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Ultrasonic vibration enhanced friction stir welding (UVeFSW) is a new variant of friction stir welding (FSW) in which a sonotrode transmits ultrasonic energy directly into the localised area of the workpiece near and ahead of the rotating tool. This study investigated the influence of ultrasonic vibration on the formation, microstructure and mechanical properties of butt welded 2024Al-T4 joints, and attempted to unveil the underlying mechanism of UVeFSW by experimental methods. Morphology inspection, X-ray detection and metallographic inspection of the welds revealed that ultrasonic vibration can improve the weld formation at higher welding speeds. The stir zone in the UVeFSW broadened, while the grains in the heat affected zone had no obvious growth contrary to that in the base metal. Results of the mechanical tests indicated that the tensile strength and elongation of joints, and the microhardness value in the stir zone increased at the same welding parameters.

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The strain and strain rate during friction stir welding was evaluated by measuring the distortion of the marker material. A thin Cu-40Zn foil as marker was inserted into the butting surface of two pure copper workpieces and the tool 'stop action' was employed. The results show that the strain in the shoulder-affected zone increases in a two stair-step shape as the material flows from the front to the rear of the tool, corresponding to the first accelerated and then decelerated flow stages. However, the strain at the two stages has opposite directions. In other words, a strain reversal occurs. Accordingly, the strain rate in the shoulder-affected zone varies in a sinusoidal shape. In the probe-affected zone, there is no obvious strain reverse occurring due to the formation of banded structures. The average strain rate during the band formation is significantly higher than the maximum strain rate in the shoulder-affected zone.

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Friction stir welding (FSW) with a relatively large heat input was applied to high purity Ag with a low stacking fault energy, which leads to profuse twinning. The microstructural evolution was examined along the material flow path during rapid cooling FSW. The frequent formation of annealing twinning induced by the heating caused the microstructural refinement, following the transformation from twin boundaries to normal high angle boundaries. The bulging of the high angle boundary, which is one of the typical mechanisms of discontinuous dynamic recrystallisation did not occur. Irrespective of the initial grain size, the grain size obtained after the FSW was approximately 8 µm due to the dynamic balance between the grain boundary migration and the annealing twin formation.