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Zhang, et al.
a large number of inner defects (mainly gas pores), the Zhang et al. [124] manufactured SLM-NiTi with
parts still have good ductility under the condition of large excellent tensile properties and recovery rate through
changes in SLM parameters. It is due to the inherent unique stripe rotation scanning strategy, as shown in Figure 25.
deformation behavior of NiTi, that is, stress-induced The part shows a tensile strain of 15.2 ± 0.8%, which is
martensite transformation or martensite reorientation almost twice the best value previously reported. It also
occurs before plastic deformation. Simultaneously, under exhibited excellent shape memory performance. After
high energy density, the defects are mainly spherical pre-deformation of 4% and 6%, the recovery rate was 97.7
pores, which effectively reduces stress concentration [121] . ± 1.2% and 92.5 ± 2.0%, respectively. Unlike traditional
The build orientation is an essential factor in the post-melting machining methods [125] , the SLM process
SLM process, affecting the mechanical strength and involves rapid solidification and repeated heating, which
fatigue properties of parts [122] . Bayati et al. [123] analyzed the gives SLM samples a unique microstructure [124] . Rapid
uniaxial tensile strength of NiTi parts manufactured in three solidification forms supersaturated vacancies, and the
different build orientations, as shown in Figure 24. The aggregation of vacancies forms high-density dislocations.
results showed that the samples made at 45° relative to the In the subsequent heating process, these dislocations
build plate exhibit the highest ultimate strength (fractured thermally move in the three directions <001>, <111>,
at ~600 MPa), while the ultimate strength of the edge and and <110>, resulting in thermal kinks, helical dislocation,
the flat specimen is relatively low (about 350–400 MPa). and wave morphology. As Figure 26 shows, while the
Through SEM observations, different structural directions dislocations are moving, the precipitated Ni Ti particles
4
3
may produce different types of defects in the part, such repeatedly nucleate and grow heterogeneously. This
as lack of fusion, entrapped gas pores, and surface flaws, repeated precipitation phenomenon does not exist in
leading to different failure modes.

A B

Figure 23. (A) The stripe rotation scanning strategy. (B) SLM-NiTi samples [115] (Reprinted from Journal of Material Science & Technology,
35(10), Z. Xiong, Z. Li, Z. Sun, et al., selective laser melting of NiTi alloy with superior tensile property and shape memory effect, 2238–
2242, Copyright (2019), with permission from Elsevier).

Figure 24. Three build orientations of SLM process [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).
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