Thermocapillary mechanism of melt displacement during keyhole formation by the laser beam

A thermocapillary model of keyhole formation in laser welding is presented. According to the thermocapillary hypothesis, a cavity forms as the result of surface tension-induced melt displacement on a non-uniformly heated surface. In this case the transition of welding into a keyhole mode is due to the change in the structure of the thermocapillary divergent flow on the attainment of threshold intensity, when vortex flow transforms into shear flow. Assessments are made of the basic parameters ( the speed of fall of the cavity bottom, the speed of thermocapillary flow, maximum depth of penetration, the threshold intensity of keyhole formation). These assessments are in good agreement with empirical data.