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 c Fig 4. Joint formation at tool entry zone and joint failure at tool exit zone. Metallographic speci- mens at longitudinal sections of the joint: a , b – sub-shoulder zone; d , e – central zone; a, d – tool inlet area; b , e – tool outlet area; c – variation of tool travel speed; 1 , 2 – an overlap at retreating side and advancing side in tool inlet zone; 9 , 10 – an overlap at retreating side and advancing side in the tool outlet area; 3 – joint packing region at tool inlet area; 4 – region of plasticized metal at tool inlet area; 5 – the stir zone at the tool inlet area, 6 – the stir zone before the joint failure at the tool outlet area; 7 – softened zone at the tool outlet area; 8 – zone of discontinous structure; 11 – the curve of the tool travel speed variation 12 – the tool travel speed curve supposed to pro- vide low structure imperfection at the tool outlet area а b d e separation of two or more parts (6’ in Fig. 4, b, e). Then the number of defects increases (7 in Fig. 4, b , e ), and a channel-type defect is formed (8 in Fig. 4, b , e ). Immediately at the tool outlet, large (9 in Fig. 4, b , e ), and small material extrusion zones are formed (10 in Fig. 4, b , e ), which are represented by significantly deformed material unlike the area of the tool inlet. The length of the material extrusion zones at the tool outlet is determined by the welding speed, besides the physical nature of the process (11 in Fig. 4, c ). During the tool inlet process, a gradual increase in the welding speed reduces defect formation and accelerates the joint formation process. But in the tool outlet area, a high welding speed, presumably, leads to a decrease in the fragmented material layer in the tool’s front and a greater deformation effect on the material in front of this layer.