Pathways to exploit the multi-spot scanning strategy in electron beam additive manufacturing for control of microstructure and defect density

The goal of this study is to investigate the impact of beam deflection pattern characteristics on microstructure and porosity formation during Powder Bed Fusion Electron Beam Melting (PBF-EB/M) processing of Inconel 718. Specifically, the complex interplay between “beam return time” and “beam jump distance” and their influence on heat accumulation, porosity, and grain formation is explored. The findings reveal that beam jump distance plays a critical role in the transition from columnar to equiaxed solidification, while beam return time is essential for achieving optimal heat accumulation and minimizing porosity. By controlling these two universal beam deflection pattern characteristics, a novel multi-spot scanning strategy was developed, enabling the formation of a fine-grained equiaxed microstructure. It is demonstrated that sufficient heat accumulation is essential for a pore-free bulk material and can be achieved by short beam return times or small beam jump distances. However, for equiaxed grain formation, a large beam jump distance, i.e., a wide distribution of melt pools, is required. Therefore, heat accumulation must be controlled by means of the beam return time if equiaxed grains are aimed for. Additionally, it is found that higher beam currents are beneficial for equiaxed grain formation, but the strongest impact on the columnar to equiaxed transition is attributed to the beam movement pattern itself. This approach offers insights into microstructure control and process reliability, regardless of the specific PBF technique employed, and enables application-specific microstructure design of Inconel 718.