# Dissimilar Rotary Friction Welding of Inconel 718 to F22 Using Inconel 625 Interlayer
## 1 Introduction  
Carbon and low-alloy steels in quenched and tempered conditions contain finely dispersed cementite in the microstructure. This dispersed cementite tends to form micro galvanic cells and has been reported to cause severe corrosion in water containing hydrocarbon fluids and dissolved CO\({}_{2}\) during oil and gas production [1]. To minimise the CO\({}_{2}\) attack of components used in oil and gas production, researchers have employed steels that form adherent high-quality chromia film with the addition of 1-5 wt% Cr. Alloy F22 is one such grade with 2-25 wt% Cr and a small amount of Mo (\(\approx\)1%), which shows moderate corrosion resistance in aqueous environments. However, under mild to severe corrosive and high-temperature conditions, nickel-based superalloys are the materials for structural designers. Inconel 718 is a widely used Ni-based alloy in various applications ranging from structural components in power generation plants to highly specialised blades in jet engines [2]. Having properties such as superplastic behaviour, ease of machinability, and weldability, although not as attractive as some other alloys, Inconel 718 displays high strength properties at unusually high temperatures (about 650 \({}^{\circ}\)C) [3]. Apart from a good strength and corrosion resistance, Inconel 718 also displays excellent toughness at cryogenic temperatures and hydrogen embrittlement resistance. These facts push Inconel 718 high up in the desirability index of most designers of very special high-end applications. Inconel 718 derives its high strength property from the age-hardening phenomenon [2]. In Inconel 718, the matrix is a solid solution of Ni and Fe. An optimum percentage of intermetallic and carbide precipitates distributed in the matrix gives the alloy outstanding properties [4]. Most important of all the precipitates are \(\gamma^{\prime}\) (Ni\({}_{3}\)(Al, Ti, Nb)) and \(\gamma^{\prime}\) (Ni\({}_{3}\)Nb). These precipitates remain coherent with the matrix, even at high temperatures, retarding the dislocation movement and thus rendering the strength properties to the alloy. Often it is required that Inconel 718 has to be joined with other materials, making it the so-called dissimilar weld. [PERSON] et al. suggested that Inconel 718 to stainless steel joints find applications in power generation plants [5]. [PERSON] et al. reported a need to obtain strong and clean joints between Inconel 718 and stainless steel to construct certain sections of the internal space station [6]. [PERSON] and [PERSON] have reported that the Inconel 718 and 3105 dissimilar joints find applications in land-based gas turbines [7]. Notwithstanding the enormous advantages that these dissimilar welds joints can offer, often it is inherently a challenge to join dissimilar materials. This is truer when the materials in question are on the opposite ends of a spectrum concerning physical and metallurgical properties. For instance, a highly conductive material is harder to join with a less conductive material. Similarly, while joining alloys with a wide difference in melting temperatures, it becomes difficult to manage the melting and solidification behaviour of the alloys. Even after assuming that the above problems could be managed by manipulating the weld setup, the metallurgical incompatibility plays the role of a spoilsport. An alloy that does not dissolve into each other in a solid state would often form hard intermetallic compounds that render the whole joint brittle. In this regard, a transition joint solves many of the problems faced in the direct welding of dissimilar alloys. A transition joint is one that has an interlayer made of the third alloy, and this layer mitigates the problems such as mismatch in physical properties, but most importantly, mismatch in metallurgical solubility [8; 9; 10]. By identifying a suitable interlayer, we may find that the harmful intermetallic phases could be significantly reduced. [PERSON] et al. welded stainless steel to copper using Ni as an interlayer [11].