Quantitative characterization of keyhole absorption mechanisms in 20 kW-class CO2 laser welding processes

Deep penetration laser welding is associated with the deposition of very high irradiation of a laser beam to a workpiece to form a slender capillary into the workpiece, denoted as a keyhole, as well as violent plasma generation which consists of metal vapour, ionized atoms, and free electrons. Plasma resides both outside and inside the keyhole, known as plasma plume and keyhole plasma, respectively. In this paper, we quantitatively characterize the plasma absorption of a laser beam inside the keyhole based on photodiode measurements and spectroscopic measurements in 20 kW-class CO2 laser welding processes. The results show that the plasma absorption of a laser beam cannot be neglected in certain welding conditions. Furthermore, this plasma absorption mainly occurs in the top portion of the keyhole, resulting in a wider weld shape in the top portion of the weld bead.