## 3 Results and Discussion
### Mechanical Properties  
#### 3.5.1 Microhardness  
Figure 14 depicts the microhardness profile across the interface of the dissimilar SS304L/IN718 ST, STA, and PWHT friction welds. The SS304L/IN718 dissimilar friction welds in the ST condition (335 \(\pm\) 4 HV) showed higher hardness values than those in the STA condition (300 \(\pm\) 5 HV) in the FDZ zone on the IN718 side near the weld interface. This is because the IN718 side of the SS304L/IN718 ST weldment experienced a greater strain hardening effect compared to the SS304L/IN718 STA weldment. It was expected that the IN718 side of the SS304L/IN718 STA would undergo a lower deformation compared to the ST condition, because of the strengthening \(\gamma^{\prime\prime}\) precipitates  
Figure 12: Scanning electron microscopy image in the secondary electron mode (**a**) inter-mixed Region (**b**) SS304L/IN718 in the as-welded ST condition and (**c**) the corresponding energy dispersive spectroscopy (EDS) line scan. in the IN718 STA base metal. As discussed earlier (Table 4), the FDZ of the IN718 side of the SS304L/IN718 ST weldment showed finer recrystallized grains than the SS304L/IN718 STA weldment. The finer grains in the FDZ of the SS304L/IN718 ST on the IN718 side resulted in a higher hardness. It was observed that the IN718 side FDZ of the SS304L/IN718 STA weldment showed lower weld zone hardness (300 \(\pm\) 5 HV) than the corresponding base material (415 \(\pm\) 5 HV) in the as-welded condition. This was due to the dissolution of the strengthening precipitates in the weld zone because of the higher peak temperatures experienced at the weld interface during the welding, while the IN718 STA base metal was in the peak hardened condition. In contrast, the IN718 side FDZ of the SS304L/IN718 ST weldment showed higher hardness values in the weld zone (335 \(\pm\) 4 HV) than the base material (305 \(\pm\) 5 HV), which could be attributed to the grain refinement of the FDZ, as a result of dynamic recrystallization. [PERSON] and [PERSON] [17] reported similar findings for the microhardness profile of IN718 linear friction welds. However, in the SS304L/IN718 dissimilar joints, the weldment with the PWHT condition (486 \(\pm\) 4 HV) exhibited the highest hardness compared to all three conditions (ST, STA, and PWHT) at the weld interface on the IN718 side of the weld. This increase in hardness at the weld interface on the IN718 side was attributed to the combined effects of precipitate formation by PWHT and the grain refinement. [PERSON] et al. [27] reported similar observations in previous investigations involving the friction welding of IN718 alloys. The hardness values of the dissimilar friction welds in the ST condition (305 \(\pm\) 10 HV) were greater than those of the welds in the STA condition (265 \(\pm\) 10 HV) in the TMAZ zone on the IN718 side of the weld. This was attributed to the relative coarsening of the grains in the STA condition compared to the ST condition (Table 4) and also to the higher strain hardening effect on the SS304L/IN718 ST samples than the STA  
Figure 13: Cross-sectional optical micrographs of dissimilar SS304L/IN718 friction welds in the PWHT condition: (**a**) FDZ, (**b**) TMAZ, (**c**) HAZ, and (**d**) BM regions. samples on the IN718 side of weldments. [PERSON] et al. [3] noticed similar variations of hardness values when studying the microstructural and mechanical properties of IN718-IN718 similar friction welded joints. The dissimilar welds in the STA condition exhibited a lower hardness (265 \(\pm\) 10 HV) in TMAZ than the BM (415 \(\pm\) 5 HV) in the as-welded condition. This was a result of the dissolution of strengthening precipitates in the TMAZ. [PERSON] et al. [32] found similar hardness patterns in the TMAZ region of the IN718 side of rotary friction welded IN718 to IN600 joints. The IN718 side TMAZ of the SS304L/IN718 ST dissimilar joints showed higher hardness values (305 \(\pm\) 10 HV) than the BM (305 \(\pm\) 5 HV).