The influence of processing conditions in Laser Powder Bed Fusion and powder recycling strategies for an extended titanium powder life-cycle

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 most widely for near-net shape generation of complex geometries, which are difficult, material-inefficient or expensive to machine subtractively. Moreover, AM also opens new routes for conceiving innovative multimaterial components, e.g. by manufacturing surface features (e.g., pins), which are beneficial for bonding with fibre reinforced plastics. However, the promised advantages often do not materialize due to the difficult processing challenges, the high costs of titanium feedstocks, or the poor material efficiency arising from degradation of the powders during builds and recycling cycles. Particularly, degradation due to pick-up of oxygen, a typical Ti alpha-stabilizer, is detrimental and can lead to ductility loss, while changes of the particle size fraction by successive sieving cycles may influence materials properties. Therefore, LPBF of critical aeronautical or space components ("class 1" parts) requires today virgin powder for every build, which is still a clear technological and commercial limitation. The current work, carried out in the Horizon 2020 project SUSTAINair, addresses the issues of powder degradation and targets improvements of the LPBF machines' process gas atmosphere and powder recycling 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. Mass spectrometric analysis is used for tracking the process gas atmospheres during builds. A comparison of properties of specimens built with virgin powder and those after reuse as well as material properties before and after the machine improvements will be presented.