A simple scaling model for balling defect formation during laser powder bed fusion

Balling is one of the cornerstone defects in laser powder bed fusion of metals, leading to high porosity of the 3D printed parts and substantially deteriorating their performance. The transition to the balling mode is commonly identified through a series of trial-and-error experiments, an extremely inefficient approach. Here, we propose an outstandingly simple thermal scaling model for predicting the threshold from balling mode to conduction mode in laser powder bed fusion. The resulting balling criterion can be expressed as a dimensionless number which combines the material properties, the powder size and the pre-heating of the substrate. The model predictions are well in agreement with our validation experiments for three different materials (copper, bronze and steel). The applicability of the model assumptions is verified through a set of suitably designed multiphysics computational fluid dynamics simulations. The combination of first-hand experiments and simulations substantiates the balling model as a readily usable and reliable scaling criterion for establishing the minimum required power for laser powder bed fusion, applicable to different materials, as well as suggesting viable strategies to adjust the operating parameters towards the defect-free regime.