| NSF Org: |
CMMI Division of Civil, Mechanical, and Manufacturing Innovation |
| Recipient: |
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| Initial Amendment Date: | June 5, 2017 |
| Latest Amendment Date: | March 31, 2020 |
| Award Number: | 1662662 |
| Award Instrument: | Standard Grant |
| Program Manager: |
Andrew Wells
awells@nsf.gov (703)292-7225 CMMI Division of Civil, Mechanical, and Manufacturing Innovation ENG Directorate for Engineering |
| Start Date: | June 1, 2017 |
| End Date: | May 31, 2020 (Estimated) |
| Total Intended Award Amount: | $188,674.00 |
| Total Awarded Amount to Date: | $212,674.00 |
| Funds Obligated to Date: |
FY 2018 = $16,000.00 FY 2019 = $8,000.00 |
| History of Investigator: |
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| Recipient Sponsored Research Office: |
2301 S 3RD ST LOUISVILLE KY US 40208-1838 (502)852-3788 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
2301 South Third Street Louisville KY US 40292-0001 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): |
AM-Advanced Manufacturing, Manufacturing Machines & Equip |
| Primary Program Source: |
01001819DB NSF RESEARCH & RELATED ACTIVIT 01001920DB NSF RESEARCH & RELATED ACTIVIT |
| Program Reference Code(s): |
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| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.041 |
ABSTRACT
Additive manufacturing (AM) has created a new paradigm of integrated materials, design and manufacturing innovations for effective product development and realization across a broad range of industries. Metal AM technologies such as selective laser melting (SLM), which uses a powder-bed and a high power laser, are especially beneficial in making complex-geometry, high-performance components without incurring tooling costs, giving early adopters a competitive advantage in the global market. One of the key challenges that hinder efficient metal AM technology implementation in industry is part quality inconsistency that is significantly affected by porosity in AM parts. Pores, with sizes of one to one hundred microns, are internal defects generated during the process and that cause large variations in part performance. Pore formation in metal AM is complex and not fully understood, making it difficult to predict for quality control. This award corroborates basic research needs of the porosity issue in metal AM parts. The project tackles original research on the coupled multi-scale and multi-physics process of heating, melting and solidification of numerous microscopic metallic particles in SLM. Research findings will not only establish correlations between the process parameters, material properties and pore attributes, but may also lead to novel techniques for mitigating pore defects, and thus, have a potential to accelerate metal AM adoption in U.S. industry. In addition, this project will contribute toward workforce training for metal AM industrial needs and attract high school students into advanced manufacturing technologies.
The objectives of this research are to distinguish pore formation mechanisms in SLM and to theoretically predict, analyze and experimentally characterize the porosity in SLM parts. In this collaborative research between the University of Louisville (UofL) and the University of Alabama (UA), a hybrid numerical modeling technique will be developed for particle-resolved simulations capable of capturing different pore formation mechanisms in SLM. The model will be validated by SLM experiments of small-scale specimen fabrications, incorporating an infrared thermal imager for process temperature and melt-pool dynamics. The fabricated specimens will be measured using micro-scale x-ray computed-tomography and analyzed to attain statistical data of detailed porosity attributes for comparison with the results from numerical modeling. The research efforts will be extended to study the relationship between process parameters and morphology and distribution of porosity in SLM, and to estimate process windows that minimize pore defects. If successful, this study will distill knowledge of a convoluted multi-physics phenomenon and will offer significant insight detailing the key to particular mechanisms of different pore formations. The research results will be included in the training materials for the Additive Manufacturing Competency Center on the University of Louisville campus, providing practitioners of the SLM technology with better understanding of the relationship between processing parameters, porosity formation, and mechanical properties of critical AM-fabricated components.
PUBLICATIONS PRODUCED AS A RESULT OF THIS RESEARCH
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PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
This project (NSF award number 1662662) researches the defect issue in one of additive manufacturing (AM) process, termed selective laser melting (SLM), which uses a high-power laser to melt alloyed particles to make a three-dimensional part in a layer by layer manner. The primary objectives are to (1) identify and distinguish pore formation mechanisms and (2) to analyze the porosity in SLM parts and correlate porosity characteristics with SLM parameters.
Metal AM has emerged as a potentially mainstream industrial technology for various product innovations. In particular, the laser/powder-based technique, i.e., SLM, has made significant progress with a potential of transforming the manufacturing capabilities. However, it is widely known that porosity in metal AM parts, one of the most problematic defects, plays a crucial role in part performance, and yet, is sharply sensitive to the parameters of the SLM process itself.
Hence, fundamental understanding of porosity formation mechanisms in SLM is necessary in order to develop innovative methods to effectively mitigate the pore defects. Through a series of experimental studies and some numerical approach, this project has achieved the following major outcomes.
First, the project makes contributions toward fundamental understanding of pore formation in SLM. In essence, confirmed by experiments, pores are formed mostly because of (1) overheating that results in vapor bubbles trapped in molten metal flow, named keyhole pores, or (2) insufficient melting of metallic particles resulting in voids between incompletely melted powder. The laser energy input to the powder plays a significant role in pore formation in SLM, possibly to other metal additive manufacturing processes as well.
Second, from experiments, with the same line energy density (LED, defined by laser power divided by the laser scan speed) input, increasing the laser power shows a distinct transition from incomplete melting, which results in lack of fusion pores, to conduction melting, and then to keyhole melting, which may result in the so-called keyhole pores. From experiments, keyhole pores are mostly close to a spherical shape, have the equivalent diameter in the range from around 10 microns to over several hundred microns, and they are over 100 to 150 microns deep beneath the scanning surface. Image 1 shows an example of keyhole pores in an SLM fabricated sample revealed using a micro-CT scanner.
Thirdly, though the keyhole porosity generally increases with increasing the energy density, such a trend is not followed in certain ranges (Image 2). Further studies reveal that the level of the laser power is highly influential towards the keyhole porosity due to its impact on the keyhole stability. For the same LED, e.g. 0.40 J/mm, the pore number and the pore volume increase with increasing the power until about 140 W and then both the pore number and volume decrease, if the power is further increased (Image 3). A numerical study shows that this may be due to the stability of the keyhole opening at a higher laser power, allowing vapor to escape from the keyhole, instead of being trapped and forming keyhole pores.
For multi-layer depositions, the porosity decreases with the decrease in the volumetric energy density (VED, defined as laser power divided by the product of scan speed, hatch spacing and layer thickness) from about 160 J/mm3 to 80 J/mm3. On the other hand, the porosity increases when the VED is further reduced below about 40 J/mm3, indicating an operating window that may prevent pore formation. Image 4 shows such a phenomenon and Image 5 presents a few examples of porosity in multi-layer samples revealed by micro-CT scanning, both overall view and 2D slice view.
Moreover, this project provided research opportunities to 5 undergraduate students, 3 of which are female, and gave a presentation and laboratory tour to a group from the Women in Manufacturing, Kentucky Chapter (https://www.womeninmanufacturing.org/kentucky). In addition, some research materials have been provided to the metal additive manufacturing professional training courses, conducted by the Additive Manufacturing Institute of Science and Technology (AMIST) at University of Louisville.
Last Modified: 09/29/2020
Modified by: Thomas L Starr
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