Fluid Dynamic Effects in Keyhole Welding – An Attempt to Characterize Different Regimes

During keyhole welding with lasers a surplus pressure has to act at the keyhole front which drives the molten material around the keyhole. Depending on the travel speed and keyhole diameter, its value can reach several bar and may, therefore, represent a significant contribution to the pressure balance in the keyhole. As a consequence, an effect also on its stability must be expected therefrom. On the basis of simplified, yet physically reasonable estimations, the pressure balance in the keyhole for a wide parameter range of travel speed and keyhole diameter is inspected in detail. By comparing the magnitude of dynamic pressure of the melt flow at the keyhole‘s side to the other contributions to keyhole pressure, i. e. ambient pressure and closing pressure due to surface tension, different parameter regimes can be identified where a keyhole with a stable geometry can exist and where not. The theoretical predictions and conclusions agree well with experimental observations of other authors.