Titanium powder reuse strategies in Laser Powder Bed Fusion

Kurzfassung

Metal-based additive manufacturing (AM) is slowly but steadily about to initiate a paradigm change across multiple industries including the aerospace sector. AM suggests major production benefits for titanium-based components by reducing manufacturing efforts and improving material efficiency compared to conventional machining. Laser Powder Bed Fusion (LPBF) is the most widely used metal additive manufacturing technology for near-net shape generation of components with high material efficiencies. However, the promised advantages often do not materialize due to the difficult processing challenges, the high costs of feedstocks, or the poor material efficiency arising from degradation of the powders during builds and recycling cycles. During LPBF processing and sieving, especially of titanium powder, material degradation due to pick-up of oxygen, a typical Ti α-stabilizer that can lead to a loss of ductility, and changes of the particle size fraction may occur. The smaller size fractions generally decrease in successive sieving cycles, which can influence the obtainable material density. Usually sieving requires removing the powders from the LPBF systems. Particularly for titanium, exposure to air and humidity must be avoided carefully when transferring and sieving the powders. Only rather recently, machines with integral closed-loop sieving options (e.g. Renishaw or SLM Solutions) have been developed that sieve the non-melted powder fraction for a direct reuse. Even in such systems powder degradation e.g. due composition change, contamination and oxygen-pickup during the melting process cannot be entirely prevented. Therefore, LPBF manufacturing of highly loaded and safety-critical aeronautical or space components (“class 1” parts) today requires the use of virgin powder, which increases costs. The current work, carried out in the Horizon 2020 project SUSTAINair, addresses the issues of titanium powder degradation and targets improvements of the LPBF machines’ process gas atmosphere and powder reuse procedures. The powder degradation depends strongly on the type of alloy, the processing parameters (particularly build temperature and the build platform occupation) and the machine-specific inert gas system. The number of permissible reuse cycles is studied based on elementary and microstructural analysis and the effects on mechanical performance, both for the original LPBF setup as well as after modification of the system’s atmosphere and process handling. Mass spectrometric analysis is used for tracking the process gas atmospheres during builds.