Recrystallization-based grain boundary engineering of additively manufactured metals

• Main Author: Gao, Shubo
• Other Authors: Wan Man Pun
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Materials::Metallic materials
• Online Access: https://hdl.handle.net/10356/164829
• Institution: Nanyang Technological University

Grain boundary engineering (GBE) is a materials processing strategy to enhance the physical and mechanical properties of polycrystalline metals by purposely incorporating special types of grain boundaries—such as twin boundaries (TBs)—in the microstructure. Conventional GBE methods involve multiple strain-annealing cycles, which change the geometry of the target material substantially. Thus, they are unfit to parts produced using near-net-shape manufacturing techniques, including metal additive manufacturing (AM). Devising a GBE method that is compatible with AM, however, would allow to further enhance the performance of topology-optimized structural components by improving their GB-controlled properties. This thesis focuses on devising an AM-compatible processing strategy to achieve this goal, which I refer to as additive-GBE (AGBE). Focusing on stainless steel 316L as a case-study material, I test and demonstrate different AGBE methodologies using different AM processes, including directed energy deposition (DED) and laser powder bed fusion (LPBF). All these methodologies rely on the ability to trigger microstructure recrystallization “on demand”. Indeed, recrystallization of austenitic steel produces microstructures containing a multitude of TBs. To drive recrystallization in DED, I apply single point incremental forming to introduce controlled mechanical strains into the build in-situ. This approach yields gradient or “sandwiched” microstructures characterized by variable TBs distributions. In LPBF, I gain control over the thermal stability of the alloy—and thus its propensity to undergo recrystallization—by tuning the residual strains in the microstructure (which mostly consist of geometrically necessary dislocations) as well as the chemical heterogeneity of the solidification structure. I demonstrate how both factors can be manipulated independently by changing the LPBF process parameters to vary the driving force for nucleation of recrystallized grains as well as their growth rate in the microstructure, respectively. Using different combinations of such process parameters, I show the possibility of producing samples of stainless steel 316L that combine arbitrary distributions of recrystallized and as-built microstructures. Finally, I analyze the mechanical behavior of the AGBE alloys produced by DED and LPBF and investigate the additional strengthening mechanisms brought about by the engineered microstructure heterogeneity. The results suggest the possibility of using AGBE as a cost-effective and practical approach for the direct production of topology-optimized parts with controlled microstructures and improved performance. As such, AGBE broadens the microstructure-based design space of engineering materials.