Drop Detachment Under Intense Laser Irradiation

Abstract

During laser processing, complex effects can occur regarding the laser-material interactions. A high laser energy input leads to surface melting and even boiling. The resulting recoil pressure can create the so-called keyhole, a vapor channel existing during welding and called cut front during laser cutting. On the keyhole front wall, the induced recoil pressure pushes the melt downwards and can ejects melt drops. Usually, those melt ejections are seen as undesired spattering or necessary waste to enable the cutting. However, outflow characteristics can tell more about the complex process behavior. Therefore, this work aimed to relate melt ejection formation effects to keyhole behavior in order to get a better understanding of the complex laser-matter-interactions and fluid flows. Axial beam shaping was used to create different energy inputs into the keyhole front walls. Beam shaping was done with an optic that can superposition up to four laser beams in axial direction, leading to varying intensity distributions on the inclined keyhole front walls. Based on high-speed image analysis, it was seen that different outflow characteristics occur depending on the beam shapes. A high intensity on the front keyhole wall could be related to high temperatures on the keyhole wall. The outflow mechanism was shown to be able to move from corrugating to atomizing drop generation at increasing temperature due to temperature-dependent material properties. The main influencing factors are assumed to be the vapor speed and the keyhole/drop diameters that define the outflow mechanism.