Relationship between formation of laminated structure and geometry of probe in friction stir welding

In this study, flow observation in friction stir welding using a transparent fluid followed by PIV analysis were conducted to discuss and verify the relationship between the thread shape of the probe and the layered microstructure. Subsequently, friction stir welding of 6061 aluminium alloy were conducted to confirm the validity of the verification. In the flow observation, the tracer swirled around the probe was observed to be discharged from the probe tip and formed a layered deposit behind the probe. This layered deposition showed different periodicity depending on the tool screw geometry. The PIV analysis showed that the tracer was discharged from the threaded part at the probe tip. Therefore, it is considered that the discharge from the probe tip contributes to the layered deposition. In the joining experiments, the spacing of the layered microstructure appeared on the cross-section altered depending on the thread shape of the tool. Therefore, it is considered that the layered microstructure is strongly related to the geometry of thread of welding tool.