Phase transformation and powder densification behavior of the polyamide-12 during powder bed fusion process

Abstract

The phase transformation and densification behaviors are two critical research topics in the semi-crystalline polymer Laser Powder Bed Fusion (L-PBF) process. This study first introduces a thermal model incorporating crystallization kinetics to study the temperature evolution and phase transformation behavior during the L-PBF of polyamide-12 (PA12). The model highlights the delayed solidification behavior represented by the overall degree of crystallization compared with the temperature evolution during the cooling stage. A thermo-hydrodynamic model is then developed based on the thermal model to predict densification behavior and investigate the contraction under various processing conditions, incorporating powder densification kinetics. Numerical studies reveal that the contraction of the melt pool lags behind the movement of the laser beam significantly because the densification is not a transient but a continuing process accompanied by the fusion phase transformation. The model is validated by comparing the melt pool contraction obtained by experimental measurement and numerical simulation on single-track sintering. The reasonable thermal interaction during multiple-track simulations is proven to improve the final density noticeably. Finally, a two-laser scanning strategy is proposed to enhance the density of polymer L-PBF components.