Detection of Transition from Keyhole-type to Heat Conduction-type Welding in CO_2 Laser Welding of Metals

摘要

In welding of metals, there are two welding types: keyhole-type welding and heat conduction-type welding. In this paper, we propose a technique to detect the transition between these two welding types in CO_2 laser welding by monitoring optical emission of a laser-induced plasma and acoustic emission from a laser irradiated point. SUS304, SS400 and A15083 were used for workpiece. Frequency analysis of the optical emission and acoustic emission was carried out using FFT program. When the welding is the keyhole-type, the frequency components of the acoustic signal at around 2-4 kHz appear strongly. When the welding changes to the heat conduction-type, the main frequency components appear at higher frequency range. Main frequency components of the signal shifts from lower frequency range to higher frequency range according to the decrease in the penetration depth, This phenomenon is observed for all three materials examined. Ratio of emission intensity at the lower frequency range to that at the higher frequency range is correlated to the aspect ratio. From this, we can evaluate the transition between the keyhole-type welding and heat conduction-type welding. The similar shift of the main frequency components is observed for the optical signal.