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 metal flow direction (Fig. 6, b-d). In the thermomechanically affected zone near the stir zone, the grain size decreases from 40–45 μm to 2-3 μm. The average grain size in TMAZ is 40.9 μm. The boundary between the stir zone and the thermomechanically affected zone is mostly smooth (Fig. 6, c-e), but differs for the advancing and retreating sides. When approaching from the thermomechanically affected zone to the stir zone, the refinement of large inclusions of secondary phases is observed (Fig. 6, e ). There are no large inclusions detected in the stir zone. The structure of the stir zone (Fig. 6, f ) is represented by finely dispersed equiaxed grains with an average size of 2.7 μm. In the structure of BFSW-joints, defects of various types and structural-scale levels can be formed (Fig. 7). The defects can be caused by an asymmetry in the material stirring process, violation of the adhesive contact, imbalance in welding and tool rotation speeds, insufficient material densification in the welding zone, etc. At the boundary of the stir zone and the thermomechanically affected zone, discontinuities of different geometries can be observed (2 in Fig. 7). Similar defects are formed in the stir zone together with defects like inhomogeneities in the grain structure (3 in Fig. 7). In some areas of the stir zone, defects as discontinuities are formed along the grain boundaries (4 in Fig. 7). The most critical defect in the structure is a defect in the form of a channel on the advancing side of the joint, which can be partially filled with the material of the stir zone (6 in Fig. 7). Such defects are formed because plasticized material enters the area behind the tool on the advancing side lastly. In combination with the continuous tool travel in the welding direction and the presence of existing discontinuities in the material, there is a constant shortage of the plasticized metal in this area. Fig. 7. Common structural defects of the BFSW-joint: a, b – the inhomogeneity in the stir zone grain structure; c-f – discontinuities and pores at the stir zone and its boundary with the thermomechanically affected zone, 1 – the grain structure inhomogeneity; 2 – discontinuities at the boundary of the stir zone; 3 , 4 , 6 – discontinuities in the stir zone structure; 5 – the deformation heterogeneity at the advancing side of the joint а c e b d f