Thermocapillary mechanism of the laser penetration welding

The discrepancies between empirical data and the notions of the evaporative hypothesis, which assumes the melt displacement by vapor recoil pressure, are sufficient to cast doubt on the legitimacy of applying the evaporative hypothesis to the simulation of hydrodynamic processes in laser welding. To resolve these discrepancies, a hypothesis and theoretical model explaining keyhole formation as being due to tangential thermocapillary forces acting on a non-uniformly heated surface and the melt being removed from the beam impact zone is proposed. The keyhole phenomenon appears when the threshold beam intensity is exceeded and the thermocapillary divergent flow becomes structurally rearranged with the flow line rupture on the transition from a vortex flow to a shear flow. The thermocapillary model of keyhole formation is verified by comparing the calculated basic parameters of the process with empirical data.