Shape of the liquid-phase surface in melting of highly absorbing media by laser radiation

The steady-state process of melting of highly absorbing media by a laser pulse of long duration is considered theoretically. The shape of the free surface of the melt, deformed by the reactive pressure of the vapor from the specimen, is determined, the equation of the surface being obtained in the explicit form z = z(r). All the parameters entering into it are expressed in terms of the laser-beam parameters and the thermophysical properties of the melt. The connection between the deformation of the free melt surface and the phenomenon of deep-penetration (dagger-like) melting of the specimen is explained. It is shown that this phenomenon involves a threshold and analytical expressions are found for the magnitude of this threshold, good agreement with experiment being obtained. It is shown that there is also a threshold of the laser-radiation intensity for the transition to the intense-evaporation regime, and the magnitude of this threshold is found in the case when the heat conduction cannot be considered as one-dimensional because the laser pulse is of long duration. It is shown that the relationship between the magnitudes of these two thresholds can be arbitrary and is mainly determined by the properties of the material being processed. A study is made of the free oscillations of the melt surface, which appear after the end of the laser pulse. It is shown that the first harmonic always has the maximum amplitude.