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. 2. Cutting scheme for investigation of a joint’s structure: CTR/BDR longitudinal section of central/sub-shoulder zone; IN/OUT – initial/outgoing section; TD – transversely-directed cross-section Results and Discussion The tool inlet area at the starting point of a permanent joint is shown in Fig. 3,  а . Under normal condi- tions, when the tool pin is fully plunged into the material, the plasticized metal is continuously transferred from the advancing side (AS) to the retreating side (RS) in front of the pin, and back behind it [20]. This is because the volume of plasticized metal is limited by the shoulder part of the tool and the unheated metal of the workpiece, as a result of which there is a directed adhesion-initiated extrusion of material from the retreating side into the area behind the tool. When the tool penetrates the material to be welded, the space behind the pin is not limited by the workpiece metal, so there is an intensive non-directional extrusion of the plasticized metal from the retreating side. It is caused by the contact of the material with the rotating tool, the cohesive interaction between the initial material and the plasticized material, and the intensive transfer of the plasticized material from the advancing side to the retreating side in the front of the tool. As a result, an area of extruded material is formed on the retreating side with a surface shape corresponding to the tool pin profile and traces of the extrusion process (1 in Fig. 3, a, b). By its nature, this process has common features with the extrusion of material from the friction zone during a dry or adhesion friction test with the formation of a metal overlap. For this reason, the friction stir processing or friction stir welding process can be represented as a dynamic extrusion of material along the retreating side, initiated by an adhesive frictional contact, from the front of the tool to the area behind the tool. At the same time, the welding pro- cess also requires a fragmented layer in front of the tool [22] that is capable of flowing around the tool pin without a direct grip and the formation of material breaks or cutting effects. Moreover, the “RS – AS” transfer behind the tool is characterized by a much smaller volume of the transferred material, because it is based only on the adhesive interaction of the material with the tool. Under these conditions, the transferred material also undergoes an initial extrusion and adhesive contact with the formation of an irregularly shaped area of adhered metal on the advancing side (2 in Fig. 3,  a ,  b ). These processes result in the depletion of the welded joint with the material on the advancing side, which leads to the formation of an extended macro defect in the structure as a channel-type one (3 in Fig. 3, a , b ). Further,