Modelling of plasma plume induced during laser welding

A theoretical modelling of the plasma plume induced during welding of iron sheets with CO2 laser is presented. The set of equations consists of the equations of conservation of mass, energy, momentum and the diffusion equation and is solved with the use of the commercially available program Fluent 6.1. The computations are made for a laser power of 1700 W and for two shielding gases—argon and helium. The results show a significant difference between these two cases. When helium is used as the shielding gas, the plasma is much smaller and burns only where the metal vapour is slightly diluted by helium. In the case when argon is the shielding gas, there are actually two plasmas: argon plasma and metal plasma. The flowfield shows that the velocity increases in the hot region but only part of the mass flux enters the plasma core. In the case when argon is used as the shielding gas, the total absorption of the laser radiation amounts to 18–33% of the laser power depending on argon and iron vapour velocities. In the case of helium the total absorption is much lower and amounts to ∼5% of the laser power.