**1. Introduction**

NiTi alloy, with a nearly equal atomic ratio, has the most excellent performance, which is the most common application among many known shape memory alloys. It has many advantages, such as superelasticity, the shape memory effect, fatigue resistance, a low elastic modulus, and biocompatibility [1–4]. As we know, the most widely used commercial methods for producing NiTi alloy parts are vacuum arc melting and vacuum induction melting, followed by hot working or cold working. It is likely that casting would lead to segregation defects [5]. Meanwhile, vacuum induction melting has the drawback of crucible contamination [6].However, when NiTi components undergo cold working, they encounter springback and make the overall dimension difficult to be shaped, hindering the application of nickel-titanium alloys.Additive manufacturing (AM), which has emerged in the past decade, is a new technology that can realize the integrated molding of complex components, and a lot of research results have been obtained [7–10]. Among them, selective laser melting (SLM) technology [11–14] and laser-directed energy deposition (LDED) technology [15–17] are typical metal additive manufacturing technologies, which could achievethe rapid formation of metal parts.

In the recent years, laser additive manufacturing has become an ideal preparation method for complex NiTi alloy components, and related technologies have developed rapidly, especially SLM technology [18–21].Dadbakhsh et al. [22] showed that SLM parameters have a great influence on the phase transition temperature and mechanical response of dense porous NiTi alloys. Meier et al. [23] studied the effect of deposition orientation on the compressive properties of SLM Ti-rich Ni50.2Ti49.8 (at.%) and found that crystal orientation

**Citation:** Yang, X.; Wang, S.; Pan, H.; Zhang, C.; Chen, J.; Zhang, X.; Gao, L. Microstructure Transformation in Laser Additive Manufactured NiTi Alloy with Quasi-In-Situ Compression. *Micromachines* **2022**, *13*, 1642. https://doi.org/10.3390/ mi13101642

Academic Editors: Jie Yin, Yang Liu and Ping Zhao

Received: 18 August 2022 Accepted: 19 September 2022 Published: 30 September 2022

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had no significant effect on the compressive properties of these samples. Yang et al. [24] investigated the additivemanufacturing process of gradient NiTi alloys and obtained the gradient martensite phase by adjusting the process parameters. Bormann et al. [25] focused on the microstructure and texture of NiTi alloys fabricated by SLM and considered the effects of different processing parameters and the scanning rate. Haberland et al. [26] researched the superelasticity and cyclic response of NiTi alloys fabricated by SLM. Lu et al. [27] learned the simultaneous improvement of mechanical properties and shape memory properties by proper precipitation of the Ti2Niphase and its mechanism of action. It can be found that a lot of research has been carried out on NiTi alloys with SLM technology, from process parameters and structure to properties.

Compared toSLM technology, LDED technology has lower cost, faster formation, and the ability to form large components and has received extensive attention and research in the industry. Bimber et al. [28] studied the spatial anisotropy of NiTi alloys prepared by LDED and explored the differences of microstructures with different deposition heights. Wan et al. [29] studied the phase transformationbehavior, microstructure evolution, and elastocaloric properties of NiTi alloys prepared by LDED. It is confirmed that NiTi alloys prepared by the LDED process haveexcellent properties and arenot inferior to as-cast NiTi alloys. Research on the compressive properties of NiTi alloys prepared by AM technology has also been reported recently, but the research results are relatively few. Marattukalam [30] prepared NiTi alloys by means of LDED technology; 8% strain was recovered under a pre-compression of 10%, and the recovery rate was further improved after heat treatment. Andani et al. [31] fabricated and designed NiTi shape memory alloys with porous forms by selective laser melting, which exhibited a good shape memory effect, with a recoverable strain of about 5% and functional stability after eight cycles of compression. In addition, the stiffness and residual plastic strain of porous NiTi alloys were found to depend highly on the pore shape and the level of porosity. Moghaddam et al. [32] fabricated Ni-rich NiTi components by using the additive manufacturing process with 250 W laser power, 1250 mm/s scanning speed, 80 µm hatch spacing and without any post-process heat treatments. The results showed superelasticity with 5.62% strain recovery and a 98% recovery ratio.Chen et al. [33] successfully fabricated uniform and graded gyroid cellular structures of NiTi alloys by laser powder bed fusion additive manufacturing. The results showed that all laser powder bed fusion NiTi structures exhibited similar nominal compressive elastic moduli (5–7 GPa) ashuman bones. The resultsof Zhang et al. [34] showed that the energy input balances against the energy output during cyclic loading of porous NiTi alloys after performing several compression cycles as 'training' and the porous NiTi alloys exhibit reliable linear superelasticity and a stable elastic modulus, with strain as high as 4%.

Most of the research on NiTi alloys with AM technology has mainly focused on their functional properties and process parameters. However, there is little systematic in-depth study on the microstructure transformation of the material after compression, especially the microstructure transformation and recovery ability after compression under the LDED process. In this paper, an in-situ compression test was carried out on LDED NiTi samples, and the microstructure transformation and recovery ability of shape memory alloyswere researched in detail.
