Material flow and microstructure evolution on the flow path of friction stir lap welded aluminium alloy

Vertical material flow and microstructure evolution were investigated by the material tracing technology during friction stir lap welding of 6061-T6 aluminium alloys from a new perspective: various angles of keyhole obtained by emergency stop technique. First, lamellar grains were transformed into equiaxed subgrains, and texture was transitioned from 110//shear direction fibre texture to strong B/(B) over bar components when materials flowed from the advancing side to the leading side. Then subgrains grew into recrystallised grains at the retreating side. After that the downward material flow at the trailing side induced plastic deformation, resulting in many subgrains with strong A/(A) over bar texture components. Finally, grains grew up and retained with strong A/A components in the stir zone.