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Zhou, et al.
It is ascribed to that the Ce with high surface activity reached 153.1 ± 3.2 MPa and 61.4 ± 1.2%, respectively,
reduces surface tension of liquid phase in LPBF process this method is difficult to customize bone implants with
and reduces the critical nucleation radius [100] . Meanwhile, porous structure [103] . In contrast, Zn prepared by LPBF
the precipitates further hinder the growth of primary with optimizing process parameters possesses relatively
grains along one single direction, thereby forming fine high elongation of 8.1 ± 0.9% and ultimate strength
grains with random orientations . of 132.9 ± 0.7 MPa . Despite that the mechanical
[79]
[82]
However, the component content of the second properties of the LPBF-processed samples are affected
phases is easily changed due to the burning loss of by sample size and defects, it still shows other excellent
elements caused by evaporation, thereby affecting the properties [104] . In addition, the porous Zn scaffolds
microstructure of Zn matrix to a large extent. According fabricated by LPBF have also great practical potential in
to a report of LPBF-processed ZK60, the mass ratio of Zn the bone repair. For instance, Montani et al. processed
[31]
element decreases from 5.2% in the alloying powders to porous Zn using LPBF method in 2017 and found laser
4.4% in the as-built part, while the Mg element content melted Zn presented more superior mechanical properties
rises from 94.0% to 94.4%, which is attributed to the than as-cast and wrought Zn material because of reduced
evaporation tendency of Zn element which is higher grain size. According to the Hall-Petch law, fine grains
than that of Mg element [101] . Since Mg has high oxidation obtained by laser rapid solidification are able to enhance
tendency, formed oxide film need to be melted by high the resistance near the grain boundary, which leads
laser energy input as compared with Zn. In this case, both to the accumulation of extensive dislocations and the
size and content of precipitates within Zn matrix increase. strength improvement [105] . Meanwhile, grains with varied
Therefore, the microstructure composition analysis for orientation against the original one can accommodate
LPBF-processed Zn needs to consider the effect of metal more deformation strain under tensile loading, which
elements on evaporation. improves the ductility. To further understand the
fracture mechanism, laser melted Zn exhibited a typical
3.2. Mechanical properties characteristic of cleavage steps surrounding the tearing
The biomedical metal with similar mechanical properties ridge. With the increase of scanning speed, the cleavage
of bone is required, which is favorable to stress stimuli steps become small and deep. Cleavage facets arrayed in
and avoidance of stress shielding [102] . Zn implants as plate a certain direction are not found, and the grains tend to be
or screw offer sufficient mechanical strength for clinical ruptured in a transgranular manner.
applications. However, Zn scaffolds, especially with high Alloying treatment is usually adopted to enhance the
porosity, still show relatively poor mechanical properties mechanical properties of Zn implants [113-115] . Zn-Mg alloy
as hard tissue repair material . An undoubted fact is that is the most studied in Zn-based medical alloys due to
[72]
the processing method directly affects the mechanical their favorable biocompatibility. For instance, the LPBF-
properties of Zn. As shown in Table 2, the as-cast Zn fabricated Zn-3Mg parts exhibit significantly enhanced
exhibits relatively poor ultimate tensile strength of 33.6 ultimate strength of 222.3 ± 8.2 MPa, which is attributed
± 0.6 MPa and tensile elongation of 1.2 ± 0.3%. Although to the grain refinement strengthening, solid solution
the ultimate strength and elongation for the hot-rolled Zn strengthening, and secondary phase strengthening caused

Table 2. Mechanical properties of Zn matrix tested for bone implant applications [32,33,79,82,103,106-112]
Material Metallurgy Grain Yield Ultimate Elongation
size strength strength
μm MPa MPa %
Zn Casting / 29.3 33.6 1.2
Zn Hot rolled / 84.2 153.1 61.4
Zn LPBF 5.9 110.3 132.9 8.1
Zn-Al LPBF 2.21 141.7 192.2 11.7
Zn-Mg LPBF 5.2 152.4 222.3 7.2
Zn-Mg Casting 65 112 120 0.6
Zn-Ce LPBF 3.9 180.6 247.4 7.5
Zn-Li Hot rolled 5.9 363.7 405.3 4.0
Zn-We43 LPBF 5.6 298.5 335.4 8.0
Zn-RGO LPBF 3.2 142.9 182.1 14.1
Human bone / / 124 – 174 150 – 180 1.4-3.1

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