## 3 Results and Discussion  
To visualize the welding appearance of samples, the top views of the joint zone for samples (A-D) are shown in Figure 2. Throughout Figure 2, no serious macro defects such as lack of connection, grooves, flashes, and warm defects appeared. Moreover, in the case of samples with rotation speed of 950 rpm, a small material outflow was observed. The cross-sections of the joints are presented in Figure 3 and the advancing side (AS) and retreating side (RS) are labeled. In all samples, AA1050 was dragged into the AZ91 layer at the center of the weld. In some samples, a hook can be observed in the retreating side in which AZ91 was dragged into the AA1050 layer. Figure 3: (**a–d**) The cross-sections of the joints for A–D samples, respectively. (Welding condition was introduced in Table 3). Figure 2: (**a–d**) Top views of the welds zone of AA1050/AZ91 joints for A–D samples, respectively. (Welding condition was introduced in Table 3). It should be noted that one way to understand the chemical bonding mechanism between AA1050/AZ91 is their properties and incompatibility aspect. Generally, an impractical feature is a limited solubility governing the formation of IMCs which is explained by the binary phase diagram for these elements, see Figure 4 adapted from Ref. [7]. Nevertheless, the welding process is non-equilibrium, and to predicate the formation phases and possible reactions during welding, the equilibrium binary phase diagram is a standard guideline. Considering the fact that measuring the exact peak temperature during welding is almost impossible, using phase diagram is an acceptable method to envisage formation IMCs which depends on local temperature and composition. Throughout Figure 4, several phenomena can be comprehended: the binary system includes two eutectic lines at 437 and 450 \({}^{\circ}\)C which are lower than melting points of pure AA1050 and AZ91. Furthermore, three stable intermetallic phases including Al\({}_{3}\)Mg\({}_{2}\) (complex cubic \(\beta\)) phase, Al\({}_{12}\)Mg\({}_{17}\) (cubic of \(\alpha\)-Mg type \(\gamma\)) phase and rhombohedral R phase exist [7; 34]. As stated, involved parameters have pivotal role on the formation of microstructure. In order to visualize heat input's influence on the fracture behavior of the welds and find a correlation between welding parameters and joint strengths, the cross sections of the welds were studied by SEM in Figure 5. The SEM image of the weld nugget zone of sample A is shown in Figure 5a. A continuous layer of intermetallic compounds is not observed at the interface of AA1050 and AZ91. In Figure 5a, an SEM image of the interface at a higher magnification reveals the presence of a liquefaction region (highlighted in pink). This region appears to consist of two distinct phases. The SEM image of sample B is shown in Figure 5b at different magnifications revealing that the lamellar eutectic structures consisting of Mg + Al\({}_{12}\)Mg\({}_{17}\) were formed at the AA1050/AZ91 interface. At the interface of AA1050 and AZ91, a region was observed which consists of a continuous bilayer IMC and a liquefaction layer. This is consistent with another report in which the eutectic layer was formed in the Mg-rich region [51]. It should be noted that the liquefaction leads to eutectic formation in the stir zone which possesses a different formation mechanism compared to IMCs [7]. The SEM interface of sample C is shown in Figure 5c, and a very thin continuous layer at the interface of AA1050 and AZ91 was observed. Figure 5d shows the SEM image of sample D, and a very thin continuous layer can be observed at the interface of AA1050 and AZ91. Comparing SEM images revealed that in sample D, a small cavity on the AS, which is colored in the orange region in the nugget zone, emerged. This is probably due to lower heat input resulting in insufficient mixing and plasticization of the materials. Figure 4: Equilibrium phase diagram of Al-Mg system [7].