Microstructure and mechanical properties of dissimilar friction stir welded AA2024-7075 joints: Influence of joining material direction

In order to assess the influence of joining material direction on the microstructure and mechanical properties of dissimilar friction stir welded joints, AA2024-T351 and AA7075-T651 sheets were joined with six different types of combination: RD‖RD, TD‖TD, RD‖TD, 45°‖45°, 45°‖RD and TD‖45° (RD, TD and 45° mean the rolling direction, transverse direction, 45° to the rolling direction, respectively). The results show that various types of welding combinations with respect to joining material direction affect the mechanical properties of joints due to the variations of material flow behavior and microstructure. Material mixed flow mainly occurs at three different regions, namely the tool shoulder domain, the tool pin domain and the transition region between them. The tunnel defect resulting from the insufficient heat input is generated with the combination type of 45°‖RD, resulting in the deteriorated tensile properties, while sound joints can be achieved through the other five types of combinations. The maximum tensile strength is acquired as 445.6 MPa for the RD‖TD joint as a result of suitable heat input and sufficient material flow. In the NZ, the average grain size in the center zone is higher than that in the bottom zone. The center interface zone of all the six joints contains B and B‾ texture components, and C texture also appears in central interface zone of the TD‖TD and 45°‖RD joints. A1∗ and A2∗ texture components can be found in the bottom interface zone of all the six joints.