The Effect of Laser Pulse Duration and Feed Rate on Pulsed Laser Polishing of Microfabricated Nickel Samples

Abstract

The objective of this work was to improve our understanding of pulsed laser micropolishing (PLμP) by studying the effects of laser pulse length and feed rate (pulses per millimeter) on surface roughness. PLμP experiments were conducted with a multimode neodymium-doped yttrium aluminum garnet (Nd:YAG) laser (1064 nm wavelength) that was focused down to approximately 50 μm diameter and scanned over the stationary workpiece surface. Simulation results presented here and previous work suggest that longer laser pulses result in smoother surfaces. Results on microfabricated nickel samples using laser pulse durations of 300 ns and 650 ns test this hypothesis. Polishing with 300 ns and 650 ns pulse durations results in an average surface roughness of 66 nm and 47 nm, respectively; reductions of 30% and 50% compared with the original surface. Furthermore, PLμP is shown to introduce a minor artifact on the sample surface whose spatial frequency (1/mm) is directly related to the laser feed rate (pulses/mm).

Pulsed laser micro polishing: Surface prediction model

“…Perry et al [1][2][3] have shown that knowledge of the maximum melt duration, t m-max , can be used to determine the minimum critical frequency, f cr , above which there should be a significant reduction in amplitude of the spatial frequency content of a surface and its asperities. The critical frequency is…”

“…Perry et al [1][2][3] have modeled PLµP as oscillations of capillary waves with damping resulting from the forces of surface tension and viscosity and a one-dimensional spatial frequency domain analysis was proposed. They have also proposed a critical spatial frequency, f cr , above which a significant reduction in the amplitude of the spatial Fourier components is expected.…”

“…Perry et al [1][2][3] have demonstrated that pulsed laser micro-polishing (PLμP) with pulse durations of 300-650 ns and a spot size of 60 µm is a method by which the surface roughness of microfabricated and micro milled parts can be effectively reduced. Improved surface finish and smoothness can be achieved on select areas of different metallic surfaces, produced by meso/micro-manufacturing processes, which is difficult via other methods (like mechanical polishing, electropolishing and chemical mechanical polishing) as the feature sizes are approaching the same magnitude as the surface roughness [4][5][6][7][8].…”

“…These models are based on spatial Fourier analysis of the surfaces and examination of Navier-Stokes equation for each Fourier component. In the present work, we extend these analytical models and numerical models of Perry et al [1][2][3] to two dimensions and use the same to predict the surface finish, given the initial surface conditions. Perry et al [1][2][3] have proposed a critical frequency, f cr , which describes the cutoff point in the spatial frequency content of the surface, above which a significant reduction in the amplitude is expected.…”

Laser Processing of Metals and Polymers

“…51 For example, laser polishing of micro-milled nickel and titanium alloy parts reduced roughness more than 85%. 52 Others 53-55 used pulsed laser micro-polishing (PLμP) to reduce the surface roughness of micro-fabricated and micro-milled parts. A significant range of pulse durations and spot sizes has been effective.…”