# Dissimilar Rotary Friction Welding of Inconel 718 to F22 Using Inconel 625 Interlayer
## 3 Results and Discussion
### Mechanical Properties  
The hardness line profiles taken across the weld interface for the Inconel 718/F22 dissimilar friction welded joints with and without the Inconel 625 interlayer are shown in Figure 7. The TMAZ on both sides of the weld interface of the Inconel 718/F22 friction weld joints displayed a decrease in hardness value compared to the corresponding base metals. However, a sharp increase in hardness at the weld interface was observed. It can be expected that the carbide phases from the F22 dissolved at high temperatures during the welding in the intermixing zone and HAZ. The dissolved carbon from the F22 layers in contact with the Inconel 718 layers tended to react with the strong carbide formers in the Inconel 718 and form carbides [16]. The carbide phases in F22 dissolved at temperatures above 1000 \({}^{\circ}\)C, and the carbon entered into the solution during the welding. Because of the higher solute diffusion in the austenite phase at higher temperatures, carbon in F22 diffused towards Inconel 718 across the weld interface. Inconel 718 contains higher Cr content compared to the F22, along with strong carbide formers such as Nb and Ti. These alloying elements in Inconel 718 resulted in lower carbon chemical potential and caused the carbon from F22 to diffuse towards Inconel 718 at the weld interface. Strain hardening of the intermixing zone and carbon enrichment of the Inconel 718 layers resulted in higher hardness values in the intermixing zone of the Inconel 718/F22 friction weld joints. The F22 side of the TMAZ showed lower hardness compared to the F22 base metal for the Inconel 718/F22 friction welded joints, as can be seen from the hardness profiles. F22 softened  
Figure 6: SEM micrograph of weldment at (**a**) 718/625 interface (5 mm burn-off) with EDS line scan and (**b**) 625/F22 (7 mm burn-off) interface with EDS line scan. at a relatively lower temperature than the Inconel 718 and thus experienced excessive plastic deformation of a wider zone. It was observed that the decrease in hardness in the TMAZ of the F22 was more significant in the region close to the weld interface. The higher strain energy and the higher peak temperatures experienced by the TMAZ regions of F22 close to the weld interface experienced recrystallisation followed by grain growth. It can be seen that the TMAZ of the Inconel 718 showed lower hardness and was much narrower compared to the TMAZ of the F22. Inconel 718 is a high-temperature alloy and experiences lower amount deformation compared to the F22 during the friction welding, resulting in a narrower TMAZ than the F22. This can be correlated with the formation of the relatively smaller flash of the Inconel 718 compared to the F22 flash in the Inconel 718/F22 friction weld joint (Figure 5b). Similarly, [PERSON] et al. reported a substantially higher flash on the EN24 side during the dissimilar friction welding of the Inconel 718 to the EN24 [16]. A decrease in the hardness of TMAZ of the Inconel 718 and F22 compared to the corresponding base metals was observed in the Inconel 718/F22 friction welded joints prepared using the Inconel 625 interlayer. However, no spikes in the hardness values were observed in the dissimilar joints prepared using the Inconel 625 interlayer. Inconel 718/F22 friction welded joints with and without the Inconel 625 interlayer failed in the base metal of the F22 during the tensile testing (Figure 8). The typical tensile plots of the Inconel 718/F22 friction welded joints with and without the Inconel 625 interlayer are shown in Figure 9. The tensile test results of the base metals and the welded joints, viz., yield strength (YS), ultimate tensile strength (UTS), and percentage elongation (%EI) values are tabulated in Table 3. The YS and UTS of the welded joints were comparable to the F22 base metal and lower than the Inconel 718 base metal. It is worth noting that the Inconel 718/F22 friction welded joints showed lower ductility compared to that Inconel 718/F22 friction welded joints with Inconel 625 interlayer.