Using the additive manufacturing process Laser Powder Bed Fusion (L-PBF), workpieces with an almost unlimited freedom of geometry are produced layer by layer from a fine metallic powder. The resulting properties of the workpieces are thereby to a certain extend determined by the process itself. Defects like pores, cracks and lack-of fusion can for example deteriorate the mechanical properties. As of today, the formation of such defects is not fully understood, making their occurrence seem

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... . The scope of this work is the investigation of mechanisms which can lead to the formation of such defects. The objectives of this work are investigations on the influence of focal shift (1) and the interaction of laser radiation with process by-products (metal condensate, splashes, ejected and entrained powder particles) (2) on the L-PBF process. Moreover, the influence of the shielding gas flow is investigated (3) since the flow is supposed to remove process by-products from the laserpowder interaction zone and is therefore a main influencing factor on the aforementioned interaction. Focal shift in L-PBF systems can be induced by the physical properties of the optics and by additional external influences. As for external influences, the contamination of the laser window from process by-products is investigated. For both influences the focal shift is measured using a non-moving laser beam and correlated with the time and space dependent relative density of test specimen made from 1.4404. While an influence of the focal shift induced by the optics itself can be proven, no correlation between the contamination and the relative density can be found. The investigations on the interaction of laser radiation with process by-products are conducted separately for metal condensate and splashes/ejected and entrained powder particles (alien particles) using the material Ti6Al4V. First, the general correlation of the emission of process by-products with laser power, scan speed and beam diameter is investigated using a high speed cam [...]