Laser Powder Bed Fusion Additive Manufacturing of High Entropy Alloys

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Embargoed until 2025-03-08
Copyright: Kong, Hui
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Abstract
The high cooling rates in metal additive manufacturing (AM) of high-entropy alloys (HEAs) can not only prohibit the formation of intermetallic compounds and detrimental elemental segregation but also facilitate microstructural refinement, thereby providing improved mechanical properties. However, there are still many challenges, including the presence of AM fabrication defects and residual stresses as well as anisotropic properties. Thus, further understanding is needed regarding applying post heat treatment to release residual stresses in as-built HEAs, and the techniques that can be employed to reduce or minimize the property anisotropy in as-built HEAs. In this thesis, a model Cantor alloy was fabricated and subject to a series of post heat treatment conditions. The results showed that annealing at a temperature lower than 900 °C gave rise to the formation of M23C6 carbides while recrystallization took place at temperatures over 900 °C. Interestingly, three distinct types of microstructures were exhibited at the annealing temperature of 900 °C. This is primarily attributable to the different thermal histories undergone through the LPBF process among these areas where heterogeneity in chemical elements and microstructure was revealed. The second group of samples were fabricated to develop a machine learning coupled microstructure mapping approach for tailoring mechanical property anisotropy. Three distinct types of microstructures were generated by varying the laser power and scanning speed, namely, herringbone, bimodal, and columnar microstructures. Epitaxial grain growth and side-branching in the melt pool center and overlapped regions are the main reasons responsible for the different microstructures. The underlying mechanisms were discussed in terms of effective grain size, heterogeneous microstructure, and the twinning induced plasticity (TWIP) effect. Subsequently, pre-deforamtion was applied before post heat treatment to study the influence of grain boundary engineering (GBE) on the recrystallization and resultant deformation behavior. Twinning-assisted recrystallization nucleation accelerated the formation of a nearly a fully recrystallized microstructure in the heavily deformed sample. As a result, the present strategy of GBE can provide further insight regarding the manipulation of the post heat treatment in as-built alloys particularly for those precipitation hardenable alloys where precipitation kinetics can be varied.
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Publication Year
2023
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Thesis
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PhD Doctorate
UNSW Faculty