The effect of molten pool evolution on the formation of surface structures during laser polishing

Abstract

As a novel surface treatment technology, in-situ laser polishing aims to improve surface finish of 3D printed parts, while laying the foundation for the integrated processing of 3D printing and laser polishing. However, the polishing process will introduce a few new surface structures (undercuts, step structures, ripples, etc.), which will affect the surface roughness. In this work, the surface roughness and surface structure after high power and low power polishing were compared respectively through experiments. In addition, a 2D transient numerical model was developed to explore the forming mechanism of different structures. The evolution of temperature field, velocity field and freedom surface of melting pool under different parameters were analyzed by the model. The results showed that the solidification induced surface structure with large roughness was generated after high power polishing. During its formation, the thermocapillary force was dominant and drove the tangential flow of fluid. While, low power polishing resulted in ripple structure with small roughness. The emergence of this structure was primarily due to the capillary force as the leading force in the melting pool flow process, which drove the normal flow of the melt, thus eliminating the large surface curvature. Furthermore, the width of the melting pool in the simulation was compared with the width of the single scanning track, and they were in good agreement. Finally, by adjusting the test strategy, it was found that the surface roughness could reach the lowest (0.88 ± 0.06 μm) only when the two structures coexisted.