Aluminum/copper dissimilar metal double-sided friction stir welding formation and mechanical properties of joint

12 mm medium-thick pure aluminum and T2 copper were welded by double-side friction stir welding, and the effects of welding speeds on weld formation and mechanical properties of the joint were studied. The results show that the welding speed has little effect on the mechanical properties of the joints when the tool rotational speed is fixed at 600 r/min and the welding speed is 30-50 mm/min, all the joints are fractured on the base material of aluminum. The defect-free welded joints are obtained at the welding speed of 30 mm/min. While the welding speed is 40-50 mm/min, some flat small void defects appear in the joints. However, the joint properties are still higher than that of the pure aluminum due to the copper mixed with the aluminum in the weld which forms a composite structure and enhances the joint properties. When the welding speed is higher than 60 mm/min, the heat input is decreased, which causes the insufficient material flow and forms a void defect through the weld, reduces the load-bearing area of the joint and decreases the tensile strength of joint. The hardness is highest in the middle of the stirring zone, which is twice thermally stirred by the tool. Furthermore, due to the welding distortion caused by the first-pass welding, the second-pass weld has a slightly lower downforce and less heat input than the first-pass weld at the same plunge depth, making the hardness at the bottom of the weld (i.e. the second-pass weld) is less than that at the top of the weld (i.e. the first-pass weld), and the second-pass weld is more likely to form void defects. Considering both the welding efficiency and joint properties, the optimum welding speed is 50 mm/min. The joint performance is comparable to the base metal of aluminum, the optimum tensile strength of 75.6 MPa and elongation of 26% are achieved at the welding speed of 50 mm/min. © 2022, China Science Publishing & Media Ltd. All right reserved.