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In the category of Additive Manufacturing (AM) processes, wire-based Laser Metal Deposition (LMD-w) offers high potential. It combines high deposition rates and accuracy with a cost-effective and easy-to-handle filler material. However, its process stability is low compared to other AM processes due to the complex wire-melt pool interaction, and its industrial application is limited. This work aimed to investigate a dual-beam process, in which the conventional continuous process laser (cw) is superimposed with a low-power pulsed laser beam (pw), for its process stabilizing effect in LMD-w. The material under investigation was a Cr and Mn-alloyed special tool steel. The procedure was composed of three sub-steps. First, the mechanisms of action of the pulsed laser beam were fundamentally investigated. It was shown that the pulsed laser-induced vaporization at the workpiece surface creates a vapor cloud, which has a significant influence on the process. In the vapor, the absorption is increased compared to the conventional LMD-w process, so that the deposition rate raises and the viscosity of the melt decreases. The expansion of the material during evaporation also creates a force that acts on the workpiece and melt pool and changes its geometry. The influence of the vapor cloud on process temperature and the resulting welding bead geometry was investigated as a function of the pw frequency and power and related to vapor-physical mechanisms. In the next step, it was analyzed whether and how the modulated laser beam leads to process stabilization during the deposition of single welding beads. It was found that the vapor cloud stabilizes the melt pool shape and dynamics and prevents interruption of the wire-melt pool connection. The process windows in which the process remains stable during parameter variation are significantly larger for dual-beam processes with low resultant force on the melt pool than in the single-beam process. The findings were transferred to models of the single effects and the overall process. In the third step, the results were applied to the production of three-dimensional geometries. Here, in addition to the increase in process stability, an improvement of surface and microstructure quality by the dual-beam process was demonstrated. In summary, the results of this work demonstrate the suitability of the dual-beam process for stabilizing wire-based LMD processes. They offer the perspective of a medium-term industrial transfer going along with higher resilience and sustainability in production. |
