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Zhou, et al.
degradation rate. In addition, Zn is a nutrient element for fast cooling cycle . Subsequently, Wen et al. unitized
[32]
human body and serves an key role in protein synthesis, LPBF to prepared Zn scaffold for biomedical application
signal transduction, and normal functions of various in 2018. The degradation behavior and cell response were
enzymes, especially in strengthening human immunity also preliminarily evaluated.
and promoting wound healing . Therefore, a series However, the evaporation of Zn easily occurs due
[11]
of Zn-based materials have been developed for clinical to relatively low melting and boiling point. The gas
application [12,13] . entrapment and the retention of slight residual porosity
Additive manufacturing, which integrates are generally observed during LPBF of Zn, and seriously
computer-aided design, material processing, and forming degrade the comprehensive properties of as-built
technology, shows great potentials in manufacturing parts . Investigations on powder properties and process
[33]
accurate devices as compared with traditional process [14-16] . parameters were carried out to address this issue, and some
According to the American Society for Testing and substantive progress was obtained [32,34] . Nevertheless,
Materials standard, it is divided into seven categories, an accurate microstructure control and a profound
including polymerization, material jet forming, material interpretation of the relationship between microstructure
extrusion, powder bed fusion, binder spray forming, and performance are still a challenge up to now. In
sheet lamination, and direct energy deposition. Among the present work, the factors that affect the formation
them, direct energy deposition and powder bed fusion are quality of LPBF-processed Zn are comprehensively
two main additive manufacturing techniques for metal reviewed. The microstructure, mechanical performance,
parts. Comparing with direct energy deposition, powder degradation behavior, and cytocompatibility were
bed fusion, particularly laser powder bed fusion (LPBF) discussed systematically. This paper aims to guide the
technology, which uses laser beam as energy source, has future research engaged in the application of Zn-based
small beam spot diameter, fine powder, and thin forming implants fabricated by LPBF.
layer, therefore exhibiting relatively high dimensional
accuracy [17-19] . Completely different from conventional 2. Factors affecting the formation quality
process like casting, LPBF is a near-net-shape fabrication 2.1. Powder properties
technology which builds parts in means of layer-by-
layer fashion [20,21] . As high-energy laser radiates on High formation quality is the prerequisite of LPBF-
the metallic powder, a microscale melt pool is formed, processed Zn parts with stable mechanical properties
which subsequently undergoes a rapid solidification. and biological behavior. In general, the formation
Usually, a sharp temperature gradient and consequent quality is primarily related to the powder properties
high cooling rate even above ~10 K/s can be achieved and processing parameters . Table 1 shows the major
[35]
7
within the melt pool . Therefore, the microstructure intrinsic characteristics of Zn powder for LPBF . Zn
[22]
[36]
containing microscale grains and precipitates is generally has high density and promotes the movement of powder
established, which endows LPBF-processed parts with during LPBF, which reduces the interference of gas flow
excellent comprehensive performance [23,24] . on powder layer. Its low surface tension and viscosity are
In the last couple of years, the combination of LPBF able to improve the densification rate of as-built parts .
[37]
and biomedical metals has received extensive attention However, Zn possesses relatively low melting and boiling
in biomedical filed [25-28] . Due to the rapid melting/ point, resulting in a small and narrow forming window
solidification and unique domain-by-domain localized during LPBF. Moreover, the relatively low specific heat
forming characteristics, LPBF is able to accurately and high laser absorptivity of Zn powder usually result in
regulate the material density, grain size, precipitate a high evaporation tendency .
[38]
distribution, and texture strength, which are important Apart from the inherent properties, the formation
for controlling the mechanical properties, degradation quality is affected by the fluidity of Zn powder, which is
behavior, and biological response of metallic implants . attributed to the powder properties such as particle shape,
[29]
On the other hand, it is able to customize the macro- and size, and chemical composition. In general, spherical
micro-structure with arbitrary complex shape and high particles with low surface friction and mechanical
precision . A three-dimensional interconnected porous interlock improve the fluidity and facilitate the uniform
[30]
structure which is similar to human bone can be easily spreading of powder . On the other hand, the particle
[39]
built by LPBF, which is conducive to nutrient transport size distribution shifting to the direction of coarse
and the regeneration of damaged tissue. Montani et al. particles cause high-energy absorption, thereby reducing
[31]
first explored the feasibility of LPBF Zn in 2017, and the laser energy arrived the underlying surface . In
[6]
some encouraging results were achieved in mechanical this case, the thermal penetration depth is reduced and
properties as compared with as-cast Zn, which is believed potentially results in inhomogeneous regions such as
to be caused by the small grain size obtained through the cracks and incomplete fusion . In contrast, fine particles
[40]
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