Product Code: PIC06_401

Basic Understanding on Beam - Plasma Interaction in Laser Welding
Authors:
Akira Matsunawa, Osaka Univ.; Hyogo Japan
Jong-Do Kim, Korean Maritime University; Korea
Presented at PICALO 2006

In early development days of laser welding, most of engineers and scientists believed that the laser induced plasma was the high temperature and pressure plasma which could reflect the incident beam by plasma due to the plasma electron frequency. However, the necessary electron number density to cut-off the incident beam are 1019 1/cm3 for YAG laser (1.06 μm) and 1021 1/cm3 for CO2 laser (10.6 μm). These electron number densities are unable to achieve in the one atmospheric pressure thermal plasma. The maximum electron number density of thermal plasma at 1 atm. is in the order of 1017 1/cm3 which is much less than the equivalent number density of cut-off frequency for infra-red lasers such as CO2 and YAG lasers. The present authors conducted spectroscopic studies of laser induced plasma in pulsed YAG laser welding. There are two types of laser plasma. The one is the metallic plasma generated from the mother plate and the another is the shield gas plasma. The measured temperature of metallic plasma in YAG laser welding of Al alloy was slightly higher than the boiling point of Aluminium and no shield gas plasma was formed. In CO2 laser welding, on the other hand, a shield gas plasma was generated in Argon and Nitrogen shielding. The temperature of Argon plasma in high power CO2 laser welding was measured by Tsukamoto and others, and the maximum temperature was around 20,000 K. Therefore, the electron number density of laser induced plasma in laser welding at normal pressure is far below the necessary cut-off density for YAG and CO2 lasers. Namely, the incident laser beam can penetrate into the laser induced plasma in laser welding. However, it is well known empirically that the beam energy is attenuated in the laser induced plasma and the melting efficiency of parent metal is greatly reduced. In particular, the phenomenon is more remarkable in CO2 laser welding than in YAG laser welding. Only possible mechanism of beam energy absorption in laser welding is the Inverse Bremsstrahlunk. The Bremsstrahlung is a process of photon emission when an electron experiences a sudden momentum change. The Inverse Bremsstrahlung is the reverse process and an electron acquires kinetic energy under the photon irradiation. The absorption coefficient of Inverse Bremsstrahlung is proportional to the square of electron number density ne2 and wave length λ2, and inverse proportional to the 3/2 power of plasma temperature T3/2. The wave length of CO2 laser is 10 times longer than that of YAG laser, Therefore, the attenuation of CO2 laser in a plasma by Inverse Bremsstrahlung is 100 times higher than that of YAG laser. This is the major reason that the CO2 laser beam is much more influenced by the laser induced plasma in laser welding than the YAG laser. The paper will describe the detailed theories and experimental results of laser plasma and the possible mechanisms of beam energy dissipation in plasma.