Study on the formation mechanism of fine equiaxed grain ribbons along the fusion lines during 2060/2099 Al-Li alloys dual laser-beam synchronous welding process

The dual laser-beam synchronous welding (DLBSW) technology is newly developed in order to improve productivity and weld seam quality. In this investigation, AA2060 and AA2099 Al-Li alloys were welded using ER4047 welding wire under different heat input in DLBSW process. The microstructure characteristics especially the fine equiaxed grain ribbons along the fusion lines of T-joints were investigated by observing the micro morphology both in cross section and vertical section. It was found that the non-dendritic equiaxed grain near the fusion line was spherical in three dimensional with small size compared with the equiaxed grain in the weld seam center. Besides, the action of thermal buoyancy flow and Marangoni convection was found to have great effect on the formation of fine equiaxed grain ribbons in the molten pool. Moreover, the energy dispersive spectrometer (EDS) results of non-dendritic equiaxed zone (EQZ) indicated that the strip-shaped intergranular precipitation phase in EQZ is rich in Cu, while the bulk-shaped intergranular precipitation phase is rich in Si.