Metal Flow during Friction Stir Welding of 7075-T651 Aluminum Alloy

Bronze foil of 0.1 mm thickness was placed between faying surfaces of two plates to be butt-welded as marker material to reveal the flow behavior of weld metal during friction stir welding of 7075-T651 aluminum alloy. By tracing the bronze foil fragments in the weld after welding, the metal flow behavior during the welding process was revealed. Besides, the tool forces in the welding process were measured by the octagonal loop resistance turning dynamometer to expound the periodic variation of metal flow pattern. Results show that the flow behavior of the weld metal is different along the thickness direction. The flow pattern presents a periodic variation, and a formula has been proposed to calculate the periodicity of the metal flow. In addition, the weld nugget zone presents a “spoon” shape and the fine grains at the spoon handle and those at the spoon bowl are originated from different zones. A plastic metal flow model in FSW was proposed based on the results. Furthermore, the formation of defects was explained by researching the weld metal flow behavior.