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,nickel titanium wire, 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,ti6al4v, 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 compounds at the interface, which can reduce the hardness of the interface microstructure and eliminate the transverse cracks near the Ti6Al4V side bond line. The tensile strength and elongation of Ni49.3Ti50.7 SMA/Cu/Ti6Al4V joint are 300 MPa and 3. 3%, respectively. In order to determine the effect of interlayer thickness on the microstructure and mechanical properties of Ni49.3Ti50.7 SMA/Cu/Ti6Al4V joint, Shojaei Zoeram et al. [13] used pure Cu foil with different thicknesses of 50 μm, 75 μm and 100 μm as the interlayer, and found that with the increase of Cu foil thickness, The content of Ni-Ti series intermetallic compounds in the fusion zone decreases due to the decrease of molten Ni content. However, the Cu element introduced into the weld results in the addition of Cu-Ti series intermetallics.When the thickness of the middle layer increases to 100 μm, the Cu-rich zone of the weld is prone to form uneven shrinkage porosity in the dendrites, which weakens the strength of the joint. The results show that the better mechanical properties can be obtained when the thickness of the interlayer is 75 μm, and the fracture of the joint occurs in the Ti-Cu intermetallic compound zone at the Ti6Al4V/Cu interface [12-13]. Chatterjee et al. [14] obtained a defect-free Ni52.49 Ti47.51 SMA/Cu/Ti6Al4V joint with a maximum tensile strength of 353 MPa by deflecting 60% of the laser spot to the Ti6Al4V side using a 100 μm-Cu foil, but pores appeared. The addition of higher melting point metals such as pure Ni and pure Nb wires for the interlayer was investigated. By manually adding pure Ni wire, Chen Yuhua et al. [15] obtained a defect-free and well-formed weld when the percentage of pulse laser power P = 18%, pulse frequency F = 3 Hz, and pulse width T = 3 ms, and the strength of the Ni50.6Ti49.4/Ni/Ti6Al4V joint reached 332 MPa. The brittle fracture occurs near the fusion line of NiTi side. Oliveira et al. [16] used Nb foil with a higher melting point as the intermediate layer. During the pulsed laser welding process, the laser spot deviated to the Ti6Al4V side. As shown in Figure 3A, the Nb foil did not melt and formed a barrier between Ni50.8 Ti49.2 SMA/Ti6Al4V to prevent Ni element from migrating and diffusing to the Ti6Al5V side. A Ni50.8 Ti49.2 SMA/Nb/Ti6Al4V joint was formed without defects and brittle intermetallic compounds in the interfacial fusion zone, as shown in Fig. 3B. (Ti, Nb) solid solution was formed at the interface of Ti6Al4V/Nb, and eutectic transformation occurred at the interface of Nb/Ni50. 8 Ti49. 2. The strength of the joint reached 300 MPa, which reached the theoretical strength of Nb foil (250 ~ 300 MPa), and the joint elongation was 2%. The brittle fracture of the joint occurs in the Ti6Al4V side fusion zone, which is caused by the absorption of oxygen or hydrogen by the liquid molten pool [15-16]. Chen Yuhua's team is committed to the research of NiTi SMA/Ti6Al4V dissimilar material welding, which involves not only binary NiTi SMA, but also ternary NiTiNb SMA with wide phase transformation hysteresis [7,15,17-22]. Like the welding of binary NiTi SMA/Ti6Al4V dissimilar materials, the welding of Ni47Ti44Nb9SMA/Ti6Al4V dissimilar materials also faces the same problems of brittle intermetallic compounds and welding crack control [17-21]. Chen Helu et al. [17, 21] analyzed the crack form of Ni47Ti44Nb9 SMA/Ti6Al4V laser joint weld in detail, and considered that the solidification crack runs through the whole weld along the center of the weld, the cooling zone forms a liquefaction crack, the reheating zone of the molten pool temperature is easy to produce hot cracks, and the molten pool zone produces crater cracks. It is considered that the cracks are formed by the tensile stress of the base metal on the weld during the cooling process of the brittle intermetallic compound Ti2Ni. To this end, the control of brittle intermetallic compounds in the weld is the key. Chen Yuhua's team [15, 18, ti6al4v eli ,ti6al4v eli, 20] tried to add Ti, Ni or Nb wires with a diameter of 300 μm as an intermediate layer between 200 μm thick Ni47Ti44Nb9 SMA/Ti6Al4V plates by adding Ti, Ni or Nb wires (see Figure 4A). The results show that the tensile strength of the joints obtained by adding Ni and Ti wires is not ideal, and it is difficult to meet the needs of engineering applications. Defect-free Ni47Ti44Nb9 SMA/Nb/Ti6Al4V joints can be obtained by adding Nb wire, and the highest strength is 740 MPa, which is 82% of the strength of Ti6Al4V base metal. The analysis shows that the melted Nb dilutes the Ni content in the weld, which reduces the Ti2Ni content in the fusion zone and effectively improves the mechanical properties of the joint. 2.3 Electron beam welding As another heat source with high energy density and equipped with vacuum chamber, electron beam has special advantages in welding active metals such as easily oxidized metals. Zhan Zilin [7] tried to solve the problem of Ni49.6 Ti50.4 SMA/Ti6Al4V dissimilar material welding by electron beam welding. The main contents include three parts: (1) direct butt welding; (2) adding intermediate layer Nb; (3) biasing the electron beam and changing the scanning form of the electron beam on the basis of adding intermediate layer Nb. In direct butt welding, a large number of cracks were produced on the weld surface by changing the welding parameters to control the heat input, and the hardness of the weld fusion zone was as high as 610 HV.When the electron beam deviated to NiTi or Ti6Al4V side for 0. 2 ~ 0.6 mm, the crack tendency of the weld was reduced, and with the increase of the offset, the size and number of cracks were reduced. The analysis shows that the offset electron beam can change the melting ratio of Ni49.6 Ti50.4 SMA/Ti6Al4V in the molten pool, improve the composition of the fusion zone, reduce the content of brittle intermetallic compounds, and reduce the hardness of the fusion zone to 259 HV. However, the interfacial hardness of Ni49.6 Ti50.4 SMA/Ti6Al4V fusion line is as high as 550 ~ 624 HV [7,22]. In order to further explore the feasibility of electron beam welding of Ni49.6 Ti50.4 SMA/Ti6Al4V dissimilar materials, Nb was added as an intermediate layer with a thickness of 0. 25 mm, 0.4 mm and 0. 55 mm, respectively.It was found that when the thickness of Nb layer was 0. 55 mm, defect-free joints could be obtained. The tensile strength of the joint is 328 MPa and the hardness is 497 HV near the fusion line of the cross section of the joint. The fracture is brittle fracture at the fusion line of Ti6Al4V side. Change the electron beam scanning mode to circular scanning wave, and bias the electron beam to the Ti6Al4V side, as shown in Figure 5A. When the thickness of Nb layer is 0.45 mm, a defect-free Ni49.6 Ti50.4 SMA/Nb/Ti6Al4V joint with a tensile strength of 520 MPa can be obtained, as shown in Figure 5B. The joint exhibits a mixed ductile-brittle fracture [7,22]. 2.4 Diffusion welding Simes et al. [6] used reaction-assisted diffusion welding to explore the weldability of NiTi SMA/Ti6Al4V dissimilar materials due to the advantage that the base metal does not melt during solid phase welding. Different from conventional diffusion bonding, reaction-assisted diffusion bonding is to deposit Ni/Ti nano-layers with the same atomic ratio of Ni/Ti and alternating phases on the butt surface of the base metal by DC magnetron sputtering to increase the diffusion coefficient of the interface during diffusion bonding, with a modulation period of 11 nm (one layer of Ni/Ti is one period) and a total thickness of 2.5 μm, as shown in Figure 6a. The results show that the interface of the joint at 750 ℃ can be divided into 8 layers, and the interface at 800 ℃ can be divided into 6 layers. From the side of Ti6Al4V, the first layer is composed of Ti2Ni and AlNi2Ti with coarse columnar grains, the second and fourth layers are Ti2Ni, and the third and fifth layers are composed of Ti2Ni and TiNi with fine equiaxed grains; The sixth layer is only 150 nm and is nanocrystalline, and the TiNi region contains TiNi austenite (B2) phase, martensite (B19 ′) phase and TiNi3. In order to further understand the process of NiTi/Ti6Al4V reaction-assisted diffusion welding, Cavaleiro et al. [23] observed the phase change in the process of interface diffusion by in-situ XRD, and found that the Ni/Ti nanolayers with modulation periods of 12 nm and 25 nm had no obvious phase change at 350 ℃, 400 ℃ and 450 ℃. When the temperature rises to 750 ℃, the diffraction peak of Ti2Ni appears at the interface, and the intensity of Ti2Ni diffraction peak increases when the temperature is kept at 750 ℃, which proves that Ti2Ni phase grows up at 750 ℃ and the content of β-Ti is also increasing. It is considered that Ti2Ni is produced by the diffusion of Ni/Ti to β-Ti or β-Ti to Ni/Ti in the reaction zone. In order to obtain better parameters such as diffusion temperature and thickness of Ni/Ti auxiliary reaction layer, 4 μm-Ni and 6 μm-Ti foils with a modulation period of 10 μm were added as the intermediate layer comparison group on the basis of the Ni/Ti layer modulation period of 12 nm and 25 nm, as shown in Figure 6B, and the diffusion temperature was subdivided into 600 ℃, 650 ℃ and 750 ℃. The results show that at 600 ℃, the Ni/Ti nano-interlayer with modulation period of 12 nm and 25 nm can be bonded by diffusion, while the Ni/Ti foil interlayer with modulation period of 10 μm can not be bonded. When the temperature rises to 650 ℃, the diffusion bonding can be achieved, as shown in Figure 7B [24]. It is proved that nanoscale Ti/Ni interlayer can reduce the diffusion temperature and welding time more than microscale Ni/Ti interlayer. The increase of diffusion temperature is more beneficial to the formation of Ti2Ni intermetallic compound by the diffusion of Ni element into the base material, and the thickness of Ti2Ni ribbon formed on the Ti6Al4V side by the interlayer with modulation period of 12 nm and 25 nm increases to 1. 2 μm and 1. 8 μm, respectively, which leads to the hardening of the joint interface. The nanoindentation hardness test of the joint interface shows that the multilayer Ni/Ti nanoscale interlayer with a diffusion temperature of 600 ℃ can obtain a better diffusion welded joint than that with a diffusion temperature of 650 ℃, and there is no obvious hardening phenomenon at the joint interface when the diffusion temperature is 600 ℃, as shown in Fig. 7c and 7d [24]. Therefore, the reaction-assisted diffusion bonding is a better method for NiTi/Ti6Al4V dissimilar materials welding. 2.5 Ultrasonic welding Ultrasonic welding is widely used in the field of polymer materials and metal foil welding. Because that kinetic energy of mechanical vibration with ultrasonic frequency can be indirectly convert into friction heat energy between workpieces to be weld under the assistance of the pressure of the Chuck, the surfaces of the workpieces to be weld are plasticized to realize connection, and an external heat source is not needed to melt local parts of the workpieces. Therefore, ultrasonic welding is an ideal welding method for the connection of sheet metal or polymer materials. Wang Litao [25] considered ultrasonic welding as a potential solution to the problem of overlapping of thin NiTi/Ti6Al4V dissimilar materials. The ultrasonic welding of 0. 2 mm thick NiTi/Ti6Al4V dissimilar materials was carried out by direct lap welding, and it was found that there was no metallurgical bonding in the joint, but a simple mechanical embedment, and the tensile shear force of the joint was 150 N. It was difficult to obtain a joint with higher mechanical properties by adjusting the process parameters. The Results show that the diffusion bonding of NiTi/Ni/Ti6Al4V joint with Ni or Al interlayer is obvious, the thickness of the diffusion layer is 2.5 μm, the maximum tensile shear force of the joint is 373 N, and the joint is brittle fracture. The thickness of NiTi/Al/Ti6Al4V interface diffusion layer obtained after the surface of Ti6Al4V base metal is infiltrated with pure Al layer is 4-5 μm, and the maximum tensile shear force of the joint is 930.8 N, showing a ductile-brittle mixed fracture [25]. / 3 Conclusion and prospect The research progress in the welding of NiTi and NiTiNb shape memory alloys (SMA) to Ti6Al4V titanium alloy in recent years was summarized. The effects of different welding methods on the microstructure and mechanical properties of NiTi SMA/Ti6Al4V joint were studied for the purpose of controlling the joint crack. The conclusions are as follows: (1) The weldability of NiTi shape memory alloy and Ti6Al4V titanium alloy was explored by brazing, laser welding, electron beam welding, diffusion welding and ultrasonic welding. (2) NiTi SMA/Ti6Al4V joint has strong crack sensitivity, and the weld is easy to crack when it is directly welded by fusion welding, so the suitable welding process window is narrow, and the mechanical properties of the joint are poor; brazing or diffusion welding can obtain lower strength of the welded joint. (3) The strength of the joint can be improved by adding Cu, Ni, Ti and Nb as the interlayer, and the tensile strength of the ternary Ni47Ti44Nb9 SMA/Nb/Ti6Al4V joint can be improved by adding Nb foil as the interlayer, reaching 740 MPa, which is 82% of that of the Ti6Al4V base metal. The mechanical properties of binary NiTi SMA/Ti6Al4V dissimilar metal welded joints need to be further improved, and there is still a lot of work to be done for the study of binary NiTi SMA/Ti6Al4V dissimilar metal welded joints. It is expected that solid phase welding, such as friction stir welding and rotary friction welding, can control the formation and mechanical properties of the joint by regulating the thickness (content) of the intermetallic compound formed at the interface due to low heat input and non-melting of the weld metal. References [1]Rao A, Srinivasa A R, Reddy J N. Design of Shape Memory Alloy (SMA) Actuators[M]. Germany: Springer-Verlag, 2015. [2] Oliveira J P, Miranda R M, Braz Fernandes F M. Welding and Joining of NiTi Shape Memory Alloys: A Review[J]. Progress in Materials Science, 2017(88): 412-466. [3] Mabe J. Variable area jet nozzle for noise reduction using shape memory alloy actuators[J]. Journal of the Acoustical Society of America, 2008, 123(5): 3871. 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Properties and interfacial microstructure of TiNi alloy/TC4 titanium alloy ultrasonic welding joint [D]. Jiangxi : Nanchang Hangkong University, 2018. Editor of this article: Tang Huang ❤ ❤ Return to Sohu to see more Responsible Editor: yunchtitanium.com