Characteristics of the Dissimilar AA7075 and Q235 Steel Joints Fabricated by Friction Stir Welding

In this study, dissimilar AA7075/Q235 steel joints were successfully fabricated by friction stir welding (FSW), and the grains of AA7075 in the nugget zone (NZ) were refined and mixed with the broken Q235 fragments (i.e., the steel particles, strips, and blocks). Meanwhile, intermetallic compound (IMC) layers were generated along the aluminum/steel (Al/steel) interface due to the occurrence of Al/steel diffusion. The results also revealed that the macro- and microstructures of the FSW joints varied depending on the welding heat input; under the cold FSW condition, micro-defects formed because of the weak and chaotic material flow in the NZ, reducing the welding heat input gave rise to inhomogeneous grain refinement; and the dynamic recrystallization of Al only occurred in the regions that lacked large steel blocks. In contrast, elevating the welding heat input led to the homogenization of the grain refinement and increased the thickness of the IMC layers. The FSW quality was controlled by both the thickness of the IMC layers and the size of the steel fragments simultaneously. Both the optimal-thickness IMC layers (about 2 μm) and fine steel particles were required to strengthen the joints, and a more than 30 MPa increment in the tensile strength could be obtained by manufacturing the above microstructures. Unfortunately, all the FSW joints failed in a brittle manner and the elongation was lower than 5%. Two kinds of fracture surfaces were observed inside the NZ: one was flat along the Al/steel interface, and the other was uneven due to the pulling out of the large steel strips and blocks.