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.