## 3 Results and Discussion
### Microstructure Characterization  
Figures 8 and 9 show the optical microstructures of the dissimilar as-welded SS304L/IN718 rotary friction welded joints with prior ST and STA treatments, respectively. The optical micrographs of the weld cross sections reveal four distinct zones as they move from the weld to the base metal on each side (SS304L/IN718) of the weldments: FDZ, TMAZ, HAZ,  
Figure 6: A visual view of SS304L/IN718 friction weld joint in the as-welded (**a**) ST, and (**b**) STA conditions. Figure 7: The macrostructure of the dissimilar rotary friction welded SS304L/IN718 ST sample shows more flash on the SS304L side. and the unaffected BM, respectively. The FDZ was located adjacent to the weld interface on both the IN718 and SS304L sides of the SS304L/IN718 ST weldment (Figure 8a,h). This is distinguishable from all other zones by its blackish color at low magnification. The FDZ on the SS304L side of the SS304L/IN718 ST weldment was \(\sim\)0.33 mm wide, while the FDZ on the IN718 side was \(\sim\)0.40 mm. Finer grains were observed in the FDZ on the SS304L and IN718 sides of the SS304L/IN718 ST weldment at higher magnification. This was due to the severe plastic deformation of the FDZ, which generated highly strained grains with a high dislocation density [25]. The highly deformed FDZ region dynamically recrystallized due to the heat generated during welding, resulting in fully equiaxed fine grains in the microstructure [26]. Similar observations were made by [PERSON] et al. during dissimilar rotary friction welding of IN718 to SS410 [5]. The average grain size of the FDZ in the SS304L/IN718 ST weldment was 12 \(\pm\) 3 \(\upmu\)m on the SS304L side and 6 \(\pm\) 2 \(\upmu\)m on the IN718 side. Elongated grains were observed on both the SS304L (Figure 8g) and IN718 (Figure 8b) sides of the SS304L/IN718 ST weldment adjacent to the FDZ; these regions are referred to as TMAZ. This zone was partially deformed and showed elongated grains orientated in a \"necklace\" pattern [3]. Due to the lower heat and plastic deformation in TMAZ compared to the FDZ, partial recrystallization occurred. Finer recrystallized grains formed along the elongated grain boundaries [16], resulting in a bimodal grain structure in the TMAZ. Similar findings were made by [PERSON] et al. [27] regarding the TMAZ zone in friction-welded alloy 718 joints. Furthermore, [PERSON] et al. [28] studied the influence of Nb-V microalloying on the hot deformation characteristics and microstructures of Fe-Mn-Al-C austenitic steel in hot compression tests at various strain rates. For a strain of 0.7, they found necklace-like distorted grains at temperatures below 1050 \({}^{\circ}\)C, whereas completely recrystallized grains were found at and above 1050 \({}^{\circ}\)C. In the SS304L/IN718 ST weldment, the average grain size of the TMAZ was 27 \(\pm\) 9 \(\upmu\)m on the SS304L side, and 14 \(\pm\) 6 \(\upmu\)m on the IN718 side. Figure 8: The optical microstructure of the SS304L/IN718 rotary friction welds in the as-welded, solution-treated condition (SS304L/IN718 ST). IN718 side: (**a**) FDZ (**b**) TMAZ (**c**) HAZ (**d**) base metal; SS304L side: (**e**) base metal (**f**) HAZ (**g**) TMAZ (**h**) FDZ. HAZ was observed on both the SS304L (Figure 8f) and IN718 (Figure 8c) sides of the SS304L/IN718 ST weldment adjacent to the TMAZ. As it is far from the weld interface, this zone experienced less heat and no plastic deformation compared to TMAZ and FDZ zones. Therefore, the HAZ did not undergo recrystallization, and the grains that were formed in this zone were coarser and had a unimodal grain structure. The average grain size values of the HAZ were 36 \(\pm\) 9 \(\upmu\)m and 25 \(\pm\) 5 \(\upmu\)m on the SS304L and IN718 sides, respectively. BM was observed on both the SS304L (Figure 8e) and IN718 (Figure 8d) sides of the SS304L/IN718 ST weldment adjacent to the HAZ. As the temperature in this area was very low for microstructural changes, the BM had an unaffected base microstructure of IN718 ST and SS304L.