*3.2. Macro and Microstructures of Dissimilar Friction Welds*

An actual Ti-6Al-4V and Nitinol joint is shown in Figure 3. Visual examination revealed no obvious macroscopic defects. The low-magnification friction welded joint made between the Ti-6Al-4V and Nitinol alloys showed a flash formation, which is a typical characteristic of the friction welding process (Figure 3). The flash predominantly occurred on the Ti-6Al-4V side, but for the Nitinol it was notably absent. From Figure 4, it is worthwhile to note that a precipitous drop in flow stresses in Ti-6Al-4V at high temperatures during friction welding makes the alloy softer compared with the Nitinol side, even though the yield strength (YS) of Ti-6Al-4V is higher than Nitinol to begin with at room temperature [28]. In addition to this, the poor heat conducting properties of Ti-6Al-4V cause the temperature to rise quickly on its side of the joint.

**Figure 3.** The visual view of a Ti-6Al-4V and Nitinol dissimilar friction welded joint.

**Figure 4.** Dissimilar friction welded Ti-6Al-4V and Nitinol sample showing the flash on the Ti-6Al-4V side.

The optical microstructure of the Ti-6Al-4V/Nitinol joint interface is shown in Figure 5a. The grains nearer to the interface on the Ti-6Al-4V side underwent refinement, whereas the Nitinol microstructure largely remained unaffected. Friction welding introduced a lot of dislocations into the materials because of the heavy plastic deformation that occurred during the process and, at the same time, it generated high temperatures close to the melting point of the base metals. The subgrain structure could also be seen if the dislocation density increased. These low-angle grain boundaries rotated to form stain-free grains, which are fine grains referred to as dynamic recrystallization (DRX) [29]. DRX was observed on

the Ti-6Al-4V side, adjacent to the joint interface when compared with the Nitinol side. This means that the deformation was more on the Ti-6Al-4V side due to the heavy plastic deformation and high temperatures encountered in friction welding. Due to dynamic recrystallization, nucleation and growth of the grains occurred; hence, a large amount of fine equiaxed grains were observed adjacent to the Ti-6Al-4V interface. A darker region (intermixed zone) was noticed between the Ti-6Al-4V/Nitinol joint interface. This may be due to the formation of intermetallics at the interface. The grains along the Ti-6Al-4V interface aligned in the rotating direction due to deformation of the material, as shown in Figure 5 b. The dark spots observed on the welded samples were referred to as etch pits, developed during etching of the sample. There was no significant change in grain size adjacent to the Nitinol interface, and the microstructure consisted mainly of austenite rather than martensite due to the heating of the material during welding.

**Figure 5.** (**a**) The optical microstructure of the Ti-6Al-4V/Nitinol joint interface. (**b**) The grains along the titanium interface, aligned in the rotating direction.

Figure 6 shows a scanning electron microscopy (SEM) image of the Ti-6Al-4V/Nitinol joint and the corresponding SEM EDS line scan, which shows the distribution of different elements across the joint. The EDS line scan revealed an interface between the Ti-6Al-4V/Nitinol joint, and here, a ~10 μm wide intermixed zone could be seen. The intermixed zone was the result of the huge amount of plastic strains and high temperatures seen in this region. It is also reasonable to expect that this intermixed zone would not be of the same width and composition between the center and the periphery [30]. This is because the intensity of rubbing differs in these zones. Ni has low solubility in Ti in its solid state and forms the brittle intermetallic compound Ti2Ni. These hard intermetallic compounds play a major role in the poor ductility of Ti-6Al-4V/Nitinol welds [9–12]. It is worth noting that even though the initial diffusion rates depend upon the alloying elements present in each of the base metals, the newly formed intermetallic phases would also start to influence

the diffusion phenomenon very quickly [30]. There is also the possibility of formation of Kirkendall voids when a particular element undergoes mass transport while it forms the reaction products in dissimilar material joining [31].

**Figure 6.** SEM image of the Ti-6Al-4V/Nitinol joint and the corresponding SEM energy dispersive spectroscopy (EDS) line scan.

The diffusion of Ni into Ti-6Al-4V was relatively higher. Figure 7 shows the X-ray diffraction analysis of the Ti-6Al-4V base metal, Nitinol base metal and fracture surface of the weld. The Nitinol base metal exhibited a B2 austenitic phase, and the Ti-6Al-4V base metal exhibited a hexagonal α phase and a weak β phase. In contrast, the fracture surface of the weld revealed a hexagonal α phase, β phase and the formation of a Ti2Ni brittle intermetallic phase.

**Figure 7.** X-ray diffraction (XRD) profiles of the Ti-6Al-4V base metal, Nitinol base metal and fracture surface of the Ti-6Al-4V/Nitinol friction welds.
