Laser -assisted machining of zirconia ceramics
Abstract
Structural ceramics, such as partially-stabilized zirconia (PSZ), are used in many high-end applications due to their improved toughness over traditional ceramics, excellent wear and chemical resistance, and the ability to retain their properties at elevated temperatures. However, shaping structural ceramics is both difficult and expensive, because of their brittleness and hardness. Because it heats the material prior to removal with a conventional cutting tool, thereby temporarily lowering the strength of the workpiece, laser-assisted machining (LAM) offers the ability to machine structural ceramics more rapidly and with greater flexibility than conventional methods. This study focuses on understanding and evaluating laser-assisted machining (LAM) of partially-stabilized zirconia (PSZ) through the synergy of experiments and modeling of heat transfer in a semitransparent PSZ workpiece. A transient, three-dimensional heat transfer model is developed, and the optically thick assumption (diffusion) to internal radiative transfer is compared with a discrete-ordinates method solution for transfer in a semi-transparent medium. Since the difference in the predicted temperatures lies within the uncertainty limits of the predictions (+40/−55°C at 400°C to +100/−120°C at 1000°C), the diffusion approximation is used. The temperature predictions are validated with single-point temperature measurements made by a long-wavelength (11–14 μm) pyrometer with a total uncertainty from −33/+22°C at 500°C to −59/+49°C at 1000°C. The temperature predictions are in good agreement with values measured during machining. The validated model is used to show that laser power and feedrate have the greatest effect on the temperature distribution and LAM. The machinability of PSZ under varying conditions is evaluated by examining tool wear, surface integrity, microstructure, and specific cutting energy. With increasing material removal temperature from 530 to 1210°C, the benefit of LAM is demonstrated by a 2.5 fold decrease in specific cutting energy and an increase in the life of a cubic boron nitride tool to 120 minutes. These machinability improvements are a result of some material removal occurring by plastic deformation, which nominally results in a finished surface without cracks and an average surface roughness of less than 1 μm. Above 1000°C cracks occasionally occur, with the scrap rate increasing to 100% at a material removal temperature of 1390°C.
Degree
Ph.D.
Advisors
Incropera, Purdue University.
Subject Area
Mechanical engineering|Industrial engineering
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