Material Flow in Friction Stir Welds

Friction stir welding generates periodic features within the weld. These “onion ring” features are associated with variations in both texture and the orientation of that texture along the length of the weld. Analysis of an AA2195 friction stir weld reveals the presence of periodic oscillations between the dominant B and \( \overline{\text{B}} \) components of the ideal shear texture, suggesting a periodic reversal in the predominant shear orientation during welding that is inconsistent with current understandings of the friction stir welding process. Microstructural features present in the weld and machine force variations during welding indicate that these textures may arise from the oscillation of an off-centered tool. Such a tool oscillation can generate a periodic extrusion of material around the tool, giving rise to the observed flow features, machine force variations, and reversals of the local shear texture orientations. A new model of material flow during friction stir welding is proposed to explain the observed features.