## 3 Results and Discussion
### Topography of the Fractured Surfaces  
Analysing fatigue-fractured surfaces using surface roughness observations and topography has several benefits, especially in comprehending material behaviour under cyclic  
Figure 21: SEM inspection of the fatigue fractured surface on the ASS side. stress and enhancing the design and reliability of components. Examining the surface topography facilitates the identification of the locations and mechanisms of fracture initiation and propagation. This comprehension is crucial for identifying the fundamental reasons for failure. The topography measurements in the current study were conducted on the LCF and HCF fractured surfaces of similar and dissimilar joints for Ti and AISI 304. Table 6 presents the results of the topography parameters. Figure 22 presents the surface texture measurement findings for the LCF and HCF fractured surfaces of similar titanium joints. In Figure 22, the blue frames denote the crack initiation zone, the propagation profile is shown in green, and the rupture profile is shown in red. The topography parameters of fatigue-fractured surfaces of the similar Ti joints (Table 6) indicate that the HCF surface exhibits lower topography roughness parameters compared to the LCF surface. The topography parameters of fatigue-fractured surfaces of the similar Ti joint confirmed that the HCF surface has smaller topography roughness parameters than the LCF surface. In the case of the HCF-fractured surface, the topographical parameters Sa, Sq, Sz, and V\({}_{\text{v}}\)(p) were 33.79, 42.89, 172.48, and 50 \(\upmu\)m, respectively. In contrast, the LCF fractured surface had Sa, Sq, Sz, and V\({}_{\text{v}}\)(p) values of 65.35, 77.39, 239.42, and 100 \(\upmu\)m, respectively. This phenomenon is explained as follows. The fatigue specimen that failed due to high-cycle fatigue was tested under lower stress conditions. This led to a lower crack growth rate, which in turn reduced the surface roughness of the crack and, ultimately, decreased the Sa, Sz, Sq, and V\({}_{\text{v}}\)(p) parameters. On the other hand, the lower-fatigue-life specimen experienced a higher applied load, resulting in greater crack growth and later increasing the Sa, Sz, Sq, and V\({}_{\text{v}}\)(p) parameters. This claim is supported by [105]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Topography** & \multicolumn{2}{c}{**Similar Grade Ti Samples**} & \multicolumn{2}{c}{**Similar AISI 304 Samples**} & \multicolumn{2}{c}{**Dissimilar Grade Ti/AISI 304 Samples**} \\ \cline{2-7}
**Parameters** & **HCF** & **LCF** & **HCF** & **LCF** & **HCF** & **LCF** \\ \hline Sa (\(\upmu\)m) & 33.79 & 65.35 & 51.72 & 56.29 & 89.29 & 106.64 \\ Sq (\(\upmu\)m) & 42.89 & 77.39 & 61.56 & 67.85 & 115.41 & 135.64 \\ Sz (\(\upmu\)m) & 172.48 & 239.42 & 133.64 & 144.10 & 412.95 & 631.49 \\ Ssk (-) & \(-\)0.73 & \(-\)0.34 & \(-\)0.84 & \(-\)0.81 & \(-\)0.10 & 0.71 \\ Sku (-) & 3.84 & 2.10 & 2.53 & 2.71 & 3.62 & 2.99 \\ Sdq (-) & 1.26 & 1.23 & 0.64 & 0.61 & 0.74 & 0.65 \\ V\({}_{\text{v}}\)(p) (\(\upmu\)m) & 50 & 100 & 60 & 70 & 160 & 220 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Topography parameters of samples determined according to standard ISO 25178-2. Figure 22: Topography textures of similar Ti grade 2 joints for **(a)** HCF and **(b)** LCF. The topography findings of similar fractured AISI 304 joints confirmed the observation noted in the Ti fatigue sample case, with the HCF-fractured surface having lower topography roughness parameters and the LCF-fractured surface having higher topography parameters, as shown in Table 6 and Figure 23. The Sa, Sq, Sz, and V\({}_{\mathrm{v}}\)(p) values for the HCF-fractured sample were 51.72, 61.56, 133.64, and 60 \(\upmu\)m, respectively. The LCF-fractured specimen had Sa, Sq, Sz, and V\({}_{\mathrm{v}}\)(p) values of 56.29, 67.85, 144.10, and 70 \(\upmu\)m, respectively.