U01 - Generation of Spherical Microscopic Samples with Single Droplet Solidification

Generation of Spherical Microscopic Samples

In subproject U01 a method for the flexible and reproducible generation of spherical micro samples for the method "Farbige Zustände" is developed. The high-temperature single-drop generator established in the project extends the temperature spectrum from 1200 °C to 1600 °C and allows for the first time the application of single-drop processes for steels and other alloys in this temperature range [Link]. For the high-throughput method "colored states", it can be used to produce spherical micro-samples of various diameters of various alloys on aluminum base (600 - 800 °C), copper base (1000 - 1200 °C) and steels (1300 - 1600 °C).

Figure 1: Selection of alloys produced by the drop-on-demand process

The synthesis of such samples with a high reproducibility requires first of all a high reproducibility of drop formation and separation.  The experimental determination and modeling of different drop formation modes is therefore an essential part of the subproject.

Video 1: Droplet formation in drop-on-demand mode (steel 100Cr6)
Video 2: Droplet formation modes, model prediction and experiment (copper)

After droplet separation, the droplet solidifies rapidly during a free fall in an inert gas atmosphere (nitrogen, argon, helium) over a distance of up to 6.5m before being collected in a liquid quenching medium (usually water or oil) and further cooled. In order to set suitable cooling conditions, a cooling model has been developed which couples the motion of the droplet in the gas atmosphere with its cooling by convection and radiation. It is to be noted here that the correlations available up to now for the drag coefficient of a sphere lose their validity at high temperature differences between the sphere surface and the gas atmosphere. By means of a CFD parameter study, a temperature-dependent correction of the established Schiller-Naumann correlation for the resistance coefficient could be worked out [Link]. It was shown that at high temperature differences the flow around a sphere is strongly influenced by the gas expansion of the surrounding gas, which cannot be represented by a choice of reference temperature for the material values between sphere surface and ambient temperature alone. For heat transfer, on the other hand, the known correlation according to Ranz & Marshall could be confirmed even for high temperature differences.

Figure 2: Modeling of droplet cooling, left: gas flow around a sphere for two different sphere surface temperatures, right: calculated droplet cooling and trajectory (top), experimental validation of droplet cooling (bottom)


On the basis of these findings, a cooling model for molten metal droplets was formulated and validated [Link]. For validation, single drops were generated under different process conditions from the model alloys AlCu4.5 and CuSn6. For these alloys, the cooling rate during solidification can be determined by evaluating the secondary dendrite arm spacing (SDAS) based on the microstructure. On the one hand, the evaluation showed a very good agreement with the prediction of the cooling rate, but on the other hand also showed a very high reproducibility of the microstructure.


In order to also confirm a high reproducibility of the samples produced with regard to their properties in the original formed state, a subset of each sample is tested before use in the "Farbige Zustände" method by micropressure testing (TP D01)  and by DSC (TP U03) with regard to its mechanical and thermal properties. Subsequently, the samples are heat treated in a defined manner in subproject U03 and then characterized by descriptors.



S. I. Imani Moqadam, M. Baune, I. Bösing, C. Heinzel, D. Meyer, A. Thomann, N. Wielki, N. Ellendt: Reproducibility of High-Throughput Sample Properties Produced by a High-Temperature Molten Metal Droplet Generator, Metals 2020, 10,  https://www.doi.org/10.3390/met10030297 .

H. Springer, C. Baron, F. Mostaghimi, J. Poveleit, L. Mädler, V. Uhlenwinkel: Additive manufacturing of high modulus steels: new possibilities for lightweight design, Additive Manufacturing 2020,  https://www.oi.org/10.1016/j.addma.2019.101033 .

M. Steinbacher, G. Alexe, M. Baune, I. Bobrov, I. Bosing, B. Clausen, T. Czotscher, J. Epp, A. Fischer, L. Langstadtler, D. Meyer, S. Raj Menon, O. Riemer, H. Sonnenberg, A. Thomann, A. Toenjes, F. Vollertsen, N. Wielki, N. Ellendt: Descriptors for High Throughput in Structural Materials Development, High Throughput 2019, 8,  https://www.doi.org/10.3390/ht8040022

S. Hussain, C. Cui, N. Temple, V. Uhlenwinkel, L. Mädler: Porosity and microstructure of steel tubes spray-formed by close-coupled atomizer, Journal of Materials Processing Technology 2020, 276,  https://www.doi.org/10.1016/j.jmatprotec.2019.116407 .

S. I. Moqadam, L. Madler, N. Ellendt: Microstructure Adjustment of Spherical Micro-samples for High-Throughput Analysis Using a Drop-on-Demand Droplet Generator, Materials (Basel) 2019, 12,  https://www.doi.org/10.3390/ma12223769 .

D. Beckers, N. Ellendt, U. Fritsching, V. Uhlenwinkel: Impact of process flow conditions on particle morphology in metal powder production via gas atomization, Advanced Powder Technology 2019, in print, https://www.doi.org/10.1016/j.apt.2019.10.022 .

S. Imani Moqadam, L. Mädler, N. Ellendt: A High Temperature Drop-On-Demand Droplet Generator for Metallic Melts, Micromachines 2019, 10, 477. https://doi.org/10.3390/mi10070477

R. Goodwins, S. Huhn, N. Ellendt: Creating the material world through data, one million inventions at a time, MongoDB Inc. Customer Success Story, https://www.mongodb.com/blog/post/creating-the-material-world-through-data-one-million-inventions-at-a-time , (2019)

N. Ciftci, N. Ellendt, G. Coulthard, E. Soares Barreto, L. Mädler, V. Uhlenwinkel: Novel Cooling Rate Correlations in Molten Metal Gas Atomization, Metallurgical and Materials Transactions B, 2019,  https://www.doi.org/10.1007/s11663-019-01508-0

J. Kämmler, N. Wielki, N. Guba, N. Ellendt, D. Meyer: Shot peening using spherical micro specimens generated in high-throughput processes, Materialwissenschaft und Werkstofftechnik 50 (2019), 5-13 https://www.doi.org/10.1002/mawe.201800068

N. Ellendt, A.M. Lumanglas, S.I. Moqadam, L. Mädler, A model for the drag and heat transfer of spheres in the laminar regime at high temperature differences, International Journal of Thermal Sciences 133 (2018) 98-105 https://doi.org/10.1016/j.ijthermalsci.2018.07.009

N. Ellendt, L. Mädler, High-Throughput Exploration of Evolutionary Structural Materials, HTM Journal of Heat Treatment and Materials 73(1) (2018) 3-12 https://www.doi.org/10.3139/105.110345

R. Drechsler, S. Eggersglüß, N. Ellendt, S. Huhn, L. Mädler, Exploring superior structural materials using multi-objective optimization and formal techniques, 2016 Sixth International Symposium on Embedded Computing and System Design (ISED), 2016, pp. 13-17  https://www.doi.org/10.1109/ISED.2016.7977046

L. Mädler, Is high-throughput screening for structural materials/metals possible?, International Conference on Nanomanufacturing, Bremen, 2014