Page 98 - IJB-8-1

Laser Additive Manufacturing of Zinc
by alloying treatment with Mg . Introducing Ag into Nano reinforcements, including nanoparticle,
[33]
Zn by LPBF also improves the mechanical strength of nanorod, and nanosheet, exhibit great potential to enhance
Zn matrix. Alloying with Ag can form the constitutional the mechanical properties of metal-based composites,
undercooling in front of the moving solid-liquid interface, because of their high specific strength and elastic
which leads to more nucleation events and the grain modulus [123-126] . The possible strengthening mechanism
refinement. On the other hand, abundant precipitates mainly includes grain refinement, load transfer effect,
(AgZn ) act as active nucleation sites to further refine Orowan strengthening, as well as thermal mismatch
3
the Zn grains, thereby effectively hindering dislocation strengthening [127,128] . Among them, load transfer effect
movement and plastic deformation . Besides, Shuai is a primary strengthening factor since the load is easily
[81]
et al. [116] introduced Al and Sn into porous Zn scaffolds transferred from the metal matrix to reinforcements
prepared by LPBF methods, and the additives effectively through the interfacial shear stress. Under this condition,
enhance the mechanical strength of Zn scaffolds. In nano reinforcements can consume massive fracture
detail, the Al phase nucleated primarily during cooling energy and prevent the crack propagation, thereby
and caused the rapid precipitation of Zn, and then, the achieving enhancement effect. Grain refinement, as
Zn-enriched phase and Sn phase could form rod-like another major strengthening factor, is attributed to the
eutectic structure of Zn-Al-Sn phase. In this case, the heterogeneous nucleation effect of nano reinforcements.
rod-like eutectic could block dislocation motion and Usually, the nucleation of new grains at the solidification
result in dislocation pile-up, thereby conducing to the frontier requires numerous nucleation sites and a
mechanical reinforcement. Besides, hot-rolled Zn-Li favorable energy condition. Nano reinforcements with
alloy even exhibits an extremely high tensile strength of high melting point lead to a significantly enhancement of
405.3 MPa [117] . supercooling at the solid/liquid interface, which provides
REs are also used to improve the mechanical an energetically favorable condition. Meanwhile,
properties of Zn. Compared with other alloying elements, the reinforcements can act as low-energy barrier
REs have better fine grain effect [79,118] . It is reported that heterogeneous nucleation sites ahead of the solidification
the nanocrystalline precipitates in Zn matrix generate front, thereby inducing the random growth of fine
a remarkable back stress for the bow dislocations grains [129] . Thermal mismatch expansion differences
around the shear resistant precipitates during stretching, between the matrix and reinforcements are also caused
thereby hindering dislocation movement [119,120] . More by massive lattice distortion at the interface to realize
significantly, the solid solution of REs in Zn matrix leads the enhancement of mechanical properties. Nevertheless,
to massive edge distortion and lattice dislocation, which Orowan strengthening only takes effect in the composites
further increases the resistance of dislocation movement. with uniformly distributed nanoparticles. The nano
As a result, the yield strength, ultimate strength, and reinforcements within the grain are able to accumulate,
elongation for Zn-Ce parts considerably enhance to 180.6 pin down, and form dislocation loops, there generating
± 7.1 MPa, 247.4 ± 7.2 MPa, and 7.5%, respectively . a back stress that hindered dislocation propagation. For
[79]
Besides, the interaction of metal matrix and multiple example, reduced graphene oxide (RGO) is used to
alloying elements can also effectively improve prepare Zn-based scaffolds [129] . The results show that
mechanical properties, thereby maintaining the initial RGO in Zn matrix remarkably enhances the strength to
mechanical stability and long-term bone osseointegration 182 MPa. Besides, nano silicon carbide was incorporated
during repairing bone defects. Alloying with WE43 into Zn matrix through LPBF, which results in high
can produce complex intermetallic compounds with Zn compressive yield strength of 121.8 ± 5.3 MPa [130] .
porous scaffolds, such as Mg Zn , YZn , and NdZn, Obviously, both the introduction of alloy elements
5
2
11
which induce a strong effect of grain refinement [33,66] . In and nano reinforcements can effectively improve the
this case, the hardness of Zn-xWE43 (x = 0, 2, 5, and 8 mechanical strength of Zn implants. However, alloying
wt.%) gradually increases from 42 ± 3 to 169 ± 8 HV, and treatment may also give rise biological problems [131] .
the tensile strength is enhanced to 335.4 ± 10 MPa [106] . Therefore, it is necessary to choose some alloy elements
However, the elongation gradually decreases due to the with high tolerance to human body. In addition, how nano
increase of precipitate content. Notably, REs can not only reinforcements are absorbed by human body during the
improve the mechanical strength of Zn matrix but also degradation of Zn is another unsolved problem. Some
enhance the creep resistance at human temperature [121] . In literature reported that carbon nanomaterials were able
this case, stacking fault energy of Zn is reduced, which to achieve complete metabolism in body fluid by human
leads to the decrease of the critical resolved shear stress myeloperoxidase, eosinophil peroxidase, and xanthine
and the improvement of lattice symmetry in Zn matrix, oxidase, and even be taken up by cells due to its nanoscale
and then benefits the activation of non-basal slip including structure [132,133] . The introduction of bioactive ceramics
sessile pyramidal slip [122] . such as hydroxyapatite and bioactive glass as reinforcing

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