Analytical Investigation of the Influence of the Spatial Laser Beam Intensity Distribution on Keyhole Dynamics in Laser Beam Welding

In deep penetration laser beam welding the shape of the keyhole is mainly determined by the equilibrium between the recoil pressure at the end of the Knudsen layer and the tension due to the surface deformation. Keyhole instabilities cause the formation of porosity in the weld seam if the keyhole is collapsing and represent a major problem to weld seam quality. A more profound understanding of the high frequency keyhole dynamics would be helpful for devising means of reducing keyhole instabilities and thus decrease the occurrence of porosity. In a simplified model of the keyhole welding process, the energy and pressure can be calculated analytically so that the keyhole radius can be derived from the energy and the pressure balance. The model also allows the analysis of the dynamic behaviour of the keyhole radius if it is perturbed from its quasi-static equilibrium. In this paper the quasi-static keyhole radius and the dynamic of the keyhole radius is investigated for thin sheet welding using laser beams with similar beam radii but different focal intensity distributions using two simplified analytical models. It will be shown that both, the quasi-static keyhole properties and especially the dynamic behaviour of the keyhole radius are significantly different for a Gaussian and a "top hat" intensity distribution of the laser beam.