Quantifying the mechanisms of keyhole pore evolutions and the role of metal-vapor condensation in laser powder bed fusion

Key to quality control for laser powder bed fusion (L-PBF) is the reduction of porosity in built parts. However, understanding the mechanisms of a complete cycle of keyhole pore evolutions, including the processes of the keyhole pore generation, collapse, and splitting, and the role of metal-vapor condensation, remains a great challenge. In this study, we employ a high-fidelity computational tool considering multiphase interactions and thermal-induced phase changes to reproduce the key observations and identify the critical physics underlying keyhole instability and the ensuing keyhole pore generation, collapse, and splitting. We demonstrate that the dynamic fluctuation of keyhole and keyhole pores is dictated by five interdependent factors: vapor condensation, liquid vortex, recoil pressure, surface tension, and keyhole morphology. The occurrence of protrusions inside the keyhole wall enhances the fluctuation of keyhole by re-directing the reflected laser rays and changing the transport of high-temperature liquid flows surrounding the keyhole. The locally generated liquid vortex joins with the overall melt pool dynamics to snap the lower portion of a keyhole to form a keyhole pore and further drive its motion, conditions for which are quantified from the numerical results. We further show that vapor condensation is the major mechanism that may cause two high-speed microjets of pores and result in pore collapse and splitting. Finally, we propose an optimization strategy based on a parametric study of the condensation rate to potentially eliminate keyhole pores during laser melting.