Influence of tool rotational speed on local microstructure, mechanical and corrosion behavior of dissimilar AA2024/7075 joints fabricated by friction stir welding

Abstract

In this investigation, the local microstructure evolution, mechanical properties and corrosion behavior of the dissimilar friction stir welded AA2024/7075 joints were evaluated as a function of rotational speed. The results showed that the rotational speed mainly affects the local microstructure and performance of the dissimilar joints. Fine equiaxed grains are formed in the nugget zone (NZ). The average grain size decreases in the sequence of shoulder zone (SZ), center zone (CZ) and bottom zone (BZ), and increases with increasing rotational speed. Different types of shear textures are formed at different positions of the NZ, which vary with the rotational speed. The maximum tensile strength of 411.4 MPa is acquired in the joint at 950 rpm, with the welding efficiency of about 87.6 % relative to the AA2024 base material. The fracture position of all the joints is in the lowest hardness zone during tensile tests. Higher corrosion current density presents in the welded zone, which results from the formation of galvanic corrosion. The corrosion behavior of the welded zone is influenced by rotational speed and higher corrosion resistance can be acquired in the joint fabricated at 950 rpm due to the appropriate precipitates density and size.