Prediction of surface layer characteristics in ultrasonic deep cold rolling process

The residual stresses generated during machining are usually distributed non-uniformly on the surface with a tensile nature, which reduces the fatigue strength of the components. The ultrasonic deep cold rolling (UDCR) technique can induce uniform compressive stresses on the machined surface with a great depth, which leads to improved surface integrity and, consequently, enhanced fatigue performance of the components. However, the latent nature of residual stresses makes it challenging to assess and measure the applied stresses accurately. In order to address this issue, a numerical approach is proposed to investigate the characteristics of the surface layer in the UDCR process. The mechanism of the UDCR process induces severe plastic deformation in the surface layer by applying static force and ultrasonic shock vibration. Therefore, taking TC4 titanium alloy as the example, the effects of UDCR parameters such as static force, ultrasonic rolling amplitude, ball diameter, feed rate, and friction coefficient on residual stress and surface deformation were investigated by finite element modeling and simulation. A validation test was conducted, and the results were compared to confirm the validity of the finite element model. The finite element simulation results show that increasing the static force and vibration amplitude significantly enhances surface deformation and the depth of the compressive residual stress layer, while the feed rate and coefficient of friction have a negligible effect on the residual stress distribution and surface deformation. Additionally, increasing the ball diameter notably reduces surface compressive residual stress, maximum compressive residual stress, and penetration depth.