Grain boundary and dislocation strengthening behavior in blue-laser-induced dual-wavelength hybrid laser welding of aluminum alloys

The influence of blue-laser-assisted near-infrared (NIR) hybrid welding on the microstructural evolution and strengthening behavior of dissimilar 6082/A356 aluminum alloy joints was systematically investigated. Electron backscatter diffraction (EBSD) was employed to characterize grain boundary features, geometrically necessary dislocations (GNDs), and crystallographic orientation distributions, while microhardness measurements were used to evaluate the local mechanical response. The results show that blue-laser assistance significantly modifies the thermal cycle and solidification behavior of the weld pool, leading to grain refinement, an increased fraction of low-angle grain boundaries (LAGBs) and coincidence site lattice (CSL) boundaries, and enhanced GND accumulation. The highest GND density (1.64 × 10 13 m⁻ 2 ) and weld-center hardness (∼106 HV) were achieved under the 1200 W NIR + 500 W Blue condition, representing an approximately 9% increase compared with conventional NIR welding. Schmid factor and Taylor factor analyses further revealed that hybrid welding optimized crystallographic orientation distribution, promoting both slip activation and orientation strengthening. More importantly, GND evolution and CSL boundary development were found to be intrinsically coupled. The increased CSL boundary fraction facilitated dislocation confinement and redistribution, while GND accumulation enhanced lattice distortion and dislocation-mediated strain hardening. This synergistic interaction established a coupled dislocation–boundary strengthening mechanism that simultaneously improved deformation compatibility, suppressed stress concentration, and enhanced resistance to dislocation glide. The findings demonstrate that dual-wavelength NIR–blue hybrid laser welding provides an effective strategy for tailoring grain-boundary character, dislocation storage, and crystallographic orientation, offering new insights into microstructure engineering and local property optimization of dissimilar aluminum alloy welded joints.