Influence of oxide film on weld characteristics of mild steel in CO2 laser welding

The influence of oxide film, such as laser cutting edge or mill scale, on CO2 laser welding phenomena and weld characteristics of mild steel was investigated. Laser welding of butt joints, which were different in location and thickness of oxide film, were performed. Examinations were made on the weld characteristics which were: bead geometry, defect, and microstructure. Light emitted from laser induced plasma was also analyzed to understand welding phenomena. The results of these experiments lead to understanding that oxide film, especially on the joint face, causes substantial spatters, an underfilled surface, the disappearance of the nail head, porosity formation, and reduction of hardness. It is considered that these are attributed to oxygen, which reduces surface tension of molten metal and reacts with carbon, manganese, and silicon. These results suggest that reduction of oxide film thickness or killing oxygen activity by supplementary deoxidizing elements may produce acceptable weld. (C) 2002 Laser Institute of America.