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Zhang, et al.
A B C

Figure 20. Cycling compression results of NiTi samples. (A) F = 63 J/mm . (B) F = 160 J/mm . (C) Recoverable strain (Shape Memory
[50]
3
3
and Superelasticity, Selective Laser Melting of NiTi Shape Memory Alloy: Processability, Microstructure, and Superelasticity, 6, 2020,
342–353, C. A. Biffi, J. Fiocchi, F. Valenza, et al. with permission of Springer).
strain is about 5%. The authors also found that as the the stress distribution more uniform during loading.
porosity of porous NiTi increases, Young’s modulus, This research has important guiding significance for
critical stress, and strain decrease. Dense NiTi can the bio-mimetic structure of SLM-NiTi in the future. In
withstand 30.2% compression deformation and fails at addition to the SLM technology to prepare NiTi, Zhou
1620 MPa. The SC structure NiTi with a porosity of 58% et al. used selective EBM (SEBM) to produce NiTi
[39]
reached 410 MPa, and it failed after 15.6% compression and then analyzed the tensile compression properties of
deformation. The BCC structure NiTi with a porosity of the parts. The result shows that the SEBM-NiTi parts
69% reaches 63 MPa and fails after 10.5% compression exhibit excellent superelasticity in cyclic compression
deformation. It seems that changes in porosity and pore tests at room temperature. The authors also observed a
shape will affect the compressive strength of parts. significant asymmetry between tension and compression,
Bormann et al. [112] used synchrotron radiation-based closely related to the (001) texture. It is worth mentioning
micro-CT and three-dimensional positioning technology that the tensile properties of SEBM-NiTi parts are better
to evaluate the internal displacement and strain field than NiTi prepared by SLM and LENS, which may be a
of the SLM-NiTi scaffold during compression. It was potential process for the production of NiTi implants in
reported that 6% of the uniaxial compression resulted the future.
in up to 15% local compressive strain and tensile strain,
which verified the finite element simulation results. The 4.3. Tensile strength
3D data obtained by 6% compression in the z-direction Previous literature [105] has made a comprehensive review
are shown in Figure 21; the compression and extension of the compressive strength of SLM-NiTi, while there is
values are as high as 15%. The maximum compressive a minor focus on the tensile strength.
strain appears along the z-direction and is located around Due to the internal defects and the unidirectional
the opening parallel of the rhombus to the z-direction and columnar grains produced in the SLM process, the reported
at an obtuse angle outside the rhombus. This research SLM-NiTi often exhibits low tensile properties [115] . There
may help optimize the design of the support structure are two main reasons for the low tensile properties of SLM-
and estimate the maximum displacement before a crack NiTi. One is that almost all SLM parts have gas pores and
occurs. cracks [116] . The other is that when stretched perpendicular
The development of bio-inspired/bio-mimetic to the build direction, the multilayer columnar grains will
designs is a potential method for porous implants to promote the propagation of cracks on the flat continent [117] .
increase strength and reduce Young’s modulus . Zhou et al. [118] found that using a proper laser scanning
[20]
Ma et al. [114] inspired by the microstructure and length can prevent the formation of pores and cracks
compression characteristics of Cancer pagurus’s claw, by increasing the remelting/reheating temperature and
prepared the bio-mimetic claw structure SLM-NiTi reducing the cooling rate. Changing the laser scanning
(Figure 22), and analyzed the influence of rotation direction can inhibit the growth of unidirectional columnar
increment mode on compression performance. The crystals by changing the heat dissipation direction.
results indicate that a larger rotation increment will Xiong et al. [115] proposed a strip rotation scanning strategy
increase the torsional stiffness and tangential force of in which, by combining the best laser scanning length
the adjacent rail interface, resulting in more significant (stripe width of 4 mm) and laser scanning direction (hatch
cumulative deformation. Besides, the spiral distribution rotation of 67°), they produced a defect-free, unidirectional
mode helps increase the in-plane isotropy, which makes columnar crystal SLM-NiTi, as shown in Figure 23. The

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