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Zhang, et al.
A B

Figure 27. (A) Schematic of successive series of cyclic loadings. (B) “n” can be sub-sectioned into three parts. The actual temperature that
includes actual fluctuations in parts I and II is shown by , while the mean temperature with no fluctuation in parts I and II is shown by [123]
(Reprinted from International Journal of Mechanical Sciences, 185, P. Bayati, A. Jahadakbar, M. Barati, et al., toward low and high cycle
fatigue behavior of SLM-fabricated NiTi: Considering the effect of build orientation and employing a self-heating approach, 105878,
Copyright (2020), with permission from Elsevier).

unit cells (octahedron, cellular gyroid, and sheet A B C
gyroid). The CAD images and their products are shown
in Figure 28. It seems that under the same volume
fraction, compared with the traditional octahedral unit
cell structure, triple periodic minimal surfaces (TPMS)
show excellent static mechanical properties and fatigue
life, and the lamellar cyclotron structure shows the
highest fatigue life. Both TPMS have continuously
varying curvatures, minimizing staircase effect, and
reducing crack initiation. Simultaneously, the residual
particles on the downward-facing surface act as stress
concentrators, allowing cracks to initiate [127] . For the
octahedral unit cell, the surface area in contact with the
powder bed during processing is the largest and more
residual particles are attached. The spiral design consists
of thicker struts at the same volume fraction, resulting
in a lower surface area. Based on the above factors, Figure 28. (A) Octahedron. (B) Cellular gyroid. (C) Sheet
the TPMS structure performs better in the practical gyroid [126] (Reprinted from Journal of the Mechanical Behavior of
application of SLM-NiTi. Biomedical Materials, 70, M. Speirs, H. B. Van, H. J. Van, et al.,
fatigue behavior of NiTi shape memory alloy scaffolds produced
4.5. Damping properties by SLM, a unit cell design comparison, 53–59, Copyright (2017),
with permission from Elsevier).
The damping capacity is the ability to eliminate sudden
shocks and oscillations [128] . It is a very critical feature in
various applications, including biomedical equipment the internal friction (tan δ) measured at an oscillation
(such as dental and spinal implants) and the automotive frequency of 1 Hz (a) and the temperature dependence of
industry (dampers) . Wang et al. used two sets of tan δ and Young’s modulus at an oscillation frequency of
[9]
[6]
SLM parameters to generate layered NiTi samples, 90 kHz (b), indicating that even without the influence of
in which alternate layers have different Ni/Ti ratios, transient effects, the layered structure samples also exhibit
so they have different transformation temperatures, good damping characteristics. Table 2 summarizes the
which lead to austenite/martensite alternating structure recent researches related to the mechanical properties of
in a specific temperature range. During the cooling SLM-NiTi.
process in a wide temperature range (~130 K), austenite After design, AM, surface modification [66,129] ,
gradually transforms into martensite, thereby obtaining drug loading [130] , and a series of characterizations of
better damping performance at both low (1 Hz) and AM-NiTi implants are required, which are summarized
high (90 kHz) oscillation frequencies. Figure 29 shows in Figure 30.

International Journal of Bioprinting (2021)–Volume 7, Issue 2 33