Development of seam imperfections due to process immanent resonances by laser beam welding of aluminium alloys.

Dependent on the aluminium alloy, weld geometry and welding parameters seam imperfections like notches or in worst cases holes in the weldment develop during laser beam welding. Although there are some process methods to minimise the number and seriousness of these imperfections, it is not possible to prevent their development at all. First experiments show, that resonances in the system laser-beam, metal vapour cloud, keyhole and molten pool could be the reason for the development of the instabilities. However, the occurrence of these resonances is not yet proofed and therefore the reason for the generation of instabilities is not fully understood yet. The experiments carried out show the interaction between the laser beam and the metal vapour cloud (often described as "plasma"). Although there is no detectable absorption of laser energy, the beam is scattered, its geometry is changed or it is moved. It was shown, that process instabilities, leading to imperfections, can be predicted by successive variations of the laser beam intensity distribution caused by the interaction with the plume. This is a proof for the hypothesis, that resonances cause instabilities and leads to arguments why some well-known methods stabilise the process and which new methods could improve the process stability.