Achieving exceptional strength-toughness synergy in aluminum/copper welded joint by coaxial infrared-blue hybrid laser

• The blue laser effectively enhanced the energy absorption of the infrared laser and formed a root-like molten pool with the effect of "canine-tooth interlocking". • The microstructure uniformity and the strain gradient at the interface of the molten pool were significantly improved by the suppression of Cu diffusion in the molten pool with infrared-blue hybrid laser. • The synergistic enhancement of strength-toughness of Al/Cu infrared-blue hybrid laser welded joint was achieved through dislocation/grain refinement strengthening at the molten pool interface and deformation strengthening in the copper matrix. For the problem of low energy coupling efficiency and weak interface bonding strength in aluminum-copper laser welding, a novel infrared-blue hybrid laser technology was adopted to conduct research on aluminum/copper lap welding. The synergistic enhancement mechanism of joint strength and toughness was revealed by combination of in-situ tensile test in SEM and theoretical calculations. The results showed that the increase of molten depth and the shape of root-like structure were closely related to the energy density of the infrared-blue hybrid laser, which indirectly confirmed the promoting effect of blue laser on the energy absorption of the infrared laser. Under the appropriate blue laser irradiation, the combined effect of the Marangoni effect and recoil pressure suppressed the diffusion behavior of Cu elements, thereby effectively improving the microstructure uniformity within the molten pool. The enhancement of Al/Cu interfacial bonding strength was attributed to the dislocation strengthening and grain refinement strengthening at the interface, while the migration of cracks into the copper matrix during in-situ tensile process was due to the driving effect of the root-like molten pool, and the deformation strengthening in the copper matrix further compensated the toughness of the joint. The molten pool microstructure formed under blue laser assistance not only enhanced the joint’s strength and toughness, but also was beneficial for improving the electrical conductivity of the joint. Among them, the maximum shear tensile strength and toughness of the joint were increased by 29.4% and 24.2%, respectively.