An improved thermomechanical model for the prediction of stress and strain evolution in proximity to the melt pool in powder bed fusion additive manufacturing

Abstract

An improved thermomechanical analysis of the evolution of the stress and plastic strain fields near the melt pool has been developed for the Electron Beam Powder Bed Fusion (PBF-EB) process. The analysis focuses on a sub-domain extracted from a larger component, which includes the sequential addition and melt/consolidation of 4 powder layers on a solid substrate. The material’s behavior was described as a function of temperature and material form (powder, semi-consolidated, bulk, and liquid). The yield stress was described as a function of temperature and strain rate to capture key phenomena related to plastic strain accumulation. The thermal component of the model has been validated using melt pool geometry. The importance of the strain rate-dependent yield stress and substrate temperature were identified. Yielding was predicted to occur in the solid directly below the melt pool in association with rapid heating and, to a lesser extent, during cooling in the wake of the melt pool as it solidifies. A linear regression model was proposed, linking the developed compressive plastic strain to substrate temperature for a single set of beam parameters. The model was validated by comparing the substrate temperatures needed to produce the same plastic strains used to predict the distortion in a component with a ledge-type feature fabricated in a commercial PBF-EB machine. It is proposed that the linear regression model may be used to estimate the strain variation in large components as a function of the varying thermal field in the newly consolidated material (the substrate) during component fabrication.