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 parallel to RD, TD parallel to TD, RD parallel to TD, 45 degrees parallel to 45 degrees, 45 degrees parallel to RD and TD parallel to 45 degrees (RD, TD and 45 degrees mean the rolling direction, transverse direction, 45 degrees 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 degrees parallel to 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 parallel to 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) over bar texture components, and C texture also appears in central interface zone of the TD parallel to TD and 45 degrees parallel to RD joints. A(1)* and A(2)* texture components can be found in the bottom interface zone of all the six joints.