Moving humps at the capillary front in laser welding

Welding is a widespread application in laser materials processing and the current development of laser systems with high beam quality helps to enhance the number of applications furthermore. However, the weld-pool dynamics is still a limiting factor leading to weld defects and it seems to be more important for lasers with high focusability and for radiation around 1 µm compared to the 10 µm radiation. In the time past a lot of investigations were performed to analyse the reasons for those instabilities: high speed imaging, X-ray analysis and also modelling. In addition to experiments with real workpieces (iron and aluminium alloys) a set of experiments with transparent materials such as water and ice were performed at IFSW in the last years. As a result, flow components of the melt parallel to the laser beam were identified to be a major reason of the unwanted behaviour of the melt, driven by humps at the capillary front. Although such humps were discovered in X-ray analyses already in the 1980s and described in some models, they were discussed relatively seldom. For this contribution, welding with tracer material in the specimen was used to identify typical flow patterns, whereas the “welding” in transparent material shows directly typical flow structures. Finally, calculations of the beam propagation within the capillary clarify interesting differences for different wavelengths and beam quality. In this context, also the role of the polarisation will be discussed.