Abstract
Laser induced plasma plume was characterized by a spectral analysis in terms of temperature and electron density in lap welding of mild steel sheet of 0.8 mm thickness by using CO2 laser in a power range from 1.4 to 2.5 kW. Spatially and temporally averaged temperature of the plasma plume was determined as a function of focal position, F, location of focal point with respect to the work surface in Ar shielding ; it was found that the temperature ranged from 7500 K to 8500 K, and increased with increasing the absolute value of F in penetration welding. Abel transformation analysis indicated that the local temperature and electron density were higher in the area near the laser beam axis than the surrounding area due to laser beam heating ; the maximum temperature and electron density were 9500 K and 1017/cm3, respectively. According to inverse Bremsstrahlung theory, this electron density corresponds to the absorption coefficient of approximately 0.2 cm-1. Although this value proved little effect on bead formation, it was high enough to heat the plasma plume depending on laser focusing conditions, indicating possibilities to monitor the laser welding by detecting plasma temperature or brightness for establishing intellectual laser welding.
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