Laser beam welding with solid-state lasers leads to the formation of defects (e.g. spatter and pore formation) at relevant processing speeds (≥ 8 m/min). This phenomenon originated from the highly dynamic interaction of melt and metal vapor inside the keyhole. Various approaches to reduce the spatter formation, like the use of a local gas flow, are already known. However, the underlying effects are not yet conclusively understood, which requires a spatial and time-dependent description of the keyhole and its stability.
The presented study characterizes spatial and time-dependent changes of the keyhole geometry during full penetration laser beam welding at welding speeds above 8 m/min. For these examinations high alloy steel AISI 304 (1.4301/X5CrNi18-10) was used. The keyhole was visualized by high-speed synchrotron X-ray imaging (frame rate: 40,000 Hz) with a beam energy up to 60 keV and a beam size of 4 mm x 4 mm at the European Synchrotron Radiation Facility (ESRF). Image processing was used to extract relevant geometric parameters over time, especially inclination, width, and length of the keyhole at weld face and root and specific positions in between. Thus, it is possible to determine position-dependent oscillation amplitudes and frequencies by means of a fast Fourier transform (FFT). Novel insights of the time-dependent formation and behavior of the keyhole during full penetration laser beam welding are provided based on these results.