Friction stir spot welding (FSSW) of dissimilar S45C steel and 6061-T6 aluminum alloy in a butt configuration is experimentally investigated. Butt spot welding is performed using a convex scrolled shoulder tool at different tool rotational speeds. FSSW butt joints are successfully fabricated by offsetting the tool to the steel side. The microstructures of the joints fabricated at three different tool rotational speeds are characterized using scanning electron microscopy and energy dispersive spectrometry. Microstructural analysis shows the presence of intermetallic compounds (IMCs) along the steel/aluminum interface. The thickness of the IMC layer and the tensile strength of the joint increase with increasing the tool rotational speed. The results of tensile tests and microstructural analysis show that the joint performance is closely related to the IMCs at the joint interface.

Three-dimensional (3D) reconstruction and finite element method are combined to study the damage behavior of aluminum alloy resistance spot-welded joints. Fatigue damage of spot-welded joints under different cyclic loading stages was obtained by X-ray microcomputed tomography (X-ray micro CT). Then, avizo software was used to reconstruct the scanned data of joints with different damage degrees, and the distribution and variation of defects in the joints are obtained. On this basis, 3D finite element damage models were established. Finite element calculations were carried out to analyze the fatigue damage of spot-welded joints by adopting the effective elastic modulus as the damage parameter. The results show that the effective elastic modulus is consistent with the experimental results. The method of combining 3D reconstruction with the finite element method can be used to evaluate the internal damage of spot-welded joints and provide theoretical basis for the prediction of fatigue life.

The microstructure and mechanical properties of electrically assisted brazing (EA-brazing) joints of aluminum alloy 6061-t6 (AA6061-t6) and S45C steel are experimentally investigated. During the EA-brazing process, an electric current is directly applied to the cylindrical specimen assembly (S45C and AA6061-t6) and fillers of 88% Al and 12% Si (in the middle of the specimen assembly). The temperature of the specimen assembly rises rapidly to the melting point of the filler and remains nearly constant for a period of time using a pulsed electric current. Two types of EA-brazing joints are fabricated, namely Joint-0s (no temperature holding time) and Joint-12s (12 s temperature holding time). The characteristics of the intermetallic compounds (IMCs) formed at the EA-brazing joint interface are analyzed using scanning electron microscopy and energy dispersive spectrometer. Compared to Joint-0s, the Fe-rich IMCs (FeAl) are observed at the interface of Joint-12s due to the 12 s temperature holding time. In addition, the microstructural analysis shows that the thickness of the diffusion layer increases with increasing temperature holding time. The mechanical properties of the EA-brazing joints are evaluated using bending tests. The results of the mechanical test show that the strength of Joint-12s is higher than that of Joint-0s.

An equivalent damage model was established to study the fatigue damage behavior of steel–aluminum friction stir welding (FSW) joints. Internal defects of friction stir welding joint under various cyclic loading stages were observed by X-ray microcomputed tomography (X-CT). For the characteristics of defects of the steel–aluminum FSW joints, a simplified method of defects considering key parameters is proposed, and then, the defect model is established. The FSW joint model was established based on the steel–aluminum boundary contour identified by using image processing techniques. Based on the defect model and the FSW joint model, the equivalent damage model was developed. The equivalent damage model was subjected to finite element analysis and compared with the test using strain amplitude as the damage variable. The equivalent damage model can be used to assess fatigue damage in steel–aluminum FSW joints, which provides some theoretical basis for fatigue life prediction.