Features of structure formation processes in AA2024 alloy joints formed by the friction stir welding with bobbin tool

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 2 2021 Fig. 8. Formation of metal flows around the tool: a , b – at defective zone; c , d , e – at non-defective zone; 1 – channel-type defect; 2 – spikes formed with the welding tool’s pin; 3 – metal flows at the stir zone; 4 – region of the “onion rings” structure formation from the metal flow; 5 – large imperfections at the stir zone; 6 – “onion rings” in the metal flow’s struc - ture; 7 – “onion rings” lines а b d c e the hardness on the advancing side of the joint is greater than on the retreating side, which is due to the above-mentioned differences in thermomechanical processes on the respective sides and, consequently, the different temperature effects on the material. The average base metal grain size is between 40–45 μm, in the thermomechanically affected zone it is 40.9 μm, and in the stir zone, it is 2.7 μm. The reduction of the grain size in the stir zone has a favorable effect on the mechanical properties, based on the Hall-Petch bar-