Ultrashort pulse laser polishing by continuous surface melting

Abstract

Enabling high precision material processing, ultrashort pulse (USP) laser beam sources are increasingly applied in the field of surface structuring. In the mold and die production, three dimensional structures with a vertical depth of a few (10–500) micrometers are created by laser ablation, resulting in a surface roughness Sa = 0.4–2 μm on the 3D structures. Because many applications require a lower surface roughness, a polishing process has to be conducted following the laser ablation. In terms of a digital photonic process chain including the workpiece’s processing in the same setting for both laser structuring and laser polishing a novel polishing process using USP laser radiation is investigated. In contrast to the customary claim of USP laser processing the aim of USP polishing is to induce a thin molten surface layer. Thus, smoothing is achieved by reallocating material from the peaks to the valleys. Unlike laser polishing with longer pulse durations melt formation is not determined by the single pulse duration but rather by the temporal and spatial energy distribution. Heat accumulation is achieved by applying a high pulse repetition rate fseed=48.92 MHz at a pulse duration of τ = 800 fs. The purpose of this work is to investigate the influence of USP polishing on surface properties of 1.2738 tool steel including surface roughness, surface topography and microstructure of the remelted surface layer. Originating form an initially ground surface with Sa = 0.6 μm a reduction to Sa = 0.3 μm is achieved revealing a polishing rate of 12.15 cm²/min per pass.