## 1 Introduction
As is well documented, increasing rotational speed eventuated in higher heat input during FSW [23; 24; 25; 26]. Therefore, to gain higher ultimate tensile strength (UTS), the heat input could be controlled by tool traverse and rotational speed [27]. However, sufficient heat input results in recrystallization through the weld zone leading to the formation desired microstructure [9]. In addition to the influence on heat input, tool rotational speed affects the fracture position of the weld, i.e., increasing rotational speed not only resulted in broadening the strained zone, but also the position of the maximum strain region moves from the retreating side (RS) to the advancing side (AS) of the weld [28]. The intermixing leads to the formation of intermetallic compounds (IMCs) in the stir zone which detrimentally affects joint quality, ductility, and the mechanical properties of the joint [29; 30]. Based on various studies, IMCs are able located in three positions: (1) coherent IMCs layer at the interface, (2) IMCs fragments in Mg matrix, and (3) IMCs fragments in the Al matrix [31]. Due to enhanced diffusion during welding, although the restriction of formation and growth in IMCs are not accessible, reducing the thickness of IMCs is one way to enhance the weld strength which could be achieved by reducing the frictional heat input [32; 33]. The formation of IMCs depends on heat input and local composition, regardless of the chemical composition of bare material. The IMCs formation at the interface of Al and Mg in FSW joints is attributed to diffusion or eutectic reaction mechanisms [34]. In addition to welding parameters, heat is governed by plastic deformation (E\({}_{\text{d}}\)), friction (E\({}_{t}\)) at the tool/workpiece interface, and viscous dissipation (E\({}_{\text{v}}\)) which is controlled by a difference in friction coefficient, liquid susceptibility, and deformability of Al and Mg [14]. For instance, the eutectic reaction (Mg + Al\({}_{12}\)Mg\({}_{17}\)\(\rightarrow\) L) resulted in the formation of liquid films causing reduced resistance to tool rotation due to tool slippage and interfering with plastic deformation [35]; consequently, due to declining E\({}_{\rm d}\), E\({}_{\rm f}\), and E\({}_{\rm v}\), heat generation is significantly decreased. Deformability strictly depends on crystal structure, e.g., in the case of Al/Mg, Al possesses a face-centered cubic (fcc) structure whereas Mg has a hexagonal close-packed (hcp) structure offering 12 and 3 slip systems, respectively. As a result, Al deformability is much better than Mg, and heat is generated through E\({}_{\rm d}\) and E\({}_{\rm v}\)[34]. Considering that the FSW process temperature remains below eutectic temperatures; therefore, the conclusion can be made that the formation of IMCs at the interface is associated with diffusion between Al and Mg atoms [36]. On the other side, despite the fact that the welding temperature is lower than the eutectic temperature, the liquation/melting due to heat produced by the rotation tool forms a thin liquid film along grain boundaries leading to growth in IMCs [37]. It is worth mentioning that the high rotational speed may cause defects formation and imperfect joining at the interface, resulting in lower tensile strength [38]; as a result, tensile strength is affected by thermal history (i.e., heat input and welding time). Considering the aforementioned approaches, maintaining the temperature below the eutectic temperature is a practical way to restrict the formation of IMCs or reduce IMCs thickness. The weld strength is mainly influenced by two phenomena: the existence and distribution of IMCs as well as the material interlocking phenomenon [39; 40]. The interlocking of materials at the interface is one way to strengthen welding; to be more accurate, since the heat input is maintained below the eutectic temperature to control IMCs formation, weld quality could improve by mechanical interlocking [41].