A multivariate meltpool stability criterion for fabrication of complex geometries in electron beam powder bed fusion

Process parameters for manufacturing of complex parts in electron beam powder bed fusion are usually derived from material-specific process windows, which are established by fabrication and evaluation of standardized cuboid specimen. The mechanisms defining low- and high-energy boundaries of the process window are well understood. The boundary emerging at high scan speeds and beam power however, was observed but the underlying mechanism is not further discussed. In addition, appropriate methodologies to transfer process parameters from standardized process windows to complex geometries are not readily available, as the meltpool geometry is significantly affected by the scan length. This work introduces and verifies a characteristic scan length dependent process parameter limit for the fabrication of complex geometries. Electron-optical process monitoring enables the surface characterization for a wide range of process parameter and the subsequent identification of the process parameter limit as a linear function of scan length. A semi-analytical heat conduction model is used to examine the corresponding meltpool geometries. The underlying mechanism is determined as the meltpool stability limit, which occurs, when the aspect ratio of the meltpool reaches the threshold for a liquid film instability. Based on the meltpool geometries for different scan lengths, an analytical relationship for the process parameter limit as a function of scan length is proposed. This relationship may be used as a guideline for the selection of process parameters and the development of new scan strategies for the fabrication of complex geometries.