## 2 Materials and Experiment  
Ti-6 Al-4V and Nitinol in plate form were used for the welding trials. The chemical composition of both the base metals is listed in Table 1. The optical microstructure of Ti-6 Al-4V base metal showed primary \(\alpha\) grains (white area) elongated in the direction of rolling in a matrix of transformed \(\beta\) (dark area), as shown in Figure 1. The average grainsize of Ti-6 Al-4V is \(67\pm 1\) um. The optical microstructure of Nitinol base metal revealed equiaxed B2 austenite grains, and the average grain size of Nitinol is 50 \(\pm\)4 um (Figure 1). The dimensions of the plates are shown in Figure 2. A 20 ton linear friction welding machine (Taylor Winfield Technologies Ltd., Youngstown, OH, USA) was employed. Forging force was applied using a hydraulic actuator. The machine had an electro-mechanical oscillation system. The amplitude could go up to a maximum of 6.5 mm. Frequencies between 25 to 60 Hz were possible. The right chuck that holds one of the plates is stationary, whereas the left chuck oscillates at high frequencies to generate the rubbing action. Each plate stuck out from the grips by 4.7 mm. At the start of the weld cycle, a scrub force of 15 MPa was used in the initial trials. Scrub force helps in cleaning off the surface's contaminants, such as oil and grease. This force also evens out the peaks on the rubbing surfaces. This will go on to help in the subsequent oscillating stage. The trials that used the lower end of the parameters (refer to Table 2) failed to produce satisfactory joints. Therefore, the parameters, especially friction pressure, were raised drastically, which ultimately helped achieve visibly strong joints. Forge pressure seemed to play no significant role, and therefore it was maintained constant throughout the trials. The highly elastic nature of the base metals, especially Nitinol, played a marked role when it comes to the repeatability of the welds: notwithstanding the higher settings, few joints turned out to be defective. This implied that a more rigid fixturing could have remedied the situation to some extent. The schematic of parent metal plates and their setup in the linear friction welding machine is shown in Figure 2a. The successful weld specimens were sectioned perpendicular to the oscillation direction; in other words, the samples were cut along the Y-direction, as shown in Figure 2b. A typical curve of how the chuck changed its position through a weld cycle is shown in Figure 3. Initially, for some time, the curve remained flat. This was the beginning of the friction stage. The heat built up slowly after the rubbing actions kicked in. After a while, when enough heat was generated, the base metals became plastically soft \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Elements** & **Ni** & **Co** & **Cr** & **Fe** & **C** & **N** & **O** & **H** & **Ti** \\ \hline Ti-6 Al-4V & 6.01 & 4.0 & 0.13 & 0.03 & 0.01 & 0.003 & 0.112 & 0.007 & balance \\ \hline
**Elements** & **Ni** & **Co** & **Cr** & **Fe** & **C** & **Nb** & **O** & **H** & **Ti** \\ \hline Nitinol & 55.6 & 0.005 & 0.003 & 0.012 & 0.03 & 0.005 & 0.032 & \textless{}0.001 & balance \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Composition of base metals (wt %). Figure 1: Optical microstructures of the base metals: (**a**) Ti-6 Al-4V and (**b**) Nitinol. Linear friction welded specimens were subjected to a PWHT at 400 \({}^{\circ}\)C (well below the \(\upbeta\) transus of the alloy) in a vacuum furnace for an hour, and furnace cooled to room temperature. The dissimilar Ti-6 Al-4V/Nitinol linear friction welded specimens in the as-weldeded and PWHTed conditions were sectioned in the transverse direction to the weld and polished for metallographic study. For microstructure examination of the Ti-6 Al-4V side, a solution consisting of a mixture of 2% HF and 3% HNO3 in 95% distilled water was used, and 40% HNO3 and 10% HF in 50% distilled water was used for etching on the Nitinol side.