Measurement of actual powder layer height and packing density in a single layer in selective laser melting

For a detailed numerical analysis of laser-material interactions and melt pool dynamics in selective laser melting (SLM), it is important to consider correct powder layer height and packing density in a single layer. Thus far, most experts assume that the powder layer height, which is equal to the leveling height of the build platform divided by the packing density of the powder bed, reaches a steady state after several layers. However, this assumption neglects the fact that a certain amount of powder is deposited (e.g., by spatter), and therefore, does not contribute to the molten powder layer height and that the packing density in a single layer is smaller than in "bulk." To determine the actual powder layer height and packing density in a single layer, experiments are conducted using two different materials (SS 17-4 PH and Ti6Al4V) and layer heights (30 and 50 mu m). The results reveal that the powder layer height is between 4 and 5.5 times the leveling height of the build platform and is, therefore, significantly larger than that assumed thus far. This is an important finding to consider when one investigates the details of the laser-material interaction and melting process in SLM, e.g., by numerical simulations. The measured packing density varied between 44% and 56%.