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Additively Manufactured NiTi Implants
aging treatment . In SMAs, the SME and superelasticity
[44]
are mutual processes. When the transformation
temperature is higher than the environment, SMA
exhibits the SME, and when environment temperatures
lower than the transformation temperature, the material
exhibits superelasticity . Therefore, the Ni/Ti ratio
[45]
must be selected according to the target characteristics
to ensure the particular mechanical behavior of the part
after fabrication. In terms of processing parameters, the
SLM parameters significantly influence the quality of the
final NiTi parts.
On the one hand, the NiTi parts produced must
be dense. Otherwise, it will seriously deteriorate the
mechanical properties, mainly related to the processing
parameters . On the other hand, the processing
[46]
parameters will affect the transformation temperature of
NiTi parts. To ensure the satisfactory behavior of the parts Figure 4. Transformation temperatures according to Ni contents
[41]
in practical applications, the transformation temperature (Lasers in Manufacturing and Materials Processing, Effects
must be controlled by controlling the processing of Composition and Post Heat Treatment on Shape Memory
parameters . In terms of gas atmosphere, an inert Characteristics and Mechanical Properties for Laser Direct
[47]
atmosphere such as argon and nitrogen or even vacuum Deposited Nitinol, 6, 2019, 41–58, J. Lee, Y. C. Shin. With
needs to be provided throughout the SLM process to permission from Springer).
minimize the oxidation and impurity absorption (such
as O and C) to improve the surface quality, density, and and the superelastic response of samples under different
2
similarity with conventionally processed NiTi samples . parameters. The results show that the laser fluence in the
[48]
3
For example, according to ASTM F2063-05, the impurity range of 63–160 J/mm can cause the highest relative
content of produced parts should be <500 ppm . density. Depending on the cooling rate and the size of
[49]
Farber et al. analyzed the influence of SLM the molten pool, the microstructure of the sample can
[48]
process parameters on the martensite transformation have different degrees of orientation. By heat treatment
temperature range. The authors believed that two main at 500°C, it can attain a fully recoverable strain up
issues are influencing the NiTi martensite transformation to 6%, which shows that the annealing treatment can
temperature during SLM. On the one hand, Ni evaporates promote the superelasticity of the sample. Yang et al.
[51]
during laser processing, and the matrix components explained that different scanning speeds affect the phase
move in the direction of high Ti content, which composition, leading to critical stress and mechanical
increases the martensite transformation temperature. recovery strain changes. Khoo et al. proposed a new
[52]
On the other hand, in Ni-rich NiTi, high temperature method – repetitive laser scanning. Figure 5 shows
may cause the formation of the Ni-rich phases. For the schematic of differences in laser absorptivity and
example, at 200–700°C, the order of precipitation in heat conductivity. The results show that the phase
NiTi is Ni4Ti3→Ni3Ti2→Ni3Ti+Ni4Ti3+Ni3Ti2. Due transformation characteristics of the sample are similar
to the precipitation phase generation, the Ni content in to NiTi powder, and it can withstand the tensile load up
the matrix decreases. As a consequence, the martensite to 5.11%, which is equivalent to that of traditional NiTi
transformation temperature increases . The lower the parts. It provides a new idea for manufacturing high-
[11]
martensite transformation temperature, the worse the performance SLM-NiTi parts.
[53]
shape memory performance. The heat treatment after SLM Saedi et al. used SLM to manufacture dense
can partially solve the problem of phase transformation and porous (32–58%) NiTi parts and the parts are then
temperature drift. Lee et al. summarized the NiTi heat-treated (solution annealing and aging). The results
[41]
transformation temperature range in different Ni content. show that SLM-NiTi with a porosity of up to 58% can
Figure 4 shows that when the Ni content is between 51 show the SME, and it can fully recover under 100 MPa
and 52 (% .at) the NiTi transformation temperature will nominal stress. The dense SLM-NiTi shows outstanding
change drastically. Therefore, the content of Ni must superelasticity after 6% deformation at body temperature,
be strictly controlled to obtain an ideal transformation with a recovery rate of 5.65%. The strain recovery of
temperature. samples with porosities of 32%, 45%, and 58% was
Biffi et al. investigated the SLM processability in about 3.5%, 3.6%, and 2.7%, respectively. The same
[50]
the production of fully dense NiTi parts, microstructure, researchers have studied the influence of SLM process
18 International Journal of Bioprinting (2021)–Volume 7, Issue 2