Formability and Grained Structure Refinement of Cold-Rolled Friction Stir Welded AA5754 Sheet

Among different welding techniques currently available in the market, friction stir welding (FSW) is surely considered as an effective and reliable joining technique. Friction stir welds are characterized by lack of voids, cracks and distortions, as the technique does not involve any material fusion. The grained structure of the weld joint is generally finer than the base material. In particular, the nugget zone (NZ) experiences a dynamic recrystallization process during FSW that generally guarantees a stable very-fine grained structure. In the present study, the effect of cold rolling (CR) on the formability limits, the resulted microstructure, and mechanical response of FSW but joint was investigated. To this purpose, an AA5754 aluminium alloy was used. The FSW was performed with rotational and welding speeds equal to 1200 rpm and 100 mm/min, respectively, and an initial tool sinking of 0.1 mm. Strips extracted from the FSWed sheets were CR, with the rolling direction perpendicular to the welding line. Two setups were used in the CR experiments. One, conventional to determine the formability limit of the FSW AA5754 sheet; a second one performed in a CR set-up designed to induce an equivalent strain ϵ ∼ 1, in a single passage through the CR gage. This was aimed to make the post-FSW CR able to induce a further grained structure reduction within the NZ.