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This work is devoted to the investigation of the laser-matter interaction during up to 20 kW ytterbium fiber laser welding of thick mild steel plates. The plume attenuation of a probe 1.3 µm wavelength diode laser beam as well as of continuous radiation in the 250-600 nm wavelength range was measured during welding with and without Ar shielding gas supply. The measured results allow the calculation of the average size and concentration of fine condensed metal particles in different plume areas using the multi-wavelength method and the Mie scattering theory. The plume temperature, which determines the condensation conditions, was measured by means of Fe I atom spectral line emission registration. The spatial distribution of the extinction coefficient in the welding plume was measured and the plume attenuation of the high-power fiber laser beam during the welding process was estimated.

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Full penetration 15 kW Yb fibre laser butt welding of thick AlMg3 (AW 5754) plates was performed in PA position. A contactless inductive electromagnetic weld pool support system was used to prevent gravity dropout of the melt. The welding speed needed to achieve 20 mm penetration was y0?5 m min 21 . An ac power supply of ,244 W at 460 Hz was necessary to completely suppress gravity dropout of the melt and eliminate sagging of the weld pool root side surface. The oscillating magnetic field can suppress the Marangoni convection in the lower part of the weld pool. The system was also successfully used in the full penetration welding of 30 mm thick AlMg3 plates.

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