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Aluminum lithium alloy is considered to be one of the ideal materials for lightweight structural materials in the aerospace field. The 2198-T851 aluminum lithium alloy 1.8 mm thickness sheet of vacuum electron beam test,analysis the different scanning modes on weld forming and the influence of mechanical properties of joints, and under the optimal welding process welding joint microstructure and mechanical properties. The results show that the working distance is 300mm, the acceleration voltage is U=60kV, the focusing current is I_f=498mA ,the welding speed V=1200mm/min and the electron beam is I_b=6.8mA, the welding joint without the addition of scanning can obtain good forming and no macro porosity defects.Under the same conditions compared to other add scan mode of welding, derived welding in the process of adding triangle wave scanning, we can not only obtain the forming good macro porosity defects in the welded joints, and the welding joint of the hardness values and tensile strength is improved. The average tensile strength of welded joint is 323.55MPa, and the fracture strength of the joint is 71.5%, The extension rate is up to 86.8% of the elongation of base metal.which is the weakest part of the weld zone.