Two kinds of stationary shoulder were designed to analyze corner joints formation and material flow characteristics

using 7A52 and 7B52 dissimilar materials

and then the effects of process parameters (rotation speed

welding speed and plunge depth) on weld formation were also analyzed. The results show that an instantaneous cavity at the top is formed owing to material flowing downward in advancing side

and if the cavity cannot be filled by material in time

the top corner defects will occur. The round corner shoulder has advantages to promote material flowing from top corner to advancing side to fill the cavity. The joints with smooth surface and no defect were obtained even if the plunge depth is 0.05mm

when rotating speed at 1000 r/min and welding speed at 100-200mm/min. Top corner defects are easily produced in joints welded by sharp corner stationary shoulder friction stir welding (SSFSW)

because there are not enough material to fill the cavity on the top of advancing side

even if the process parameters are changed.