Authors:
Hiroshi Honda
Susumu Tsukamoto
Isao Kawaguchi
Goro Arakane
Structural Metals Center, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan

A slender keyhole formed by deep penetration laser welding fluctuates violently and is apt to form porosity. To reveal the keyhole depth behavior, the authors observed the keyhole images simultaneously by using a microfocus x-ray transmission imaging system and plasma plume images in deep penetration CO₂ laser welding at various welding speeds. Plasma light emission was also measured by using photodiodes. The upper part of the keyhole was found to fluctuate largely and its motion corresponds to the large plasma plume generation. The keyhole depth fluctuation corresponding to the light emission coefficient of the plasma inside the keyhole was also observed.