## 3 Results and Discussion
### Microstructure Characterization
Figure 9 depicts the optical microstructures of an as-welded SS304L/IN718 STA dissimilar rotary friction welded joint. On both the IN718 and SS304L sides of the SS304L/IN718 STA weldment (Figure 9a,h), the FDZ was observed next to the weld interface with an average grain size of 7 \(\pm\) 1 \(\upmu\)m on the IN718 side and 10 \(\pm\) 3 \(\upmu\)m on the SS304L side. The FDZ was observed next to the weld interface on both the IN718 and SS304L sides of the SS304L/IN718 STA weldment (Figure 9a,h), with an average grain size of 7 \(\pm\) 1 \(\upmu\)m on the IN718 side and 10 \(\pm\) 3 \(\upmu\)m on the SS304L side. The TMAZ was observed beside the FDZ (Figure 9b,g), with an average grain size of 19 \(\pm\) 4 \(\upmu\)m on the IN718 side and 27 \(\pm\) 9 \(\upmu\)m on the SS304L side. The HAZ was observed adjacent to the TMAZ (Figure 9c,f), with an average grain size of 28 \(\pm\) 7 \(\upmu\)m on the IN718 side and 37 \(\pm\) 9 \(\upmu\)m on the SS304L side. To summarize, the zones and mechanisms of zone formation in the dissimilar friction-welded SS304L/IN718 ST were identical to those in the dissimilar friction-welded SS304L/IN718 STA. When comparing the SS304L sides of the SS304L/IN718 ST and SS304L/IN718 STA weldments, it can be observed that they had almost the same grain sizes in both the SS304L/IN718 ST and SS304L/IN718 STA weldments, in all zones except the FDZ (i.e., TMAZ, HAZ, and BM). This can be attributed to the fact that SS304L is not heat-treatable. Furthermore, the FDZ grain size on the SS304L side of the SS304L/IN718 ST weldment was coarser than that of the SS304L/IN718 STA weldment. When comparing the SS304L  
Figure 9: The optical microstructure of the SS304L/IN718 rotary friction welds in the as-welded, solution-treated, and aged conditions (SS304L/IN718 STA). IN718 side: (**a**) FDZ (**b**) TMAZ (**c**) HAZ (**d**) base metal; SS304L side: (**e**) base metal (**f**) HAZ (**g**) TMAZ (**h**) FDZ. sides of SS304L/IN718 ST and STA, it was observed that the SS304L side of SS304L/IN718 STA experienced higher deformation than the SS304L side of SS304L/IN718 ST. Thus, the relatively higher strain hardening in the FDZ of the SS304L side of SS304L/IN718 STA resulted in finer recrystallized grains compared to the SS304L side of SS304L/IN718 ST weldment. When comparing the IN718 side of the SS304L/IN718 ST and SS304L/IN718 STA weldments, significant grain coarsening was observed in all zones (FDZ, TMAZ, HAZ, BM) of the SS304L/IN718 STA weldments. The IN718 side of SS304L/IN718 ST underwent higher deformation and strain hardening compared to the SS304L/IN718 STA weldment, resulting in finer recrystallized grains in the FDZ of the IN718 side of the SS304L/IN718 ST weldment. The average grain sizes are summarized in Table 4. Figures 10 and 11 show scanning electron micrographs of the weld interface of the rotary friction welded dissimilar SS304L/IN718 ST and SS304L/IN718 STA weldments, respectively. The \(\xi\)- phase particles in the IN718 were dissolved at the weld interface during friction welding, due to the high temperatures developed during the process. Faster cooling rates during the welding arrested the reprecipitation of the second phase [29]. It is notable that the intermixing zone was prominently observed for the SS304L/IN718 ST weldment, while SS304L/IN718 STA showed no intermixing zone. IN718 in the ST condition was softer and underwent mechanical mixing during welding with SS304L, while the harder IN718 STA sample stayed stronger and did not show prominent intermixing zone formation at the weld interface. An intermixed zone was found in several regions of the weld interface of the SS304L/IN718 ST weldment, as shown in Figure 12a. Hot plasticized metal at the interface became mechanically mixed, because of the relative motion of the dissimilar base metals and the heat generated by friction during the welding. The alternate layers of the SS304L and IN718 can be seen in Figure 12a, suggesting purely mechanical mixing.