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 Thereby, the formation of a large-sized channel-type defect in the stir zone on the advancing side indicates a significant deviation of welding mode parameters, and the presence of a void filled with the material with a high content of structural discontinuities indicates an understatement of mode parameters. As a result, the formation of structural defects has a constant character and is accompanied by the formation of the thermomechanically affected zone, which repeats the contour of the tool pin with non-uniform deformation of the material (5 in Fig. 7). Fig. 8, a , b shows the stir zone of the welded joint section with pronounced structural defects. The above features of the structure formation lead to the formation of a zone of structural discontinuities (1 in Fig. 8), which can be divided into two parts and contain large pores (1, 1’ and 5 in Fig. 8). The metal flows along the tool contour in the area between the defect and the retreating side (I, II, II’, II’’ in Fig. 8) form a non-uniform structure with a “helical structure”, consisting of zones with a different material deformation direction, which merge with each other. However, the weld area without pronounced structural defects is significantly different (Fig. 8, d ). In the joint structure, metal flows (I-IV in Fig. 8) reproduce the contour of the tool and form a system of parallel zones of material deformation. On the advancing side of the joint, the boundary of the stir and thermomechanically affected zones also repeats the shape of the tool (2 in Fig. 8). On the retreating side, the shape of the boundary is close to a straight line (3 in Fig. 8). At optimal parameters of the CFSW process in the welded joint formation of a monolithic core with the structure, in the literature called “onion rings” (concentric and similar in shape areas of deformed material) occurs, otherwise, formation of separate disconnected metal flows occurs [23]. Areas formed by individual metal flows can be observed clearly, and areas with “onion rings” features can be distinguished in each of them (Fig. 8, e ). This structure indicates that at each metal flow a process occurs, which is similar to the processes occurring in the stir zone at the CFSW. As noted earlier, the joint formation is accompanied by the extrusion of the fragmented material on its retreating side from the area in front of the tool to the area behind the tool. If the pressure exerted by the extruded material on the material behind the tool determines the structure formation process, its greatest influence will be in the area where the temperature at the time of the local material transfer process is the maximum, that is, in the center of the flow (since the heat dissipation conditions in the center are significantly more difficult than at the edges). As a result, at each stage, the local metal flow will have a shape close to a dome. In this case, in the cross-section, the resulting structure will have the form of concentric rings of irregular shape in separate metal flows (Fig. 8, d , e ). These features of the stir zone structure cannot be explained exclusively by the adhesive interaction be- tween the tool and the welded material, which is currently accepted as the main mechanism of the structure formation in friction stir welding [23]. One of the possible ways of such structure formation is the stepwise extrusive formation of the stir zone, which is consistent with the data of structural studies. The material extrusion manifests itself already at the initial stage of joint formation (Figs. 3–5). The extruded material structures at the tool inlet and the joint material are practically identical, with the presence of fine grains and a gradient transition to the base metal through the thermomechanically affected zone (Fig. 5). Presum- ably, the joint formation is accompanied by a grain size refinement to values conducive to deformation by the grain sliding [24] followed by a superplastic material flow of an extrusive nature. This makes friction stir welding and friction stir processing (FSP) similar to the superplastic deformation process, which is per- formed under conditions of a grain size refinement in the material to 1-10 μm at the appropriate temperature [25]. Consequently, the data obtained indicate the dual physical nature of the stir zone structure formation based on the adhesive transfer and extrusion of the material from the tool front area to the tool rear area. The data on microhardness and grain size of the welded joint characteristic zones in its cross-section were compared (Fig. 9). The average value of microhardness in the stir zone equals НV SZ = 1.43 GPa, in the heat-affected zone it equals НV HAZ = 1.17 GPa, and in the thermomechanically affected zone, it equals НV TMAZ = 1.21 GPa. The minimum values of microhardness correspond to a heat-affected zone that is caused by overaging of the material after the welding and by a weakening of a solid solution of the base metal. The hardness on the advancing side is more than on the retreating side, which is connected with the above-mentioned differences of thermomechanical processes on the corresponding sides and, consequently,