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 distinguished (Fig. 5, b , f , h , l ) than on the retreating side (Fig. 5, a , e , g , k ). This can be explained by differences in the local thermomechanical processes. The material of the advancing side of the joint is less exposed to a temperature and deformation influence, as at welding there is a primary deformation of only a small part of it. On the retreating side, besides a similar deformation process, almost the entire volume of the plasticized material is extruded. For this reason, there is a sharper structural gradient in the retreating side region in the direction from the base metal to the stir zone. In the area of the structural gradient at the inlet (Fig. 5, h ) and at the outlet (Fig. 5, j ) of the tool, defects are observed as discontinuities in the stir zone. In the shoulder effect zone of the welded joint (Fig. 5, a-f ), the size of the main structural zones is significantly higher than in the tool pin effect zone (Fig. 5, g-l ). The material of the main structural zones in the sub-shoulder area is characterized by less defectiveness, which is caused by the process of “clamping” behind the tool of the material extruded from the friction interaction area. Besides the smaller size of the main structural zones, a sharper gradient in the structure of the thermomechanically affected zone can be observed on the advancing side compared to the structure of the retreating side (TMAZ in Fig. 5). A thin zone of thermomechanical influence is observed in the tool inlet and outlet areas. (Fig. 5,  h ,  j ). The material of the heat-affected zone is clearly distinguished from the base metal area in most images, which is due to its increased etchability (e.g., HAZ in Fig. 5, b , j ). The typical structure of the main structural zones of the joint produced by the BFSW is shown in Fig. 6. The base metal zone in this section is represented mainly by large equiaxed grains (Fig. 6, a ). The average grain size in the base metal zone and the heat-affected zone is 40–45 μm. The material in the thermomechanically affected zone is represented by grains elongated and strongly deformed towards the а c e b d f Fig. 6. Characteristic zones in the structure of the BFSW-joint: a – base metal, b – thermomechanically affected zone, c, d, e – boundary between thermomechani- cally affected zone and stir zone, f – stir zone