Spiking behavior observed during welding of ice

Longitudinal sections of seams produced by partial penetration laser-beam welding of steel often show penetration depth fluctuations, called spiking. This phenomenon can hardly be observed during welding of metals. Even state of the art X-ray technique with a frame rate of more than 1 kHz and a spatial resolution of 100 µm cannot visualize the details of this process. In welding experiments with transparent materials, however, the temporal and spatial resolution is high enough to see a lot of details. High-speed videos with frame rates much higher than 10 kHz can be produced. Simultaneously a very high spatial resolution can be achieved. Therefore, welding experiments with a commercial 5 kW CO2 laser were performed in plates of pore-free ice. The spiking process was observed with high-speed video technique for different process parameters, such as laser power, welding velocity and focal position. In this paper, the focus is on the change of the spiking process by varying the focus position. Two variants of the spiking process could be identified. On the one hand the behavior of the capillary and the formation of the spike in the solid material (here ice) is synchronous. On the other hand the frequency of capillary fluctuations is much higher compared to the frequency of the spike generation. Actual investigations of capillary depth measurements with inline coherent imaging during welding of steel seem to confirm this behavior. After the discussion of the spiking process, the behavior of the flow field will be elucidated. It can be stated that the change in the spiking behavior is accompanied by a change of the flow field behind the capillary. When the frequency of the capillary fluctuations is high, pores and an eddy at the tip of the capillary can be produced. For the generation of the eddy, however, it is necessary that a collapse of the capillary occurs outside of the spike. Finally, mechanisms provoking the spikes will be discussed. In order to do this, the role of the geometry dependent beam propagation within the capillary will be discussed and the frequency of steps running downward the front of the capillary will be compared with the frequency of the spikes.