Friction stir welding (FSW) was employed to achieve a reliable joining of 2 mm thick dissimilar metals, 6061 aluminum alloy and AZ31B magnesium alloy. This study revealed the evolution of interfacial interlocking features and their impact on the mechanical properties of the joints under different welding speeds (25–35 mm/min). The results indicate that the Al/Mg FSW joint interface exhibits a strip-like interlaced structure, the morphological characteristics of which are closely related to the welding speed. For quantitative analysis, the ratio of interlocking length to plate thickness (embedding ratio) was used as a quantitative indicator of the structural interlocking feature. As the welding speed increased from 25 mm/min to 35 mm/min, the embedding ratio decreased from 13.2 to 7.9, and the average thickness of the intermetallic compound (IMC) layer decreased from 2.71 μm to 2.19 μm. Transmission Electron Microscopy (TEM) results confirmed that the Al/Mg FSW joint interface consists of a bilayer of IMCs, specifically Al₃Mg₂ and Al₁₂Mg₁₇, with thicknesses of 220 nm and 470 nm, respectively. Tensile testing of joints with different embedding ratios demonstrated that the tensile strength of the welded joint exhibits a positive correlation with the embedding ratio, reaching a maximum of 178 MPa.

For the thick plate aluminum (Al)/magnesium (Mg) friction stir weld (FSW), the kissing bond easily appears at the bottom of the joint due to the uneven heat distribution along the thickness direction. In this paper, three sets of pin geometries are proposed: three-start thread pin (A-Pin), one flat pin (B-Pin), and double helical groove pin (C-Pin), and their effects on the thermal cycle and intermetallic compounds (IMCs) of joints are studied. The results show that pin geometry has little effect on the peak temperature and temperature difference along the thickness direction. But the pin geometry can play an important role in the elevated temperature residence time (t) at the bottom of the interface. This variation of the t is related to mass transfer and heat transfer caused by pin geometry. Additionally, the upper of the interface obtained by three sets of pins is composed of eutectic layer (Mg+Al12Mg17) and Al3Mg2 layer. Nevertheless, the bottom of the interface for A-Pin exists a kissing bond with a width of 3.4μm. Under the action of B-Pin and C-Pin, the bottom of this interface is well metallurgically bonded, and both have a two-layer structure composed of Al12Mg17 layer and Al3Mg2 layer. The thickness of these two-layer structures for B-Pin and C-Pin reaches 7.9μm and 4.9μm, respectively. This is attributed to the eccentric stir of the B-Pin and the mass and heat transfer of the C-Pin.

Stepped-lap and dovetail interfaces were designed for friction stir welding of 8-mm-thick AA5083 and TA2 plates. The stepped-lap joint showed upper and root unbonded regions, whereas the dovetail joint largely retained its non-planar profile and mechanical interlocking. EDS point analysis at the upper dovetail interface revealed a local Al/Ti atomic ratio close to the nominal stoichiometry of Al₃Ti. DF-TEM and SAED characterization of the middle dovetail interface showed that both the Al-side and intermediate regions were predominantly amorphous-like, whereas crystalline Ti₃Al was identified near the Ti side. The tensile strength increased from 39.86 MPa for the stepped-lap joint to 58.03 MPa for the dovetail joint, corresponding to an improvement of 45.6%. This improvement was attributed mainly to the geometrical constraint, mechanical interlocking and extended load-transfer path provided by the dovetail interface.