Growth mechanism and motion trajectory of lazy “S” in friction stir welding joint of 6082-T6 aluminum alloy

Abstract Samples of 4-mm-thick 6082-T6 aluminum alloy were subjected to friction stir welding (FSW), and the different lazy “S” morphology of joint cross section and weld surface was obtained corresponding to five groups' welding parameters. Based on the diffusion and kinetics equation of the oxygen–aluminum interface film, a vortex current model of lazy “S” motion trajectory composed of (A → B → C → D → E) five points which was affected thermomechanical action by temperature field, non-Newtonian fluid and stress field was provided. It was found that the oxygen–aluminum interface film theory not only explained the growth mechanism of lazy “S” and the phenomenon that the lazy “S” on the weld surface disappeared as the welding speed increases, but also clarified the reason why there is no significant difference in cross-section lazy “S” width. In addition, the vortex current model of the accumulation area at the back of the stirring tool illustrated the regularity of the movement trajectory forming of the cross-section lazy “S” and the variation of the position of the lazy “S” of the weld surface deviated from the weld centerline.