materials and inhibiting the retention
a computer-aided design file. However, the influence of of gas in the molten pool. Meanwhile, the flowing argon
different scanning strategy on formability is rarely studied atmosphere is able to provide appropriate pressure and
for LPBF of Zn, which should be one issue worthy of prevent the formation of oxide layers on the pool, thereby
attention in the future research. Particularly, LPBF of Zn, reducing the fluctuation of molten metal in the molten
instead of other metals such as Fe or titanium, is more pool and consequently improving the formation quality.
likely to cause gasification and smoke [32,72] . Therefore, a As a result, the relative density of as-built Zn is >99% .
[79]
more appropriate scanning strategy can be considered to
reduce the interference of smoke on the laser beam, so as 3. Microstructure and mechanical properties
to improve the stability of molten pool.
3.1. Microstructure
2.3. Gas flow The optimized laser energy input condition and gas
LPBF is conducive to adjusting the performance of metal shielding are used to fabricate Zn-based implants with a
implants as compared with traditional manufacturing satisfactory quality. They possess special microstructure
process . However, during laser process, the evaporation characteristics, such as fine grain, random grain orientation,
[73]
of Zn easily results in strong and uneven vapor flow in the and homogeneous texture. Usually, the fastest heat
molten pool . This situation can accelerate the irregular dissipation direction of the molten pool is perpendicular to
[74]
jump of molten liquid, resulting in serious spatter and the the substrate, which leads to a high cooling speed of ~10 5
formation of pores, thus reducing the formation quality – 10 K/s as compared with the traditional casting process
6
and corrosion resistance of the LPBF-processed samples, (~10 K/s) . Therefore, a typical feature like the columnar
[80]
2
as displayed in Figure 4. In fact, numerous small grain along the building direction is present in the LPBF-
particles exist in the evaporation fume of melted Zn. Such fabricated Zn samples, although significant diversity in
an evaporation fume will undoubtedly contaminate the the microstructure decided by the process parameters is
transmission mirror and thereby disturb the propagation involved. Shuai et al. found that with the increase of
[81]
of the laser beam. As a result, the inaccurate laser beam laser energy density, the grains coarsened and changed
with unstable laser energy or deviated direction sharply from columnar into equiaxed grains. Qin et al. showed
[82]
deteriorates the formation quality. A typical example is that as the laser scanning speed increases, the number of
that a large number of keyholes are observed in the Zn columnar grains growing along the building direction will
matrix . decrease. It is reported that the formation of columnar
[75]
To reduce or even eliminate the impact of metal grain structure is able to be explained by the epitaxial
vapor on formation quality, Wen et al. developed an solidification and the grain growth competition [83,84] .
[77]
International Journal of Bioprinting (2022)–Volume 8, Issue 1 79