Friction stir welding (FSW),a highly efficient solid-state joining technique,was invented at The Welding Institute,UK,in 1991.Utilizing frictional and adiabatic heating,plastic deformation and solid-state diffusion,FSW has been widely accepted as a “green” welding technology owing to its energy efficiency and environment friendliness.Now,FSW has achieved broad application in various industries,such as aerospace and transportation,and is considered the most significant development in the field of material joining over the past two decades.Friction stir processing (FSP),later developed based on the fundamental principles of FSW,has been demonstrated as an effective and versatile metal-working technique for modifying and fabricating metallic materials.