## 3 Results and Discussion
### Mechanical Properties
#### 3.3.2 Tensile Properties  
Tensile tests were conducted for the dissimilar Ti-6 Al-4V/Nitinol joints in the as-welded and PWHT conditions. The tensile plots are shown in Figure 11. The base metal tensile curves have been included in Figure 11 for comparison. Three samples of each were tested. The dissimilar weldments in the as-welded and PWHT conditions exhibited lower strength and ductility values compared with the wrought base metals (Ti-6 Al-4V: UTS = 1065 MPa, 15% elongation; Nitinol: UTS = 883 MPa, 17% elongation). This could be attributed to the formation of intermetallics of Ti and Ni, such as Ti\({}_{2}\)Ni at the weld interface. As discussed earlier, the XRD data of the fracture surface showed the presence of Ti\({}_{2}\)Ni intermetallic phase in the as-welded and PWHT conditions (Figure 9). The dissimilar Ti-6 Al-4V/Nitinol linear friction welded joint exhibited a slightly higher ultimate tensile strength (UTS) and lower ductility (UTS = 282 MPa, 3.9% elongation) compared with PWHT condition ((UTS = 250 MPa, 5.9% elongation). The presence of the strain hardening effect at the weldment could be the reason for the higher strength in the as-welded condition. However, PWHT could result in a reduction in strain energy and cause a slight decrease in the strength of the weldment. Further, the improvement of the weld ductility in the PWHT condition, compared to the as-welded condition, could also be attributed to the reduction in residual stress and strain energy. PWHT was found to have significantly improved the tensile strength and fracture elongation in similar joints of Nitinol [21]. When the PWHT temperature was just about right (350 \({}^{\circ}\)C), the tensile strength went up to 550 MPa from about 500 MPa (as-welded condition). Any further increase in the PWHT affected the weld properties negatively. This kind of behaviour is in line with the results seen in the current work. The strength values in the current study (282 MPa) are a little below the values reported by [PERSON] et al. [31] (300 MPa) and [PERSON] et al. [13] (480 MPa). Nevertheless, it has to be remembered that both of these studies incorporated a third material, such as Cu and Nb, to stop intermixing of the Ti and Ni. In both studies, direct joints (without filler material) failed immediately after welding. Joints that involve a third alloy might create  
Figure 10: Hardness distribution across the interface of dissimilar Ti-6 Al-4V/Nitinol linear friction welds in the as-welded and PWHT conditions. challenges such as galvanic corrosion. Therefore, whenever a direct joint between Ti-6 Al-4V and Nitinol is desired, the current study results show a promise that linear friction welding might be a better route compared to fusion ones. The dissimilar Ti-6 Al-4V/Nitinol joints in the as-welded and PWHT conditions failed at the weld interface after tensile testing, as shown in Figure 12a,b, respectively. According to XRD patterns, the fracture surface of dissimilar Ti-6 Al-4V/Nitinol welds in the as-welded and PWHT conditions contains Ti\({}_{2}\)Ni intermetallic phase at the weld interface. This could be the main reason for the failure of welds at the weld interface in both conditions. Upon tensile testing, the deformation of the weld is restricted due to the early cracking of the Ti\({}_{2}\)Ni intermetallic phase. Further, mechanical constraint provided by the relatively non-deforming Ti-6 Al-4V and Nitinol base metals at the stresses of consideration restricts the deformation of the weld interface region [32]. Hence, this constraint at the weld interface results in the triaxial state of the stresses at the weld interface and reduces the deformation tendency. As a result, fracture occurs at the weld interface, which is the weakest part of the weld joint  
Figure 11: The typical tensile properties of base metals and dissimilar Ti-6 Al-4V/Nitinol friction linear friction welds in the as-welded and PWHT conditions. Figure 12: The failure location of the dissimilar Ti-6 Al-4V/Nitinol joints in the (**a**) as-welded and (**b**) PWHT conditions.