Applicability of a Picosecond Laser for Micro-Polishing of Metallic Surfaces

An increasing number of recent technological advancements is linked to the adoptions of ultra-short pulsed picosecond (ps) lasers in various material processing applications. The superior capability of this laser is associated with precise control of laser-material interaction resulted from extremely short interaction time. In this context, the present study explored the applicability of a ps laser in laser micro-polishing (LµP) of Inconel 718 (IN718) and AISI H13 tool steel. The melting regime ‒ a mandatory phase for LµP ‒ was determined experimentally by the variation of focal offset to attain desired laser fluence. The finite element formulation of heat transfer equation and its solution were also estimated in order to develop a theoretical foundation for the heat transfer mechanism in ps laser-material interaction. The initial one dimensional (1D) line polishing experiments were performed on ground IN718 and H13 tool steel samples with the parameters related to the melting regime of corresponding material. The knowledge of this initial experimental investigation was later utilized to prepare the surface topography by micromilling with a specific step-over and scallop height, followed by LµP experiments with the same set of aforementioned parameters. The performance of LµP was evaluated by average surface roughness (Ra) spectrum at different spatial wavelength intervals along the laser path trajectory. Additionally, statistical measures, such as power spectral density (PSD) function, transfer function (TF) and material ratio (MR) curve were analyzed in order to establish the process parameters resulting the best possible surface quality. From the analysis of this experimental investigation, surface quality improvement up to 78.5% and 75.7% were reported for the spatial wavelength interval of 50‒100 µm for IN718 and H13 tool steel respectively. As a next step, two dimensional (2D) areal polishing of micromilled IN718 and H13 tool steel were performed, where surface quality improvement up to 69.32% and 77.28% were observed for the spatial wavelength interval of 50‒100 µm for IN718 and H13 tool steel, respectively. Overall, ps LµP was found to be an effective way of enhancing desired surface quality as demonstrated by the reduction of surface asperities as well as their volumetric uniform redistributions