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.

Monitoring Variability in Melt Pool Spatiotemporal Dynamics (VIMPS): Towards Proactive Humping Detection in Additive Manufacturing

“…The process conditions that trigger these factors are still unclear. In keyhole printing, with high energy intensity and deep depression in the laser-material interaction zones, some hypothesize that the lateral oscillation of the keyhole triggers humping [13,14]. However, humping is not exclusive to the high-energy-intensity keyhole printing mode but is also observed for conduction-mode welding with lower energy intensity and shallow melt pools.…”

Supercritical melt flow in high-speed laser welding and its interdependence with the geometry of the keyhole and the melt pool

“…It also has been observed that the melt flow in the weld pool significantly impacts the advent of these defects. Especially in laser beam welding, the vapor keyhole and the melt flow surrounding this keyhole have been identified to have a strong impact on the occurrence of humping and undercuts at high welding velocities [19][20][21][22]. Previous works [7,[23][24][25] showed that the keyhole's geometry elongates backwards with increasing welding velocity and that this is associated with the occurrence of undercuts and humping.…”