Adhesion effects on spreading of metal powders in selective laser melting
Selective Laser Melting (SLM) is a rapidly developing and advanced manufacturing method for fabricating complex products. In SLM, the powder spreading process is crucial to ensure that the right amount of material can be fully melted by a certain laser energy input in order to minimise defects and achieve the desired microstructure. The packing density and homogeneity of the formed powder bed are of interest when comparing melting efficiency and quality of SLM processes with different metal powders or different spreading methods. Particle-based numerical studies are required for identifying the powder bed structure and particle dynamical behaviours which are affected by particle adhesion. In this work, experiments on powder packing density and repose angle for different particle size distributions are carried out. The discrete element method (DEM) model is validated and calibrated based on experimental results. The DEM is then used to examine the powder spreading process, focusing on the effects of particle adhesion and particle-based behaviours. Effects of spreader type, adhesion magnitude and particle size distribution are analysed. The results show that particle adhesion can reduce powder packing density and smoothness of the powder bed surface. Proper adhesion effects can improve powder bed homogeneity. Powder bed structure is determined not only by adhesion effects but also by particle rearrangement during spreading. Regarding spreader type, the roller can spread a better powder bed than rigid blade due to different particle contact force distributions and particle velocities in the powder pile and powder bed, which lead to different particle rearrangements and particle contact conditions.
Keywords: Metal powder, Selective laser melting, Adhesion effect, Spreader type, DEM