  Metal additive manufacturing has gained extensive attention from research
institutes and companies to fabricate intricate parts and functionally graded
materials. However, the porosity of the as-built part deteriorates the
mechanical property and even hinders the further application of metal additive
manufacturing. Particularly, the mechanisms of keyhole pores associated with
the keyhole fluctuation are not fully understood. To reveal the mechanisms of
the keyhole pores formation, we adopt a multiphysics thermal-fluid flow model
incorporating heat transfer, liquid flow, metal evaporation, Marangoni effect,
and Darcy's law to simulate the keyhole pore formation process, and the results
are validated with the in-situ X-ray images. The simulation results present the
instant bubble formation due to the keyhole instability and motion of the
instant bubble when it pins on the solidification front. Moreover, the unevenly
distributed recoil pressure on the keyhole surface is an important factor for
keyhole collapse and penetration. Furthermore, comparing the keyhole pore
formation under different laser scanning speeds shows that the keyhole pore is
sensitive to the manufacturing parameters. The keyhole fluctuation features and
energy absorptivity variation on the rear keyhole wall could be metrics to
evaluate the likelihood of the keyhole pore formation. Additionally, the
simulation under a low ambient pressure shows the feasibility of improving the
keyhole stability to reduce and even avoid the formation of keyhole pores.
