Mg content optimization inducing precipitation of strengthening phases to enhance laser butt-welded 6D10/A356 dissimilar joints

To address joint softening in the welding of dissimilar aluminum alloys, laser welding is performed on 6D10 and A356 using a magnesium foil interlayer. The effects of controlled Mg microalloying on weld porosity, microstructure evolution, and mechanical properties are systematically investigated. The porosity increases with the increase of Mg. Progressive Mg addition refines fusion-zone grains from 114.93 μm to 70.62 μm by increasing the growth-restriction factor Q, which intensifies constitutional undercooling and suppresses dendritic growth, yielding a uniform equiaxed structure. Moreover, Mg addition decreases the activation energy for β" precipitation from 170 to 134 kJ/mol, accelerating the formation of β"-Mg₅Si₆ while suppressing Q″-AlCuMgSi and θ′-Al₂Cu. A peak yield strength of 220.6 MPa (+44.4 MPa) is achieved at 2.37 wt% Mg, governed predominantly by precipitation strengthening via a dense distribution of β" precipitates. At excessive Mg content (>2.37 wt%), β" precipitates appear coarser and porosity increases, which together deteriorate the strength and ductility. These results demonstrate that controlled Mg microalloying can mitigate weld softening and enhance joint performance in dissimilar aluminum alloys, where the fusion-zone chemistry is Al–Si due to dilution from the A356 side, providing compositional guidance for the development of 4xxx-series aluminum alloy welding wires.