## 3 Results and Discussion  
### Macrostructure  
Defects such as cracks, solidification cracking and incomplete bonding were not observed upon visual inspection (Figure 4). Flash was seen on all sides of the interface. Nevertheless, the shape was not uniform all around. The flash projected more on the top side than the bottom in Figure 5. This indicates that there was a slightly non-symmetrical situation during the oscillating stage. Either the center lines of the specimen did not align perfectly, or the oscillation mechanism itself was biased more towards one side. [PERSON] and [PERSON] suggested even minor misalignments between the base metal plates could lead to perceptible changes in the way a flash emerges [20]. Even a minor disturbance in the initial stages will first affect the distribution of the temperature, which leads to a non-uniform material extrusion, and this, in turn, affects the temperature profile, thus settling down into a cause-and-effect loop. Figure 4: A successfully bonded Ti-6 Al-4V/Nitinol dissimilar linear friction weld. Two mechanisms play a role in the flash formation during friction welding [23]. The first one is because of the oscillation motion between the workpieces: the material is transported from the interiors to the exterior. The material then piles up more at the edges (along the oscillating direction) than at the centre. The second mechanism that influences flash formation is the forging pressure. This is the pressure applied across the cross-sectional area of the weld. The forging pressure pushes material outward more in the central region than at the edges, opposite to what was seen in the first mechanism. For the above reasons, it can be said that the geometry of the flash differs for different weld settings. The shape of the flash was not uniform along its length. For instance, the outermost edge of the flash was seen to have ridges, whereas the root seemed to be smoother. At the beginning of the weld cycle, when the equilibrium conditions were yet to be established, the base metals would not have become sufficiently soft because the temperature were not high enough. This would be the time when the flash had ridged edges. As the friction stage progressed, a continuous supply of fully plasticised material ensured that the subsequent flash was smoother. The extent to which Ti-6 Al-4V and Nitinol deform as flash (Figure 5) depends upon their yield strength at elevated temperatures. The formation of more flash on the Ti-6 Al-4V side and little flash on Nitinol side can be attributed to drop-in flow stresses of Ti-6 Al-4V at high temperatures. In addition to this, the poor heat conducting properties of Ti-6 Al-4V cause the temperature to rise quickly on its side of the joint. The preferential deformation of one alloy over the other gives rise to the problem of insufficiently cleaned surfaces. One of the advantages of friction welding is its less stringent requirements of the joining surfaces. The rubbing action and the subsequent flash expulsion ensures the impurities, such as oxide films and contaminants, on the surfaces get pushed out, and therefore only virgin surfaces come into contact with each other, ultimately ending up with a strong weld. However, this idealistic scenario breakdown when two materials with different flow properties are brought together. Lack of flow, manifested by material on one side, hinders the much-desired cleaning action. Nevertheless, in the present case, Nitinol has also deformed to some extent (Figure 6), aliasing the self-cleaning concern more or less. The same phenomena have been observed during rotary friction welding of dissimilar welds, namely titanium to stainless steel [24] and titanium to Nitinol [16]. Figure 5: Macrostructure of the dissimilar linear friction welded Ti-6 Al-4V/Nitinol sample showing more flash on the Ti-6 Al-4V side. In Figure 5, a greater volume of material was seen on one side (bottom) than the other.