Suppression mechanism of collapse defects and microstructural evolution during full-penetration laser welding assisted by the following bottom airflow

Collapse has consistently posed a critical bottleneck in the laser penetration welding of thick plates. This study proposed a novel method involving the application of bottom airflow to suppress collapse. The suppression mechanism was elucidated through high-speed imaging and numerical simulations, and effects of airflow on the microstructure and properties of welded joints were clarified. Firstly, during the welding process, the keyhole underwent periodic changes in a "penetration-closure-penetration" cycle. With the application of bottom airflow, the cycle duration was reduced from 5 ms to 0.4 ms. The more frequent opening of the keyhole facilitated the timely release of pressure and heat from within the molten pool, thereby avoiding the downward movement even the loss of melts. Secondly, the application of bottom airflow provided an upward force of approximately 7.7 mN at the backside of the molten pool, compensating for insufficient surface tension and helping to further suppress the downward collapse of the molten pool. Finally, the bottom airflow introduced beneficial disturbances in flow and cooling, refining the bainite and enhancing the strain degree in ferrite. The changes contributed to strengthen the tensile properties of the welds. This study validated the effectiveness of bottom airflow in suppressing collapse during laser penetration welding, offering a significant solution to this technical bottleneck and presenting a new approach for large-scale equipment manufacturing.