Porosity formation mechanism and reduction method in CO2 laser welding of stainless steel

Abstract

A large amount of porosity is liable to be formed in high power CO2 laser welds of stainless steels. The formation mechanisms of porosity have not been satisfactorily understood up to the present. In this study, therefore, microfocused X-ray transmission in-situ observation system was utilized to observe the behavior of a keyhole and the formation situations of bubbles and pores during high power CO2 laser welding. It was observed that a keyhole fluctuated in geometry unstably and vibrated up and down dynamically, and accordingly many bubbles were frequently generated in the bottom part of a molten pool from the tip of the deep keyhole. Almost all bubbles were captured into spherical pores by solidifying solid-liquid interface during floating up near the bottom part of the weld pool. Moreover, it was revealed that partial penetration welding with a forward inclination of a laser beam or in N2 shielding gas, and full penetration welding in any shielding gas were beneficial to the reduction and prevention of porosity because the formation of bubbles was suppressed or prevented.