Effect of shielding gas on keyhole stability and pores in laser-CMT hybrid welding of ultra-high strength steel

Laser-CMT hybrid welding (LCHW) is a highly efficient and high-quality welding technique, showing promise for welding ultra-high strength steel (UHSS) in the aerospace industry. However, the presence of pores significantly limits its application in this field. It has been observed that using an argon-rich gas can effectively stabilize the keyhole and prevent the formation of pores. Therefore, this study aims to investigate the effect of different shielding gases on keyhole stability and porosity. The weld profile, pores, electrical signal, droplet transfer, and molten pool internal flow were carefully characterized and compared with various shielding gases. The results show that increasing the CO₂ content gradually reduces weld porosity. The optimal weld formation and porosity were achieved at a CO₂ content of 25 %. With the CO₂ content increases, the droplets shift from off-axis upwards to downwards. This shift increases the distance between the droplet and the keyhole, reducing the time for droplet growth and promoting droplet transfer. Furthermore, the surface tension pressure gradually decreases from 3.86 Kpa to 1.1 Kpa, and at Ar + 25 % CO₂, itpromote the opening of keyhole and further suppress the formation of the protrusion on the keyhole rear wall. In addition, when the CO₂ content reaches 25 %, the top metal of the molten pool changes from counterclockwise flow to clockwise flow, reducing the likelihood of keyhole collapse. These effects stabilize the keyhole and suppress the formation of pores.