Additive manufacturing of high-strength alloys for marine and offshore applications
The rise of additive manufacturing (AM) brings forth a vast array of advantages for the manufacturing sector, including the designing of complex geometries, formulating custom alloys, and remanufacturing worn-out parts. The widespread adoption of AM has proven to be instrumental for a diverse range...
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| Format: | Thesis-Doctor of Philosophy |
| Language: | English |
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Nanyang Technological University
2025
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| Online Access: | https://hdl.handle.net/10356/182820 |
| _version_ | 1826130365823057920 |
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| author | Lek, Yung Zhen |
| author2 | Zhou Kun |
| author_facet | Zhou Kun Lek, Yung Zhen |
| author_sort | Lek, Yung Zhen |
| collection | NTU |
| description | The rise of additive manufacturing (AM) brings forth a vast array of advantages for the manufacturing sector, including the designing of complex geometries, formulating custom alloys, and remanufacturing worn-out parts. The widespread adoption of AM has proven to be instrumental for a diverse range of industries, including the biomedical, aerospace, and marine and offshore sectors. However, the limited understanding of the microstructure–property–process relationship of AM-fabricated corrosion-resistant alloys has hindered the adoption of these alloys within the marine and offshore industry. This Ph.D. research intends to provide a deeper understanding of the relationship between AM processes and their resulting microstructure and material performance of corrosion-resistant alloys.
Laser-based directed energy deposition (DED-LB/M) was utilized to fabricate near-fully dense corrosion-resistant maraging steel parts to evaluate the process-microstructure-property relationship of DED-LB/M-printed M789 steel. The as-fabricated microstructure exhibits martensite aligned to the build direction with retained austenite along the melt pool boundaries. The direct aging (DA) treatment retained the as-fabricated microstructural features, while solutionizing and aging (SA) treatment eliminated the melt pool boundaries and austenite. The SA samples achieved a 55% enhancement in ultimate tensile strength (UTS) with a 73% reduction in elongation. Additionally, corrosion tests reveal a higher pitting potential than DED-LB/M-printed 15–5 PH steel. The results show that SA-treated DED-LB/M-printed M789 steel possesses both high strength and good corrosion resistance at the cost of its elongation.
Transformation-induced plasticity (TRIP) is an effective approach to address the strength–ductility dilemma. However, TRIP alloys require extended heat treatment strategies and specific alloying compositions that incur additional costs. In-situ alloying with DED-LB/M provides an alternative approach to alter the alloy composition. The addition of 316L increases the austenite volume fraction within the martensite matrix, resulting in enhanced uniform elongation (UE). The in-situ alloyed with 6 wt% of 316L achieved a well-balanced strength and ductility of 1638 MPa and 5.5%. In comparison to heat-treated M789 steel, the UE was enhanced by 76% while maintaining high strength. In contrast, the 8 wt% of 316L in-situ alloyed sample experienced a 10% reduction in tensile strength due to the increased volume fraction of austenite. In addition, the pitting resistance of in-situ alloyed samples was enhanced by 55%. Therefore, this work demonstrates the ability to tailor the chemical composition via in-situ alloying to improve the UE and corrosion resistance of alloys simultaneously.
Alternatively, mimicking the brick-and-mortar architecture of gastropods with AM offers an opportunity for enhanced mechanical performance in alloys. The precise control capabilities of the laser powder bed fusion (PBF-LB/M) enable a two microstructure-one alloy integration. Leveraging the control capabilities of AM processes by strategically altering the scanning vectors achieves regions with varying mechanical anisotropy to synergistically strengthen the alloy. The bioinspired architecture achieved a 32% increase in yield strength as compared to the uniform microstructure, owing to the hetero-deformed induced strengthening. Demonstrating the universality of the bioinspired approach, the heterogeneous architecture concept was applied to M789 steel fabricated by PBF-LB/M, resulting in a 50 MPa enhancement in tensile strength. The results provide a pathway for the design and development of heterogeneous structured high-performance alloys with enhanced mechanical properties by controlling the scanning patterns and anisotropy at site-specific regions.
The design approach of in-situ alloying and site-specific control with AM technology provides insights into strategies for enhancing the performance of corrosion-resistant alloys for the marine and offshore industry. The findings contribute to the expanding knowledge of process optimization and microstructure manipulation of AM technology, which are associated with improving the performance of M789 and IN 725 alloys. |
| first_indexed | 2025-03-09T15:20:28Z |
| format | Thesis-Doctor of Philosophy |
| id | ntu-10356/182820 |
| institution | Nanyang Technological University |
| language | English |
| last_indexed | 2025-03-09T15:20:28Z |
| publishDate | 2025 |
| publisher | Nanyang Technological University |
| record_format | dspace |
| spelling | ntu-10356/1828202025-03-03T02:29:28Z Additive manufacturing of high-strength alloys for marine and offshore applications Lek, Yung Zhen Zhou Kun School of Mechanical and Aerospace Engineering Singapore Centre for 3D Printing kzhou@ntu.edu.sg Engineering High strength alloys Additive manufacturing Microstructure control In-situ alloying The rise of additive manufacturing (AM) brings forth a vast array of advantages for the manufacturing sector, including the designing of complex geometries, formulating custom alloys, and remanufacturing worn-out parts. The widespread adoption of AM has proven to be instrumental for a diverse range of industries, including the biomedical, aerospace, and marine and offshore sectors. However, the limited understanding of the microstructure–property–process relationship of AM-fabricated corrosion-resistant alloys has hindered the adoption of these alloys within the marine and offshore industry. This Ph.D. research intends to provide a deeper understanding of the relationship between AM processes and their resulting microstructure and material performance of corrosion-resistant alloys. Laser-based directed energy deposition (DED-LB/M) was utilized to fabricate near-fully dense corrosion-resistant maraging steel parts to evaluate the process-microstructure-property relationship of DED-LB/M-printed M789 steel. The as-fabricated microstructure exhibits martensite aligned to the build direction with retained austenite along the melt pool boundaries. The direct aging (DA) treatment retained the as-fabricated microstructural features, while solutionizing and aging (SA) treatment eliminated the melt pool boundaries and austenite. The SA samples achieved a 55% enhancement in ultimate tensile strength (UTS) with a 73% reduction in elongation. Additionally, corrosion tests reveal a higher pitting potential than DED-LB/M-printed 15–5 PH steel. The results show that SA-treated DED-LB/M-printed M789 steel possesses both high strength and good corrosion resistance at the cost of its elongation. Transformation-induced plasticity (TRIP) is an effective approach to address the strength–ductility dilemma. However, TRIP alloys require extended heat treatment strategies and specific alloying compositions that incur additional costs. In-situ alloying with DED-LB/M provides an alternative approach to alter the alloy composition. The addition of 316L increases the austenite volume fraction within the martensite matrix, resulting in enhanced uniform elongation (UE). The in-situ alloyed with 6 wt% of 316L achieved a well-balanced strength and ductility of 1638 MPa and 5.5%. In comparison to heat-treated M789 steel, the UE was enhanced by 76% while maintaining high strength. In contrast, the 8 wt% of 316L in-situ alloyed sample experienced a 10% reduction in tensile strength due to the increased volume fraction of austenite. In addition, the pitting resistance of in-situ alloyed samples was enhanced by 55%. Therefore, this work demonstrates the ability to tailor the chemical composition via in-situ alloying to improve the UE and corrosion resistance of alloys simultaneously. Alternatively, mimicking the brick-and-mortar architecture of gastropods with AM offers an opportunity for enhanced mechanical performance in alloys. The precise control capabilities of the laser powder bed fusion (PBF-LB/M) enable a two microstructure-one alloy integration. Leveraging the control capabilities of AM processes by strategically altering the scanning vectors achieves regions with varying mechanical anisotropy to synergistically strengthen the alloy. The bioinspired architecture achieved a 32% increase in yield strength as compared to the uniform microstructure, owing to the hetero-deformed induced strengthening. Demonstrating the universality of the bioinspired approach, the heterogeneous architecture concept was applied to M789 steel fabricated by PBF-LB/M, resulting in a 50 MPa enhancement in tensile strength. The results provide a pathway for the design and development of heterogeneous structured high-performance alloys with enhanced mechanical properties by controlling the scanning patterns and anisotropy at site-specific regions. The design approach of in-situ alloying and site-specific control with AM technology provides insights into strategies for enhancing the performance of corrosion-resistant alloys for the marine and offshore industry. The findings contribute to the expanding knowledge of process optimization and microstructure manipulation of AM technology, which are associated with improving the performance of M789 and IN 725 alloys. Doctor of Philosophy 2025-03-03T02:29:28Z 2025-03-03T02:29:28Z 2024 Thesis-Doctor of Philosophy Lek, Y. Z. (2024). Additive manufacturing of high-strength alloys for marine and offshore applications. Doctoral thesis, Nanyang Technological University, Singapore. https://hdl.handle.net/10356/182820 https://hdl.handle.net/10356/182820 en This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC 4.0). application/pdf Nanyang Technological University |
| spellingShingle | Engineering High strength alloys Additive manufacturing Microstructure control In-situ alloying Lek, Yung Zhen Additive manufacturing of high-strength alloys for marine and offshore applications |
| title | Additive manufacturing of high-strength alloys for marine and offshore applications |
| title_full | Additive manufacturing of high-strength alloys for marine and offshore applications |
| title_fullStr | Additive manufacturing of high-strength alloys for marine and offshore applications |
| title_full_unstemmed | Additive manufacturing of high-strength alloys for marine and offshore applications |
| title_short | Additive manufacturing of high-strength alloys for marine and offshore applications |
| title_sort | additive manufacturing of high strength alloys for marine and offshore applications |
| topic | Engineering High strength alloys Additive manufacturing Microstructure control In-situ alloying |
| url | https://hdl.handle.net/10356/182820 |
| work_keys_str_mv | AT lekyungzhen additivemanufacturingofhighstrengthalloysformarineandoffshoreapplications |