In this paper, the effect of alternating longitudinal electromagnetic fields on the microstructure, composition segregation and solid solubility of solute elements in aluminium alloy weld beads is studied. The results show that the X-ray diffraction peaks of the deposited aluminium alloy layer under the external magnetic field are much less intense than those for the alloy prepared without the external magnetic field. The diffraction peak intensities from different crystallographic planes in the structure slightly differ under the action of different magnetic field frequencies. As the excitation current increases, the increase in Mg segregation is obvious. With an increase in the magnetic field frequency and excitation current, the external magnetic field promotes the solid solubility of solute Mg in Al.