Understanding humping formation based on keyhole and molten pool behaviour during high speed laser welding of thin sheets

Abstract In this study, the high speed camera is used to observe humping formation process as well as the corresponding behaviour of the keyhole and the molten pool in the high speed laser welding with full penetration. The results show that, as the increasing of the welding speed and laser power, and the decreasing of the defocusing distance, the keyhole and molten pool behaviour changes and the humping is apt to form. Humping formation undergoes five successive physical phenomena: elongate keyhole, the lateral oscillation of the keyhole wall, local closure of the keyhole, swelling initiation and swelling evolution. Elongate keyhole is certain to lead to the lateral oscillation of the keyhole wall. Then the contact of the lateral oscillations is certain to cause the local closure of the keyhole, which gives rise to the swelling initiation. Subsequently, the swelling has two evolving modes which are ‘flat out model’ and ‘be filled up model’, and humping will form through the ‘be filled up model’. The modification of the keyhole and molten pool behavior can control the swelling initiation and evolution process. Therefore, a trailing beam, adding on the oscillations of the keyhole, can modify the keyhole behaviour and resist swelling initiation. What’s more, a trailing beam, adding on the back edge of the swelling, can modify the molten pool behaviour and suppress the swelling evolving into humping. Both of the two methods can suppress humping.