Grain boundary engineering in additively manufactured polycrystalline materials

Additive manufacturing (AM) offers exceptional advantages over conventional manufacturing methods, including unparalleled flexibility, design freedom, and the capability to produce complex near-net-shape components with minimal material waste. Grain boundary engineering (GBE) is a well-established approach for enhancing mechanical properties and reducing corrosion susceptibility in polycrystalline materials by increasing the fraction of low-energy grain boundaries. However, conventional GBE relies on complex cycles of mechanical deformation and annealing, which is incompatible with the precision-engineered geometries of near-net-shape AM parts. Consequently, integrating GBE into AM materials and optimizing AM processes to improve grain boundary-related properties remain critical challenges. This thesis investigates strategies to control powder characteristics, processing parameters, and post-AM heat treatments to tailor the microstructure and properties of stainless steel and Ni-based superalloys. It introduces an approach to control the strain energy generated during AM by adjusting processing parameters, specifically, by reducing the volumetric energy density to create sufficient driving force for recrystallization, eliminating the need for mechanical deformation. Additionally, a novel in-situ GBE strategy is proposed, whereby controlled powder recycling promotes the development of a fine-grained 316L microstructure with a high fraction of Σ3 twin boundaries, achieved without mechanical deformation or annealing. This approach not only improves the microstructure and properties of AM materials but also underscores the potential for environmentally sustainable manufacturing through powder recycling. This thesis further extends the GBE methodology to the Ni-based superalloy INC738, demonstrating that grain boundary precipitation (e.g., carbides) and segregation can be manipulated by optimizing scanning strategies from linear to random patterns. These modifications enhance mechanical properties and reduce cracking in this traditionally non-weldable alloy. Overall, this thesis highlights the potential of AM to produce complex parts with superior microstructure and enhanced performance while enabling cost-effective and sustainable manufacturing. By expanding the microstructure design space of engineering materials, AM-based GBE presents great promise for accelerating the industrial adoption of AM technologies.

Grantor dc:publisher

UNSW, Sydney

Year dc:date

2025

Author dc:creator

• LUO, Ming

• Additive Manufacturing
• anzsrc-for: 401607 Metals and alloy materials

Statement dc:rights

• open access
• CC BY 4.0
• free_to_read

Licence

• https://purl.org/coar/access_right/c_abf2
• https://creativecommons.org/licenses/by/4.0/

Language dc:language

en

Identifier

https://doi.org/10.26190/unsworks/31292

OAI identifier oai:identifier

oai:unsworks.library.unsw.edu.au:1959.4/105299