Research On Fabrication Of Nanocrystalline And Ultra-fine Grained Chip In Large Strain Machining

Nanocrystalline and ultra-fine grained chips with high strength can be produced in a single stage of deformation with significantly larger shear strains in machining which is regarded as a novel approach to manufacture nanostructure materials, while simultaneously overcomes the limitations of sever plastic deformation. It is confirmed that chip materials with ultra-fine grained and high hardness can be created with more negative tool rake angle at some low cutting velocity.As a technology to manufacture nanocrystalline and ultrafine grained structure materials, large strain machining has been developed to some extent. While deformational behaviors has not been researched systematically in large strain machining. At first, refinement and strength mechanism of nanocrystalline and ultra-fine grained chips in large strain machining are lack of systematic research. Second, although the cutting velocity has an important effect on plastic flow, microstructure and properties of nanocrystalline and ultra-fine grained chip, it has not been researched sufficiently. Third, the uniform of distribution on shear strain, microstructure and properties of nanocrystalline and ultra-fine grained chips across vertical section of chips has not been researched. Last, the dimensions of the chips are usually too small for fabricating any structural or mechanical components directly. The chips need to be consolidated by some thermal or thermo-mechanical processing. It is necessary to characterize the annealing behavior of the machining chips to provide background information for preparing bulk nanostructure materials successfully.The influence of machining parameters such as rake angle, cutting velocity and cutting thickness on variables in machining deformation area, microstructure and properties of chips are conducted in detailed. The annealing behavior of nanocrystalline and ultra-fine grained chips of copper is studied. The main research work for the dissertation focused on five aspects of nanocrystalline and ultra-fine grained materials. (1)The principle of machining and machining primary parameters are analyzed based on the orthogonal cutting model of metal which is offered by Oxley. The upper-bound solution of deforming force imposed on chip in machining is deduced based on upper bound theorem. (2)The effect of machining parameters such as rake angle, cutting velocity and cutting thickness on variables in machining deformation area such as stress, strain, strain rate and temperature is analyzed by finite element modeling. Average friction coefficient model based on experiments is adopted in finite element modeling. The formation mechanism of ultra-fine grained chips and machining parameters needed by nanocrystalline and ultra-fine grained chip formation in machining is discussed. The cutting force and effective shear strain imposed on chips are increased and effective strain rate is decreased with the decreasing rake angle and cutting velocity. Although rake angle have more important effect on the shear strain imposed on chip than the cutting velocity, cutting temperature is increased significantly by increasing cutting velocity which will influence the microstructure refinement and properties improvement of chip. The mean effective strain is increased and distributed evenly accros the chip with the increasing of deformation times in large strain machining. Especially the variation is significant at the rake angle with-20°. In addition, the mean effective strain rate is also obviously increased while the stress and temperature are not obviously increased. (3) Effect of machining parameters such as rake angle, cutting velocity and cutting thickness on extrusion shape of chip is studied by scanning electron microscopy (SEM). The deformation ability of chip influenced by machining parameters is discussed. (4) The refinement ability of microstructure, the effect of machining parameters on microstructure and mechanical properties of chips and the relations between microstructure and mechanical properties were studied by Optical microscopy (OM), SEM, transmission electron microscopy (TEM) and Vickers hardness tester. The refinement and strength mechanism of ultra-fine grained chips in large strain machining are discussed on the basis of above studies. (5)The annealing behavior of metal and alloy chips is influenced by deformation extent of materials, annealing temperature, annealing time and so on. Progressive relation between thermal stability of ultra-fine grained chip and deformation extent, annealing temperature and annealing time is researched from microstructure and properties of nanocrystalline and ultra-fine grained chips by SEM and Vickers hardness tester. Experimental results indicated that chips produced by the tool of rake angle with-20°are composed of grains less than 1μm and the increase in hardness of chip is 77% compared bulk material after an hour annealing under 360℃.