Surface Removal of Ductile Metal Using Cyclic Low-Frequency Impact of a Discrete Particle-Less Waterjet

The discrete waterjet (DWJ) offers advantages in material removal because of the cyclic impact loading, but the specific removal characteristics and corresponding mechanisms have not been yet fully understood. Therefore, the flow field and material removal characteristics of the DWJ were studied through numerical and experimental methods, including the velocity and impact pressure distribution, surface morphology, quantitative statistics of erosion crater, and fractography under different jet pressures and exposure times. The results indicate that mechanical interruption has a positive impact on increasing the maximum velocity (53.0% at 30MPa) of the waterjet head and altering the impact pressure distribution, resulting in an asymmetric erosion crater with a tapering groove along the edge. Meanwhile, the DWJ has superior working efficiency (higher erosion area, depth and volume) and energy utilization (lower specific energy) than the continuous waterjet (CWJ), while avoiding the unfavorable high crater lips. A large amount of material removal does not necessarily occur in the initial stage of waterjet impact, and it is related to material properties, waterjet parameters and specific evaluation indexes. The increase of waterjet pressure accelerates the material removal process and improves the energy consumption efficiency. The materials removing process is divided into four stages: surface deformation due to the cyclic impact pressures, surface break-ups localized near the surface unevenness, independent fragment forming mainly by water wedge pressure, and larger removal of materials due to these cyclic processes.