## 3 Results and Discussion
### Microstructure
One of the drawbacks of linear friction welding compared to rotary friction welding is, while oscillating, the base metal rubbing surfaces--hot as they are--will be exposed to ambient conditions, even if that were for a fraction of a second [29]. This is when oxides would form. The greater the amplitude, the higher the chances of oxide formation. This is not seen in rotary friction welding, wherein the hot metal is always shielded from the ambient air. In addition to oxides, it is possible for chemical elements such as Ni and Ti to have formed intermetallics between themselves. During the friction stage, when there are higher temperatures present and the materials are under severely plasticised conditions, it is highly possible that there is a mechanical intermixing of the elements as well as the diffusion of these elements across the interface. Figure 8a shows that there is a sudden compositional change at the interface. This suggests that Ni and Ti would have formed intermetallic phases. According to the phase diagram, NiTi\({}_{2}\) and Ni\({}_{3}\)Ti are the most prominent phases that would result if Ni and Ti were brought together at high temperatures. In the line scan, it is seen that the percentage of Ti is higher whether it is on the Ti-6 Al-4V side or the Nitinol side; the chance of formation of Ti-rich NiTi\({}_{2}\) is higher. The microstructural analysis (Section 3.3) suggested that it was Ti-6 Al-4V that deformed more. So, it is highly likely that the phases formed at the interface were rich in Ti rather than Ni. Since NiTi\({}_{2}\) is a highly brittle compound [30], its presence in the welds indicates the possibility of a brittle joint. Figure 7: The optical microstructure of the Ti-6 Al-4V/Nitinol linear friction welds in the post-weld, heat-treated (PWHT) condition. Nevertheless, since the thickness of this intermixed zone is quite low (\(\sim\)10 \(\upmu\)m), its effect would be subdued on the joint strength. Figure 8b shows the same interface after PWHT. It is not clear from the line scan if there were any chemical compositional changes, but at least there was no sign of a major thickness increase. This means the joint strength should remain more or less the same. The structure of the intermediate zone could be seen to have a grain-based microstructure. Before PWHT, the grain structure was finer, whereas, after PWHT, the grains became broader. It looks like even though PWHT has not altered the overall chemical composition nor the thickness of the zone, it surely has affected the grain morphology of the intermediate zone. It is highly possible that the intermetallic zone consisted of Ti\({}_{2}\)Ni as suggested by the EDS line scan. Ti\({}_{2}\)Ni was seen to form readily in the laser weld of Ti64 and NiTi [31]. The X-ray diffraction (XRD) patterns of the Ti-6 Al-4V parent metal and Nitinol parent metal are presented in Figure 9. This also shows the tensile fracture surface of the Ti-6 Al-4V/Nitinol linear friction welds in the as-welded and PWHT conditions. XRD analysis of the Ti-6 Al-4V base metal revealed the presence of the hexagonal \(\upalpha\) phase along with the \(\upbeta\) phase, while Nitinol base metal showed the austenitic (B2-cubic) phase in the microstructure. In contrast to the base metals, the fracture surface of the Ti-6 Al-4V/Nitinol  
Figure 8: Scanning electron microscopy image in secondary electron mode showing (**a**) Ti-6 Al-4V/Nitinol weld interface in the as-welded condition (left) and the corresponding energy dispersive spectroscopy (EDS) line scan. (**b**) Ti-6 Al-4V/Nitinol weld interface after heat treatment (left) and the corresponding energy dispersive spectroscopy (EDS) line scan. linear friction welds in the as-welded and PWHT conditions showed a hexagonal \(\alpha\) phase and the presence of a Ti\({}_{2}\)Ni intermetallic phase. It can be seen from Figure 9 that the PWHT does not alter the phase constitution of the as-welded sample. A slight increase in the Ti\({}_{2}\)Ni intermetallic phase diffraction peak intensity was observed after the PWHT. [PERSON] et al.