Aspetti microstrutturali di un giunto friction stir welded tra una lega di magnesio (AZ31) e una di alluminio (AA6061)

This work presents a microstructure characterization of dissimilar FSW of AA6061 and AZ31B. Metallographic inspections were carried out either by optical microscopy (OM), and by scanning electron microscopy (FEG-SEM) techniques. OM was performed to obtaining the FSW panoramic survey and to determining the mean grain size in the nugget zone. To determining the chemical nature of the detected secondary phase particles within both AA6061 and AZ31B, in the nugget zone, FEG-SEM was coupled with energy-dispersive spectrum analysis. Microhardness measurements were carried out along the welded joint in the advancing, retreating, and centreline sides. Nugget was characterized by diffuse dynamic recrystallization phenomena, inducing a refined equiaxed grained structure. The fine grains in the nugget zone were formed by the rotating pin by the combined effect of shear deformation and high-temperature exposure followed by slow cooling to room temperature after the pin passage into the nugget. Microhardness vales were observed to be lower in the nugget, although fine grains characterized the microstructure. The FEG-SEM inspections, and the microanalyses data, clearly revealed a diffuse presence of secondary phase fine particles in the nugget zone, either within the AA6061, and within the AZ31B deformed grains. These were induced to form and precipitate during the passage of the pin (that is during the high-temperature excursion and the shear plastic deformation, followed by a slow room temperature cooling).