Elucidation of Laser Welding Phenomena and Porosity Formation Mechanism(Physics, Processes, Instruments & Measurements, INTERNATIONAL SYMPOSIUM OF JWRI 30TH ANNIVERSARY)

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Imperfections, especially porosity, are easily formed in deeply penetrating weld fusion zones produced with high power lasers. To investigate the mechanism of porosity formation, welding phenomena, such as laser-induced plume, keyhole behavior, bubble formation and melt flow were observed during CO_2 or YAG laser welding of aluminum alloy and stainless steel plate by using high speed video camera and X-ray transmission real-time imaging systems. It was revealed that the keyhole fluctuated violently in depth and size at slow speeds during continuous wave welding and that the liquid chiefly flowed backwards near the bottom part of a molten pool from the tip of the keyhole. Such keyhole instability and liquid flow induced by evaporation enhanced the generation of bubbles and pores including the shielding gas. On the other hand, at high welding speeds, different main melt flows toward the upper keyhole inlet were induced by metallic vapor jets from the front wall of the keyhole, leading to the reduction in porosity. Consequently, porosity formation mechanism and laser welding phenomena were clarified in terms of keyhole dynamics, vapor jet directions and liquid flows.