Electron beam powder bed fusion for direct microstructure design - In-depth analysis of prospects and limitations of the multi spot scanning strategy

The electron beam powder bed fusion process generally results in a strong (001) texture alongside the build direction for Inconel 718. This is a result of prevailing high thermal gradients between the melted surface and the solid material. However, based on specific beam movement patterns, the solidification behaviour can be altered towards an equiaxed, isotropic microstructure. The present study focuses on the "multi spot scanning" strategy directly provided by the machine manufacturer. The operation mode of the multi spot scanning strategy of an Arcam A2X was investigated in depth by means of high-speed imaging of the beam movement pattern and direct correlation to the process parameters considered. As a hypothesis, it was thought that this strategy is effective for direct microstructure design as well. Results indicate that a target-oriented adaption of the scanning strategy in different part cross-sections is feasible. Microstructural investigations including scanning electron and optical microscopy revealed that the multi spot scanning strategy generally allows establishing a fine-grained, isotropic microstructure. However, if the scanning strategy parameters are not adapted to the part cross-section, a randomized electron beam movement pattern can occur, which negatively affects the microstructural evolution and relative density of the processed material.