**2. Materials and Methods**

For the current investigations, three different materials namely 1060 Al (thick), 2219 Al (thin), and pure Cu as listed in Table 1 and three different tools such as high-speed cobalt taper-pin (HSCo) tool (HSCo), high-speed cobalt (HSCo) square-pin tool, and tungsten carbide (WC) square-pin tool were employed as shown in Figure 1 and specifications listed in Table 2. The stress–strain curve of each of the materials is shown in Figure 2 and the mechanical properties are presented in Table 3. The blank from each material was cut to the size of 100 mm × 70 mm, as shown in Figure 3. The surface oxides from the plates were removed using the abrasive paper. The plates were firmly held in a fixture shown in Figure 4. The FSW was performed along the long dimension of plates (i.e., rolling direction) utilizing the BYJC vertical milling machine. The tilt angle was kept constant at 2◦ and the dwell time ranged from 20–35 s. After welding, the strength of lap joints was characterized by conducting lap shear tests on the Universal Testing Machine 5567 (Instron Corp., Norwood, MA, USA): The geometry of the test sample is shown in Figure 5. To examine the microstructure and defects, samples (size: 16 mm × 16 mm) were cut using a CNC EDM wire cut machine and were ground with 220, 320, 500, 800, 1000, 2400, and 4000 abrasive papers of Silicon Carbide (Figure 6). The samples were thoroughly observed with the TESCAN scanning electron microscope (TESCAN, Brno, Czech Republic) and OLYMPUS B061 optical microscope (OLYMPUS, Tokyo, Japan). OMEGA infrared thermometer (OMEGA, Norwalk, USA) was utilized during friction stir welding to measure the temperature at the center on the top surface of the upper plate as shown in Figure 4.

**Figure 1.** Tool geometries employed in the present study: (**a**) HSCo tapered tool; (**b**) HSCo squared tool; and (**c**) WC squared tool.

**Figure 2.** Stress–strain curve for: (**a**) 2.15 mm 2219 Al, (**b**) 4 mm 1060 Al, (**c**) 3 mm Cu, and (**d**) 1.65 mm Cu.

**Figure 3.** Schematics of friction stir welding (FSW) lap sample.

**Figure 5.** Lap shear sample.

**Figure 6.** Microscopy and micro-hardness sample.



**Table 2.** Specifications of FSW tools.


**Table 3.** Mechanical properties of base metals.


### **3. Results and Discussion**
