## 2 Materials and Methods  
Here, aluminum AA1050 and AZ91 magnesium with thicknesses of 3 mm were used and the chemical composition of the bare sheets are given in Tables 1 and 2. To remove surface oxide layers and contaminations, the AA1050 and AZ91 sheets were cleaned with acetone followed by scratch wire-brushing. The sheets were placed in an AA1050/AZ91 lap joint configuration and the welding process was carried out with different welding traverse speeds and tool rotation speeds; the whole process is depicted in Figure 1. The used tool was made of an H13 tool steel containing the shoulder with diameter of 18 mm and the pin diameter of 5 mm and length of 4.7 mm. The tool tilt angle was maintained at 2.5\({}^{\circ}\) for the tool during the joining process and the formation of welds. At the constant inclination angle, to investigate the influence of tool traverse and rotational speed, the samples with different tool rotation speeds and the traverse speeds were fabricated and listed in Table 3. In the welding process, as schematized in Figure 1, the pin was penetrated into the bottom layer (AZ91).  
To visualize the morphology and chemical analysis of samples, scanning electron microscopy (SEM, model: Mira2 TESCAN) equipped with an energy-dispersive X-ray spectrometer (EDS) was used. To this end, samples were prepared using a standard metallographic procedure, which involved cutting, grinding, and polishing the samples to a mirror finish. In order to investigate shear-tensile test, different specimens were prepared for, as depicted in Figure 1.  
To prepare samples for tensile experiments, ASTM D3164 was considered as a guideline. Shear-tensile tests were carried out with a constant crosshead velocity of 2 mm/min. As shown in Figure 1, during the shear-tensile test, to adjust the offset in the lap joint, spacer  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Al** & **V** & **Zn** & **Mn** & **Fe** & **Si** & **Cu** \\ \hline Balance & 0.010 & 0.050 & 0.050 & 0.42 & 0.2 & 0.08 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Chemical composition of AA1050 (wt%).  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & **Welding Speed (mm/min)** & **Tool Rotation Speed (rpm)** \\ \hline A & 23.5 & 950 \\ B & 37.5 & 950 \\ C & 23.5 & 600 \\ D & 37.5 & 600 \\ \hline \hline \end{tabular}
\end{table}
Table 3: The welding parameters used for friction stir lap welding of AA1050/AZ91.  
Figure 1: An illustration of the welding process representing the geometry of the tool and the location of the samples for the performed tests. The lap configuration was Al/Mg and the tool was inserted into Al. The advancing side was placed in aluminum. The length of the weld was 100 mm.  
sheets were used. To avoid experimental errors and ensure repeatability, three shear-tensile specimens were examined. The offset of lap joint was achieved by using spacer sheets located between the clamps and the specimen ends, as seen in Figure 1.  
The Vickers hardness test method was utilized, which is a widely accepted method for measuring the hardness of metallic materials. This method involves applying a load to the specimen using a diamond indenter and measuring the size of the resulting impression to calculate the hardness. The microhardness measurement line for samples in AA1050 and AZ91 sides is depicted in Figure 1.