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.

An account is given of a series of experiments directed at examining the variation with velocity of depth and volume of hole produced in semi-infinite targets of aluminium by impact at normal of long rods of hardened and tempered steel.

No abstract

No abstract

The keyhole mode of selective laser melting (SLM) of metal powder bed in additive technology is characterized by an intensive hydrodynamic process in a thin molten layer. Such a mode is widely used also in laser and electron beam welding indicating the similarity of hydrodynamic processes in these technologies despite a significant difference of operating parameters. The threshold conditions of thermocapillary keyhole mode transition for various metals (Cu, Fe, Ti) in a wide range of beam parameters used for selective laser melting of metal powder layer and laser welding are investigated. The condition of threshold for thermocapillary transition into keyhole mode by sticking of viscous layer to the solid boundary is formulated. The fulfillment of this condition is confirmed by the convergence of estimated values of the viscous layer thickness and the molten layer depth during the transition to a keyhole mode. Analytical estimates of keyhole threshold and comparisons with experimental values of beam power and spot size corresponding to the transition in keyhole mode for SLM processes and laser welding are presented. The correlation of these values confirms the thermocapillary mechanism of cavity formation and the similarity of hydrodynamic processes in laser welding and SLM processing in keyhole mode at wide range of operating parameters.