The effect of preferred orientation in the single point diamond turning of polycrystalline materials

Abstract

Single-point diamond turning (SPDT) is a machining technology which can be used to manufacture products with submicrometer form accuracy and surface finish. In SPDT, depth of cut is usually less than the average grain size of a polycrystalline aggregate, and cutting is generally performed within a grain. The crystallographic orientation of the substrate material being cut exerts a great influence on the cutting physics, the cutting forces and hence the surface finish. Most previous studies on the effect of crystallographic orientation in SPDT were conducted mainly on single crystal materials. The effect is more obscure in polycrystalline materials as various crystallographic textures or preferred orientations exist in engineering materials which can be completely random or, on the other hand, can behave as a single crystal. A systematic study of the effect of preferred orientations in the diamond turning of polycrystalline aggregates is of both scientific and technological importance. This thesis is in two parts. The first part seeks to demonstrate the effect of preferred orientations in the industrial diamond turning of materials possessing different textures. The second part deals with the fundamental aspects of shear angle prediction in micro-scale machining and compares this with the Merchant type shear angle equations. In the first part of the study, a range of materials ranging from amorphous state, wrought forms to single crystals were used in experiments. Wrought aluminium alloy plates each with the same chemical composition and similar hardness were thermal-mechanically processed with different crystallographic textures. A series of cutting tests were conducted and the micro-cutting forces were measured by a piezoelectric force transducer. The variation of surface roughness of the workpieces along different directions were assessed by their Degree of Surface Roughness Anisotropy (DRA). The result reveals that there is a systematic variation between different types of texture patterns and the roughness parameters. Materials with a texture having a higher symmetry tend to have better surface finish after diamond turning, whereas amorphous material produces a faithful reproduction of the periodicity of the cutting tool width in its machined surface. The shear angle in metal cutting is an important parameter as it is related to the cutting force and surface finish that could be obtained. In the second part of this study, the mesoplasticity model of Lee and Zhou (1993) is extended to cover the prediction of ideal shear angles in polycrystalline materials based on the instability criteria of shear formation in materials deformed under large plane strain compression. The shear angle for a-brass and oxygen-free high conductivity copper could be obtained from their orientation distribution functions determined from X-ray diffraction without the need of any trial cutting experiments. The theoretical angles thus obtained were compared with the ones calculated from the experimental cutting data by the author as well as from published literature. Close agreement between the predicted ones and the experimental ones determined from the Merchant type of shear angle equation was observed. In addition, it was found that among the various Merchant type shear angle equations which attempt to correlate the shear angle, friction angle and rake angle in orthogonal cutting, the Ernest and Merchant equation is closer to the experimental cutting data. Further work is needed to test the applicability of the mesoplasticity theory of shear angle formation in other alloy systems and extend the theory to cover other geometrical configurations of the tool and work material, as well as to test it under frictional conditions.