# Dissimilar Rotary Friction Welding of Inconel 718 to F22 Using Inconel 625 Interlayer
## 1 Introduction
Porosity and liquidation cracking problems present in the direct joints disappeared with the use of Ni interlayer. Ni also proved helpful while joining the stainless steel to a high melting point metal, namely, molybdenum [12]. Some authors have used even multiple interlayers to stop the inter-diffusion of highly incompatible base materials into each other. [PERSON] et al. used a Cu/Ta multi-interlayer to weld highly incompatible Inconel 718 and Ti-6 Al-4V joint combination [13]. Similarly, [PERSON] et al. positioned Ni/Cu in the middle of aluminium to steel weld joints [14]. Metallurgical compatibility of the interlayer with both the base metals is a prime criterion for selecting the interlayer while preparing the dissimilar weld joints using the interlayer. The shift to advanced ultra-supercritical (AUSC) plants for power generation is mainly because of the higher power generation efficiency and lower carbon footprint [15]. Ferritic/martensitic steels are used for the thicker sections of the boiler structure. In contrast, the use of super austenitic steels and nickel base superalloys is limited to thin sections because of the high cost of these alloys. Joining of superalloys to the high strength steels can be done by mechanical fastening for turbine engine applications [16]. However, fusion welds display superior strength and fatigue performance compared to mechanical joining, especially in applications such as power generation [16]. Re-heater tube, superheaters, and boiler header components employ dissimilar steel/superalloy joints for improved performance and economics [15]. 2.5 Cr-1 Mo (F22) steel is the most commonly used power plant steel, showing good strength and resistance to long-term high-temperature exposure. However, the dissimilar superalloy/high strength steel fusion weldments showed enrichment of C and Fe because of the dilution and displayed solidification-related cracking behaviour [16]. The development of dissimilar joints between Inconel 718 and F22 is advantageous technically and economically in applications such as nuclear power generation and repair welding [17]. However, the carbon content in F22 is higher compared to the min imum requirement in the Inconel 718. Carbon, being an interstitial alloying element, shows higher diffusion from the F22 to Inconel 718, compared to the other heavy elements during the joining process. It was reported that the NbC and Laves phases promote microfissuring and hot cracking in the Inconel 718 welds by forming the incipient liquid films at the intergranular regions during the weld thermal cycle and solidification, respectively [18]. The results of the spot varestraint tests revealed that there was an 18% increase in cracking susceptibility with the increase in carbon content (0.02 to 0.06 wt%) in cast Inconel 718 alloys. An interlayer that shows metallurgical compatibility with F22 and Inconel 718 can be suggested as a suitable solution for preparing Inconel718/F22 dissimilar joints. Nickel-based corrosion-resistant alloys are used as coatings on alloy steels for oil and gas industry applications. Interestingly, the Inconel 625 is commonly used for the weld overlay on high-strength alloy steels such as F22 because of its good metallurgical compatibility. Inconel 625 is a weld overlaid on F22 for subsea oil platform applications to provide corrosion resistance [19; 20]. As a nickel-based superalloy, Inconel 625 is a suitable candidate material as an interlayer to join Inconel 718 to the high-strength alloy steel F22. Good weldability, high-temperature oxidation, and corrosion resistance in hostile environments make Inconel 625 a Ni-based solid solution strengthened superalloy, a popular candidate for marine, petrochemical, refinery, and aero industries [21]. High-temperature strength of this alloy is improved by adding Mo and Nb, while Cr results in resistance to oxidation and corrosion. Additional strengthening of the Inconel 625 is achieved by carbide-forming elements such as Nb, Ti, and Mo.