Advanced Study of Pulsed Laser Micro Polishing.

摘要

A fundamental study of pulsed laser micro polishing (PLµP) was conducted as a method for reducing surface roughness of micro/meso-scale metallic parts. Although PLµP was shown to be an effective process at these scales, the knowledge of the process physics lacked thoroughness. The goal of this work, therefore was to improve upon the existing knowledge of PLµP, with focus on developing physics based models, understanding the effects of various process parameters and developing strategies for selection of process parameters and laser scan trajectories.;In PLµP, a small area on a surface is irradiated with laser pulses to melt roughness features. The molten features are smoothed out by surface tension and viscous forces. Two polishing regimes for PLµP, namely thermocapillary regime and capillary regime, were defined based on whether the temperature gradient of surface tension driven thermocapillary flows are dominant or negligible. Dominant thermocapillary flows, in the thermocapillary regime, result in significant smoothening of both low and high spatial frequency features, but introduce residual low-amplitude high spatial frequency features. In the capillary regime, thermocapillary flows are negligible and the molten features oscillate as stationary capillary waves that damp out due to the viscosity of the molten metal. The capillary regime is only effective at smoothening higher spatial frequency features. A physics based surface finish prediction model was developed and validated for the capillary regime.;The fundamental understanding of the two polishing regimes lead to the development of a two-pass multi-regime PLµP method that takes advantage of both the polishing regimes. In this method, the first pass, in the thermocapillary regime, results in significant reduction of the surface roughness. The second pass, in the capillary regime, removes the residual process features introduced in the first pass. Finally, an artificial potential fields based method was proposed to generate laser scan trajectories for PLµP that can overcome the limitations of a zigzag trajectory. In this method, artificial attractive fields are assigned based on surface condition and repulsive fields assigned to regions where polishing is not desired. The generated trajectories are irregular, smooth and adapt to the changing surface condition of surface being polished.