Material Flow and Mechanical Properties of a Non-Keyhole Friction Stir Welded Aluminum Alloy with Improved Sleeve Bottom Geometry

Non-keyhole friction stir welding (N-KFSW) is a technique that can fabricate a welding joint without a keyhole through a one-time welding process. The Al–Mg–Si alloy was chosen as a research object, and the N-KFSW technique was investigated by numerical and experimental methods. Firstly, the sleeve bottoms of the N-KFSW welding tool system were innovatively optimized in this study. The optimal sleeve bottom with an 80° angle between the spiral groove and the sleeve inner side wall allowed avoiding the incomplete root penetration defect at the bottom of the stir zone (SZ), which was verified by numerical results and the C-shaped line height. Then, using a 3 mm-thick aluminum alloy as the experimental material, the material flow and joint formation characteristics and mechanical properties at 110, 150 and 190 mm/min welding speeds were studied and compared. The results showed that the SZ presented a drum shape due to the action of the clamping ring and the threads on the side wall of the sleeve. The SZ width decreased from 7.17 to 6.91 mm due to the decreased heat input. From 70 to 210 mm/min welding speed, the maximum tensile strength of the joint was 250 MPa at 190 mm/min, and the joint with relatively higher strength fractured at the heat-affected zone.