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Laser Additive Manufacturing of Zinc
breaks melt track into droplets, which eventually result for the remelting of previous layer. As a result, the freshly
in the Plateau–Rayleigh instability and the formation of melted material cannot be well bonded to the previous
opening defects . When the scanning speed decreases, this layer, and more easily cause melt splashing and porosity.
[58]
instability is alleviated. However, the molten metal solidifies Considering the relatively thick powder layer that leads
before filling the pores due to the high thermal conductivity to highly irregular and unstable melt pools, selecting the
of metal powders, thus resulting in many keyhole pores and appropriate powder layer thickness is the key to controlling
inverted triangle pores . With the further decrease of the the porosity and improves the formation quality .
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[63]
scanning speed, the powder layer inside the laser radiation The hatching space also plays an important role in the
area is completely melted and then forms a complete molten formation quality of LPBF-fabricated samples . The
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pool. Specifically, the low scanning speed corresponding hatch spacing is closely related to the laser spot diameter.
to the high laser energy input per unit length contributes In general, relatively small hatching space means massive
to sufficient metallic liquid and high peak temperature overlapping remelting between scanning passes, leading
in molten pool, so as to reduce the viscosity and surface to a large amount of evaporation, thus destroying the
tension. Flow capacity of melts is evidently enhanced surface structure and reducing the formation quality. On
due to low melt viscosity and surface tension caused by the other hand, excessive hatching space leads to non-
high working temperature . Consequently, the metallic overlapping area, which cannot be melted completely and
[60]
liquid spreads wider and fills in the gaps between particles
efficiently, which contributes to the decrease of the porosity greatly reduces the formation quality.
and the improvement of formation quality. To optimize the process parameters, the input of
The influence of powder layer thickness on surface laser energy is expressed by the laser energy density (E )
v
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structure and porosity rate should also be marked, which as follows :
is realized by its influence on melt flow behavior . The
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powder layer thickness mainly affects the melt behavior E =P/(V∙d h ) (1)
v
s
s
by influencing the amount of material melted by laser
beam. As the powder layer thickness increases, the melted Where, the P, V, d , and h are laser power, scanning
s
s
powder materials increase and the melt surface area rate, hatching space, and layer thickness, respectively.
enlarges accordingly, which leads to high evaporation Basing on the Equation 1, the process window for Zn
and Marangoni force . In this case, the melt flow powder with varied P and V through massive tests is
[62]
velocity increases, which destroys the stability of melt obtained, as displayed in Figure 3A. When the laser
flow. In addition, relatively thick powder particles largely energy density is 60–135 J/mm , the Zn evaporation
3
consume the input laser energy, reducing the heat used decreases and the density of laser melted parts reaches
A B

Figure 3. (A) The densification under different laser energy input for Zn. Reprinted from Materials & Design, 155, Wen P, Voshage M,
Jauer L, et al., laser additive manufacturing of Zn metal parts for biodegradable applications: Processing, formation quality and mechanical
properties, 36-45, Copyright (2018), with permission from Elsevier . (B) The processing map of LPBF experiments and corresponding
[48]
surface morphology for Zn-Al parts. Reprinted from Journal of Alloys and Compounds, 798, Shuai C, Cheng Y, Yang Y, et al., laser additive
manufacturing of Zn-2Al part for bone repair: Formability, microstructure, and properties, 606-615, Copyright (2019), with permission from
Elsevier .
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78 International Journal of Bioprinting (2022)–Volume 8, Issue 1