## 3 Results and Discussion
### Fracture Behaviour Observations
All fatigue tests reveal the same failure mechanism, IF, independent of the welding case and the parameters utilised. Hence, only one specimen was used to investigate the fractured surface's metallographic characteristics. Figure 20a illustrates the results of the SEM examination of the interface failure on the titanium side, which reveals that a brittle fracture occurred due to the presence of a cleavage facet on the fracture surface. Figure 20a shows that there are many micro-cracks and fatigue striations included in the fractured surface. Each grain in the fusion zone (FZ) has a different orientation, therefore, the fracture will usually divide into stages that resemble terraces. One kind of trans-granular brittle fracture that characterises these crack processes is cleavage fracture due to the presence of cleavage  
Figure 19: Fatigue–fractured surface of LCF stainless steel welded joint. facets on the surface (Figure 20b). As seen in Figure 20c and validated by the XRD results in Figure 12, there are elongated dimples that are believed to be the primary cause of the increased shear force in this research. The presence of pores on the fracture surface, as shown in Figure 19d, proves beyond a reasonable doubt that the welding defects are caused by different solidification shrinkage rates. The presence of zones with apparent striations suggests gradual fracture formation under cyclic stress, as shown in Figure 21a. Figure 21a reveals that there are specific areas that seem rough, perhaps indicating ductile tearing from titanium, while other zones look smoother, showing brittle fracture characteristics, probably from the stainless steel or intermetallic phases. The smoother regions may signify a transition zone, where fracture deflection possibly occurred due to microstructural differences. Crack propagation likely slowed or changed its path at this point, indicating variations in hardness across the contact. Figure 21b shows regions with elongated ductile dimples that imply material deformation before final failure. Some areas of the fractured surface exhibited ductile fracture, whereas others failed in a brittle state owing to carbide precipitation, which is the source of the crack initiation region. Because of this, welded joints are brittle. Since each metal has its  
Figure 20: SEM inspection of the fatigue fractured surface on the Ti side. own melting point, it is important to note that AISI 304 melts at around 1400-1450 \({}^{\circ}\)C, while titanium melts at about 1668 \({}^{\circ}\)C. Furthermore, these metals differ in how much they expand thermally, meaning that AISI 304 expands about twice as much as titanium. All of these factors contributed to uneven thermal stress and volume expansion, which ultimately resulted in brittle fractures. Furthermore, Ti and St have different resistivities, which results in differing rates of shrinkage and the formation of pores and fractures after solidification is complete. In summary, the above-observed microstructural changes in fatigue-fractured samples, including the existence of slip bands, equiaxed grains, cleavage facets, elongated dimples, and intergranular cracking, have significant implications for the enduring functionality and the long-term performance of these joints. The microstructural characteristics have a direct influence on the mechanical qualities and longevity of the welded material. Slip bands in fatigue-fractured samples are a visible sign of material deterioration owing to cyclic stress. Their presence degrades joint performance by encouraging fracture formation, expediting fatigue failure, and reducing the material performance [100]. The fatigue performance of welded joints is often improved by equiaxed grains because of their isotropic and homogeneous characteristics. Crack resistance, ductility, and toughness are all improved by fine equiaxed grains, however, fatigue resistance might be decreased by coarse grains [101; 102]. Cleavage facets, a form of brittle failure in fatigue-fractured materials, adversely affect the RSW joint performance [103].