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THERMOFLUID MODELLING OF THE POWDER BED FUSION PROCESS: A CFD-DEM APPROACH

Due to a limited understanding of the underlying physics, the components produced by Additive manufacturing suffer from quality issues such as porosity defects, delamination and lack of fusion. Powder Bed Fusion (PBF) is one of the seven categories of additive manufacturing where the thermal energy from a Laser/Electron Beam is used to fuse the metal powder particles. Some of the mechanisms involved are difficult to observe or capture using experiments due to the spatial and temporal scales. This study focuses on the development of a numerical model to predict the behaviour and flow of the material during Powder Bed Fusion process using a Laser Beam (PBF-LB).

The Discrete Element Method (DEM) is used to model the stochastic and discontinuous powder bed. In DEM, the trajectory of powder particles due to the contact forces is solved using Newton's Equations. The powder bed is generated using the Rain-Drop model, where the particles are created and allowed to free-fall until the desired layer thickness is achieved. The particle size distribution is chosen such that the volume density versus the particle size for different layer thickness agrees with the physical sample. A packing density of 57.4% was achieved for a layer thickness of 75 microns which is similar to the predictions from the literature. The size and position of each particle from the DEM model is then transferred to a background mesh in OpenFOAM.

The melting, flow, and solidification of the metal are simulated by a custom OpenFOAM solver that includes modified momentum and energy equations. The Volume of Fluid approach is used to model the interaction between the metal and gas phases. The melting and subsequent solidification processes are included using the Voller method. Buoyancy driven flow, Marangoni convection, and heat loss due to radiation and convection are also included in the solver. The developed solver is then used to simulate single line tracks and the results are validated with the experimental data. In the current study, SS 316L and Argon are chosen as the metal and gas phases respectively.

Preliminary investigation of the powder melting using OpenFOAM provides a better understanding of the mechanisms involved in PBF process. The findings from the current study can provide a process-property-structure map to the feedback system of the PBF printers, thus improving the overall quality of the finished product.