08-16-2017, 09:26 PM
pulsed leaser micro polishing seminar report and ppt
Abstract:
A fundamental study of pulsed laser micro polishing (PL P) was conducted as a method for reducing surface roughness of micro/meso-scale metallic parts. Although PL P 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 PL P, 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 PL P, 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 PL P, 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 PL P 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 PL P 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.
Introduction
This project is focused on developing physics based models to predict the outcome of pulsed laser micro polishing (PL P). Perry et al. [1 3] have modeled PL P as oscillations of capillary waves with damping resulting from the forces of surface tension and viscosity. They have proposed a critical spatial frequency, fcr , above which a significant reduction in the amplitude of the spatial Fourier components is expected. The current work extends the concept of critical spatial frequency to the prediction of the spatial frequency content and average surface roughness after polishing, given the features of the original surface, the material properties, and laser parameters used for PL P. The proposed prediction methodology was tested using PL P results for Nickel, Ti6Al4V, and stainless steel 316L with initial average surface roughnesses from 70 nm to 190 nm. The predicted average surface roughnesses were within 10% to 15% of the values measured on the polished surfaces. The results show that the critical frequency continues to be a useful predictor of polishing results in the spatial frequency domain. The laser processing parameters, as represented by the critical frequency and the initial surface texture therefore can be used to predict the final surface roughness before actually implementing PL P.