Formation mechanisms and control strategies of geometric errors induced by edge bumping during laser powder bed fusion

Abstract

Edge bumping, a typical abnormal surface feature during the laser powder bed fusion (LPBF) process, can significantly affect the geometric accuracy of the final product. In a representative case, edge bumping induced severe geometric errors in lattice structures, including both strut necking and out-of-tolerance deviations. Despite the critical influences, the formation mechanisms and control strategies of edge bumping remain unclear. This study comprehensively investigated the characteristics of edge bumping for both standard octagonal specimens and general samples (such as topological features and overhang structures) with various geometries and dimensions, utilizing in-situ monitoring, ex-situ characterization and numerical modelling approaches. The results showed that edge bumping manifested as edge protrusions on the part top surface, exacerbated by higher laser power, slower scanning speeds, and increased laser rotations at edges. The formation mechanisms of edge bumping were revealed for the first time in this work, which comprised spatter knockdown by the laser, extra powder entrainment into the molten pool, and molten material flow and solidification at the rear of the molten pool. To mitigate the geometric errors, control strategies of edge bumping considering LPBF energy densities and inter-track cooling intervals were developed. Efficient suppressions were achieved, with edge bumping height reduced to 0.04 mm for the standard octagonal specimens, and the dimensional accuracy of lattice structures increased significantly from 68.0 % to over 96.9 %. The novel findings provide valuable insights for understanding the complexity of the transient processes, and improving the LPBF quality of engineering structures.