Impact of Overlapping Trajectories in Laser Micro-Polishing

The quality of a laser micro-polished (LμP) surface is dependent upon the part material, initial surface topography and roughness, and the energy density of the beam used to produce a thin molten layer on the finished part. Prior research has shown that the amount of energy delivered to the work piece can also be controlled by adjusting the “focal offset distance” (FOD) of the laser optics. In this paper, the impact of using multiple overlapping trajectories for laser polishing on the average surface roughness (Ra) is investigated. The analysis is performed on ball-end micromilled H13 tool steel specimens polished using a nanosecond Q-switched Nd:YAG laser. The initial surface roughness is dependent on the height and width of micromilled scallops. The trajectory overlap percentage (TO%) is an advanced machining parameter used to describe the ratio between the size of the polishing step and the molten LμP diameter. The average surface roughness of the “best” experimental specimen was reduced from a Ra of 746 nm to 205 nm (~73% improvement) using a short distance polishing regime (1.3 mm <; FOD <; 1.8 mm) with a TO% = 96.8% (or 2 μm step trajectory). Similar, but less dramatic, reductions in profile Ra were also observed for the longer polishing regime (FOD > 2.2 mm). The experiments also illustrate that the quality of the final surface finish can be further improved by combining continuous wave laser polishing (CW-LP) and LμP.