The development of process parameters for electron beam powder bed fusion (PBF-EB) is usually made with simple geometries and uniform scan lengths. The transfer to complex parts with various scan lengths can be achieved by adapting beam parameters such as beam power and scan speed. Under ideal conditions, this adaption results in a constant energy input into the powder bed despite of the local scan length. However, numerous PBF-EB machines show deviations from the ideal situation because the beam diameter is subject to significant changes if the beam power is changed. This study aims to demonstrate typical scaling issues when applying process parameters to scan lengths up to 45 mm using a fourth generation γ-TiAl alloy. Line energy, area energy, return time, and lateral velocity are kept constant during the additive manufacturing process by adjusting beam power and beam velocity to various scan lengths. Samples produced in this way are examined by light microscopy regarding lateral melt pool extension, melt pool depth, porosity, and microstructure. The process-induced aluminum evaporation is measured by electron probe microanalysis. The experiments reveal undesired changes in melt pool geometry, gas porosity, and aluminum evaporation by increasing the beam power. In detail, beam widening is identified as the reason for the change in melt pool dimensions and microstructure. This finding is supported by numerical calculations from a semi-analytic heat conduction model. This study demonstrates that in-depth knowledge of the electron beam diameter is required to thoroughly control the PBF-EB process, especially when scaling process parameters from simply shaped geometries to complex parts with various scan lengths.

Electron Beam Powder Bed Fusion (PBF-EB) is an Additive Manufacturing (AM) method that utilizes an electron beam to melt and consolidate metal powder. The beam, combined with a backscattered electron detector, enables advanced process monitoring, a method termed Electron Optical Imaging (ELO). ELO is already known to provide great topographical information, but its capabilities regarding material contrast are less studied. In this article the extents of material contrast using ELO are investigated, focusing mainly on identifying powder contamination. It will be shown that an ELO detector is capable of distinguishing a single 100 μ$\mathrm{m}$ foreign powder particle, during an PBF-EB process, if the backscattering coefficient of the inclusion is sufficiently higher than its surroundings. Additionally, it is investigated how the material contrast can be used for material characterization. A mathematical framework is provided to describe the relationship between the signal intensity in the detector and the effective atomic number ${\mathrm{Z}}^{\mathrm{eff}}$ of the imaged alloy. The approach is verified with empirical data from twelve different materials, demonstrating that the effective atomic number of an alloy can be predicted to within one atomic number from its ELO intensity.

The dual microstructure concept for gamma titanium aluminides (γ-TiAl) processed via electron beam–powder bed fusion (PBF-EB) provides a huge potential for more efficient jet turbine engines. While the concept is feasible and the mechanical properties are promising, there are still some challenges. For an industrial application, the heat treatment window has to match the conditions in industrial furnaces. This study shows how the required heat treatment window can be achieved via advanced PBF-EB technology. Through using an electron beam with 150 kV acceleration voltage, the difference in aluminum between the designed aluminum-rich and aluminum-lean regions of the part is increased. Moreover, the aluminum content within each of these regions, respectively, is more homogenous compared to the 60 kV acceleration voltage. This combination provides a heat treatment window of 25 °C, enabling the industrial application of the dual microstructure concept for γ-TiAl.