During shot peening, high-speed flow impacted the surface of Al-Li alloy. With the cumulative impacts of a
number of shots, the specimen surface generates uneven plastic deformation and produces surface residual
compressive stress. The plastic deformation gradually decreases inwardly from the surface layer. Because of the
presence of plastic deformation, the compressive residual stress caused by shot peening produces the occurrence of
slack. So that the compressive residual stress of surface inward appears an increasing trend and then the trend turn
into a decreasing trend.