Laser incidence angle influence on energy density variations, surface roughness, and porosity of additively manufactured parts

Abstract

The use of additive manufacturing (AM) technologies for fabricating structural and functional parts is rapidly rising due to their transformative design flexibility and customizability. However, maintaining the uniformity and consistency of the fabricated part across the build plate is a critical challenge. One of the main sources of variability across the build plate is laser spot elongation in laser beam powder bed fusion (LB-PBF) AM which typically occurs when the beam moves from the center of the plate to the edge at different incidence angles. This laser spot elongation may then result in the loss of laser power density which can potentially cause elongated melt pools, poor dimensional accuracy, and formation of lack of fusion and more gas entrapped pores in the final part. Here, we report both geometrical calculations and experimental evidence to show the behavior of the laser in the different areas of the build plate. A pulsed laser is first used to show and calculate the geometrical footprint of the beam elongations at different incidence angles. Then, the surface roughness, melt pool size, and porosity variations are investigated for varying laser interaction cross-section across the build plate by manufacturing AlSi10Mg (an aluminum alloy) parts and correlating them to theoretical calculations. It is found that for given optimized process parameters at the center of the build plate, as the beam goes farther away from the center, it becomes elongated. As the power density falls, melt pool width becomes wider, melt pool depth becomes shallower (depending on the elongation orientation with respect to the scan direction), and the number of lack of fusion defects increases. This work provides fundamental insight into possible sources of variations across the build plate in an LB-PBF process that may help design new strategies to compensate for such variations to enhance uniformity and reliability of AM parts.