**4. The material**

In both the cases, aluminum alloys were tested. In the first methodology, a heat-treatable AA6000-series alloy (AA6082) was used; whereas in the second methodology, a nonheattreatable AA5000-series alloy (AA5754) was welded.

The AA5000-series alloys, such as the AA5754, are widely used in automotive, aerospace, marine, and military applications. They are characterized by a good strength-to-weight ratio, an appropriate weldability, and a good corrosion resistance. This class of aluminum alloy is difficult to join by conventional fusion welding techniques. This is mainly due to a dendritic structure, which typically forms in the melted zone and it seriously weakens the mechanical properties of the joint AA5000 series alloys. In this context, FSW has emerged as a promising solid-state process with the successfully overcome the fusion welding problems, making the welding process of AA5000 series alloys a sound one [23]. FSW of AA5754 is thus a promising technique to obtain sound welded joints, either in similar [24, 25] and dissimilar [26, 27] welding combinations (using AA5757 or AA5083).

In the present case, the AA5754 sheets were produced by twin roll continuous casting followed by cold rolling to give a H111 (EN485) metallurgical initial status and a thickness of 2.5 mm.

The AA6000-series are widely used because of their good weldability, corrosion resistance, and immunity to stress-corrosion cracking. These are known to be among the most used aluminum alloys for extruded components [28]. In fact, AA6082 (Al-Mg-Si) typical application include aeronautics, automotive, and recreation industries. In the present case, cold-rolled sheets of AA6082 were used to show the soundness of the DS-FSW.
