P 1215 – Distribution of intermetallic phases during laser beam welding of press hardened steels


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P 1215 – Distribution of intermetallic phases during laser beam welding of press hardened steels

The state of the art for joining ultra-high-strength steels shows that when welding already press-hardened components, the base material properties are not retained after welding and only reduced mechanical joining properties can be realized. Recent results from application research on the subject show that the introduction of intermetallic phases is particularly critical in laser beam welding. These accumulate at the fusion line and have a crack-initiating effect under load. If the strength-limiting effect of the intermetallic phases can be reduced or completely avoided during laser beam welding, the application potential of press-hardened components could be significantly increased.
The aim of the project was therefore to increase the strength of laser-welded joints of press-hardened manganese-boron steels by minimizing the crack-initiating effect of intermetallic phases introduced into the weld. This was to be achieved by homogeneous distribution of the intermetallic phases in the weld metal using beam modulation strategies or targeted ultrasonic coupling.
Necessary fundamental investigations were able to show, among other things, that a partial decoating of one or both contact surfaces can significantly reduce the extent and size of the agglomeration of the layer constituents.
The application of a targeted ultrasonic coupling did not lead to the prevention of the agglomeration formation and a considerable wear of the sonotrode could be determined, so that no economic application could be implemented for this. With regard to the use of local beam modulation, which was extensively investigated on the basis of 1-D and 2-D oscillation patterns, no agglomerations could be detected with a suitable choice of parameters, which is why the method was identified as target-oriented for an industrial application to prevent agglomeration formation. By means of energy dispersive X-ray spectroscopy (EDX analysis), it was shown that the coating residues are finely distributed in the weld structure. It was also shown that instead of a fracture along the agglomerations (crack-initiating effect), a preferential shear fracture is achieved. Furthermore, the seam width can be increased for all modulation strategies compared to linear welding at the same welding speed, which has a positive effect on the maximum force that can be applied to the welded joint. Furthermore, it was found that the strength of the joints is influenced less by the gglomerations of the intermetallic phases, but in particular by the general layer introduction on the weld metal as well as the hardness of the microstructure, and thus a homogeneous distribution of the layer residues is not sufficient for an increase in strength. The applicability of local beam modulation was demonstrated for industry-related laser deep penetration welding of hat profiles. The objective of the research project, to homogeneously distribute intermetallic brittle phases and thus minimize the crackinitiating effect, was achieved.

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Published in:
November 2022

Sc. M. Möbus, M. Sc. T. Mattulat