The availability of lasers with highest beam quality at laser powers of 1 kW or more (such as single-mode fibre laser, which nowadays come close to the theoretical limits) provides a unique tool to investigate a wide range of welding process phenomena with penetrations of some 10 µm to penetrations of some mm. Thus covering the field of micro welding as well as of macro welding, scalability of welding processes as well as size effects associated with the underlying physical phenomena may be of significance.

In this paper, the humping effect will be given a closer look, as this periodic melt pool instability is an important limitation to welding speed both in the micro and the macro range. Based on experimental investigations with a single-mode fiber laser (YLR-1000, laser power 1 kW, M2 < 1.05), a model including a modification of Rayleigh’s considerations on the stability of an inviscid incompressible fluid which is freely suspended in space and maintained only by surface tension is developed and discussed. It is shown that, within the scope of the investigations, humping to a considerable extent can be explained by a modified version of Rayleigh’s theory, permitting to neglect a direct influence of three-dimensional melt flow.

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