## 3 Results and Discussion
On the other hand, the fracture on the AZ91 side was completely brittle and from the grain boundary. The failure of sample D occurred between the Mg and Al\({}_{12}\)Mg\({}_{17}\) intermetallic compound, which is continuously formed at the weld boundary, as shown in Figure 5d. Overall, the presence of intermetallic compounds and the lamellar structure resulted in brittle fracture in some areas, whereas the presence of ductile fracture in other areas is likely due to the plastic deformation of the aluminum matrix. These findings suggest that the microstructure and composition of the materials play a crucial role in determining the fracture behavior of the welded samples. From the shear-tensile test and fracture morphology, it could be concluded that the relation between brittle fracture and crack propagation is associated with the formed brittle IMCs at the stir interface of AA1050/AZ91, which is consistent with other reports [52; 53; 54]. The microhardness distribution across the weld line is shown in Figure 9. The microhardness results show an increase in hardness within the welding zone, followed by a  
Figure 3:0 SEM image of (**a\({}_{1}\)**–**d\({}_{1}\)**) Al side and (**a\({}_{2}\)**–**d\({}_{2}\)**) Mg side of the fracture surface from samples A-D, respectively. rapid decrease as one moves away from the central weld location. Two distinct hardness distribution profiles were evident on either side. Throughout Figure 9, the results indicate that the Al side of sample A has the highest hardness. The presence of intermetallic compounds in the nugget zone caused a sudden increase in hardness in the weld zone, and the accumulation of these compounds resulted in higher hardness compared to the base metal. Furthermore, Figure 9a,b revealed that there was a significant increase in hardness in some locations, which is probably due to the formation of eutectic in the stir zone. The coincidence of hardness values in AA1050 and AZ91 is attributed to drag of the layers into each other. Though SEM analysis revealed the presence of intermetallic compounds such as Al\({}_{2}\)Mg\({}_{3}\) and Al\({}_{12}\)Mg\({}_{17}\) at the welding boundary in samples C and D, the hardness values are lower in these samples. This is due to two factors. First, the volume of IMCs formed in these samples is lower and second, the IMCs layers are thin and dispersed in the matrix. Considering the discussed results, the correlation between mechanical properties of the joint and welding parameters can be explained as follows:  
At a low rotation rate of the tool, a thin intermetallic compound forms at the Al-Mg interface due to the low heat generation. Therefore, a high level of weld strength can be obtained. In our cases, the fracture surface is almost composed of brittle material. At a high tool rotation speed, a severe mixing of Al and Mg occurs in the stir zone. The severe mixing along with the high local temperature results in local diffusivity of Al and Mg which in turn promotes liquid Al\({}_{12}\)Mg\({}_{17}\)-Mg eutectic compound formation. The formation of this liquid compound decreases the friction coefficient between the tool and the material; consequently, the temperature during welding decreases. This decline in  
Figure 3:1 (**a–d**) Hardness profile of the joints for sample A–D. The insect picture in panels show the microhardness measurement line of specimens (red line: AA1050 side and blue line: AZ91 side). the intermetallic growth leads to the formation of a very thin layer of Al-Mg intermetallic compounds at the interface. Whereas this eutectic layer consists of brittle (Al\({}_{12}\)Mg\({}_{17}\)) and ductile (Mg) layers, the fracture takes place in this region. The EDS study of the fracture mode confirmed the presence of a mixture of ductile and brittle materials.