Porosity Formation Mechanism and Suppression Procedure in Laser Welding of Aluminum Alloy

Abstract

It is well known that a lot of porosity is liable to be formed in high power CO₂ laser welding of aluminum alloys. At present, the formation causes and mechanisms of such porosity have not been satisfactorily understood. In this study, therefore, a microfocused X-ray transmission in-situ observation system was developed and utilized to observe the keyhole behavior and the formation situations of bubbles and pores during laser welding. It was observed that a keyhole fluctuated unstably in geometry and vibrated up and down dynamically, and accordingly many bubbles were frequently generated in the bottom part of the keyhole front wall. Bubbles were pushed into the liquid pool, and floated up quickly in a complex manner. Some bubbles disappeared into the keyhole or from the molten pool surface. However, most of bubbles were captured at the solidifying wall in the rear part of the weld pool to form pores in the spherical or elongated shapes. Moreover, it was revealed that partial penetration welding with a proper pulse modulation power input, 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.