The effect of cyclic heat treatment on microstructure evolution during Plasma Arc Additive Manufacturing employing an SEM in-situ heating study

Processing of materials far from equilibrium conditions, i.e., at extreme temperature gradients (up to 10⁶ K/m) and/or rapid temperature changes (up to 10⁵ 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.

To capture these structural variations and identify significant trends, new approaches in material characterization are required. In the present work, in-situ SEM methods to understand the complex spatial-temporal thermal transients experienced by AM components during fabrication and post-processing have been developed. 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 rapid thermal conditions and to 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 microstructures development during the Plasma Arc Additive manufacturing process.