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. 2018 Dec 26;12(1):77.
doi: 10.3390/ma12010077.

Effect of Heat Treatment on Repetitively Scanned SLM NiTi Shape Memory Alloy

Zhong Xun Khoo  1 , Jia An  2 , Chee Kai Chua  3 , Yu Fang Shen  4   5 , Che Nan Kuo  6   7 , Yong Liu  8
Affiliations

Effect of Heat Treatment on Repetitively Scanned SLM NiTi Shape Memory Alloy

Zhong Xun Khoo et al. Materials (Basel). .

Abstract

Selective Laser Melting (SLM) has been implemented to address the difficulties in manufacturing complex nickel titanium (NiTi) structures. However, the SLM production of NiTi is much more challenging than the fabrication of conventional metals. Other than the need to have a high density that leads to excellent mechanical properties, strict chemical compositional control is required as well for the SLM NiTi parts to exhibit desirable phase transformation characteristics. In addition, acquiring a high transformation strain from the produced specimens is another challenging task. In the prior research, a new approach-repetitive scanning-was implemented to achieve these objectives. The repetitively scanned samples demonstrated an average of 4.61% transformation strain when subjected to the tensile test. Nevertheless, there is still room for improvement as the conventionally-produced NiTi can exhibit a transformation strain of about 6%. Hence, post-process heat treatment was introduced to improve the shape memory properties of the samples. The results showed an improvement when the samples were heat treated at a temperature of 400 °C for a period of 5 min. The enhancement in the shape memory behavior of the repetitively scanned samples was mainly attributed to the formation of fine Ni₄Ti₃ metastable precipitates.

Keywords: 3D printing; 4D printing; NiTi; Selective Laser Melting; additive manufacturing; shape memory alloy.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Micrograph of the NiTi powder used.
Figure 2
Figure 2
Fifth cycle of the differential scanning calorimetry testing of the (a) NiTi powder, (b) NT, (c) H400, (d) H500, (e) H600, (f) H700 and (g) conventionally optimised single scanned samples.
Figure 3
Figure 3
XRD patterns of NiTi powder, NT, H400, H500, H600 and H700 samples.
Figure 4
Figure 4
Micrographs of (a) NT, (b) H400, (c) H500, (d) H600 and (e) H700 samples under 50 times magnification with their typical grain shape highlighted. Scale bar: 20 µm.
Figure 4
Figure 4
Micrographs of (a) NT, (b) H400, (c) H500, (d) H600 and (e) H700 samples under 50 times magnification with their typical grain shape highlighted. Scale bar: 20 µm.
Figure 5
Figure 5
Micrograph of the H400 sample with its typical grain shape highlighted and with the schematic of laser scanning strategy.
Figure 6
Figure 6
Schematic of the overall effects of heat treatment on the shape memory responses of repetitively scanned NiTi samples.
See this image and copyright information in PMC

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