Mechanical Micromachining-Effect of Crystallographic Anisotropy on Machining Forces
With increased application of mechanical micromachining for creating small features with complex geometries on a broad range of materials, the need for understanding the mechanics of machining at the micro-scale has been recognized. During mechanical micromachining of metals, the tool-workpiece interaction occurs entirely within either a single crystal or a few crystals of the workpiece material. Consequently, the crystallographic properties (e.g., anisotropy) of individual crystals strongly affect the machining response, including micromachining forces and resulting surface finish. Hence, the crystallographic effects that are generally neglected (due to the perceived isotropic nature of the workpiece) in macro-scale machining need to be studied both experimentally and theoretically for gaining a better understanding of the micromachining process.
This thesis aims to understand the effects of crystallographic anisotropy on machining response of face centered cubic metals through physics-based modeling and experimental analysis. The thesis begins with an introduction to micromachining and the associated crystallographic effects on the micromachining response. Subsequently, a literature review is presented and the shortcomings of the available research are identified. In particular, (a) the lack of physically realistic machining force models incorporating the effects of anisotropy, and (b) a necessity for experimental data analyzing the effect of anisotropy over a broad range of machining conditions, are addressed. The work is performed in three stages, with the first two addressing the former, and the last one addressing the latter shortcoming.
First, a simplified machining force model incorporating the effects of anisotropy is developed by combining a plasticity theory and the Merchant's machining model. Since the deformation geometry is unknown a-priori in machining operations, a shear angle determination scheme is necessary before predicting the forces. For a given crystallographic orientation, the model considers the minimization of the total power, including the shearing (plastic) and rake-face friction power, to determine the shear angle and predict the machining forces. The calculation of shearing power is performed using the Bishop and Hill's plasticity theory, thus incorporating the effects of anisotropy. The model is calibrated and validated using the available (but limited) machining force data from the literature. An analysis of the model is also performed to observe the effects of orientation, friction angle and rake angle. The simplified model neglected the effects of hardening and lattice rotation observed during large strain deformation (such as that experienced in machining).
Second, a more physically realistic rate sensitive plasticity-based machining (RSPM) force model is developed to enhance the simplified model by incorporating the hardening and lattice rotation effects. Similar to the simplified model, minimization of the total power (sum of plastic and friction power) is used to determine the shear angle. When calculating the required plastic power, rate-sensitive constitutive equations with hardening and kinematics of single crystal deformation (including lattice rotation) are used. The obtained shear angle is then used to predict the machining forces. The RSPM model is calibrated using the Kriging-algorithm-based SuperEGO (efficient global optimization) code to obtain the five material parameters required. Both the calibration and the subsequent validation are performed using the machining force data available in the literature. Use of the RSPM model improved the match with the experiments over the use of the simplified model. The RSPM model is then used to analyze the effects of orientation, rake angle, coefficient of friction and material properties on machining forces.
Third, to address the need for comprehensive experimental data and analysis, a precision turning and a precision planing apparatus are designed and constructed. Initial machining experiments performed on single crystal and coarse-grained polycrystal aluminum showed that the machining force and surface finish values vary strongly with crystallographic orientations. A measurement of deformation below the cut surface also indicated the importance of measuring the subsurface deformation in future studies. Subsequently, a comprehensive study on the effect of anisotropy over a range of cutting parameters is performed for coarse-grained polycrystal aluminum. In these experiments, in addition to the machining parameters, the effect of subsurface deformation is studied by comparing experimental results from cases with and without cleanup cut. The results from these experiments quantified the effects of crystallographic anisotropy, its interaction with machining parameters and the effect of sub surface deformation on machining forces and surface finish.
The thesis concludes with a discussion of future work covering both modeling and experimental aspects of the research. The future work is divided into near term and long term future work, where the near term work includes planing and plunge turning experiments on single crystals and the extension of the RSPM model to oblique machining. In the longer term, modifications to the machining force model to include the non-homogeneity of the shear zone, and extension of the model to three dimensional machining operations like milling are proposed.
The fundamental contributions of this thesis research are focused on modeling and experimental investigations on single-crystal and coarse-grained materials. Specific contributions include; (1) A simplified machining model that includes the crystallographic anisotropy; (2) A comprehensive rate-sensitive plasticity-based machining force model including hardening and crystal rotation effects and large deformations; (3) An experimental infrastructure, including precision planing and plunge-turning testbeds, to facilitate experimental investigations and model validations in the presence of crystallographic effects; and (4) An experimental understanding on the effects of crystallography when micro-machining single-crystal and coarse-grained materials.