Investigation of the Flow Stress Model for Cr4Mo4V Bearing Steel under Ultrasonic Vibration Conditions

To investigate the mechanisms behind the effect of ultrasonic vibration on the plastic deformation of materials, the flow stress model of Cr4Mo4V was established according to the dislocation dynamics and thermal activation theory, which considers the effect of dislocation density evolution on plastic deformation under ultrasonic vibration conditions. The effect of amplitude and strain rate on the flow stress was analyzed by fitting the stress-strain data obtained from an ultrasonic vibration-assisted tensile (UVAT) single-factor test. To investigate the influence of strain rate and vibration duration on the acoustic effect, comparative tests with and without vibration were performed for various strain rates. The results showed that the flow stress decreased significantly in the tensile test with ultrasound compared to the test without ultrasound, and the degree of reduction increased with increasing amplitude. In addition, the nonlinear relationship between the acoustic softening effect and the strain rate was analyzed. The result demonstrates that the dislocation density absorbs the ultrasonic vibration energy, which results in slip and proliferation. Macroscopically, due to a greater susceptibility to plastic deformation, the dislocation density shows residual hardening at the end of the ultrasound. Finally, the average absolute relative error (AARE) between predicted flow stresses and experimental results under three ultrasonic conditions using the developed model were 4.49%, 1.27%, and 5.64%, which proved the validity of the model.