Elucidation of laser irradiation behaviors associated with the keyhole dynamics during laser powder bed fusion

The keyhole phenomenon, frequently found in LPBF, is critically responsible for the formation of internal porosities which degrades the part quality. Owing to the ultra-high transience and complexity of the LPBF process, the underlying mechanisms during the keyholing process have not been well understood. In this work, a Multiphysics-based CFD model is developed to simulate the keyhole-penetration process in LPBF of CuCrZr alloy, and the ray-tracing laser heat source is implemented to visualize the multiple laser reflections. The simulation results are well validated in terms of the track size, porosity distribution and laser absorption by conducting the single-track experiment, X-CT scanning and calorimetric measurement, respectively. The flow patterns, driving forces and laser irradiation behaviors in the keyhole mode are collectively elucidated to understand the formation mechanisms of the keyhole-induced pores. The reduction of laser ray entries into the emerging cavity with a shrinking opening is noticed and contributes to the pore formation.