High Frequency Vibration Effects Inrotary Ultrasonic Machining Hard-brittle Material

As the core component of the micro-target assembly, the preparation quality of the hohlraum became the key constraint to the ignition success in the Inertial Confinement Fusion (ICF). Processing of the glass mandrel was the core aspects in the hohlraum preparation process, and its shape accuracy and surface roughness directly determined energy conversion rate of the hohlraum and the pellet energy gain. Due to its the small size, low bearing capacity and subsurface crack sensitivity, the precision target micro-structure could fracture easily during the processing, which presented a challenge to the Rotary Ultrasonic Machining (RUM).When machining the target structure with the micro-tools, the abrasives were easily fell off, due to the limited abrasive number involved in machining and the large loading acted on the abrasives, which made it difficult to achieve the desired surface roughness and shape accuracy. Therefore, in the present work, based on the actual needs of the target structure machining, the unique dynamics characteritics of the abrasive were investigated. Also, the material removal mechanisms, surface formation process, sub-surface damage characteristics, the cutting force and tool wear characteristics were comprehensively, systematically explored, which would provide the reliable technical and theoretical guidances for the requirements on the high quality target structure. Detailed research work includes the following aspects:Microscopic observations of the RUM surface presented that pulverization could be another material removal mechanism besides brittle fracture and plastic deformation. The damage characteristics of the RUM scratching surfaces were investigated using the dynamic fracture mechanics of the brittle material, and it was revealed that incipient cracks acted as the initial stage of the material pulverization were resulted from the great vertical inertia force of the abrasive and the inertial effects of the material. Based on the analyses of the specific kinematics principles of the abrasive, a nondimensional parameter K was defined to quantitatively characterize the effects of the ultrasonic vibration in RUM process. A fresh analytical model of surface formation process involved in the RUM process was proposed, which incorporated the effects of the larger inertia force of the abrasive, the cyclical variation in the effective work angles of the abrasive, the lower dynamic fracture toughness of the material.With the bonded interface sectioning technique, the ultrasonic effects on the subsurface cracking patterns were investigated, and it was found that the micron/submicron fragments just smeared on the top surface forming the pulverization layer. Based on the fracture dynamic theory of the brittle materials, it was proposed that the superimposition with the ultrasonic would increase the material strain rate together with the decreased dynamic fracture toughness, which pulverizing the mateiral in consequence. At the bottom of the abrasive trajectory, the increased cutting depth would increase the extending depth of the lateral cracks and the median/radial cracks, which was the root cause of the worsened subsurface quality on the RUM specimen.The impact process of the abrasive on the specimen surface was simulated by means of the Smoothed Particle Hydrodynamics (SPH) method, proposing a new method to simulate the crack formation and expansion within the brittle material. Based on indentation fracture mechanics of brittle material, the correlation between the median and lateral crack systems aroused by a sharp indenter was analyzed. By incorporating the combined effects of the elastic and plastic stress fields on the lateral and the median/radial cracks, a nonlinear theoretical model between the surface roughness and the subsurface damage depth was proposed, with which the depth of the subsurface damage could be effectively predicted.Based on the analyses of the specific kinematics principles of the abrasive and dynamic indentation fracture mechanics of the material, the formula of the material strain rate during abrasive loading process was established, which could quantitatively characterize the strain rate effects. Brittle-ductile transition critical condition suited for the RUM process was developed with the incorporation of the strain rate effects on the abrasive-material interactions. Through the indentation experiments at the central of the incipient cracks, it was found that their nucleation provided a shielding effect to the increased cutting force. The relatively loose structural characteristics of the incipient crack made it easy to extend forward, which could increase material removal rate and further reduce the abrasive cutting force. These effects of the incipient cracks supplied a theoretical basis for the reduction of the cutting force by optimizing the processing parameters.Effects of the ultrasonic superposition on the wear regional characteristics, tool contour wear and the abrasive number were investigated, and it was found that the ultrasonic vibration by reducing the cutting force of the abrasive suppressed the tool contour wear, ultimately improving the shape accuracy and surface quality of the specimen. Microscopic fracture morphology of the abrasives on the tool end face revealed that the conchoidal chipping defects (microscopic fracture) were generated by the shallow fracture. Dynamic mechanical properties of the material were investigated by means of the impact experiments of the Hopkinson bar, which exhibited that the high strain rate effects would increase the material elastic modulus, thus reducing the crack nucleation depth in the internal abrasive, resulting in microscopic abrasive fracture.