**2. Materials and Methods**

Ti-6Al-4V and Nitinol rods 100 mm in length with a cross-sectional diameter of 10 mm were used in the present investigation. The chemical compositions of these alloys are

listed in Table 1. The rods of Ti-6Al-4V and Nitinol alloys were face turned and cleaned with acetone before friction welding. Continuous drive friction welding with a capacity of 150 kN was employed for the friction welding of dissimilar materials (Figure 1). Ti-6Al-4V was held in the non-rotating vice and Nitinol in the rotating chuck. At the start of the friction welding process, the rotating spindle quickly reached a set speed (spindle speed). The non-rotating specimen was then pushed toward the rotating one under high pressure (friction pressure). Due to the relative motion between the two specimens under pressure, frictional heat was generated at the interface. This heat very quickly plasticized both of the base metals, resulting in a flash. The flash, in fact, helps by ejecting out impurities and oxide layers from the surface of the specimens. The loss in the overall length of the rods was monitored by a sensor, and when the set burn-off length was achieved, the rotating chuck was suddenly brought to rest using a brake. This ended the first stage of the weld cycle, called the friction stage. In the next stage (the upset stage), the pressure was further increased (upset pressure) and held constant for a certain length of time (upset time). The bond consolidation between the two specimens was thus completed.


**Table 1.** Composition of base metals (wt%).

**Figure 1.** Friction welding machine used in the current study.

The initial parameter window considered is listed in Table 2. At first, one parameter was varied at a time to see how it affected the quality of the welds. Here, quality was assessed by two simple and quick methods: visual inspection of the flash and how the welds survived a drop test. The upset pressure, spindle speed and upset time did not have a significant effect, whereas the friction pressure and burn-off length noticeably influenced the weld quality. In the second stage, just these parameters (friction pressure and burn-off length) were changed by two levels each. Again, based on visual inspection, a final set of parameters was selected, and the same set was used for all the subsequent welds in the current work (Table 2).

**Table 2.** Welding parameters.


A solution containing 2% HF and 3% HNO3 in 95% distilled water was used to etch the Ti-6Al-4V alloy weld, and 40% HNO3 and 10% HF in 50% distilled water was used to etch the Nitinol weld. The chemical compositions of the Ti-6Al-4V alloy to the Nitinol rods were analyzed by employing a LECO TCH 400 instrument (LECO Corporation, St. Joseph, MI, USA). The low-magnification macrostructure of the friction welds was observed using a Nikon SMZ745T stereo microscope (Nikon Instruments Inc., New York, NY, USA). The microstructural features of the friction welded samples were observed using a Leitz optical microscope. The microstructure and energy dispersive spectroscopy (EDS) line scan were investigated using a VEGA 3LMV, TESCAN scanning electron microscope and oxford instruments, respectively (TESCAN ORSAY HOLDING, Brno, Czech Republic). X-ray diffraction (XRD) analysis (PANalytical, Malvern, UK, X'pert powder XRD) was used to identify phases in the base metal and weld. Vickers microhardness measurements were carried out as per the ASTM E384 standard (West Conshohocken, PA, USA) across the weld region by using a diamond pyramid indenter under a load of 500 g for 15 s (MMT-X Matsuzawa, Akita Prefecture, Kawabetoshima, Japan). Transverse weld specimens of a 25 mm gauge length and 4 mm diameter were machined from friction welded samples. Tensile tests were carried out according to the ASTM E8 standard on the base metal, as well as dissimilar friction welded samples using a servo hydraulic testing machine at a constant displacement rate of 0.5 mm/min.
