## 1 Introduction  
Friction welding is a solid-state welding technique that does not require external heat but generates heat to raise temperatures by mechanical friction between the relative motion of the contact surface of the workpiece. With the addition of external force, the workpieces are subsequently plastically deformed to combine the materials. Extreme plastic deformation makes it possible to combine dissimilar and similar materials without melting the workpiece. Because no melting occurs, friction welding is a solid-state welding technology that does not relate to the fusion welding process. This method is different from intermetallic bonding/compound where intermetallic compounds are formed from electropositive and electronegative metals which chemically bond to form compounds with a specific composition and crystalline structure. Friction welding minimizes the likelihood of welding failures and influences the material over a small surface area, hence reducing stresses, flaws, and material loss [1]. Friction welding has a minor effect on the heat-affected zone because the heat generated by friction is below the melting point of the base metals [2] and has low residual strains and deformations [3, 4]. This approach also allows the connecting of similar and differing material qualities [5, 6, 7] and consumes less electricity than conventional electric welding. These benefits make FRW useful for a number of applications [1, 8, 9]. Thereafter, several varieties of friction welding are created, such as continuous or direct drive friction welding [8, 10, 11], inertia-friction welding [12, 13], linear friction welding [10, 14, 15], and orbital friction welding [16, 17]. Reviewing the present research, several advantages and downsides of the methodologies were outlined and analyzed. Continuousor direct drive friction welding, usually referred to as rotary friction welding (RFW) and linear friction welding (LFW), are two prevalent friction welding processes. A number of metallic structural joints with comparable and dissimilar materials have been successfully welded by using RFW and LFW. These well-established procedures with diverse applications have been widely utilized to link a wide variety of components [10; 14; 15]. Reviewing the present research, besides having several advantages which are mentioned above, friction welding methods still have some downsides and limitations, which were outlined and analyzed. The welding of parts needs relatively large cross-sectional areas due to the limited available power input of the spindle motor; therefore, welding of thin-wall structures such as tubes and plates remains a challenge. Moreover, the process of welding is successful for the material of at least one of the components to be plastically deformable. The fundamental idea of rotary friction welding is to separate the two components to be welded into one component that is held stationary and then pushes in the normal direction to rub against the other component that is rotating at a specified speed. In general, friction pressure comprises two stages: the friction stage, in which the two materials undergo an increase in temperature due to friction, and the forging phase, which consolidates the weld. As a result of the hot work, a flash is produced and the grain structure is polished at the contact interface. This is because heat is only created at the interface of the joints. Researchers have been studying the numerous factors influencing the properties of friction welding joints, such as pressure and friction time, pressure and forging time, rotational speed, oscillation frequency, oscillation amplitude, friction pressure, and their correlation with microstructure characterization, microhardness fluctuation, interfacial phase formation, and optimal welding parameters and mechanical properties [1; 10; 18; 19].