Surface/subsurface formation mechanism of tungsten during ultrasonic elliptical vibration cutting

The ultrasonic elliptical vibration cutting (UEVC) has been proved to be a valid method to improve surface quality and reduce subsurface damage of tungsten. However, it is still a huge challenge to elucidate the surface/subsurface formation mechanism of tungsten during UEVC. Hence, molecular dynamics simulations were employed to reveal the formation mechanism of surface/subsurface during UEVC of tungsten from the atomic level, and the effects of ultrasonic amplitude and frequency on the surface/subsurface formation were explored. The results showed that compared with general cutting (GC), the improved surface quality was attributed to the smaller actual cutting depth and more amorphous tungsten in chips caused by cyclic loading-unloading during UEVC. Moreover, the reversing friction direction facilitated the formation of surface chips during UEVC as well. The thermal-activated dislocations caused by high transient cutting temperature could result in the wide distribution of subsurface dislocations, and the dislocations were usually expanded by the way of crystal plane slip to suppress generation of long slip plane, which could reduce the subsurface damage of tungsten during UEVC. In comparison with ultrasonic amplitude in cutting depth direction, elevated ultrasonic amplitude in cutting speed direction or ultrasonic frequency could reduce depth of subsurface damage layer, surface roughness, and dislocation density, which derived from the small transient cutting force and stress as well as high strain rate. This study provides a new insight into the surface/subsurface formation mechanism of tungsten during UEVC, which can offer a theoretical basis on the selection of ultrasonic parameters for achieving high surface/subsurface quality for tungsten and other materials machined by UEVC.