Development of an additive manufacturing processing route for high entropy alloys using powder bed fusion

Chen, Peng ORCID: 0000-0002-7771-7072 (2022). Development of an additive manufacturing processing route for high entropy alloys using powder bed fusion. University of Birmingham. Ph.D.

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Laser powder bed fusion (LPBF) is an additive manufacturing (AM) technique that can produce components from digital models in a layer-by-layer fashion using metallic powders. The customisation of pre alloyed powders used by LPBF is expensive and time-consuming, making LPBF not ideal for alloy development. In-situ alloying approaches using blended powders as raw materials are therefore carried out to shorten the alloy development process. Recently, high entropy alloys (HEAs) have drawn growing scientific attention. The HEA concept is of great compositional flexibility, allowing vast composition spaces and advanced properties to be explored. The CoCrFeMnNi HEA has been widely studied as a representative face-centred-cubic (FCC) HEA. The feasibility of the LPBF in-situ fabrication of the CoCrFeMnNi HEA and its body-centred-cubic (BCC) variation, AlCoCrFeNi, is the subject of this study.

This study aims to develop an AM processing route for HEAs through LPBF in-situ alloying. Elemental Mn and Al powders were blended with pre-alloyed CoCrFeNi powder for quasi-equiatomic composition, respectively. The in situ alloying printability was evaluated via the parametric study based on densification and defect assessments. The chemical homogenisation and phase formation in the as-built samples was examined and correlated to the laser heat input. The results showed that Mn could be in-situ alloyed into the FCC CoCrFeNi matrix with homogeneity, indicating good printability of the CoCrFeMnNi HEA. However, the attempt to in-situ fabricate the AlCoCrFeNi HEA failed to produce samples free of cracking/porosity, despite the investigation of a wide range of parameters. The resultant defects and Al segregations suggested that the BCC HEA cannot be realised using this approach.

The tensile and compression properties of the in-situ alloyed CoCrFeMnNi HEA were compared with the LPBFed CoCrFeMnNi HEA fabricated using pre-alloyed powder. The tensile strength was reinforced by the oxide-dispersion-strengthened (ODS) effect, because Mn oxides were introduced during the process. Submicron voids were observed around the oxides in the deformed samples, which were responsible for the early failure during the tensile deformation. The Mn oxides were identified as MnO and Mn2O3 and their forming mechanisms were analysed.

To understand the underlying mechanisms of the LPBF in-situ alloying processes, elemental homogenisation and grain development were further investigated through single-track, single-layer and three-layer experiments. The experimental meltpool dimensions were compared with the predicted ones, showing that the processing window for in-situ alloying was operated in the keyhole mode. Remelting was found to be the main mechanism of elemental homogenisation. Crystalline characteristics were found to be inherited during the accumulation of tracks, reflecting parameter structure correlations.

In conclusion, the results of this study have shown that LPBF in-situ alloying had the potential for HEA development, and raised topics for further research. A comprehensive understanding of the process will help to shorten the period from alloy design to microstructural tailoring.

Type of Work: Thesis (Doctorates > Ph.D.)
Award Type: Doctorates > Ph.D.
Licence: All rights reserved
College/Faculty: Colleges (2008 onwards) > College of Engineering & Physical Sciences
School or Department: School of Metallurgy and Materials
Funders: Other
Other Funders: Southern University of Science and Technology (SUSTech)
Subjects: T Technology > TN Mining engineering. Metallurgy


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