Metal flow in laser additive manufacturing using x-ray imaging and machine learning

Project Information

MFILAMUXIAML

Grant agreement ID: 896742

DOI

10.3030/896742

Project terminated on 10 May 2022

EC signature date 11 March 2020

Start date 1 February 2021

End date 30 September 2023

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 212 933,76

EU contribution

€ 212 933,76

Coordinated by

UNIVERSITY COLLEGE LONDON
United Kingdom

Periodic Reporting for period 1 - MFILAMUXIAML (Metal flow in laser additive manufacturing using x-ray imaging and machine learning)

Reporting period: 2021-02-01 to 2023-01-31

Summary of the context and overall objectives of the project

Problem/issue being addressed: Laser powder bed fusion (LPBF) technology fabricates parts layer by layer via a focused laser beam to fuse the loose powder along the pre-designed path, is promising in aerospace, automotive and medical for its customization and free of geometric limitations that can’t be realized by the traditional technologies. A keyhole pore forms when gas is trapped by the cavity. In LPBF, when the melt pool transforms form conduction mode to keyhole mode, which is beneficial for low laser-absorptivity material, it is likely to generate a keyhole pore. Therefore, conducting in-depth research on the formation mechanism of keyhole pore is needed.

Importance: AM is a collection of emerging manufacturing techniques, e.g. Laser powder bed (LPB), laser metal deposition (LMD), electron beam powder bed (EBPD), and has the potential to make greater breakthrough than conventional methods. AM offer many advantages, e.g. of making complex components with relatively low cost. A fundamental understanding of the laser-metal powder interaction is critical to improving the quality and efficiency in the laser AM process. Our research provides a strategy to reduce or remove the pore in laser powder bed fusion, which is extremely important for the mass production of additive manufacturing technology.

Overall objectives: a three-dimensional thermal-mechanical-fluid coupled model is established via finite volume method, considering the heat transfer, fluid flow, recoil pressure as well as solidification drag model, and reproduces the formation processes of three representative pores. The proposed model is validated by the in-situ X-ray imaging results and reveals the fluid flow, keyhole fluctuation, three types of pores formation process and mechanism under a range of high-speed welding and powder bed fusion conditions. The goal of this research is to provide a comprehensive understanding of keyhole induced-porosities in laser powder bed fusion of aluminium and suggests the strategy towards pore-free laser fusion.

Conclusions: Through the research of the action, we clearly clarify the mechanism for pore formation in laser additive manufacturing and suggests the method to avoid the pore. Our work is beneficial to improving the quality of AM and promote its application in many fields, e.g. aerospace, transportation, automobile, and so on.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Work performed and main results:
A three-dimensional multi-physics thermal-mechanical-fluid coupled model is implemented to investigate the keyhole pore formation mechanism during the laser is running, and validated by the ultra-high-speed in-situ X-ray imaging results. The following main results are achieved:
1) A new Keyhole pore formation mechanism is uncovered. It happens when the keyhole penetrates the molten pool due to their fluctuation and results in the keyhole tip to be captured prematurely by the L/S interface, finally forming a pore.
2) The RF-pore and R-pore are reproduced, which has been revealed before as that R-pore is generated due to hump or budge forming at rear keyhole wall and the wall collapsing and pinching off bubble. While the RF-pore formation results from the fluctuation and the bridge formed between the rear and front keyhole wall, leading to the transient bubble formation and then captured by the solidification front.
3) The depth of the molten pool and the keyhole, as well as the pore size decrease with the introduction of powder, while the number increases, which can be attributed to the smaller thermal conductivity of the powder compared with the bulk material. The existence of the powder influences the laser irradiation path, increasing the oscillation of the molten pool and keyhole.
4) The study upon the effects of input energy density uncovers that the oscillation frequency, KP-pores and total pore counts increase with the increase of energy density in power function. The input-energy-density thresholds for the penetration and pore occurrence are in the following order: ETotal pore counts < EOscillation frequency ≈ EMaximum penetration depth < EKP-pore counts. Pores will no longer be formed when the input energy density is below 0.02 MJ/cm3.

Overview of the results and their exploitation and dissemination:
To sum up, our work reveals the formation mechanism of pores and found that the different formation mechanisms of the three types of pores make their shapes and distributions unsimilar. R-pore is small and unstable, usually distributed in the upper part. RF-pore is medium-sized and nearly spherical, mainly in the middle part. While KP-pore is large and unregular, with its morphology similar to that of the keyhole tip captured by the solidification interface. Most of the KP-pores are located at the bottom.

The following measures have be adopted to exploit the results and contribute to the commercialization:
The simulated results about the mechanism of pore formation have guided the optimization of additive manufacturing process, which has been used in producing the AMed parts in both Europe and China (the researcher’s mother countries).
The research results supported by this funding have be disseminated with the styles of papers, reports, and presentation in the conferences, seminar, short meeting, and so on.
The simulation program has be expanded to predict the printability of various materials. With the development of the AM, various new materials will be used and assessing the printability of new material should be a hot topic. The developed program for predicting the printability of the new materials has great potential to be commercialized by providing the service to test the printability of the new material.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The fellowship can enhance the future career of the researcher in the following aspects:
The research stay at UCL is a major step in the researcher’s future career because it will enable the applicant to gain crucial knowledge in understanding the mechanism in AM and strengthen the applicant’s comprehensive abilities towards an excellent academic group leader. This research experience and the research achievement contribute the applicant to find a faculty position in a top Chinese university, Shanghai Jiao Tong University.
China has one of the largest AM markets in the world, while the experienced researcher with the diverse international background is scarce. The researcher gets a good chance to build an independent research group at a Chinese university, focusing on the AM process. The knowledge and skills obtained during the fellowship contribute to the development of the research group.
The knowledge and skills about simulation developed during the fellowship can speed up the application of simulation in AM. The researcher will continue this interdisciplinary research after the fellowship, train students in this interdisciplinary field, and contribute to the application of this technique in the industry.
The grant proposal writing skills obtained from preparing this proposal and training during the fellowship stage is crucial for successfully attracting external funding, which is an essential part of building up an own research group. The researcher will cooperate with the host to apply for the Joint Funds between China and the EU in the future, contributing the collaboration in the AM and providing supports for the industry in both areas.

Fluid flow and formation of KP-pore in the longitudinal plane.