Generation mechanism and suppression method of landing error of two successively deposited metal droplets caused by coalescence and solidification


3D printing has emerged as a promising technology for manufacturing various components due to its low energy consumption and affordability. With the continuous advancement of 3D printing, metal droplet-based 3D printing has drawn significant research attention for rapid manufacturing. Notably, obtaining the desired component configuration and functionality using this approach requires the inclusion of continuous features accurately printed on the substrate. However, in most cases, the surface tension-driven flow may shift the droplets from their original location before solidification. The displacement of droplets, also known as the landing error, results in unpredictable droplets movements leading to several geometric defects, including irregular waved surfaces, broken traces, and collapsed structures. Such defects present a huge challenge in metal droplet-based 3D printing because they significantly lower the forming accuracy and mechanical performance integrity of the printed parts.

Efforts to eliminate or minimize the landing error focus on enhancing the droplet deposition accuracy and achieving continuous track printing through effective control of the key process parameters like overlap rate, droplet temperature, and substrate temperature. Additionally, since the error emanates from the initial two successive droplets, there is a need for a thorough investigation of the coalescence of the deposited droplets to gain more insights into the underlying mechanism behind the formation of the landing error. Unfortunately, the coalescence of metal droplets with phase transition is highly susceptible to solidification, presenting another challenge. Additionally, there is a general lack of adequate studies on the role of coalescence and solidification in the development of geometric defects in metal droplet-based 3D printing.

To address these challenges, Mr. Yibo Dou, Associate Professor Jun Luo, Professor Lehua Qi, and Dr. Hongcheng Lian from Northwestern Polytechnical University, together with Professor Xianghui Hou from the University of Nottingham, proposed an effective strategy for determining the generation mechanism and suppressing the landing error of two metal droplets caused by coalescence and solidification. First, the authors performed an overlapping experiment of the deposited droplets to cross-examine the landing error. Next, a numerical model based on Volume of Fluid (VOF) method was proposed to analyze the evolution of solidification and formation of the error, while the mass conservation model was constructed to predict the dependence of the substrate temperature on the landing error. Finally, the model was validated, and an effective strategy to suppress the landing error was proposed. The work was published in the International Journal of Heat and Mass Transfer.

The research team showed that the two droplets had distinct final shapes because the second one absorbed more mass from the first one to become bigger when the substrate temperature was increased. The authors identified two landing errors at various substrate temperatures: spread error and retraction error. The former was due to the coupled spread of the second droplet which led to the blockage of the solidified layer, while the latter was due to over-remelting and slow speed of the obstructed solidified layer. Furthermore, it was worth noting that the landing errors were further divided into precision, spread and retraction regions, and increasing the substrate thermal conductivity emerged as an effective strategy for suppressing the landing error.

In summary, the authors successfully developed an effective strategy to suppress the landing error which has been a big challenge in 3D printing involving metal droplets. The effectiveness of the conservation model in predicting the shapes of the overlapped droplets was demonstrated, and the predictions agreed well with the experimental data. Further, increasing the thermal conductivity of the substrate allowed for expanding the processing window for accurate deposition, thus suppressing the occurrence of the landing error. In a statement to Advances in Engineering, the authors pointed out that the study findings provide the required guidance for accurate metal 3D printing of high-quality components using metal droplets.

Generation mechanism and suppression method of landing error of two successively deposited metal droplets caused by coalescence and solidification - Advances in Engineering

About the author

Yibo Dou is an Ph.D. candidate in metal 3D printing and additive manufacturing. He received his Bachelor of Mechanical Engineering from Zhengzhou University (ZZU), China. He then enrolled in the master and doctoral programs of Mechanical Engineering under the supervision of Associate Professor Jun Luo and Professor Lehua Qi at Northwestern Polytechnical University (NPU), China. His research interests include coalescence of metal droplets, heat and mass transfer of overlapping metal droplets, and advanced components fabrication-based 3D printing. Mr. Yibo Dou has done seminal research work in metal 3D printing, and he has published a series of research papers on some high-level journals, such as: International Journal of Heat and Mass Transfer, Journal of Materials Processing Technology.

More information about Mr. Yibo Dou can be found at Researchgate.
Email addresses: [email protected](Y. Dou);

About the author

Jun Luo is currently an Associate Professor at School of Mechanical Engineering in NPU. He obtained his Ph.D. in 2010 in Northwestern Polytechnical University, under the supervision of Professor Lehua Qi. He did his post-doctoral research in NPU from 2010 to 2012. In 2015, he visited the Physics of Fluids (POF) group as a visiting scholar at the University of Twente in the Netherlands.

For many years, he concentrates in developing metal droplet-based printing technologies. In NPU, he and his colleague keep developing molten metal droplet printers. Now the accomplished printers can work at the temperature beyond 1000 oC. Solder, aluminum and copper droplet printing have been successfully tested. He also has a keen interest in the fundamental research about molten metal droplet ejection, impact, and deposition behaviors. In POF group, he coordinated with Assistant Professor Claas Willem Visser, under the supervision of Prof. Chao Sun (now at Tsinghua University) and Prof. Detlef Lohse, to explore the first multi-material nanosecond laser-induced forwards transfer (ns-LIFT) system for printing micron-scaled functional metal structure. Based on this collaboration, several interesting topics on the metal droplet impact are still undergoing.

He has published papers in journals such as Small, Int. J. Mach. Tool and Manuf., J. Mater. Process. Technol., Appl. Therm. Eng., Mater. Lett., etc. His a peer reviewer for over 10 journals, such as Int .J. Mach. Tool. and Manuf., Addit. Manuf., Science reports, Int. J. Multiphase Flow, J. Mater. Process. Technol., powder technology, Chinese Chemical Letters etc.

More information about associate professor Luo Jun can be found at Researchgate.
Email addresses: [email protected];


Yibo Dou, Jun Luo, Lehua Qi, Hongcheng Lian, Xianghui Hou. Generation mechanism and suppression method of landing error of two successively deposited metal droplets caused by coalescence and solidification. International Journal of Heat and Mass Transfer, 172, 2021: 121100.

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