Effect of overhanging surfaces on the evolution of substrate topography and internal defects formation in laser powder bed fusion

Overhanging (a.k.a. down-facing) surfaces are typically found in complex metal parts built with laser powder bed fusion (L-PBF). When these surfaces exceed a certain extension or inclination with respect to the build plate, they need to be supported with external structures to avoid failure and macro-geometrical errors. However, a relatively large portion of the slice gets printed directly over loose powder, thus facing a substrate with significantly different wetting and heat transfer characteristics from solid/bulk. Several quality aspects (e.g., internal defects, surface topography) can be affected by the presence of overhangs, but their evolution during the process is still a relatively unexplored field. In this work, a new strategy based on process interruption is proposed for analyzing the evolution of defects produced during the printing of down-facing surfaces. Ex-situ high-accuracy characterization equipment was used to study their effect on the evolution of printed surface topography, internal defects, melted and sintered thickness. Results show that the process gradually recovers from the disturbance introduced by the overhang, but the peculiar structure of the internal defects observed in those regions reveals that even small unsupported areas can be detrimental to the as-built quality of the part. The combined use of surface topography data and volume reconstruction also allowed developing and validating a physics-based model for predicting the evolution of surface topography and effective layer thickness in overhangs.