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Additively Manufactured NiTi Implants
300 MPa. According to the SEM and XRD results, the played an essential role in this field. For AM-NiTi,
microstructure gradient characterizes by increasing the whether its compressive strength will be affected due to
martensite B19’ along the grain direction. In the gradient the AM process parameters is worthy of attention.
region produced by higher repetitive laser power, there Dadbakhsh et al. [111] used two groups of laser
forms more martensite B19’ and less B2 phase. When the parameters named high parameters (HP) and low
applied stress exceeds the elastic limit, the gradient region parameters (LP) to produce octahedral porous SLM-NiTi
with more B19’ phase will be deformed preferentially scaffolds and analyzed their compression performance.
due to the reorientation of the pre-existing B19’ twin. Compared to the LP sample, the compressive strength
Since the reorientation is irreversible after the stress of the HP sample was almost 4 times greater than that
is released [109] , the loading and unloading paths almost of the LP sample. On the one hand, because the HP
overlap (0.5%< strain <1.5%). As the applied stress sample has a higher substantial volume fraction in the
increases, more B2 phases include, and the gradient loading direction (Figure 19), the bending of the pillar
region with lower laser power is repeated and then is suppressed. On the other hand, the higher cooling
deformed by stress-induced deformation of martensite rate of the HP parameters produces finer grains, which
B19’. This combination of multiple deformation leads to a higher overall strength. Biffi et al. realized
[50]
mechanisms increases mechanically recoverable strain the integration of multiple laser parameters by defining
and an excellent strain hardening effect [108] . fluence. The authors performed loading/unloading
4.2. Compressive strength compression tests on SLM-NiTi samples prepared
with a fluence of 63 and 160 J/mm , respectively. The
3
The main load of orthopedic implants during service in trends of strain recovery and applied deformation are
the body is cyclic compressive stress [110] . Therefore, the shown in Figure 20. When the applied strain is small,
compressive strength of the implant material has always the lower fluence can obtain higher superelasticity (the
stress is almost twice that of the higher fluence when the
A deformation is 3%). However, when the applied strain
exceeds 6%, the increase of fluence will bring better
superelasticity. Due to the accumulation of irreversible
plastic deformation, the recovery deformation curve of
low fluence has a plateau, while the recovery deformation
curve of high fluence is almost linear.
Andani et al. [113] prepared dense and porous
SLM-NiTi, conducted a routine test, and analyzed its
B compressive strength. They found that both the dense and
porous SLM-NiTi have a good SME; the compressible

C

Figure 18. (A) The first laser scan with a constant laser power of
60 W. (B) The repetitive laser scan with the varied laser power
from 5 W to 95 W. (C) Two typical graded NiTi specimens [108] Figure 19. Optical microscope images of the scaffolds [111]
(Reprinted from Scripta Materialia, 188, Y. Yang, J. B. Zhan, J. B. (Reprinted from CIRP Annals - Manufacturing Technology, 64(1),
Sui, et al., functionally graded NiTi alloy with exceptional strain- S. Dadbakhsh, M. Speirs, J. P. Kruth, et al., Influence of SLM on
hardening effect fabricated by SLM method, 130–134, Copyright shape memory and compression behavior of NiTi scaffolds, 209–
(2020), with permission from Elsevier). 212, Copyright (2015), with permission from Elsevier).

28 International Journal of Bioprinting (2021)–Volume 7, Issue 2