Numerical modelling of surface morphology in selective laser melting

Abstract

Selective laser melting (SLM) is a promising additive manufacturing technology, which involves complex physics such as heat and mass transfer, phase transformation and molten pool flow. In this study, a three dimensional numerical model is developed to model the thermal-fluid flow and to predict the surface morphology for the SLM process. Particularly the laser ray tracing method is coupled with the VOF method to reproduce the multiple reflections of laser between the randomly packed powder particles and highly dynamic molten pool. Two sets of experiments are used to validate the model: 1) single tracks on a bare plate with different scan strategies, and 2) single tracks on a powder layer with different scan speeds. For the bare-plate single tracks, the width and surface elevation, including their variation along the track distance, are well reproduced in the simulations. For the powder-layer single tracks, the experimentally-observed balling, distributed and smooth tracks with varied scan speeds are all reproduced in the simulations. The results illustrate the complex flow pattern of the molten pool, particularly the effect of partially melted particles: 1) the semi-melted particles, drive the molten fluid flow from the molten pool center towards the unmelted particle, leading to the single track non-uniformity; 2) the near-fully melted particles, drive the molten fluid flow down into the melt pool, increasing the single track uniformity.