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in 1991 by the Welding Institute, friction stir welding (FSW) has received much attention from both industry and academia.[1] Nowadays, FSW has been successfully applied in the aerospace, ship manufacturing, and rail transport industries.[2–4] Compared with traditional fusion welding, FSW is considered to be an energy efficient, environmentally friendly, and versatile joining technology.[3–5] Although FSW has found great success, there are still some technical issues. The joint performance is greatly affected by welding defects.[6,7] Various welding defects such as voids and kissing bonds—weak interfacial bonds that are present when the materials are in intimate contact—may be produced in the welding process if inappropriate FSW parameters are used.

X.H. ZENG is with the Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, P.R. China and also with the University of Chinese Academy of Sciences, Beijing, 100049, China. P. XUE, D. WANG, D.R. NI, B.L. XIAO, and Z.Y. MA are with the Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences. Contact e-mails: [email protected] and [email protected] Manuscript submitted April 23, 2017.

METALLURGICAL AND MATERIALS TRANSACTIONS A

It is well accepted that the macroscopic structure of the FSW joints, welding defects, and characteristic microstructures, such as the ‘‘S’’ line (explained next) and onion rings [the circular or semi-circular bands in the stir zone (SZ)], are controlled by the material flow in the FSW process.[8] So improving the understanding of how the key factors affect the material flow, microstructural evolution, and defect formation is important for the control and optimization of the quality of weldment.[9] However, due to the instantaneity and complexity of the FSW process, there has been no unanimous understanding of the material flow until now. For example, Heurtier et al.[10] reported that the material flow during FSW was composed of circumventing motion, torsional motion, and vortex motion. On the other hand, Schneider et al.[11] suggested that the material flow in FSW was composed of rigid body rotation, uniform translation, and ring cortex. Clearly, the material flow during FSW still needs further investigation. Usually, welding parameters (rotation rate and traverse speed) are considered as the primary factor affecting the material flow during FSW.[12,13] For most FSW joints, the morphologies of the SZs change from basin shape to elliptical shape with increasing rotation rate, and a hierarchical-shaped SZ can be observed at very high rotation rates.[14] Arbegast[8] pointed out that the welding parameters affected the flow partitioning and the volume of material flowing through the different

zones of the SZ during FSW of an aluminum alloy. Meanwhile, Xu et al.[15] and Pan et al.[16] found that the spacing of the bands on the longitudinal and horizontal cross-sections, which were in the form of an onion ring structure on the transverse cross-sect