EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Alloy Additive Manufacturing (AM) results in the formation of metastable hierarchical microstructures due to the highly non-equilibrium processes that occur during fabrication. Currently, most experimental/modeling efforts are aimed at studying the role of melt-pool dynamics and rapid solidification on the microstructure formation during AM. We are interested in studying the microstructure evolution occurring after solidification and until the end of the AM process, i.e. the long period during which the solid material is subjected to multiple heating-cooling cycles, called Solid-State Thermal Cycling (SSTC) or intrinsic heat treatment. During SSTC, a plethora of mechanisms could occur such as dislocation dynamics and defect interactions, precipitation, micro-segregation, solid-state phase transformation, recrystallization, grain growth, etc., which can result in a significant evolution of the microstructure, which can affect the mechanical properties of the AM parts.

Microstructure characterization is typically performed via techniques such as electron microscopy. However, it is impractical to probe microstructure evolution due to SSTC during AM via such conventional techniques. As an alternative, instead of probing microstructure evolution during AM, we propose to subject pre-built AM samples to SSTC under controlled conditions, in this case, inside a Transmission Electron Microscope (TEM).

In this work, an investigation of the precipitate microstructure in a 316L Stainless Steel (316L SS) manufactured via Laser Metal Deposition (LMD), a Directed Energy Deposition (DED)-type process, is reported. In situ high-resolution TEM SSTC experiments were conducted on thin film lamellae extracted from the LMD 316L SS. Results show that oxide and non-oxide precipitates can form due to the LMD process, where the material can be subjected to cooling rates as high as ~1e4 – 1e5 K/s. When the lamellae are subjected to SSTC inside TEM, both type of precipitates exhibit evolution in size, morphology and number.