Prediction of the laser-induced plasma characteristics in laser welding: a new modelling approach including a simplified keyhole model

The characteristics of the laser-induced plasma encountered in laser welding are investigated using a new three-dimensional modelling approach. A simplified keyhole model is employed to couple with our previous plasma plume model, and thus both the plasma inside a blind keyhole and the plasma plume issuing from the keyhole can be treated simultaneously. Investigations include the effects on the laser-induced plasma characteristics of many factors, including the velocity of metal vapour leaving from the keyhole bottom, the velocity of the shielding gas injected coaxially with the laser beam, the velocity and location of the assisting gas injected laterally with respect to the workpiece, and the energy absorption and radiation heat loss of the laser-induced plasma. Typical computed distributions of temperature, velocity and vapour concentration within the plasma are presented with the continuous-wave CO2 laser welding of iron workpiece as the calculation example. It is shown that the high-temperature core of the laser-induced plasma is mostly located inside the blind keyhole or near the keyhole top for the cases under study. The metal-vapour/shielding-gas momentum ratio plays an important role in determining the height of the plasma plume, and the plume height decreases with increasing shielding-gas velocity. The laterally injected assisting gas may also significantly affect the laser-induced plasma characteristics and thus can be used to control the unfavourable effect of the laser-induced plasma on the laser welding process. The predicted temperatures of the laser-induced plasma are reasonably consistent with corresponding experimental data.