Experimental study on surface integrity changes during turning-ultrasonic impact of nickel alloy 718

Ultrasonic impact strengthening is a kind of “no cutting” processing technology. It is widely used in surface strengthening of mechanical parts. In this research, the surface integrity changes the mechanism in the composite process of turning-ultrasonic impacting of nickel alloy 718 was studied and analyzed. The turning surface quality was significantly improved when treated with the same set of ultrasonic impact parameters due to the extrusion and impacting of ultrasonic impact head. The average value of surface roughness Ra was about 0.2 μm. The profile valley effective radius was about 5 μm. The value of surface stress concentration factor was within a small range from 1.07 to 1.11. During the process of ultrasonic impact, severe plastic deformation occurred in the surface material, the dislocation density increased rapidly, and the grains were refined. This resulted in the improvement of the mechanical properties of the surface material. The axial residual stress was around − 1800 MPa, and its influence depth increased by nearly 10 times. The maximum value of surface microhardness was 593.28 HV, and its influence depth was 120 μm. The distribution of residual stress and microhardness along the depth direction has been characterized by cosine decay function and exponential decay function. The thickness of the plastic deformation layer increased from 3 to about 19 μm. EBSD test showed that the grain size decreased by nearly 90%. In general, the surface integrity was significantly improved after ultrasonic impacting treatment regardless of the initial turning surface conditions.