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. 3. Tool entry zone at the beginning of joint: а , б – general view, в , г , д – cross-sectional views in region of 30–40 mm/min welding speed; 1 – region of upset metal with a complex shape; 2 – region of adhered metal with an irregular shape; 3 – channel-type defect; 4 – discontinuity flaws at channel extension zone а b c d e defects in the joint structure can be formed as discontinuities with a localization in the area of the initial channel formation at the tool inlet (4 in fig. 3, c , d , e ). These defects are also associated with insufficient material densification in the welding zone, which causes the material extrusion and the formation of pores and discontinuities. In the tool inlet zone, the material overlap formed on the retreating side (1 in Fig. 4, a, d) has a complex shape. As will be shown below, the metal structure in the overlapping zone is similar to the metal structure of the stir zone and, partially, the thermomechanically affected zone. The material adhered from the advancing side (2 in Fig. 4, a , d ) also has a structure similar to that of the stir zone, but, due to the different formation nature, it is smaller than that from the retreating side. As the tool continues to move along the butt line, the material extruded from the retreating side partially occludes to form the highly defective structure of the stir zone (3 in Fig. 4,  a ,  d ). In the sub-shoulder zone, because of the larger amount of material involved in the process and the more limited formation zone, the “occlusion” of the material occurs much earlier than in the central part of the joint. At the initial stages, the volume of plasticized material is clearly visible in the boundary and central areas of the joint, which is caused by significant specific heat input from the tool rotation due to the low speed of its movement (4 in Fig. 4, a , c ). With a gradual increase in the movement speed to values of 20 mm/min and above, the specific heat input decreases. That leads to a decrease in the amount of plasticized metal and a decrease in the thermomechanically affected zone. Also, with the further tool movement, gradual densification of the material behind the tool and defect reduction is observed. This situation is caused by the intensification of the material extrusion process (Fig. 4, a , d ). According to the literature data [21], the lower the rotation and tool movement speed ratio in a certain value range, the more effective is the transfer mechanisms between the retreating and advancing sides of the joint. In Fig. 4, a, c , d , it is seen that as the tool travel speed increases, the material amount on the advancing side of the joint increases, and a stir zone with a denser and less defective structure is formed (5 in Fig. 4, a , d ). Closer to the tool outlet zone (Fig. 4, b , e ) there is deconsolidation of the stir zone with the formation of a more defective structure (6 in Fig. 4, b , e ) with