**2. Material and Experimental Details**

Two technologically important age hardenable Al alloys, 7A52 and 6061, were employed in this study. These alloys were received in the form of 5 mm thick sheets. The chemical compositions of these alloys were determined, as listed in Table 1.


**Table 1.** Chemical composition of 7A52 and 6061 aluminum alloy.

The FSW were carried out while using a commercial gantry type FSW machine (FSW-LM-025-2030,Jiangsu Ruicheng Machinery Co., Ltd, Yixing, China) with position control mode. The friction stir butt welds between the rolled plates of 7A52 and 6061 aluminum alloys were obtained at a welding speed of 90 mm/min. and rotational speed of 500–700 rpm, while employing a single pass welding procedure. The plates were held firmly, covered by thermal insulation board, and then preheated at 150 ◦C for 2 h before welding. Figure 1 shows the welding direction. The FSW tool was made of die steel and had flat shoulder with truncated conical pin having anticlockwise thread of 1 mm pitch. Figure 1 demonstrates the profile of the FSW tool. The depth of shoulder plunge was kept 0.2–0.3 mm from workpiece surface.

**Figure 1.** Schematic illustration of the friction stir butt welding of 7A52/6061 dissimilar Al alloys and an image of actual friction stir welding (FSW) tool used in all welds.

Before welding, the as-received alloy plates were treated as solid solution (SS): 475 ◦C for 1.5 h for 7A52 alloy and 535 ◦C for 1 h for 6061 alloy, or further treated as T6-state after SS: aging at 120 ◦C for 24 h for 7A52 alloy and 180 ◦C for 30 min. for 6061 alloy. The SS treatment was carried out in a box-type furnace and the age treatment was carried out in oil bath. The SS treated samples and T6 treated samples were joined by FSW, respectively. After welding, different heat treatments were employed to strengthen the weld joint. They were solid solution treated again or aged directly. The parameters in the post-heat-treatment are the same as the pre-heat treatment. If the two side of the weld joint were aged respectively, one side being immersed into hot oil and the other side is uncovered.

The visually sound joints were selected by visual inspection or while using optical microscopy for hardness test and microstructure analysis. Microhardness variations across the joint were measured on a HMV-G 21DT Micro Vickers Hardness Tester with an applied load of 100 g and a holding time of 10 s, each value was obtained by averaging at least five successive measurements. With the help of indentation of the hardness test, the specimens for microstructure characterization were accurately extracted from specific zones. A focal-ion beam (FIB)/SEM system (Carl Zeiss Auriga 45-66, Jena, Germany) was employed to examine grains of FSW joints in electron back-scattered diffraction (EBSD) (Oxford Instruments, Oxford, UK) mode. An FEI Titan G2 60-300 transmission electron

microscope (TEM) (Thermo Fisher Scientific, Waltham mass, Massachusetts, USA) was used to examine the precipitates in different zones. Specimens for EBSD were prepared by mechanical polishing and then electrochemical polishing in a solution mixed of 10% perchloric acid and 90% ethyl alcohol at the voltage of 15 mV and at the temperature of −30 ◦C. The specimens for TEM were prepared as the following: the specimens were thinned down to 100 μm and then punched to form a disc with the diameter of 3 mm. These discs were mechanically ground and subsequently electro-polished while using a twinjet unit (TJ100-SE produced by LEBOscience) with an electrolyte of 30% nitric acid in methanol at −25 ◦C and 20 V. The specimens for microstructure examination were sectioned from the transverse section perpendicular to the welding direction, because the transverse section comprises all of the zones with different microstructures and associated mechanical properties.
