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Original Title: [Heavy Paper] Journal of Electric Welding Machine, Volume 50, Issue 400 | Research Progress of NiTi Shape Memory Alloy/Titanium Alloy Dissimilar Material Welding Citation format: Chen Yuhua, Deng Huaibo, Xu Mingfang, et al. Nickel-titanium shape memory alloy Research progress of dissimilar material welding of/titanium alloy [J]. Electric welding machine , 2020, 50(9): 177-185. Authors: Chen Yuhua, Deng Huaibo, Xu Mingfang, Ji Di (Jiangxi Key Laboratory of Aeronautical Component Forming and Connection, School of Aeronautical Manufacturing Engineering, Nanchang Aeronautical University) Abstract: NiTi shape memory alloy (Nitinol shape memory alloy, NiTi SMA) has excellent shape memory effect (Shape memory effect, SME) and super-elastic (SE). It has been widely used in aerospace, artificial intelligence and biomedical fields. The welding of NiTi SMA/Ti6Al4V dissimilar materials has attracted much attention in order to control the cost, obtain multi-functional complex components or develop new products. Aiming at the key problems in the welding process of NiTi SMA/Ti6Al4V dissimilar materials, the research progress of fusion welding, diffusion welding and brazing were summarized, and the effects of different welding methods on the welding results were compared. It is considered that the key factor affecting the mechanical properties of NiTi SMA/Ti6Al4V dissimilar material joint is to inhibit the formation of brittle intermetallic compounds at the interface, and the addition of intermediate layer is beneficial to reduce the formation of brittle intermetallic compounds. Key words: shape memory alloy; titanium alloy; microstructure; mechanical properties / Ti6Al4V alloy is widely used in aerospace, biomedical, shipbuilding, nuclear industry, chemical industry, sports equipment and other fields because of its good comprehensive mechanical properties, such as high specific strength, good corrosion resistance and good biocompatibility; NiTi series shape memory alloy, because of its excellent shape memory effect and recoverable deformation up to 8%, has become the most widely used shape memory alloy, and is considered to be one of the potential intelligent materials in the new century. Therefore, based on their excellent mechanical properties and functional characteristics, researchers are committed to designing and manufacturing components or products with potential and competitive advantages [2]. Mechanical fasteners of NiTi SMA in combination with Ti6Al4V are promising for turbine gas nozzles [3]. For civil use, the combination of NiTi SMA and Ti6Al4V materials can better improve the performance of golf clubs, titanium alloy with high specific strength and excellent corrosion resistance has become the best choice for golf club materials, and the use of NiTi SMA with high damping and superelasticity to prepare rackets can achieve higher and farther hitting effect under the same hitting force. Reduce energy loss [4]. However, the physical and chemical properties of Ti6Al4V and NiTi SMA are not matched, which makes the welding of NiTi SMA/Ti6Al4V dissimilar materials a challenge, and then leads to the difficulty of large-scale application of NiTi SMA/Ti6A14V dissimilar material components. Hinder the design and development of NiTi SMA/Ti6Al4V dissimilar material components. Therefore, researchers are committed to solving the problem of NiTi SMA and Ti6Al4V dissimilar material welding. Based on the existing research results, the welding method, the microstructure and mechanical properties of the joint, and the existing problems were described, and the possible methods and possibilities to solve the problem in the future were prospected. / 1 Existing problems The difficulty of welding is an inevitable and urgent problem in the manufacturing of NiTi SMA/Ti6Al4V dissimilar material composite components. NiTi SMA/Ti6Al4V joints were obtained by brazing, diffusion welding, laser welding and electron beam welding. It was found that the Ti2Ni brittle intermetallic compound at the interface of NiTi SMA/Ti6Al4V not only affects the weld formation, but also seriously deteriorates the mechanical properties of the joint. It can be seen that NiTi SMA/Ti6Al4V dissimilar material welding has the following problems: (1) adjust and controlling that intermetallic compound at the interface of the NiTi SMA/Ti6Al4V dissimilar material weld joint. Expand the full text And (2) the mechanical property of the NiTi SMA/Ti6Al4V dissimilar material welding joint is improved. (3) Explore the better welding method and structure of NiTi SMA/Ti6Al4V dissimilar materials. / 2 Crack control method NiTi SMA and Ti6Al4V alloy are easy to be oxidized, so they should be welded in protective atmosphere. Therefore, the current welding methods of NiTi SMA/Ti6Al4V dissimilar materials mainly include brazing, diffusion welding, laser welding and electron beam welding [2,5-7]. 2.1 Brazing In 2005, Shiue et al. [5] considered that the welding of NiTi shape memory alloy and Ti6Al4V alloy had important application potential, and used infrared brazing to weld Ni50Ti50 SMA/Ti6Al4V dissimilar materials with a size of 10 mm × 10 mm × 2.5 mm. BAg-8 (71% ~ 73% Ag/Cu at 780 ℃) with thickness of 50 μm was used as the filler metal. As a result, it was found that Ag in the brazing filler metal did not react with the base metal on both sides, but formed a hypoeutectic structure with Cu and gathered in the middle of the brazing zone, as shown in fig. 1A. At the same time, strong metallurgical reactions occur at the interface of Ni50Ti50 SMA/BAg-8 and BAg-8/Ti6Al4V, and the reaction at the interface of BAg-8/Ti6Al4V is more intense, resulting in a large number of intermetallic compounds. The phase distribution formed at the Ni50Ti50 SMA/BAg-8/Ti6Al4V interface at different brazing temperatures and holding times is shown in Table 1. Droplet test at 1 000 ℃ shows that the wettability of BAg-8 filler metal with Ni50Ti50 SMA is poor, while the wettability of BAg-8 filler metal with Ti6Al4V is good. Ti element in Ti6Al4V dissolved in the liquid filler metal can activate the activity of the filler metal and enhance the wettability between the filler metal and Ni50Ti50 SMA, thus achieving brazing. The average shear strengths of Ni50Ti50 SMA/BAg-8/Ti6Al4V joints obtained at 800 ℃ and 850 ℃ + 180 s were 206 MPa and 192 MPa, respectively, and the joints failed at the CuNiTi layer at the Ni50Ti50 SMA/BAg-8 interface, as shown in Fig. 1b. Therefore, it is considered that the CuNiTi phase weakens the Ni50Ti50 SMA/BAg-8/Ti6Al4V brazing joint. When the brazing temperature is equal to or higher than 900 ℃, the liquid brazing filler metal will seriously erode the Ti6Al4V base metal, and when the brazing temperature is 900 ℃ and the holding time is 60 s or more, the Ti2Ni brittle intermetallic compound will be produced at the interface [5]. In 2013, Quintino et al. [8] used silver nano-paste and silver foil as brazing filler metal to weld NiTi/Ti6Al4V with a thickness of 1 mm and a lap size of 10 mm × 3 mm, and found that the brazing filler metal could not bond with the base metal at room temperature without pressure or at room temperature with a pressure of 200 N and a holding time of 60 min; When the Nd-YAG laser is added to melt the solder,titanium seamless tube, the bonding between the solder and the base metal is very poor, and the diffusion bonding between the solder and the base metal does not occur. When brazing NiTi/Ti6Al4V dissimilar materials, the wettability between the filler metal and the base metal should be considered, and intermetallic compounds are easily produced in the brazing zone, which not only affects the corrosion resistance of the joint, but also weakens the mechanical properties of the joint. Therefore, the NiTi/Ti6Al4V brazed joint is suitable for the service condition without requirements for the mechanical properties and corrosion resistance of the joint. 2.2 Laser welding Laser is a kind of high energy density heat source, which can be used to weld NiTi/Ti6Al4V dissimilar materials to obtain small weld pool size, large penetration ratio and easy to add protective atmosphere. Researchers have mainly tried two ways: (1) direct welding of NiTi/Ti6Al4V dissimilar materials; (2) controlling the metal composition of the molten pool, including adding an interlayer and biasing the laser beam. First of all, there are two types of joints for direct welding: butt joint and lap joint. Quintino et al. [9] used Nd: YAG pulsed laser to try to butt the cold-rolled Ni50.8 Ti49.2 SMA/Ti6Al4V dissimilar materials with a thickness of 1mm. After optimizing the welding parameters, there were still cracks in the Ti6Al4V side fusion line of the weld cross section, and there were a large number of Ti2Ni intermetallic compounds in the fusion zone. In order to improve this phenomenon, the laser spot was deviated from the NiTi/Ti6Al4V interface to the Ti6Al4V side by 0.2 mm, and the interface gap between the two plates was adjusted to 1 mm to obtain a defect-free Ni50.8 Ti49.2 SMA/Ti6A14V joint, but the mechanical properties of the joint were not given [9]. It is considered that the effect of cooling rate on the formation of intermetallic compounds at the joint interface, especially Ni3Ti and Ti2Ni, should be considered in laser welding of Ni50.8 Ti49.2 SMA/Ti6Al4V dissimilar materials. It is believed that the intermetallic compound formed on the Ti6Al4V side is formed by the migration and diffusion of Ni element into β-Ti [9]. In order to realize the adaptive control of the cooling rate of the joint after welding by controlling the heat input, Miranda et al. [10] used a high-energy fiber laser to control the heat input at 360 ~ 559 J/cm by changing the laser power and welding speed, and explored the butt joint of Ni50.8 Ti49.2 SMA/Ti6Al4V dissimilar materials with a thickness of 1 mm. The results show that the weld cracks are serious during the cooling process. When the welding speed is 16.7 mm/min and the laser power is 900 W, the joint does not crack due to incomplete penetration. Increasing the laser power to 1 100 W can eliminate the incomplete penetration, but there are cracks between the heat affected zone and the fusion zone on the Ti6Al4V side, as shown in Figure 2a; There are two failure modes of the joint: transgranular fracture along the brittle zone of Ti2Ni and ductile fracture along the solidified dendrite. In order to further control the formation of brittle intermetallic compounds in the fusion zone to obtain a defect-free joint: (1) an intermediate layer can be added as a barrier to prevent the migration and diffusion of Ni element; (2) the Ti6Al4V side is biased by laser to reduce the melting of Ni and inhibit the formation of brittle intermetallic compounds [10]. Lap joint is a common welding joint form. Song et al. [11] used fiber laser to weld 1.2 mm thick Ni51Ti49 and 1.5 mm thick Ti6Al4V dissimilar materials through Ni51Ti49 in the upper and lower forms, and found that cracks occurred in both joints,Titanium welding pipe, which was considered to be caused by residual stress. The crack types are divided into transverse crack and central crack, and the weld surface is shown in Fig. 2b. The analysis shows that the Gibbs free energy for generating Ti2Ni is -59.23 kJ/mol < 0, which is less than the Gibbs free energy for generating NiTi, so the molten pool is more likely to generate Ti2Ni intermetallic compound. Secondly, when direct welding of NiTi/Ti6Al4V dissimilar materials can not prevent the formation of intermetallic compounds in the weld, controlling the composition of the weld pool to achieve reliable bonding of NiTi/Ti6Al4V dissimilar materials is another solution. Under the welding parameters of 450 V, 0.5 mm defocus distance, 2 ms pulse interval, 7.7 Hz frequency, 1.5 mm/s welding speed, 0.4 mm spot diameter, 100 W average laser power and unbiased spot, Shojaei Zoeram et al. [12] compared the Ni49.3Ti50.7 SMA/Cu/Ti6Al4V joint with the Ni49.3Ti50.7 SMA/Ti6Al4V joint obtained by double-sided welding and adding 75 μm pure Cu as an interlayer. The Results show that the addition of 75 μm pure Cu interlayer can reduce the content of Ti2Ni intermetallic compounds and the formation of Cu-Ti series intermetallic co