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Keyhole is a defect which seriously restrains the industrial application of friction stir welding (FSW) technique. In this study, a new technique of non-keyhole friction stir welding (N-KFSW) was proposed, which could eliminate the keyhole during the in-situ welding process. Experiments were performed using a 6 mm thick 6061-T6 aluminum alloy plate under different rotating velocities. Due to the unexpected consumption of rotating sleeve during N-KFSW, the N-KFSW tool was simplified to a system consisting of a hollow sleeve and a clamping ring based on the working principle of N-KFSW. Compared with the solid sleeve, this hollow sleeve promoted the material flow below the sleeve. Therefore, the depth of the sleeve-stirring zone (SSZ) was equal to the thickness of the plate when the plunging depth was only 4.2 mm. The SSZ was divided into the sleeve directly affected zone (SDAZ) and the sleeve indirectly affected zone (SIAZ). The material in the SIAZ was transferred by the indirectly stirring effect of hollow sleeve, so the butt interface in the SIAZ was distributed as the S-shape line at the bottom of the SIAZ. The maximum tensile strength of 217.3 MPa was obtained at 1200 rpm, which was 71.5% of that of the base material. The height of the S-shape line shrunk with the rotating velocity increasing, and then governed that the fracture position of the joint transferred to the HAZ from the SSZ. The fracture surface morphology exhibited typical ductile fracture.

A new technique named rotating extrusion was proposed that uses rotating extrusion action to rectify residual distortion of aluminum alloy thin-plate weldments to improve mechanical properties of welded joints. The basic principle and device of rotating extrusion were introduced. The residual distortion and stresses in rotating extrusion weldments were compared with those in conventional weldments. The differences in microstructure and mechanical properties between conventional welded joints and rotating extrusion welded joints were investigated and analyzed in order to make clear the effect of rotating extrusion on the performance of aluminum alloy weldments. Experimental results show that rotating extrusion can enhance the hardness and tensile strength of aluminum alloy welded joints evidently. This method has also potential effect on extending the life of welded structures.