A fundamental study of the pulsed laser micro-powder (PLμP) was carried out as a method to reduce the surface roughness of the micro / meso-scale metal parts. Although PLμP proved to be an effective process on these scales, knowledge of process physics was lacking in rigour. The objective of this work was, therefore, to improve the existing knowledge of PLμP, focusing on the development of models based on physics, understanding the effects of several parameters of the process and developing strategies for the selection of process parameters and trajectories of laser. In PLμP, a small area on a surface is irradiated with laser pulses to melt roughness characteristics. The molten characteristics are softened by surface tension and viscous forces. Two polishing regimes for PLμP, namely the thermocapillary regime and the capillary regime, were defined, based on whether the temperature gradient of surface-tension thermo-capillary flows is dominant or insignificant.
The dominant thermocapillary fluxes in the thermocapillary regime result in a significant softening of low and high spatial frequency characteristics, but introduce high spatial frequency characteristics of low residual amplitude. In the capillary regime, thermo-capillary flows are insignificant and the molten characteristics oscillate as stationary capillary waves that are cushioned due to the viscosity of the molten metal. The capillary regime is only effective in smoothing higher spatial frequency characteristics. A physics-based surface finish prediction model for the capillary regime was developed and validated.
Laser pulses are used to create surface fusion pools with a controlled size (eg, depth) and duration to allow surface tension forces to "drag" roughnesses with a small radius of curvature. No ablation occurs in the process being modeled. The depth and duration of the melt is predicted with a transient two-dimensional axisymmetric heat transfer model with temperature dependent material properties. The surface of the melt bottom is analytically modeled as stationary capillary wave oscillations with damping resulting from surface tension and viscosity forces. Above a critical spatial frequency, a significant reduction in the amplitude of Fourier spatial components is expected. The proposed prediction methodology was validated using line polishing data for the results of 316L stainless steel polishing and surface polishing for pure nickel, Ti6Al4V and Al-6061-T6. The expected average surface roughness was within 12% of the measured values on the polished surfaces.
The motivation for micro polished laser polishing is to reduce the roughness of the surface of the parts whose surface texture can approach the size of the feature. Being able to predict the magnitude of the polishing and the frequency (wavelength) of the surface content will help in the design of optimal processing parameters with minimal experiments. Laser pulses are used to create surface fusion pools with a controlled size (eg, depth) and duration to allow surface tension forces to "drag" roughnesses with a small radius of curvature. No ablation occurs in the process being modeled. The depth and duration of the melt is predicted with a transient two-dimensional axisymmetric heat transfer model with temperature dependent material properties. The surface of the melt bottom is analytically modeled as stationary capillary wave oscillations with damping resulting from surface tension and viscosity forces. Above a critical spatial frequency, a significant reduction in the amplitude of Fourier spatial components is expected.