Abstract
The layer-wise deposition of materials by means of Additive Manufacturing (AM) enables the development and production of components with unparalleled complexity. Laser-based powder-bed fusion (PBF-LB) and direct energy deposition (DED-LB) are prominent examples of fabrication technologies in the field of metallic materials. Due to the repeated remelting of previously applied layers, these processes promote epitaxial grain growth, which can lead to the formation of long grains with a pronounced <001> texture. Particularly for the DED-LB, the insulating effect of the surrounded atmosphere only allows heat to be dissipated through the generated structure. The resulting temperature gradient further enhances the formation of a textured and columnar-grained microstructure with respect to the building direction. However, recent studies have shown that coupling ultrasonic excitation with the DED-LB processes is one of the most promising methods to counteract epitaxial grain growth during AM. In addition, this technology can also be used to systematically manipulate the microstructure of individual layers by triggering the ultrasonic excitation at specific points in time within the process. This represents a novel approach for the additive production of metallic geometries with graded properties. 
The presented study seeks to expand the understanding of potential process limitations by analyzing the influence of ultrasonic excitation parameters and build envelope sizes on the microstructural evolution of stainless steels fabricated by powder-fed DED-LB using a force-fit excitation source attached to the build platform. The results of the electron backscatter diffraction analyses reveal a pronounced influence of build envelope size and ultrasonic excitation parameters, thus providing new possibilities for the potential use of ultrasonic excitation in future AM applications.