Numerical simulations of the LPBF process ‒ LMTM ‐ EPFL

Numerical simulations of the Laser Powder Bed Fusion (LPBF) process are performed with a dedicated in-house finite element (FEM) code by Dr. Eric Boillat. The heat transfer is described by the evolution of two fields: the enthalpy per unit mass and the temperature.

The laser beam is modelled as a surface heat source for bulk and liquid and a volume heat source for powders. The powder bed is considered as a homogeneous medium with averaged (so-called effective) properties such as absorptivity, thermal conductivity, optical penetration depth, density.

Parameters used for the simulation — Fig. 1. Schematic of the LPBF process. P=laser power, v=laser speed, w=laser spot size, a=absorptivity, l=light penetration depth, k=effective thermal conductivity, e=emissivity, k =convection coefficient, r=density, c_p=specific heat

The mesh consists of a fixed coarse mesh on the whole domain and a refined moving mesh around the laser spot to get the needed accuracy for the computation of thermal conduction within the powder and at the interface between the powder and the bulk solid or liquid.

Meshing used for simulating the LPBF process — Meshing used in the FE simulations of the LPBF process. Five zones are visible inside the moving mesh (the zone indicated by the green border), which follows the laser scanning during simulations.

For a precise description of the LPBF process a new field is introduced. This field is called sintering potential and contains the information about the state of the material during LPBF. Its value is 0 in loose powder; 1 corresponds to a fully dense bulk material.

The effective thermal conductivity k depends on the sintering potential: k=k(Φ). During LPBF the effective thermal conductivity of a medium evolves from the value k_p of a loose powder to the value k_b corresponding to a bulk material and can be connected to the sintering potential Φ by interpolating between k_p and k_b :

Simulation results for temperature and beam energy absorption — Simulation results of the LPBF process for bronze and red gold, in optimal conditions. (A-B) temperature field in bronze and red gold (°C), respectively, (C-D) laser absorbed intensity for bronze and red gold, respectively.

Simulated temperature and energy absorption in bronze — Simulated temperature distribution (right) and absorbed power intensity in powder bed and melt pool (left) in bronze, during the LPBF process. The laser beam size (1/e²) is represented by the yellow circle. A Gaussian profile schematically indicates the beam intensity distribution corresponding to the laser beam size.

Related publications

B. Meylan, A. Masserey, E. Boillat, I. Calderon, K. Wasmer. Thermal Modelling and Experimental Validation in the Perspective of Tool Steel Laser Polishing. Applied Science, 2022, 12, 8409. https://doi.org/10.3390/app12178409.

H. Ghasemi-Tabasi, C. de Formanoir, S. Van Petegem, J. Jhabvala, S. Hocine, E. Boillat, N. Sohrabi, F. Marone, D. Grolimund, H. Van Swygenhoven, R. E. Logé, Direct observation of crack formation mechanisms with operando Laser Powder Bed Fusion X-ray imaging, Additive Manufacturing, Volume 51, 2022, 102619, ISSN 2214-8604, https://doi.org/10.1016/j.addma.2022.102619.

N. Sohrabi, J. Jhabvala, G. Ku., M. Stoica, A. Parrilli, S. Berns, E. Polatidis, S. Van Petegem, S. Hugon, A. Neels, J. F. Löffler, R. E. Logé, Characterization, mechanical properties and dimensional accuracy of a Zr-based bulk metallic glass manufactured via laser powder-bed fusion, Materials & Design, Volume 199, 2021, 109400, ISSN 0264-1275, https://doi.org/10.1016/j.matdes.2020.109400.

H. Ghasemi-Tabasi, J. Jhabvala, E. Boillat, T. Ivas, R. Drissi-Daoudi, R. E. Logé, An effective rule for translating optimal selective laser melting processing parameters from one material to another, Additive Manufacturing, Volume 36, 2020, 101496, ISSN 2214-8604, https://doi.org/10.1016/j.addma.2020.101496.

T. Polivnikova, E. Boillat, R. Glardon, Study and Modelling of the Melt Pool Dynamics during Selective Laser Sintering and Melting, EPFL PhD thesis, 2015. https://infoscience.epfl.ch/record/213654.

S. Kolossov, E. Boillat, R. Glardon, Non-linear model and finite element simulation of the selective laser sintering process, EPFL PhD thesis, 2005. https://infoscience.epfl.ch/record/33647.

S. Kolossov, E. Boillat, R. Glardon, P. Fischer, M. Locher, 3D FE simulation for temperature evolution in the selective laser sintering process, International Journal of Machine Tools and Manufacture, Volume 44, Issues 2–3, 2004, Pages 117-123, ISSN 0890-6955. https://doi.org/10.1016/j.ijmachtools.2003.10.019.

E. Boillat, Finite element methods on non-conforming grids by penalizing the matching constraint, ESAIM M2AN. 37 (2003) 357–372. https://doi.org/10.1051/m2an:2003031.