## 3 Results and Discussion
### Fracture Behaviour Observations  
The morphology characteristics of the fractured fatigue specimens were acquired by SEM in order to examine the fatigue crack initiation and propagation behaviour of similar Ti grade 2 and AISI 304 joints and dissimilar Ti grade 2/AISI 304 resistance spot welded joints. This was done in order to confirm the mechanisms of fatigue crack initiation. The present study observed that fatigue cracks of similar welded titanium and stainless steel joints under HCF and LCF regimes generally initiated primarily in the BMs, propagated around the fusion zone, and eventually fractured through the BM of the opposite side. On the other hand, the failure behaviour was seen to be an interfacial failure of the dissimilar Ti grade 2/AISI 304 joint, and the fracture behaviour is transferred via the interface welding area between the two welded metals under HCF and LCF regimes. The SEM results showed that there are three main categories of fracture sources that might lead to fatigue failure: dislocation-slip defects, pores, and an uneven lack of fusion, which are all regarded as fatal flaws. It should be noted that only inner failure shows pore-induced crack nucleation, while the other two kinds of deadly flaws are indicative of surface failure. This research examined the SEM images of fatigue-fractured samples under both the LCF and HCF regimes. The SEM inspections of HCF samples for similar titanium joints are presented in Figure 16. It can be noted from Figure 16a that failure initiated at the surface of the BM and progressed rapidly through the small crack propagation area, which failed under overloading conditions, as evidenced by the presence of dimples in the high-magnification image of the fracture surface in Figure 16b. Also, Figure 16b demonstrated the presence of a short crack in the fractured surface. Figure 16c demonstrates the presence of slip bands, which are associated with plastic deformation before fracture and were considered to be caused by the martensitic & phase [88; 89; 90]. The presence of the fine \(\hat{\alpha}\) phase results in the formation of many grain boundaries. Meanwhile, they prevent plastic from slipping until the load is capable of initiating localised slipping [91]. One factor that will contribute to early localised plastic slip development is the similar crystal orientation of the \(\hat{\alpha}\) grains in the thicker columnar matrix [88; 92]. When the number of cyclic loading cycles increases, the plastic slip expands and builds up, eventually creating a fatal crack from the localized slip bands. The SEM analysis of the HCF fractured specimen shown in Figure 16d verified that the formation of intergranular fracture surfaces typically exhibits a more uniform look with fewer steps compared to the LCF fractured specimen (Figure 17). Moreover, equiaxed grains often produce wider angles between intergranular facets on fracture surfaces in contrast to the cleavage facets seen with LCF (Figure 17), which are parallel to low-index crystallographic planes. Furthermore, compared to cleavage, intergranular cracking is more likely to produce secondary cracks along grain boundaries, as seen in Figure 16d, that intersect the primary fracture path. The microstructure of the grain boundaries, the method of fracture, and the surrounding environment determine the presence or absence of fine-scale characteristics on intergranular facets. Typical forms of these features include steps, dimples, fatigue striations, and crack-arrest markings. Conversely, the fatigue-fractured surface of the LCF titanium specimen shown in Figure 17a exhibits a cleavage facet, pores, and a lack of fusion. Figure 17b illustrates the onset of cracks in HCF. Additionally, it is seen that the formation of trans-granular fracture surfaces, associated with cleavage failure, exhibits non-uniformity, in contrast to the intergranular surfaces observed in LCF noticed in the current study. Furthermore, the fractured surface seen in Figure 17b has short cracks.