Visualization of vertical transfer of material through high-velocity rotating flow zone during friction stir welding.

• A novel simulation method of FSW based on two-phase flow is proposed. • High-velocity rotating flow zone is the channel for vertical migration of material. • The accuracy of the simulation results is verified by experimental measurement. In this letter, combined experimental and computational analysis has allowed new understanding of the vertical material flow behaviors during friction stir welding. It is found that a high-velocity rotating flow zone exists near the welding tool, where horizontal and vertical flow are both significant. Material entering the high-velocity rotating flow zone migrates to the lower location of the workpiece after multiple rotations. Material outside the high-velocity rotating flow zone bypasses the tool and migrates towards the upper locations. The findings of this research offer insights into the regulation of microstructures and the formation of defects in FSW. [ABSTRACT FROM AUTHOR]