Laser Powder Bed Fusion (LPBF) has become an extremely attractive additive manufacturing process for industrial applications, since it is suited to produce small to medium components (~ 1-50 cm) of high complexity and added value. Basically, a laser source of few hundred watts, small focal spot (~ 100 µm) and operating in the near infrared (~ 1.06 µm), interacts at high velocity (~ 1 m/s) with a powder bed which consequently melts, vaporizes and gives rise to complex hydrodynamic phenomena that should be understood to control the process stability and repeatability. In that regards, the growing interest on numerical work attests that multiphysical modelling becomes an essential tool for the researchers as well as for the industrials.

     The laser-material interaction in LPBF regime as described above is typical to that encountered in laser micro-welding, i.e. near the keyhole threshold, when the incident laser beam interacts mainly with the front wall of the incipient keyhole. However, recent works suggest that depending on the keyhole opening – which is determined by the whole melt pool hydrodynamics – the incident ray might be reflected to the rear wall of the keyhole, causing its instability and in the worst case, leading to the occurrence of porosities. Such phenomena were observed in laser micro-welding, so LPBF could also be affected. Thus, the aim of this work is to provide a deeper insight on the transient keyhole formation and to assess the conditions of its stability near the occurence threshold.

     In that purpose, a multiphysical finite element model of laser-induced keyhole has been developed with the commercial software COMSOL Multiphysics^®. It includes all the relevant physics necessary to describe the melt pool hydrodynamics – surface tension, thermocapillary stress, vaporization-induced recoil pressure – and a self-consistent ray-tracing algorithm to treat the local variations of absorbed intensity due to the “beam trapping” effect. In the present paper, the simulation method is first validated thanks to experimental results, including dynamic x-ray images of keyhole available in the literature. Then, transient fluctuations of the keyhole walls are investigated, by looking at the absorbed intensity inhomogeneities resulting from multiple reflections and by investigating the dynamics of penetration of the keyhole. The contribution of the incident and the reflected rays on the intensity distribution along the keyhole walls is also discriminated and finally, more general considerations regarding the energy coupling as a function of the keyhole geometry are discussed.