Characterizing keyhole plasma light emission and plasma plume scattering for monitoring 20 kW class CO2 laser welding processes

In very deep penetration, the plasma inside the keyhole exhibits complicated behavior and the plasma plume covering outside the keyhole can also affect the observation of keyhole plasma. Measuring the light emission from keyhole plasma is an effective method to monitor laser welding processes in thin sheets; however, in welding thick sheets, several critical phenomena must be observed. In this article, we provide experimental investigations using a 20 kW CO2 laser to characterize the wavelength contents of the keyhole plasma light emission. We also demonstrate that the plasma plume outside the keyhole can scatter the light emission from the keyhole plasma significantly. A scattering model is proposed and verified experimentally. The particles responsible for scattering are also identified. (C) 2002 Laser Institute of America.