Processing of materials far from equilibrium conditions, i.e., at extreme temperature gradients (up to 10^6 K/m) and/or rapid temperature changes (up to 10^5 K/s), as in additive manufacturing (AM), causes the formation of metastable phases and induce directional microstructural alterations with a significant impact on overall part properties. While a part is subsequently being built layer-by-layer, previously formed layers are subjected to cyclic heat input that can result in element diffusion, phase transitions, and/or modifications of the grain morphology. As a result, the microstructure of near-net-shape AM components varies across the component leading to local- and design-specific variations in application-critical properties.

New approaches in material characterization are required to capture these structural variations and identify significant trends. In the present work, in-situ SEM methods have been developed to understand the complex spatial-temporal thermal transients experienced by AM components during fabrication and post-processing. This study is critical for understanding and utilizing the influence of variable heat input to control the microstructure of AM-manufactured components, the optimization of process parameters as well as the design of new alloys by allowing to simulate the far-from-equilibrium processing conditions.

Supported by COMSOL simulations, in-situ SEM heating studies using a MEMS heater were performed to mimic AM's rapid thermal conditions and understand dynamic solid-state processes during AM. The microstructural changes were investigated by electron backscatter diffraction (EBSD) and compared to the final microstructure of different layers. The conducted study allows drawing conclusions on the feasibility of SEM-based heating experiments to reproduce microstructure development during the Plasma Arc Additive manufacturing process.