Increased Productivity of Ferrous Alloys Produced by Powder Bed Fusion - Laser Beam

Abstract: Low production speeds limit wider adoption of powder bed fusion – laser beam (PBF-LB) in cost sensitive industries as it correlates directly with high production costs. The main process parameters such as scan speed, hatch distance and layer thickness can be scaled up to increase build speed, but it often comes at a trade off in lower quality, i.e. surface roughness, porosity/density and mechanical properties. The objective of this thesis study was to investigate how large build rates influences microstructure and processability of two low alloy steels (4130 and 4140) and 316L stainless steel by detailed characterization of defects and porosity present in relation to process parameters applied. The initial study found that achieving consistent processability of low alloy steels across layer thicknesses, as indicated by the as-printed density, is better correlated with surface energy density (SED) J/mm2 rather than volumetric energy density (VED) J/mm3. Regions with high densities above 99.8% exhibited similar ranges of SED at different layer thicknesses, explained by the resulting melt pool depths being more similar than at similar ranges of VED. The second study addressed the impact of three- and four-factor increase in hatch distance and layer thickness compared with state of the art. It was observed that the influence of VED on density was poorly described as differences of up to 7.5% in density were measured at the same VED. More accurate representations of impact on main print parameters on density was found by regression analysis which also captured the interaction between laser power, scan speed and hatch distance at different layer thicknesses. Build rate increase can be realized by numerous combinations of basic laser parameters, however this results in distinctive porosity characteristics and even at the same levels of build rate increase, pore characteristics, such as orientation, aspect ratio and size, can differ significantly.