**6. Concluding remarks**

In this contribution, two novel approaches and methodologies of friction stir welding on aluminum alloys were presented. The first approach consists of a double-side FSW (DS-FSW). The second approach is represented by a radial deviation of the rotating pin from its centerline, during FSW (RT-FSW). Both new methods were tested in a conventional pin and nonconven‐ tional pinless configuration. Several interesting achievements, from a technological point of view, were obtained and are here summarized.

DS-FSW:

*5.2.4. Defect and void formation during FSW*

reduction ranging from 76 to 30%.

tional FSW), 0.5, and 1 mm.

centerline, in the case of non-annealed AA5000 FSW.

FSW.

22 Joining Technologies

Traces of the presence of oxide layers (the lazy S-lines) are evident in the NZ microstructure (**Figures 12** and **13**). These, actually, follow the location of the fine grain strips. It thus appeared that the fine grains are formed where the oxide layers, the lazy S-line oxides, are present, and were formed at *R* = 1 mm of RT-type FSW. Thus, it appeared that the fine grain strips, at *R* = 0.5 mm, are being formed along already existing lazy S-line oxide, which formed during

*5.2.5. Mechanical properties and hardness modifications induced by pre- and post-welding annealing*

Typical stress-strain curves are shown in **Figure 14**. It appeared that the closest mechanical response to the unwelded annealed AA5754 sheet is obtained by welding with *R* = 0.5 mm in the PWA condition, where UTS differed only by 5%, and ductility differed by 30% with respect to the ductility of the unwelded annealed condition. In the other conditions, the UTS remained within a range of 14% of difference, with respect to the annealed sheet, with a ductility

**Figure 14.** Tensile stress-strain curves for RT-FSW, in the AA5754-O stare and in the PWA condition, at *R* = 0 (conven‐

Therefore, based on the microstructure evidence, and the obtained hardness and mechanical response, the use of a RT-type welding motion is justified when the plate is homogenized prior, or, even better, after FSW. Conversely, there is no need to deviate the pin, from its welding

DS-i: the elastic modulus and the hardness showed a larger uniformity across the sheet section, with respect to the FSW;

DS-ii: A better formability of the DS-FSW, compared to the conventional pin and pinless FSW, was obtained;

DS-iii: The DS-FSWed joints are characterized by LDH, and FLC values higher than those measured on conventional FSW.

RT-FSW:

RT-i: The RT setup, for a pin rotation radius of 0.5 mm, induced a low reduction of the mechanical response, compared to the conventional T setup FSW (i.e., with no pin deviation from the welding line). Accordingly, both the microstructure and the hardness profiles of all the characteristic welded zone were quite similar;

RT-ii: The post-weld annealing (PWA) showed the best mechanical response respect to the unwelded annealed AA5754 sheet. The best experimental setups were obtained setting a pin rotation radius *R* = 0.5 mm. In this configuration, UTS was 15% higher, and a ductility reduction of up to 30%, respect to the unwelded annealed sheet. In this condition, the micro‐ structure of the NZ appeared to be characterized by very coarse grains. These coarse grains were generated by geometric dynamic recrystallization (GDR), which is induced by the combined effect of shoulder pressure (heat input), and post-welding annealing (PWA) thermal energy.
