Influence Of Subsurface Defects On Surface Integrity During Machining Process

Defects such as inclusions, voids and cracks in materials can not only affect their functionality and performance but also may cause their eventual failure. When these defects are located near a material surface subject to cyclic contact loading, their damage effect becomes more prominent. When it comes to the field of mechanical processing, the defects in subsurface divided into two categories:pre-existed defects and machining induced defects. The defects affect the quality of the processed surface to a great extent. Among the numerous defects, the most critical issue is the large surface damage after machining, which should be removed by the subsequent grinding and polishing. As a result, the production cycle is prolonged and production costs are increased and the processing efficiency is reduced. It can be said that the influence of the subsurface defects on the mechanical processing is an unavoidable problem.Machining is essentially a large number of influencing factors, strong nonlinear, strong thermal coupling process, and is also an integrated physics, chemistry, mechanics and other interdisciplinary complex engineering problems. In order to obtain the mechanical processing method of low damage, in this paper, the influence of two kinds of subsurface defects on the subsurface quality and the evolution process of subsurface damage are studied systematically in this paper:(1) Compared to without considering defects, the finite element simulation results show the various sizes and heights of inclusion bonded in the matrix can affect the cutting force, residual stress to a various degree. With the increasing height of the inclusion, both of the cutting force and the residual stress increase or decrease in the surface subjected to the inclusion. Additionally, larger and closer to uncut surface inclusions result in the increasing of the residual stress in the surface and the cutting force during the machining process, indicating that the inclusion effects play a key role in the cutting process. More interesting still, the cutting force and the residual stress are related to the stress magnitude exerted on the inclusion as the tool moving close to the inclusion. Hence, these results may give a potential way to acquire a better machined surface and improve the machining process.(2) The author proposed a mechanical and numerical study of fracture mechanics from the perspective of external loading and indentation geometry in brittle machining. Stress intensity factors are computed to analyze various impacts of external loading and indentation configuration on subsurface crack propagation. Results indicate that the main fracture mode for inclined crack is shear rather than opening and the apex angle of the indentation plays an important role in fracture behavior. As a certain external loading is exerted to the surface of the silicon, a large apex angle of indentation may lead to strong shielding effect on mode II crack propagation. A relationship between critical value of external loading to the crack propagation and the apex angle of the indentation is given in this paper that shows quantitative indication for suppression of crack growth.(3) The results of MD simulations show that the hardness, elastic recovery ratio and temperature of those three nanocrystalline copper strongly depend on crystal structure and twin-lamellae-thickness. It is also revealed that as nanoindenter goes deeper, the extent of plastic zone becomes substantially larger. Initial dislocation always nucleates at the beneath of indenter, and the discrete drops of indentation force observed at certain indentation depths, indicates dislocation bursts during the indentation process. Furthermore, we find that plastic deformation has a strong dependence on crystal structure. The plastic deformation of the single crystalline copper relies on the generation, propagation and reaction of dislocations, that of the polycrystalline copper depends on the dislocation-grain boundary (GB) interactions, and that of the nanotwinned polycrystalline copper relies upon the dislocation-twin boundary (TB) interactions as well as twining/detwining. This work not only provides insights into the effects of crystal structure and two-lamellae-thickness on the mechanical properties of copper under nanoindentation, but also shed lights onto the guideline of understanding other FCC nanocrystalline materials.