THERMOCAPILLARY INSTABILITY IN A DEEP-PEN ETRATION WELD KEYHOLE

A study is made of the origin of thermocapillary instability on the wa lls of a weld keyhole forming during the deep-penetration welding of m etals by high-power laser radiation. Main physical factors generating this instability have been identified and the conditions under which i t occurs have been established in quantitative terms. The neutral (mon otonic) stability curve relating the Marangoni critical number (or las er radiation intensity) to wave disturbance parameters and to properti es of the medium has been analytically calculated. lt is found that th e simultaneous account of the thermocapillary effect and capillary dis turbances of the free surface results in lowering of the melt stabilit y threshold with respect to comparatively monotonic disturbances, espe cially in the region of small wave numbers, Estimates of the critical laser radiation intensity fall within the range 3-6 GW/m2 correspondin g approximately to the conditions of deep-penetration laser welding of metals.