## 3 Results and Discussion
### SEM and XRD Examinations  
Welding raises the temperature beyond the beta-transus temperature, approximately 882 \({}^{\circ}\)C, leading to an increase in the grain size, even though the area outside the molten pool remains unmelted. Concurrently, the microstructure approaches a pure \(\beta\) phase as all the martensitic \(\alpha\) in this area vanishes and the \(\alpha\) is converted into \(\beta\). Figure 7a,d demonstrate that dendritic formations primarily compose the WZ of similar titanium RSW joints. The microstructure of the welding zone (WZ) in Figure 7c reveals a predominantly aligned acicular composition. However, upon cooling, the phase transforms into the alpha phase. The high cooling rate of the RSW process can trap some beta-phase grains in a metastable form, as shown in Figure 7d, potentially leading to fine, acicular alpha structures (martensitic alpha). This acicular alpha structure is generally harder and more brittle than the parent alpha phase [82]. As the temperature rises throughout the RSW process, the \(\alpha\) phase in the HAZ progressively converts to the \(\beta\) phase, resulting in an increase in the grain size of the \(\beta\) phase. Concurrently, when the heat input increases, the displacement and absorption of the \(\beta\) phase border become more pronounced. Consequently, increased optimum heat input during the RSW process results in microstructures of the HAZ and WZ becoming more similar, producing an unseen fusion line as shown in Figure 7a-d; this is supported by [37]. The XRD patterns of the similar RSW titanium joint are presented in Figure 8. The predominant phase is alpha-titanium (\(\alpha\)), as apparent from the XRD inspections. This is explained by the fact that titanium grade 2 does not contain significant alloying elements to stabilize other phases, and \(\alpha\)-Ti is the predominant phase in the weld, the HAZ, and the BM. Even though the SEM inspection confirmed the presence of the beta phase (\(\beta\)) in a small amount, the XRD results did not prove the existence of this phase, meaning that this phase appeared in a small percentage, and the XRD test wave was unable to detect it. Another explanation of the presence of a tiny amount of \(\beta\) the phase is the fact that  
Figure 6: Shear strength of RSW specimens of titanium-stainless steel for dissimilar E, F, I, and J cases. titanium grade 2 does not contain significant alloying elements to stabilise other phases like \(\beta\) when it cools to room temperature, and \(\alpha\)-Ti is the predominant phase in the weld, HAZ, and BM. Figure 8: XRD analysis of the RSW titanium joint (case A). Figure 7: SEM of the similar titanium RSW joint with different magnifications. (**a**) 500\(\times\); (**b**) 1000\(\times\); (**c**) 2000\(\times\); (**d**) 8000\(\times\). Under equilibrium solidification, the microstructure of 304 stainless steel is mostly made up of austenite (\(\gamma\)-Fe), which is a well-known fact. It is certain that some metastable Fe will be present under non-equilibrium fast solidification circumstances like the RSW process due to the high cooling rate, which might cause incomplete transformation [83; 84]. It is well known that the ratio of ferrite to austenite in 304 stainless steel in the RSW process is affected by the amount of heat used. Therefore, the RSW process produces a slightly different ratio of ferrite to austenite in the fusion zone. The 304/304 nugget showed the ferritic-to-austenitic mechanism of solidification. Due to the non-equilibrium rapid solidification condition in the RSW process, the peak temperature in the fusion zone is much higher than the upper limit of the phase balance between the \(\delta\)-Fe and \(\gamma\)-Fe phases. The austenite starts to precipitate at the ferrite grain boundaries during the cooling of joints in the temperature range 1573-1073 K [85]. Since the \(\delta\)-\(\gamma\) transformation is a diffusion-controlled process, the fast cooling in the RSW process does not offer sufficient time to complete the phase transformation.