Impact of Beam Characteristics in Electron Beam Powder Bed Fusion

As an important additive manufacturing (AM) technology, Electron beam powder bed fusion (PBF-EB) is pushing toward broader industrial applications. Some unique possibilities, for instance, high-temperature processing, vacuum atmosphere, inertia-free deflection for innovative scanning strategies, and robust process monitoring via electron optics, make it an intriguing technology for high-performance industries like aerospace or medical applications. In recent years, more machine manufacturers, for example, probeam, Freemelt, JEOL, or Wayland Additive, have entered the market, accelerating the technological advancements of PBF-EB. These machines provide a broad range of unique features. One example is the first PBF-EB machine with an acceleration voltage of 150 kV. To investigate the opportunities provided by 150 kV, compared to the state-of-the-art 60 kV, the beam diameter has to be accounted for. However, commercially available PBF-EB machines only disclose a minimal beam diameter under optimum conditions. This is insufficient for precise process control because the beam diameter also depends on other parameters, for example, the beam current. Unfortunately, no data is specified for the power-dependency of the beam diameter. Additionally, the beam diameter can be adjusted by the focus current, for which also no data is provided. Another issue is the difficulty of measuring the beam diameter due to the closed machine software of most machine manufacturers. As a result, the beam diameter has been neglected as a process parameter in PBF-EB, and almost no research has been published. The lack of knowledge about the beam diameter is astonishing since the electron beam is the tool used to produce high-quality parts such as turbine blades via PBF-EB. Therefore, the impact of the beam diameter on melting during PBF-EB is investigated in detail. This dissertation reveals that a larger beam diameter directly results in less aluminum evaporation and more gas porosity in γ-TiAl. With the insights gained from these experiments, the impact of the acceleration voltage is explored. For the first time 150 kV acceleration voltage is used for PBF-EB. A higher acceleration voltage possesses some inherent advantages for the PBF-EB process. It enables higher beam powers, making the process more productive. At the same time, less beam current is needed to set a predefined beam power for preheating and melting, ensuring improved process stability. The impact of the acceleration voltage on beam characteristics, melt pool dimensions, defect density, and evaporation is analyzed based on γ-TiAl and Ti-6Al-4V. On the one hand, the preheating is stabilized, allowing processing without any process gas. On the other hand, 150 kV enables melting with less area energy and a more homogeneous aluminum distribution compared to 60 kV. These insights are only possible because the beam diameter is precisely controlled, and its effects are accounted for. Hopefully, this thesis is a starting point for more research on the next generation of PBF-EB machines with higher acceleration voltages and a wake-up call to treat the beam diameter with the necessary care for high-quality processing.