Ultrasonic impact treatment (UIT) is a type of surface strengthening technology that can improve the fatigue properties of materials by improving the surface quality,residual stress,and other aspects.In this study,the influence of ultrasonic impact parameters on the surface integrity of nickel alloy 718 was studied.The micro stress concentration caused by the surface morphology was also explored.The cosine and exponential decay functions were used to fit and characterize the distribution of residual stress and work hardening in the surface material.The results showed that the feed rate had the greatest influence on surface roughness,stress concentration,and surface residual stress.It was not appropriate to evaluate the surface hardening effect only by the number of impacts per unit area,the ultrasonic impact parameters such as feed speed and pre extrusion depth should also be considered.The grain refinement was obvious after UIT.The multiobjective optimization of machining parameters was performed with the objective of surface stress concentration and residual stress.A surface with a smaller surface stress concentration factor and larger compressive residual stress can be obtained simultaneously using medium linear velocity,medium pre extrusion depth,and smaller feed rate.