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During laser microwelding, the melt pool behavior and the formation of the weld seam depend on various process parameters. In this paper, the authors performed tests to clarify the influence of laser power P, the feeding rate vf, the focal diameter df, the foil thickness s, and the thermophysical material properties. Ultrathin metal foils such as stainless steel, aluminum, and titanium in thicknesses of 50 and 100 μm were welded in bead-on-plate welds in order to generate a full penetration weld. For this purpose, focal diameters between 25 and 78 μm were applied. By means of high-speed videography and micro-cross-sections, the observations were analyzed depending on the feeding rate. Imperfections such as root defects, surface structures, and humps were described and evaluated. Moreover, the influence of melt pool behavior prior to the appearance of humping is illustrated for full penetration microwelding in contrast to insufficient welds. The Rayleigh theory of the instability of a free suspended liquid cylinder is discussed. A correlation between the length of the melt pool L and the spot size df is examined to define the humping threshold for stainless steel. Due to the different thermophysical properties of the applied materials, weld seam formation and process behavior are changed.

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