Investigation of thermodynamic and hydrodynamic processes governing laser sintering of metal powders
Final Report Abstract
In the framework of the present project a particle-scale numerical model describing heating, melting, coalescence and solidification of two powder particles has been developed. This model can be applied to powder particles of equal size or different sizes and to time-dependent heating of the particles (for example, to simulate the pulsed laser heating). The developed numerical model has been used for investigation of the influence of different governing parameters on the evolution of the particles shape, the evolution of the solid-melt interface and the coalescence dynamics. It has been found that the coalescence dynamics is governed by the time scales of melting, conduction and coalescence. If the hydrodynamic time scale of coalescence is much larger than the time scale of melting, then the coalescence dynamics of two particles is close to coalescence of two completely viscous particles. Generally, each coalescence curve (the dependence of interparticle neck size on time) in the case of constant heating consists of two parts: (i) initial coalescence of partially melted particles, in which the solid cores decelerate the coalescence process, and (ii) acceleration of coalescence at the time instant when the solids cores are so small that they don’t affect the hydrodynamics of coalescence. A macroscale numerical model of radiation-conduction heat transfer in a simulated powder bed has been developed. The localized heating is induced by moving laser beam. In this model the ray tracing method has been used for description of transport of laser radiation energy to individual particles. The ray tracing method has been also used for computation of the view factors between the particles in the powder bed with random packing geometry. The model allows describing the temperature evolution of each particle and estimating the dimensions of the melt pool at each time instant. However, at the present stage the model does not take into account the effect of the particles coalescence on heat transfer, which leads to limitation of the accuracy of the model predictions. The numerical model has been used for simulation of Selective Laser Sintering process induced by powder irradiation by a laser beam moving along a straight line with a constant scan speed. The total laser power and the scan speed have been varied in the simulations. Experiments on Selective Laser Sintering have been performed, in which the laser power and the scan speed varied in a wide range. The microscopy images of the resulting tracks have been analyzed to determine the topology of the track (continuous track, fragmentation, balling) as well as the track width for the continuous tracks. This analysis resulted in a process parameters map showing the topology as a function of the combination of governing parameters (laser power and scan speed). The topology of the tracks has been also predicted from numerical simulations on the basis of the Rayleigh-Plateau stability analysis of the melt pool. The agreement between the experimental data and the numerical predictions is good. In addition, it has been established experimentally that the track width increased with increasing of the laser power and decreased with increasing of the scan speed. The same trends have been predicted by numerical simulations. The results of numerical simulations overestimate the track width. This discrepancy can be reduced by taking into account the shrinkage of the track due to the particles coalescence. In the future, the model for description of thermodynamic and hydrodynamic processes governing laser sintering of metal powders should be further developed by combining the macroscale heat transfer model in a powder bed with the microscopic model describing the simultaneous heat transfer, phase change and coalescence of individual particles. The results of the project can be used in designing the Selective Laser Sintering process parameters for arbitrary kinds of metal powders. Therefore, the exploitation perspectives of the project results in the industry are straightforward.
Publications
- 2012 Heat transfer, phase change and coalescence of particles during selective laser sintering of metal powders, Computational Thermal Sciences, v. 4, pp. 411-423
Dayal, R., Gambaryan-Roisman, T., Abele, E.
(See online at https://dx.doi.org/10.1615/ComputThermalScien.2012005795) - Numerical investigation of coalescence of viscous particles with solid cores, Proceedings of the 9th International Conference on Nanochannels, Microchannels and Minichannels, ICNMM2013-73189, June 16-19, 2013, Sapporo, Japan
Dayal, R., Abele, E., Gambaryan-Roisman, T.
- 2014, Numerical Modelling of Processes Governing Selective Laser Sintering, PhD Thesis, TU Darmstadt
Dayal, R.
- Heat transport phenomena governing selective laser melting: Numerical investigation and experimental validation, Proceedings of DDMC 2014 – Fraunhofer Direct Digital Manufacturing Conference, March 12-13, 2014, Berlin
Dayal, R., Stoffregen, H.A., Fischer, J., Gambaryan-Roisman, T., Abele, E.
- Melting, solidification and coalescence of metallic particles during phase change invoked by laser heating, Proceedings of the 15th International Heat Transfer Conference, IHTC15-9456, August 10-15, 2014, Kyoto, Japan
Dayal, R., Gambaryan-Roisman, T.
(See online at https://dx.doi.org/10.1615/IHTC15.tpm.009456) - 2016, A novel numerical method for radiation exchange in granular medium, Heat and Mass Transfer, v. 52, pp. 2587-2591
Dayal, R., Gambaryan-Roisman, T.
(See online at https://doi.org/10.1007/s00231-015-1738-5) - 2017, Heat transfer in granular medium for application to laser sintering: A numerical study, International Journal of Thermal Sciences, v. 113, pp. 38-50
Dayal, R., Gambaryan-Roisman, T.
(See online at https://doi.org/10.1016/j.ijthermalsci.2016.11.014)
