**1. Introduction**

Titanium and titanium alloys are being widely used in several industrial applications because of their attractive properties, with a low density being one of them [1]. The most widely used α+β titanium alloy is Ti-6Al-4V (the workhorse grade in the titanium alloy group). This alloy finds extensive use in the medical industry and aerospace applications, due to its high specific strength, corrosion resistance, good fracture toughness, fatigue resistance, elevated temperature strength up to 500 ◦C, biocompatibility and weldability [1–4]. Nitinol, or titanium nickelide, belongs to the category of shape memory alloys, and it consists of an equiatomic alloy of nickel (Ni) and titanium (Ti). Nitinol is applied in the biomedical, aerospace, sensorics, fashion and automotive industries, as well as in structural elements and actuators, owing to its shape memory, biocompatibility and pseudoelasticity properties [5,6].

The joining of Ti-Al-4V to Nitinol would be of great interest for many applications, including the hybrid welded structure of an adaptive gas turbine engine's toothed nozzle [7]. A joint made of Ti-Al-4V joined to Nitinol adds the superelastic behavior of Nitinol to the excellent biocompatibility and corrosion resistance properties of these alloys. However, welding Ti-Al-4V to Nitinol is challenging because of differences in their chemical and physical properties [8–10]. The functional behavior of NiTi alloys is strongly influenced by the chemical compositions and heat input of welding processes. Fusion welding readily forms stable intermetallic compounds (Ti2Ni, Ni3Ti) when titanium alloy is welded to Nitinol. The migration of Ni from Nitinol to the liquid titanium leads to the formation of

**Citation:** Rehman, A.U.; Babu, N.K.; Talari, M.K.; Usmani, Y.S.; Al-Khalefah, H. Microstructure and Mechanical Properties of Dissimilar Friction Welding Ti-6Al-4V Alloy to Nitinol. *Metals* **2021**, *11*, 109. https://doi.org/10.3390/met11010109

Received: 9 December 2020 Accepted: 2 January 2021 Published: 7 January 2021

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Ti2Ni. These intermetallic compounds contribute to degradation of the mechanical properties of the welded joints [9–12]. These problems can be avoided by using an interlayer in between the Ti-Al-4V and Nitinol that inhibits or decreases the formation of intermetallic compounds in the weld zone [13].

NiTi was joined to another shape memory alloy, CuAlMn, by laser welding. A complex microstructure with islands of base metal deep inside the fusion zone were seen [14]. Datta et al. [15] did a feasibility study on the dissimilar joining of NiTi to Ti by laser welding. They reported that dissimilar welds exhibited poor strength and ductility due to the formation of Ti2Ni phases and transverse cracks in the weld [15]. Similar problems occurred in Ti-6Al-4V-NiTiNb welds, too [9,16]. Though intermetallics in general reduce weld ductility in dissimilar metal combinations, some types of intermetallics seem to be less harmful. In NiTi-Cu laser welds, a larger Cu dilution means a greater amount of Cu-based intermetallics in the weld metal, and these Cu-based intermetallics prove beneficial for the weld ductility when compared to Ti-based intermetallics [17,18].

The problem of brittle phases was overcome by placing an Nb interlayer with a thickness of 50 μm in between the Ti-6Al-4V and NiTi in laser welding [19]. The interlayer acted as a diffusion barrier between the two base metals. By varying the laser power and thereby controlling the Nb dilution, Zhou et al. could control the overall amount of intermetallics within the weld [20]. When the Nb melted completely, the joint strength shot up to as high as 82% of the Ti-6Al-4V base metal strength. Even electron beam welding produced similar effects [21].

Another possibility is to explore the solid state welding process to join Ti-Al-4V to Nitinol, in which the joining of materials takes place without melting and without the use of any filler. Solid state welding, like friction welding, can alleviate to some extent the fusion welding problems described above. Friction welding is a solid state welding process in which joining is achieved by one placing work piece in the rotating fixture and the other in a stationary fixture, and frictional heat is generated by the pressure and relative motion between these work pieces. Senkevich et al. [22] studied the joints of Ti-54.2% Ni with VT6 titanium alloy by the diffusion bonding technique. They observed a transition zone between the connected alloys, and this zone exhibited a higher hardness compared with the two base metals due to enrichment of the titanium. It was reported that a maximum shear strength of 170 MPa was achieved at 950 ◦C and a 30 min holding time. Wei Zhang et al. [23] investigated the microstructure and mechanical properties of Nitinol-Nitinol joints by ultrasonic welding, using Cu as an interlayer. They found that no intermetallic layer was observed at the joint interface. It was reported that the ultimate shear load increased with the increasing weld energy from 500 J to 1000 J due to metallurgical bonding at the interface.

There were quite a few successful attempts at joining Nitinol to Nitinol, whether it was through fusion welding [24,25] or solid-state welding [26,27]. However, when it came to joining Nitinol to Ti-Al-4V, the reports were few and far between, and even then, the welds turned out to be substantially poor in quality. All these published works dealt with fusion welding only [9–21]. This knowledge gap threw open the opportunity for us to attempt the same weld combination through a solid state joining technique like friction welding. Friction welding had already proved itself as a promising technique in several other dissimilar metal combinations. To the authors' best knowledge, this is one of the first attempts at joining Nitinol to Ti-Al-4V. This study demonstrates how, by choosing weld parameters carefully, we could obtain significantly strong welds between Nitinol and Ti-Al-4V. At the same time, some of the challenges that still need to be overcome before this promising combination of dissimilar metals could be used in wide-ranging applications have been identified.
