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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 novel non-keyhole friction stir welding technique was proposed to weld the butt joint of 6061-T6 aluminum alloy with the thickness of 6 mm. A sound joint was obtained by this technique, simultaneously eliminating the flash, shoulder mark and keyhole defects. The sleeve directly affected zone (SDAZ) and the sleeve indirectly affected zone (SIAZ) were divided into the joint according to the plunging position of the hollow sleeve. The lack of root penetration defect was avoided when the plunging depth of the hollow sleeve was only 4.2 mm, because the hollow part inside the sleeve improved the material flow below the sleeve. An S-shaped line was left at the SIAZ, and the height of it had the minimum value of 1.47 mm at 20 mm/min. Whether the failure location of the joint was in SIAZ/SDAZ or the heat-affected zone (HAZ) depended on the height and bonding strength of the S-shaped line. The joint fracture location changed from the SIAZ/SDAZ at 35 mm/min to the HAZ at 20 and 30 mm/min. The maximum tensile strength of 224.3 MPa was obtained at 30 mm/min which was 73.7% of that of the base material. The fracture surface morphology exhibited the typical ductile fracture.

Ultrasound assistance was applied to friction stir transient liquid phase spot welding (FSTLPSW) to attain a high-quality dissimilar copper-aluminum joint in this study. Different ultrasound-assisted times were selected for comparing with the conventional condition without ultrasound. The interfacial microstructure evolution of conventional joint was analyzed combined with the measured temperature cycle. The ultrasound assistance improved the joint quality by regulating the interfacial microstructure. Cavitation erosion, broken IMC particles and thicker eutectic layer appeared in ultrasound-assisted joint. The highest tensile shear strength of 7.38 kN was obtained under the ultrasound-assisted time of 60 s which was 43.1 % higher than that of the conventional joint. The fracture location transferred from IMC layers to eutectic layer under the ultrasound assistance.