The laser-induced plasma which is essential for the laser welding process absorbs a fractional part of the beam power above the workpiece as well as inside the "keyhole". The absorption above the surface of the workpiece always reduces the welding efficiency. It is demonstrated at which processing parameters this absorption takes place and how it can be reduced by shielding gases. Inside the keyhole, different absorption mechanisms could take place. 1st: Fresnel-absorption at the keyhole walls is the substantial absorption mechanism. In this case the direction of the beam polarization shows a strong effect on the welding result. The absorption length of the laser induced plasma Labs is well above the welding depth d (Labs >> d) 2nd: Plasma absorption inside the keyhole takes place. In this case, the welding depth is a function of the plasma absorption length. These absorption mechanisms are described and examples are shown.
"Plasma Absorption Effects In Welding With CO2 Lasers", Proc. SPIE 1020, High Power CO2 Laser Systems and Applications, (24 March 1989); doi: 10.1117/12.950052; https://doi.org/10.1117/12.950052