**3. Results and Discussion**

Heat-treatable aluminium alloys derive much of their strength from the presence of fine precipitates. As residual stresses and microstructures are of great significance in determining weld performance, they were studied together for thick 6005A-T6 aluminium alloy FSWs.

## *3.1. Microstructural Evolution*

Figure 4 shows the macrostructure of the 6005A-T6 single-sided and double-sided FSW joints. The NZ, thermo-mechanically affected zone (TMAZ), HAZ and BM are divided by the dotted lines. The TMAZ is more optically distinct on the advancing side and more diffuse on the retreating side in both specimens. The microstructures, including grain size and precipitation at the marked positions in Figure 4, are discussed further.

**Figure 4.** Macrographs (optical) of a single-sided FSW (**a**) and a double-sided FSW (**b**).
