Lawrence Livermore National Laboratory

The powder scale model uses the input laser beam characteristics to transform a particle bed through the dynamics of the molten state into solidified material. The model is initialized with powder particles of the desired size distribution and bed thickness, often on a uniform substrate or on a previously processed layer. The combined thermal and hydrodynamic simulations model the appropriate distribution of the laser's energy as it interacts with the powder particles, the substrate, and the melt pool. The deposited energy from the laser heats the powder above the melting point, where the model coalesces the particles into a melt pool that flows under the influence of surface tension and vaporization recoil. The powder model tracks the various modes of heat loss, including conduction and evaporation, until the melted material solidifies onto the existing substrate. The contributions of the powder model to the overall additive manufacturing modeling effort come in four areas: laser interaction with the powder bed, powder response, melt pool characterization, and the build quality metrics of surface finish and final part density.

The total energy absorbed from the laser is an important integrated quantity that must be provided by the powder model. Another important quantity, the net energy deposited into the part, accounting for losses including evaporation and thermal radiation, will also come from the powder model simulation results. The net deposited energy plays an important role in the part scale model simulations, particularly as the details of individual layers are abstracted away for computational efficiency. The powder model has the capability to include effects of the laser beam geometry, including spot size and shape and various option for the distribution of power within the beam, such as Gaussian, top hat, or "donut". Through modeling of various arrangements of individual powder particles, the powder scale simulations are used to determine their integrated effects. The particle size distribution is used by the powder model to initialize the geometry, thereby affecting a number of model outputs, including the powder bed packing density and the effective thermal conductivity of the unconsolidated powder bed. The thickness of the powder layer has significant effects on the part quality, including the obtainable density and surface roughness, which can be investigated with a powder scale model. The powder model includes the formation, evolution, and eventual solidification of the melt pool. Single-track (Yadroitsev et al.: 2010) parameters such as width, height, and depth can be compared with experimental data for validation of the model. The powder model can determine the uniformity of the track/bead, which is useful for creating maps of optimal process parameters. As part of the hydrothermal calculations of the melt pool motion and solidification, the powder model can generate temperature-time history data for use in models of microstructure evolution.

The role of the powder model also includes build-quality measures such as surface roughness and obtainable density. Multi-track simulations will give the solidified shape of many overlapping or overlaying melt tracks, giving the roughness of top, bottom, and side-facing surfaces of the part. These simulations can also be used to study the formation of voids in the final part structure. The powder model may be used to investigate mitigation strategies to improve these quantities by such techniques as laser power or speed variations.

Modeling approach

Modeling approach:

  • Mesoscale model of discrete powder particles in an ALE framework
  • Laser beam energy deposition on particles and melt pool
  • Covers time scales on the order of fractions of a second and length scales of a few millimeters

Contact: Andy Anderson

Click for Powder Model inputs/outputs

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Watch a technical presentation on the powder model by Saad Khairallah at the National Academy of Sciences Workshop on Predictive Theoretical and Computational Approaches for Additive Manufacturing that was held Wednesday, October 7 and Thursday, October 8, 2015.