Keyhole oscillations during laser deep penetration welding at different spatial laser intensity distributions

Laser deep penetration welding processes are highly dynamic. Due to a multitude of influences like the varying laser power and intensity or temperature dependent absorption of the laser energy in the material, process oscillations are produced. The keyhole radius and the pressure inside the keyhole oscillate at high frequencies. These instabilities are assumed to support the unwanted process pore formation during welding. In this paper the influence of different spatial laser intensity distributions on the process dynamics and the resulting pore formation is theoretically and experimentally investigated. Optical and acoustic measurements have been taken during laser welding. A Fourier transformation of the time signals calculates frequency spectrums of the emissions. Variation of process parameters like welding velocity from 1 m/min to 6 m/min or laser power from 3 kW to 4 kW have a minor influence on the frequency spectrum while different spatial laser intensity distributions significantly affect the keyhole oscillation frequencies. The frequency spectrum shows higher values when changing the intensity profile from a Gaussian-like to a top hat profile. Analytical keyhole modelling shows similar tendencies. Smaller but more pores can be found in the weld seam produced by a top hat profile.