Thermal cycles and deformations during high-speed micro friction stir welding (μFSW) under different welding conditions were studied by experimental methods. The results show that the peak temperature and elevated-temperature exposure time (t₁₅₀) increased with the increasing of rotational speed and decreased with the increasing of welding speed. Increasing rotational speed or welding speed led to an increase in both heating and cooling rates. The joint fabricated by the pinless tool experienced a lower peak temperature, a shorter elevated-temperature exposure time, and a larger temperature gradient than that by the pin tool. The welded sheet presented an anti-saddle deformation character, with convex bending in a longitudinal direction and concave angular bending in a transverse direction. In comparison to the pin tool, the longitudinal maximum bending deformation, Z_max, of the joint fabricated by the pinless tool was reduced by 12.35%, and the transverse angular deformation, α, was reduced by 6.67%. In comparison to the steel backing plate, the Z_max of the joint produced using a copper backing plate was reduced by 40.66%, but the α was increased by 53.27%.

In this paper, the friction stir welding (FSW) of single-curvature and double-curvature "S" curved surface lap joints were realized using split type pressing blocks fixture and robotic system with welding parameter 1400rpm, 150mm/min. By Finite Element Method (FEM) and axial force analysis, the effects of curvature direction on friction stir welding lap joint of 5083 Aluminum Alloy "S" Curved surface is explored. The results show that Z-axis curvature direction has important influence on joint forming. By split type pressing blocks fixture, the material thermal expansion causes upward force F’ in Z-axis positive curvature position, but causes downward force F’ in Z-axis negative curvature position. So Z-axis positive curvature makes the actual axial compressive force F_R (F_R=F_N-F’) smaller than the device input F_N. Which decreases the interface stress of the lap joint and increases the tendency of upper plate edges bending deformation and the probability of interface defects. Finally, the joint mechanical properties are reduced, the tensile load can be as low as 1.5 kN. While Z-axis negative curvature makes the actual axial compressive force F_R (F_R=F_N+F’) greater than the device input F_N. Which increases the interface stress of the lap joint. Meanwhile it decreases the tendency of upper plate edges bending deformation and leads to the interface defects disappearing. So the joint mechanical properties are good, the tensile load can be more than 2.4 kN. For Y-axis curvature direction, its impact is mainly shown in changing the direction of resultant force F_R. In Y-axis positive curvature position, the force F_R lies between Y positive direction and Z negative direction. While in Y-axis negative curvature position, the force F_R lies between Y negative direction and Z negative direction. The inclination of force F_R is helpful to eliminate the interface defects of the joint. At the same time, it will cause inconsistent stress between the advancing side (AS) and the retreating side (RS) of the joint interface, which is the reason that the double-curvature lap joint welding process is unstable.