Abstract: Single-sided and double-sided welding of 10 mm 6063-T6 aluminum alloys are carried out by friction stir welding, and the effect of welding speed on the microstructure and mechanical properties of the joints are investigated. The results indicate that the shape of the "S" lines of single-sided and double-sided welded joints are essentially the same and concentrate on the advancing side of the shoulder affected zone, meanwhile the single-sided welded joints show more obvious onion ring shapes. The equiaxed grain size in the central region of the weld nugget is smaller than that in the upper shoulder affected area, and the grain size of the single-sided weld nugget is larger than that of double-sided welding. When the rotational speed is 1 500 r/min and the welding speed increased from 300 mm/min to 1 400 mm/min, the mechanical properties of the joints first undergo an increase and then a decrease. During the process, the mechanical properties of the double-sided welded joints are always better than single-sided welded joints. For all the welded joints, the maximal tensile strength is 187 MPa, and the fracture position is mostly located in the heat-affected zone on the receding side, which is exactly the same position of the joint with lowest hardness. Besides, the main fracture type is ductile fracture.

Key words: 6063-T6 aluminum alloy, friction stir welding, microstructure, mechanical properties