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One of the major problems by laser beam welding of aluminium alloys is still the appearance of seam imperfections. This problem, mainly due to weld instabilities can be reduced by the processing method. Nevertheless the origin of these welding process instabilities is not fully understood.The presented paper shows that the oscillation of the keyhole and the melt pool are in resonance dependent on the melting and vapourising process. Due to the intensity of this resonance surface ripple irregularities, notches and blow-holes appear. Therefore it can be claimed, that resonant oscillations of the keyhole and the melt pool are one reason for the appearance of imperfections like blow-holes.The resonances were generated by artificial laser beam power modulations. Using this method imperfections like blow-holes were reproducibly produced.

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