**1. Introduction**

SLM is the most common metal additive manufacturing technology. In this technology, a high energy density laser is utilized as a heat source. The laser spot is concentrated in a very small range to selectively melt the spherical metal powder on the powder bed, by which parts with quite complex shapes can be obtained [1]. It is widely used in aerospace and medical fields because of high forming accuracy [2–4]. However, SLM is mainly used to manufacture single metal parts.

SLM of single material has been unable to meet the performance requirements of people with the continuous development of its commercial application. Therefore, the research of SLM of two or even multiple materials has appeared in recent years. Chen et al. [5] studied the forming of CuSn10/316 L dissimilar materials and found that the generation of interfacial defects was relevant to process parameters of the interface. The defects near the interface were the main factors affecting the bonding strength. Finally, dissimilar materials' tensile parts with tensile strength close to that of CuSn10 single alloy tensile part were formed successfully. Liu et al. [6] formed 18Ni300 maraging steel on traditional machined Cr8Mo2SiV cold work die steel by SLM. They found that a toothed mosaic structure could be formed at the interfacial area, which could promote interfacial bonding. Tan et al. [7] studied the manufacturing of maraging steel/copper dissimilar materials by the combination of selective laser melting and subtractive process. They found that the improvement of interfacial bonding performance was related to the intense Marangoni flows at the interface. This kind of manufacturing can combine the advantages of multiple metal materials, and therefore is very promising.

Shape memory alloy (SMA) refers to the alloy with SME due to the reversibility of phase transition [8], among which NiTi alloy is the most widely used. It has been applied

**Citation:** Song, C.; Hu, Z.; Xiao, Y.; Li, Y.; Yang, Y. Study on Interfacial Bonding Properties of NiTi/CuSn10 Dissimilar Materials by Selective Laser Melting. *Micromachines* **2022**, *13*, 494. https://doi.org/10.3390/ mi13040494

Academic Editors: Jie Yin, Yang Liu and Ping Zhao

Received: 18 February 2022 Accepted: 20 March 2022 Published: 22 March 2022

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in sensors, actuators, medical devices, and many other fields [9,10]. The integrated forming of NiTi alloy and other materials can combine the functionality of NiTi with the excellent properties of other materials, thus having broad application prospects. Some studies on the combination of NiTi alloy with other alloys have been conducted in recent years. Li et al. [11] studied the laser welding process of NiTi SMA wire and stainless-steel wire using nickel as the interlayer. It found that appropriate Ni interlayer thickness could reduce the brittle intermetallic compounds and pores, thus improving the bonding properties. Gao et al. [12] proposed a laser welding method using Nb/Cu multi-interlayer. Through a single pass welding, a hybrid joint composed of three metallurgical bonding zones was obtained according to different welding mechanisms. This completely prevented the formation of Ti-Fe and TixNi<sup>y</sup> intermetallic compounds, and thus significantly improved the mechanical properties of the joint. Panton et al. [13] studied the laser welding of Nitinol wire and MP35N wire and found that the position of the laser beam controlled the composition of the molten pool, while the intensity of the laser beam affected the size and mixing of the molten pool. These variables greatly influenced the hardness and cracking sensitivity of the interfacial region, and thus seriously affected the bonding strength. At present, one of the research hotspots of NiTi alloy-related dissimilar materials is the combination of Ti6Al4V and NiTi alloy. Because it can be used in biomedical and aerospace fields, thus it is quite promising. The main problem of NiTi bonding with Ti6Al4V is that brittle intermetallic compounds can be easily formed during the process, which affects the bonding properties. Oliveira et al. [14] used niobium as an interlayer to prevent the generation of brittle phase during laser welding of NiTi and Ti6Al4V. The melting temperature of the niobium interlayer is much higher than that of the matrix material. As a result the bulk niobium did not melt during the joining process, which played a role in the diffusion barrier between these two materials. It ensured that no cracks in the welds and brittle intermetallic compounds were discovered. Zoeram et al. [15] utilized thin copper as an interlayer to conduct laser welding of NiTi and Ti6Al4V. It found that selecting an appropriate interlayer thickness could inhibit the formation of Ti-Cu intermetallic compounds and shrinkage cavities, thus improving the interfacial bonding properties. Xie et al. [16] studied the ultrasonic spot welding of Ti6Al4V/NiTi dissimilar joints with Al coating as an interlayer and found that the firm bonding of Ti6Al4V/NiTi dissimilar materials can be realized by selecting appropriate welding pressure. Bartolomeu et al. [17] fabricated NiTi-Ti6Al4V multi-material cellular structures with a Ti6Al4V inner region and a NiTi outer region by SLM. The bonding between NiTi and Ti6Al4V in the transition region was declared to be successful by morphology analysis. There were also some studies on other Ni-based alloys. Tabaie et al. [18–20] studied joining SLM Inconel 718 with forged AD730 TM Nickel-based superalloy using linear friction welding (LFW). Joints free of micro-porosity, micro-cracks, and oxides were obtained. It was found that the strength of AD730 TM alloy depended significantly on the grain size while that of SLM Inconel 718 was dominated by shape (or size) and the presence of secondary phases (γ 0/ γ" and Laves). However, both the size and the volume fraction of all phases affected the hardness and mechanical properties of the joint. After welding, a new post-weld heat treatment cycle was designed, resulting in uniform hardness across the dissimilar joint. It can be found from the above research that welding is a quite commonly used technology for the combination of NiTi alloy and other Ni-based alloys related dissimilar materials, in which an interlayer is often applied. SLM technology is seldom used for the combination. This is partly due to the forming of the NiTi alloy by SLM is still immature [21]. Conversely, the studies about the combination of two kinds of materials using SLM are still in their nascence; there are still many mechanisms that need to be researched.

CuSn10 is a type of tin bronze with good ductility, electrical conductivity, and thermal conductivity. NiTi/CuSn10 dissimilar materials parts can combine the excellent shape memory performance of NiTi alloy with the advantages of CuSn10, which has a wide application prospect. The technical experts of NASA are developing a sort of smart radiator that controls heat-loss rate flexibly by deforming. The thermosensitive materials can be utilized to realize the change of shape. When the thermosensitive material experiences the temperature change, the radiator can automatically change its shape to dissipate heat or store heat. This expectation can be achieved by using NiTi alloy to achieve the shape change and using CuSn10 alloy to dissipate heat. As for electrical conductivity, an intelligent fuse can be realized based on the mobile deformation of NiTi alloy and the high electrical conductivity of CuSn10 alloy, which can control the circuit on and off in real-time, according to the temperature to achieve protection. There are also many other possibilities. However, there are few studies on the bonding between NiTi alloy and CuSn10 alloy. This is because the interface is a part of the dissimilar materials parts at which defects are most likely to occur and a significant factor determining the reliability of the whole part. The bonding properties of the interface are mainly affected by the differences between the physical and chemical properties of the two materials, which were found to be large [6,22,23]. In this study, the effects of adding CuSn10 interlayers with different process parameters between NiTi alloy matrix and CuSn10 alloy by SLM on the defects, microstructure, phases, and properties at the interface were investigated, providing a theoretical basis for forming NiTi/CuSn10 dissimilar materials structural parts. The results showed that appropriate Ni4Ti<sup>3</sup> content and columnar grain at the interface can be realized by selecting appropriate parameters, the combination of which can improve the interfacial bonding properties. Refined grains in the interfacial region can also play a strengthening role.
