Analyse und Modellierung der Keyhole-Dynamik beim Laserstrahlschweißen von Aluminiumlegierungen

Abstract

The subject of this thesis is the behaviour of the keyhole in the laser welding process. In deep penetration laser welding there exists a complex system of weld pool, keyhole, and solid material under the influence of the laser beam. The dynamics which are regarded are primarily the changes in the shape of the keyhole. The dynamics determine the intensity in radiation of the plasma plume which exists above the keyhole. The first part of this thesis focuses on the analysis of time series which contain the experimentally recorded radiation intensity. Since classical techniques like simple Fournier transformation don´t give crucial results, these time series are analysed with methods arising from the theory of nonlinear dynamics. The methods include singular value decomposition, the calculation of correlation sums, and the test of determinism in the origin of the time series by use of surrogate data. The results of this analysis shall give valuable information for the modelling of the keyhole dynamics. The content of the second part is the development of a model for keyhole dynamics from first principles. Several basic assumptions for the physical properties of the weld pool and the keyhole wall lead to a system of nonlinear differential equations. The numerical integration of the model system shows non-trivial behaviour in a wide range of the parameter space. The existence of an attractor, which consists of a complex periodic orbit, dominates the phase space. The model presented here can reproduce qualitatively some aspects of the real laser welding process such as the sensitive dependence of the weld process on the laser power.