Formation mechanism and reduction method of porosity in laser welding of stainless steel

Abstract

It is generally acknowledged in any steels or alloys that porosity is liable to be formed in a keyhole type of deeply penetrated fusion zones made with a high power laser. Therefore, this investigation was carried out with the objectives of elucidating the formation mechanism of porosity and developing preventive procedure of pores in high power laser welding. The formation behavior of keyhole, bubbles and porosity was observed during butt-joint or bead-on-plate CO₂ laser welding of stainless steel by microfocused X-ray transmission insitu imaging system with high speed video camera. The influence of various laser welding conditions on porosity formation was investigated. As a result, it was observed that many bubbles were predominantly frequently formed at the bottom of a molten pool from the tip part of a deep keyhole in consequence of its dynamic motion due to intense evaporation, and some bubbles were formed from the middle part of the keyhole in the case of focal point under the plate surface or high power laser irradiation. It was also seen that most of the bubbles were soon captured or trapped into pores by the solidifying solid-liquid interface during floating up in a stainless steel. SEM observation result of a fractured surface demonstrated traces that liquid was penetrated and solidified inside pores, probably because evaporated materials trapped in the bubble solidified and/or the temperature dropped to render their inside pressure lower. According to Q mass-spectroscopic analysis of gas content of porosity, it was revealed that He shielding gas and H₂ gas were included inside a pore near the bottom of the weld fusion zone. It was observed to be feasible to produce a sound full-penetration weld bead without porosity in steel plates of 10 mm thickness, since no bubbles were formed from the bottom part of the fully penetrated keyhole. Bubbles, porosity or pores were confirmed to be reduced by utilizing N₂ shielding gas and forward keyhole-inclination welding in He shielding gas even in the case of partially penetrated weld.