## 3 Results and Discussion
### Mechanical Properties
#### 3.5.2 Tensile Properties  
Figure 15 depicts typical room temperature tensile graphs of the three base metals and the IN718/SS304L rotary dissimilar friction welds in the ST, STA, and PWHT conditions. The SS304L/IN718 dissimilar friction welds in the STA condition exhibited lower yield strength (YS) and ultimate tensile strength (UTS) values (YS: 497 \(\pm\) 2 MPa, UTS: 670 \(\pm\) 3 MPa) and higher ductility values (Elongation %: 16 \(\pm\) 2) when compared to welds in the ST condition (YS: 573 \(\pm\) 4 MPa, UTS: 713 \(\pm\) 2 MPa, Elongation %: 10 \(\pm\) 1). This could have been due to the grain refinement and dissolution of strengthening precipitates at the weld zone [3]. Furthermore, when evaluating the IN718 sides of the SS304L/IN718 ST and STA weldments, the weldment in the STA condition showed a lower strain hardening effect compared to ST, due to the grain refinement at the weld interface. The SS304L/IN718 ST and STA welds had lower strength and ductility values than the IN718 base materials with prior ST and STA (ST condition: YS: 938 \(\pm\) 4 MPa, UTS: 1312 \(\pm\) 2 MPa, Elongation %: 30 \(\pm\) 2; STA condition: YS: 1075 \(\pm\) 5 MPa, UTS: 1375 \(\pm\) 10 MPa, Elongation %: 28 \(\pm\) 1). This was because the IN718 was rotary friction welded to the SS304L, a low-strength material. Therefore, the strength of the SS304L/IN718 dissimilar welded sample was reduced compared to the IN718 base metal under the ST and STA conditions. [PERSON] et al. [32] made similar findings in their investigation of the rotational friction welding of IN718 to IN600. In comparison (Table 5), the SS304L BM exhibited lower strength (YS: 460 \(\pm\) 3 MPa, UTS: 615 \(\pm\) 5 MPa and higher ductility (Elongation %: 51 \(\pm\) 4) values. The SS304L/IN718 PWHT welds exhibited higher strength values (YS: 604 \(\pm\) 5 MPa, UTS: 721 \(\pm\) 4 MPa) and lower ductility values (Elongation %: 07 \(\pm\) 1) in comparison to all other welds (ST, STA, PWHT) (Table 5). Figure 15: The typical tensile properties of the base metals and dissimilar SS304L/IN718 rotary friction welds in the as-welded ST, STA, and PWHT conditions. This was most likely due to the fact that SS304L is not heat-treatable, and hence the PWHT only improved the strength in IN718. This increase in strength in the PWHT sample on the IN718 side was due to the formation of strengthening precipitates \(\gamma^{\prime}\) and \(\gamma^{\prime\prime}\). Similar observations were made by [PERSON] et al. [35] during the friction welding of IN718 and EN24 dissimilar metal combinations. Table 5 summarizes the average YS, UTS, and % Elongation values. Figure 16 depicts the failure position of the SS304L/IN718 transverse tensile welded specimens in the ST, STA, and PWHT conditions. The SS304L/IN718 ST and SS304L/IN718 STA dissimilar welds were observed to fail near the HAZ of SS304L because of the grain coarsening in the HAZ zone. Similar results were reported by [PERSON] et al. [31] in rotary friction weld joints of alloy-800H. It was expected that the mechanical intermixing would result in better interfacial bonding compared to the weld interface without intermixing zone. As discussed earlier, the SS304L/IN718 ST showed prominent mechanical intermixing at the weld interface compared to the SS304L/IN718 STA sample. Moreover, the SS304L/IN718 STA also exhibited good weld joints due to the expulsion of the oxide layer and atomic diffusion as a result of the plastic deformation and higher temperatures experienced during friction welding. It is worth noting that in the present study, both samples failed in the HAZ of the SS304L side, due to the sound weld interface in both conditions. The SS304L/IN718 PWHT welded samples were observed to fail in the base of the SS304L side, due to the grain coarsening-induced softness on the SS304L side caused by the increased temperature exposure throughout the aging process. Similar observations were made by [PERSON] et al. [24] during the friction welding of Alloy 718 and SNCRW stainless steel.