Additive manufacturing, also known as 3D printing, is a method of manufacturing parts typically from powder or wire using a layer by layer approach. Interest in metal based additive manufacturing processes has taken off in the past few years. The three-major metal additive manufacturing processes in use today are powder bed fusion (PBF), directed energy deposition (DED) and binder jetting processes. FLOW-3D provides unique simulation insights for each of these processes.
In powder bed fusion and directed energy deposition processes either a laser or an electron beam can be used as a heat source. In both cases, metal, in the form of powder for PBF and either powder or wire for DED processes, is completely melted and fused together to form parts layer by layer. In binder jetting, however, a resin acting as a binding agent is selectively deposited on metal powders to form parts layer by layer. These parts are then sintered to achieve better densification.
FLOW-3D’s free surface tracking algorithm and its multiphysics models can simulate each of these processes with high accuracy. The steps in modeling laser powder bed fusion (L-PBF) processes are discussed in detail here. A couple of proof-of-concept simulations for the DED and binder jetting processes are also shown.
Laser-powder bed fusion processes
L-PBF processes involve complex multi-physics phenomena such as fluid flow, heat transfer, surface tension, phase change and solidification that significantly influence the process and ultimately, the build quality. FLOW-3D’s physical models simulate the melt pool phenomena at the meso-scale by accounting for particle size distribution and packing fractions while simultaneously solving for the mass, momentum and energy conservation equations.
FLOW-3D’s add-on modules DEM and WELD are used to simulate the full powder bed fusion process. The various stages in a L-PBF process are powder bed laying, powder melting and solidification and subsequently, the laying of fresh powder on the previously solidified layer, and once again melting and fusing the new layer to the previous layer. FLOW-3D can be used to simulate each of these stages.
Powder bed laying process
Using the DEM module that is integrated with FLOW-3D, it is possible to simulate the powder bed laying process by dropping a randomized distribution of particles and letting them pack, as shown in this video.
One way to achieve different powder bed compactions is to choose different particle size distributions while laying the bed. As seen below, there are three different sized particle size distributions, which result in varying powder bed compactions with Case 2 giving the highest compaction.
Particle-particle interactions, fluid-particle coupling and particle-moving objects interactions can also be analyzed in detail with the DEM module. Additionally, it is also possible to specify an inter-particle force to study powder spreading applications with more accuracy.
This FLOW-3D simulation uses the discrete element method (DEM) to study powder spreading due to a counter-rotating cylindrical roller. At the beginning of the video, the powder reservoir shifts down while the build platform moves upward. Immediately after, the roller spreads the powder particles (which are colored according to their initial location) to the build platform in preparation for the next layer to be melted and built. Such simulations can provide additional insights into the preferred sizes of powder particles being transferred from reservoir to the build platform.
Powder bed melting
After laying the powder bed, laser beam process parameters can be specified in FLOW-3D WELD to perform high-fidelity melt pool simulations. Plots of temperature, velocity, solid fraction, temperature gradients and solidus velocity can be analyzed in detail.
Melt pool analysis of the powder bed under a laser power output of 200W, scan speed of 3.0m/s and a spot radius of 100μm.
Once the melt pool has solidified, the FLOW-3D pressure and temperature data can also be imported into an FEA tool such as Abaqus or MSC Nastran to analyze the stress contours and displacement profiles.
Multi-layer additive manufacturing
When the first melt layer has solidified, a second layer of particles are deposited on the solidified bed. Melt pool simulations are again performed by specifying the laser process parameters on the new layer of powder particles. This process can be repeated several times to evaluate the fusing between consecutively solidified layers, the temperature gradients within the build while also monitoring the formation of porosity or other defects.
Binder jetting simulations provide insights into the spreading and penetration of the binder in the powder bed, which are influenced by capillary forces. Process parameters and material properties directly influence the deposition and spreading process.
Directed energy deposition
FLOW-3D’s built-in particle model can be used to simulate directed energy deposition processes. By specifying the powder injection rate and the heat flux incident on the solid substrate, the solid particles can add mass, momentum and energy into the melt pool. In the following video, solid metal particles are injected into the melt pool and the subsequent solidification of the melt pool on the substrate is observed.