Metallographic and morphological characterization of sub-surface friction stirred channels produced on AA5083-H111

The friction stir channelling process derives from friction stir welding, where the process itself is modified in order to produce sub-surface channels by removing material from the bulk of the workpiece and leaving its upper surface closed and at the same level it had before being processed. In the present study, sub-surface friction stirred channels were produced on rolled plates of AA5083-H111. Channels' metallography and morphology were characterized using optical, scanning electron and confocal laser scanning microscopy. The microhardness profile at mid-thickness between the channel top and the upper processed surface, perpendicular to the channelling direction, was determined and the influence of the friction stir channelling process parameters on the microhardness distribution, near the channel, was assessed. It was observed that, the channel inner surfaces are topographically different: the retreating side is rougher than the advancing side and the channel bottom is smoother than its top inner surface. The average microhardness in the dynamically recrystallized zone (nugget), thermo-mechanically affected zone and heat affected zone is lower than that of the unprocessed base material. It was also found that the average microhardness of the nugget is not very sensitive to the tool rotation speed variation.