Melt removal in pulsed laser action of millisecond range

The transformation of melt hydrodynamics from thermocapillary to forced convection induced by reactive pressure of evaporation and corresponding deformation of free surface are analysed. The range of parameters corresponds to pulse laser action of nearly millisecond duration and absorbed energy density flux of about 10⁵ W/cm². On initial stage of laser action the flow pattern corresponds to thermocapillary convection: vortex type flow with melt moving from the centre to the pool’s edges on the free surface, and near the bottom in the opposite direction. An increase of pulse duration and therefore surface temperature leads to an increase of reactive pressure of evaporation and corresponding deformation of free surface. The flow pattern varies qualitatively: melt near free surface and near pool’s bottom starts to move in one direction: from the centre of laser action towards its boundaries. It is shown that the dependence of melt thickness versus time may have a maximum, that corresponds to the beginning of intensive melt removal. The model predicts that on initial stage melt rises near the edges of pool rather slowly (about a few μm), later the process increases sharply and the altitude of melt rise on the pool’s periphery may increase twice the pool’s depth. In practice it corresponds to melt splashing out of pool. The dependence of threshold time for melt removal versus energy density flux is estimated.