Effect of shoulder geometry and clamping on microstructure evolution and mechanical properties of ultra-thin friction stir-welded Al6061-T6 plates

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Friction stir welding (FSW), especially high-speed FSW, was introduced to provide an optimum welding method for ultra-thin Al6061-T6 plates that would improve their mechanical properties. The influence of the shoulder geometry and clamping distance on the joint forming, microstructure evolution, and mechanical properties of a 0.8-mm-thick FSW Al6061-T6 joint was investigated. The joint forming, grain structure evolution, precipitate distribution, and tensile properties of the joint were obviously affected by the shoulder geometry and clamping distance. Sound FSW joints were obtained at both a conventional speed and high speed using the tool with a three-helix concave shoulder. In addition to forming a large number of high-density dislocations and substructures, a large number of Mg2Si, Al2CuMg, and Al8Fe2Si precipitates were reprecipitated in the nugget zone during high-speed FSW, and the number of precipitates was significantly higher than at the conventional speed. As the clamping distance increased, in addition to poor joint forming, the effective thickness of the FSW joint was also gradually reduced. The microhardness distributions were mainly affected by the process parameters and hardly affected by the shoulder geometry and clamping distance especially during high-speed FSW. A combination of the tool with a three-helix concave shoulder and the small clamping distance was suggested to be the optimal path to produce excellent mechanical properties in a high-speed FSW joint.