**2. Materials and Methods**

Rolled AA5A06-H112 and AA6061-T651 plates with a thickness of 5 mm were employed as base materials, and the chemical compositions obtained by energy dispersive spectroscopy (EDS, Carl Zeiss SMT Pte Ltd., Oberkochen, Germany) are listed in Table 1. The plates were cut into small rectangular plates measuring 165 mm × 5 mm. These rectangular plates were welded in butt joint configuration using FSW by placing AA5A06 on the advancing side and AA6061 on the retreating side. A welding machine HT-JL10X12/2H (Shanghai Aerospace Equipments Manufacturer Co., Ltd., Shanghai, China) with a forced air cooling system was employed to carry out FSW. Both natural cooling (NC) and forced air cooling (FAC) conditions were considered in the FSW process. For FAC, forced air was blown on the welded area through a nozzle, as shown in Figure 1a. The rectangular nozzle with a size of 10 mm × 2 mm was placed 20 mm behind the tool and about 20 mm above the surface of the materials. The pressure of the forced air was 0.5 MPa, and the blowing direction was along the welding direction and had an intersection angle of 30◦ with the surface of the materials. The tapered left-hand threaded cylindrical tool used for FSW was made of "H13 steel", and the dimensions are shown in Figure 1b. During the welding process, the tool was plunged into the butt surface of the two base materials, with a tilt angle of 2.8◦ and a depth of 4.96 mm. Three different tool rotational speeds (RS)—i.e., 600, 900, and 1200 rpm—and two welding transverse speeds (TS)—i.e., 100 and 200 mm/min—were applied. It has been reported that the left-hand thread pin tool rotating clockwise generates better FSW joints [25]. Thus, a clockwise rotation direction of the tool was used in the experiments. As shown in Figure 1c, the welded AA5A06-AA6061 plates were processed into dumbbell-shaped specimens for uniaxial tensile tests and small rectangular specimens for nanoindentation and microstructure characterisation.

**Materials Si Fe Cu Mn Mg Cr Zn Ti Al** AA5A06 0.40 0.26 0.06 0.86 5.37 - 0.10 0.11 92.84 AA6061 0.79 0.70 0.35 0.08 1.46 0.17 0.08 0.21 96.16

**Table 1.** Chemical composition of AA5A06 and AA6061 (wt. %).

**Figure 1.** (**a**) Schematic of friction stir welding (FSW) with forced air cooling, (**b**) geometry of the tool, (**c**) specimens for uniaxial tensile tests, nanoindentation, and microstructure characterisation.

For microstructure characterisation, the side surface of the specimen was polished to a mirror finish using an automatic polishing machine (Shenyang Kejing Auto-instrument Co. Ltd., Shenyang, China) with 0.3 μm alumina powder. The surfaces were anodic coated and then observed using a polarizing microscope (Carl Zeiss AG Co. Ltd., Guangzhou, China). To obtain the hardness distribution in the joint area, nanoindentation tests were implemented in an Agilent Nano Indenter G200 system with a modified Berkovich indenter (Agilent Technologies, Oak Ridge, TN, USA). The nanoindentation hardness mapping method was used and an array consisting of 56 × 10 indents was carried out on the side surface of each specimen, as shown in Figure 1c. For each nanoindentation test, the maximum indentation load was 120 mN, and the thermal drift was controlled within 0.05 nm/s.

For uniaxial tensile tests, the dumbbell-shaped specimens had 1 mm of thickness machined away from both the top and bottom surface to ensure they had a uniform cross section. A material testing system, MTS CMT4204 (MTS System Co. Ltd., Shanghai, China) was employed to conduct the tensile tests. All the specimens were stretched to break at a tensile strain rate of about 0.04 s−<sup>1</sup> at room temperature. The average value of the ultimate tensile strength was used for analysis. Scanning electron microscopy (SEM, Carl Zeiss SMT Pte Ltd., Oberkochen, Germany) was employed to analyse the microstructures of the fracture surface.
