Three-dimensional modelling of the laser-induced plasma plume characteristics in laser welding

Modelling results are presented concerning the spatial distribution of plasma parameters in a laser-induced plasma plume with laser welding as the research background. In the modelling, the plasma plume characteristics are affected by many factors, such as the temperature and flow velocity of the metal vapour leaving the welded workpiece surface, the velocity of the shielding gas injected coaxially with the laser beam, the velocity of the assisting gas injected laterally with respect to the workpiece, and the energy absorption and radiation heat loss of the plasma plume. Typical computed distributions of temperature, velocity, vapour concentration, absorption coefficient and the refraction index within the plasma plume are presented with the continuous-wave (CW) CO₂ laser welding of an iron workpiece as the calculation example. The predicted temperatures of the plasma plume are shown to be reasonably consistent with the corresponding experimental data. It is also shown that the metal-vapour/shielding-gas momentum ratio plays an important role in determining the height of the plasma plume formed in the laser welding. Due to the cooling effect of the shielding gas, the dimensions of the plasma plume will become smaller and thus laser absorption and refraction by the plasma plume can be reduced by increasing the shielding-gas velocity. The laterally injected assisting gas may also significantly affect the plasma plume and thus can be used to control the effect of the laser-induced plasma plume on the laser welding process.