Science of laser welding - Mechanisms of keyhole and pool dynamics

Establishment of mathematical modeling and process monitoring of keyhole laser welding is a key issue for further application of laser welding in industries. However, the actual phenomena in keyhole laser welding are extremely complex and hence there exists no perfect theory though many models of keyhole laser welding have been proposed. Most of the models are based on the assumption of quasi-stationary state. In recent years, however, the present author and his collaborators have clarified that the keyhole is no more in the stationary state but fluctuate dynamically in space and time. It has been also revealed that there exists a non-stationary complex flow in the weld pool. The keyhole instability and swift liquid flow lead to generate the characteristic porosity that has never seen in other fusion welding. These experimental observations show the necessity of constructing the dynamic model of keyhole laser welding, however, only a few groups are trying to construct the dynamic models. There is also no model that is taken into account the convective heat transfer in the weld pool. The keyhole instability is primarily associated with the intense localized evaporation of metal at the front keyhole wall. The evaporation site changes its position with time in a very short period, and the recoil force as well as the dynamic pressure of metal vapor jet enhances the keyhole instability and induces the complex liquid flow in the weld pool. The unstable keyhole phenomena occur particularly in high power CW laser welding of any metal. The motive forces of liquid motion are not well understood at the moment, but it seems that the evaporation phenomena in the keyhole play the most important role. In the paper, the author will describe 1) the unstable keyhole phenomena in laser welding and their mechanisms, 2) suppression methods of porosity caused by keyhole instability, 3) possible motive forces of liquid motion in laser welding and 4) requirements to mathematical modeling and process monitoring.