Abstract <p>The problem of adequate reproduction by numerical models of hydrodynamic processes in the keyhole mode in laser and electron beam welding and related technologies is considered. The problem manifests itself in the contradictory conclusions of models concerning the relative contributions of thermocapillary (TC) and ablative mechanisms in the keyhole formation. The solution of the problem involves bringing the results of the numerical model in line with the experimental data, according to which, under technological conditions, the main contribution to the keyhole formation is made by the TC mechanism with insignificant evaporation. The distortion of these contributions is a consequence of inadequate reproduction of the TC mechanism’s specific features in the keyhole mode. This is due to the choice of the grid step without taking into account the real scale of the TC flow in the keyhole mode. A method for choosing a physically justified spatial and temporal step of the computational grid is proposed. Estimates are made for the recommended discretization for characteristic spot sizes used in laser welding and additive manufacturing technologies by selective laser melting. The recommended grid step is compared with the discretization commonly used in models.</p>