## 1 Introduction  
Nitinol (equitatomic alloy of nickel (Ni) and titanium (Ti)) alloys are widely used in aerospace, automobile, biomedical, actuators and robotics applications owing to their pseudoelasticity properties, shape memory, excellent corrosion resistance and biocompatibility [1, 2]. Titanium and titanium alloys are broadly used in aerospace, marine, chemical, military and medical applications owing to their high strength to weight ratio, excellent corrosion resistance, good fatigue properties and good toughness. \(\alpha\)+\(\beta\) titanium alloys are increasing applications in the aerospace and medical fields due to their good corrosion resistance, high specific strength, excellent fracture toughness, and weldability [3, 4, 5, 6]. Several aerospace applications, such as a variable area fan nozzle, require Nitinol to be joined with other predominant aerospace alloys such as Ti-6 Al-4V. The welding of Ti and Nitinol alloys is problematic because it readily picks up interstitial elements such as hydrogen, oxygen and nitrogen from the atmospheric gases at high temperatures. An increase in hydrogen, oxygen and nitrogen in the weld metal results in increased strength at the expense of toughness. The melting and solidification of titanium alloys should be carried out in a vacuum or under an inert atmosphere (gas shielding) to avoid these problems. The low thermal conductivities of titanium (17 W/mK) and Nitinol (18 W/mK) leads to longer welding times for high heat input processes like gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW). The long welding time leads to slow cooling and promotes coarse grains in the weld microstructure. In contrast, low heat input processes, such as electron beam welding (EBW) and laser beam welding (LBW), accelerate cooling and promotes fine grains in the weld microstructure [7; 8; 9; 10]. Several researchers have attempted to join Nitinol to titanium and its alloys through fusion welding routes, laser welding being the most prominent of those. Despite the fact that laser welding is a highly energy-dense process, the weld pool quality suffered from problems of solidification cracking, non-uniform microstructures and uneven depth of melting between the two base metals. Most notably, it was intermetallics that have rendered the joints extremely brittle [11]. Even when an alloying element such as Nb was added to Nitinol, which ideally should have made the intermetallic formation sluggish, the brittleness of the joint remained the same [12]. When the same Niobium was situated as an interlayer between Nitinol and Ti-6 Al-4V, the situation improved. However, the whole welding process had to be precisely controlled, wherein the melting of Nb was carefully avoided by focusing the laser beam more toward one of the base metals [13]. As can be noted, this was more of a design substitute rather than a technological opportunity. The problem of copious amounts of intermetallic formation in laser welding continued to show up in other combinations as well (Nitinol to copper) [14]. Solid-state welding trials showed a few promising results as compared to fusion welding. For instance, diffusion bonding of Nitinol alloy to a titanium alloy, when carried out at the right settings of pressure, temperature and time, produced joints of respectable strength [15]. Even though intermetallics of type Ti\({}_{2}\)Ni were seen to have formed in this type of bonding too, the relatively thin nature of such a layer (\(\sim\)40 \(\mu\)m) seems to have a not-so-deleterious effect on the joint strength. When the same interlayer grew thicker (\(\sim\)130 \(\mu\)m), because of higher temperature settings, the strength fell precipitously. Similarly, ultrasonic bonding of Nitinol, another type of solid-state welding, did not yield intermetallics when joined in the presence of a copper interlayer. It has to be noted here that the same copper produced plenteous amounts of intermetallics in laser welding, as discussed in the preceding paragraph.