For CO2 laser welding of large output, when a deep penetration welding in single pass is done in the ambient atmosphere, it is known that blowholes may occur because of the in-keyhole gas being entrapped in molten metal. Keeping this problem in mind and changing the parameters, we conducted welding tests with full-penetration bead-on-plate welding, and checked by radiographic test for welding defects.
The experimental results demonstrated that the larger is the specimen thickness, the more frequently occur welding defects, and that generation of welding defects depends upon the amount of welding heat input.
Welding defects such as blowholes remain in metal, because the gas once entrapped into keyhole floats up in molten metal, and it is enclosed in the course of solidification. From this, we can verify the theory that a larger welding heat input, in the case of the full-penetration bead-on-plate welding, may be favorable for preventing welding blowholes. It can be assumed, therefore, that a larger heat input may hinder cooling of molten metal, and need a longer time for metal solidification; in this longer span of time, in-molten metal gas may escape while the metal is sufficiently heated.