Autocorrelation analysis of plasma plume light emissions in deep penetration laser welding of steel

The light emissions of the plasma plume in the deep penetration laser welding of metals typically have the character of irregular short-time pulses. Their nature indicates that the size of the plume significantly fluctuates and the plasma flows out of the keyhole in the form of short bursts rather than a continuous flow. In this paper, the authors study the plasma plume light emissions using an autocorrelation analysis. The authors show that it is an efficient tool for the detection of the plasma bursts period which is typically in the order of milliseconds. The authors compare the autocorrelation characteristics and the geometry of the welds made on a 2 kW ytterbium-doped yttrium aluminium garnet (Yb:YAG) fiber laser welding machine for the X5CrNi 18-10 stainless steel and the S235JR carbon steel. The welding parameters settings is varied over a range of laser power (1–2 kW) and welding speed (10–30 mm/s) usually used in industry. As a result, the authors identify a linear dependence between the plasma bursts period and the weld depth.