3.3.2. Tensile Properties

The tensile properties of dissimilar Ti-6Al-4V/Nitinol friction welds are shown in Figure 9. The important objective of the present investigation was to determine whether the joint strength of a dissimilar weld shows any improvement in its weld zone tensile properties. The data of two base materials' tensile properties have also been included in Figure 8 for comparison. It is revealed from Figure 9 that the dissimilar Ti-6Al-4V/Nitinol friction weld exhibited a lower ultimate tensile strength (UTS) and ductility (UTS = 589 MPa, 3.9% elongation) compared with the base metals (Ti-6Al-4V: UTS = 1073 MPa, 15% elongation; Nitinol: UTS = 980 MPa, 16% elongation). This could be because of intermetallic formations (Ti2Ni) at the interface which are brittle in nature. Similar results were observed in laser welding of dissimilar Ti-6Al-4V to NiTi joints [10]. These dissimilar welds exhibited a UTS value of 250 MPa and 3% elongation. The authors attributed the reason for the inferior properties to the formation of a Ti2Ni phase and transverse cracks in the weld metal. Electron beam welding was better in that the UTS was 480 MPa with 2.3 % elongation [21]. Three tensile tests were conducted for each condition, and the average of these specimens was taken. The dissimilar weld failed in the intermixed zone, except for the Ti-6Al-4V and Nitinol base metals. Failure in the intermixed zone indicates that the weld region was weaker than the base metals in the dissimilar welded joint. The tensile fracture surfaces of the Ti-6Al-4V base metal, Nitinol base metal and dissimilar Ti-6Al-4V/Nitinol friction welds are shown in Figure 10. Very fine and ductile fracture features were observed for the Ti-6Al-4V and Nitinol base metals in Figure 10a,b respectively. It is well established that the fine equiaxed dimples are observed in the base metal samples because of microvoid

formation and joining. Figure 10c shows the cleavage fracture (brittle) features of dissimilar Ti-6Al-4V/Nitinol friction welds.

**Figure 9.** The tensile properties of base metals and dissimilar Ti-6Al-4V/Nitinol friction welds.

**Figure 10.** Fracture surfaces of the base metals and dissimilar welded joint for (**a**) Ti-6Al-4V, (**b**) Nitinol and (**c**) dissimilar Ti-6Al-4V/Nitinol friction welded joint.
