## 1 Introduction
#### Schematic of Base Plates and Their Setup in the Linear Friction Welding Machine  
Linear friction welding consists of four phases, as described initially by [PERSON] and [PERSON] [20]. In the first stage, when the oscillation of the workpieces begins, the asperities on both surfaces come in close contact. At the same time, heat is generated due to friction. When the conditions are about right--sufficient relative speed and pressure--the heat will soften the materials at the interface. The plasticised material starts to extrude out as a flash. In the subsequent stage, equilibrium conditions are established with regard to the rate of heat generated versus the rate of material extruded. In the final stage, the oscillation is quickly stopped, and an extra pressure called forge pressure is applied on the now stationary workpieces to consolidate the materials into a final joint. The main controllable process parameters are friction pressure, oscillation frequency, amplitude, friction time (all of these during the oscillating) and forge pressure (after the oscillation stops) [9]. In general, heat treatment of a weld improves the joint properties by reducing the residual stresses that might have originated due to the thermal cycles of a welding process. Sometimes, heat treatment is implemented to obtain the desired microstructure in the weld. When it comes to heat treatment of the Nitinol-based weld, one has to be careful not to stimulate the formation of harmful intermetallics. Heat treatment of a Nitinol weld can also alter the alloy's transformation temperature, especially in the region adjacent to the interface. Nevertheless, heat treatment did show positive results when the conditions were just right. [PERSON] et al. [21] showed how with the right post-veld heat treating conditions, one could improve Nitinol laser welds. The authors attributed the improved weld properties to the precipitation of Ni\({}_{4}\)Ti\({}_{3}\) during the heat treatment, which arrested the dislocation motion. The benefits of Ni\({}_{4}\)Ti\({}_{3}\) were further confirmed by [PERSON] et al. [22]. This time, the Ni\({}_{4}\)Ti\({}_{3}\) was reported to benefit the fatigue properties. It has to be noted that the eventual size of the precipitating intermetallics was critical. Only when they were fine (\(\sim\)10 nm) could they become coherent with the matrix and improve the fatigue strength. When the same precipitates were allowed to get bigger--because of higher temperature settings--the properties suffered. Therefore, it could be deduced that lower temperatures, in the range of 350-400 \({}^{\circ}\)C, are optimal when heat-treating Nitinol welds. Having said that, not much is known about the effect of heat-treatment on dissimilar welds involving Nitinol, as the literature on this topic is sparse. As evidenced in the previous paragraphs, fusion [11; 12; 13; 14; 17; 18] and solid-state welding [9; 15; 16; 19; 20] techniques were shown to have been quite successful in joining Nitinol to Nitinol, whereas in dissimilar combination welds, Nitinol was poor at bonding with other materials, such as Ti-6 Al-4V [11; 12; 13; 14; 15; 16]. It has to be noted here that most of these studies concentrated mainly on fusion welding techniques, but not solid-state welding, [PERSON] et al. [16] were some of the first to employ rotary friction welding to join Nitinol to Ti-6 Al-4V. The current study could be considered an extension of that particular study, wherein linear friction welding has been used instead of rotary friction welding. As a matter of fact, linear friction welding is a more suitable joining technique as compared to rotary welding because the joint designs could be of any configuration in linear friction welding, whereas rotary friction welding is restricted to cylindrical configurations only. This study involves welding experiments using linear friction welding to achieve comparatively sound joints and, after that, subject those joints to stress-relieving heat treatment cycles.