# Dissimilar Rotary Friction Welding of Inconel 718 to F22 Using Inconel 625 Interlayer
## 3 Results and Discussion
### Mechanical Properties
Further, a narrow region that was not deformed can be observed at the interface of the Inconel 718/F22 friction welded joints without the Inconel 625 interlayer (Figure 8a). The formation of carbides and strain hardening in the intermixing zone resulted in the lower plastic deformation of the intermixing zone of the Inconel 718/F22 friction welded joints. It can be suggested that the Inconel 625 interlayer present between the alloys being joined, viz., Inconel 718 and F22, arrested the formation of the carbides and subsequent hard intermixing zone. The Inconel 718/F22 friction welded joints with the Inconel 625 interlayer displayed higher %EI values (14 \(\pm\) 0.2%) compared to the weld joint without the interlayer (12 \(\pm\) 1.5%). This higher  
Figure 7: The hardness distribution across the weld interface of the 718-F22 friction weld joint with and without 625 interlayer. ductivity of the weld joints prepared using the Inconel 625 interlayer could be attributed to the absence of the weld interface's embrittlement because of the interlayer's presence between Inconel 718 and F22. From the failed tensile samples (Figure 8) it can be seen that the tensile deformation was mainly on the F22 side of the weld joint. F22 displayed lower strength compared to the Inconel 718 and plastically deformed at lower stresses during the tensile testing. Figure 8: Tensile failure location of the (**a**) 718-F22 friction weld joint and (**b**) 718-F22 friction weld joint with 625 interlayer. Figure 9: Typical tensile curves of base metals and friction weld joints. Charpy impact energy values of the base metals and the weld joints are shown in Table 3. F22 base metal absorbed higher energy during the impact testing compared to the other samples. The Inconel 718/F22 weld joint without the interlayer absorbed lower energy compared to the either of the corresponding base metals. This embrittlement of the Inconel 718/F22 weld joint at the weld interface can be attributed to the carbon diffusion at the intermixing zone and strain hardening. It is interesting to note that the tensile weld samples failed in the F22 base metal during the tensile testing of the weldments prepared with and without the interlayer, and hence the elongation values were close to the F22 base metal. Alloy F22 yielded at lower tensile stresses compared to the Inconel 718, and as a result, the tensile deformation was observed predominantly on the F22 side, as seen in Figure 8. However, the notch was machined at the weld interface for the Charpy impact testing of the weldments. The Inconel718/F22 weld joint absorbed the lowest energy among all the samples due to the embrittlement of the weld interface by the carbide formation. For the welds prepared using the interlayer, the notch was positioned at the Inconel 718/Inconel 625 interface for half of the samples, while the other half notch was at the Inconel 625/F22 interface. It can be seen that the Inconel 625/F22 interface displayed higher impact strength compared to the Inconel 718/Inconel 625 interface for the Inconel 718/F22 weld joints prepared using the Inconel 625 interlayer. Further, the impact strength of the Inconel 718/F22 weld joints prepared using the Inconel 625 interlayer was significantly higher compared to the dissimilar welds prepared without the interlayer. The presence of the Inconel 615 interlayer had eliminated the formation of the embrittleled region at the weld interface. It improved the impact strength of the weldment when Inconel 625 interlayer was used for the weld joint preparation. The tensile fracture surface of the base metals at weld joints is shown in Figure 10. Inconel 718 fracture surface showed fine dimpled fracture features indicative of the ductile fracture. However, the carbides present in the Inconel 718 showed brittle cleavage fracture features. F22 base metal and the welded joints showed fibrous and dimples fracture features indicative of the ductile fracture.