Detecting Shrinkage-Related Defects

You can use the Macro Porosity model to detect shrinkage-related defects in a cast part. When isolated liquid melt regions occur during the solidification process of a cast part, shrinkage cannot be compensated for by supplying extra melt, and pores form.

The Macro Porosity model is available in two versions:

Macro Porosity (fully coupled)
Used in a full VOF simulation that models flow, pressure, and thermal effects.

This model uses an auxiliary void phase to model the shrinkage-related cavities. The cavities can be vacuum bubbles, or they can be filled with gas that is dissolved in the liquid phase (hydrogen, for example). In either situation, you specify the appropriate void pressure. The shrinkage is compensated for through the gaseous void phase such that the liquid mass remains constant.
Macro Porosity (pure thermal)
Used with a pure thermal simulation to model only the solidification process of a liquid with temperature-dependent density. The solution of the flow equations is deactivated and only thermal effects are modeled.

This model applies only when the solidifying melt is fully enclosed by either wall boundaries or by flow-stopped cells. The shrinkage or expansion is compensated for by phase replacement in the appropriate cells to ensure that the mass is conserved.
- To model shrinkage, the liquid phase is replaced with void phase.
- To model expansion, the void phase is replaced with liquid phase.
  If no void phase is available in the liquid zone cells, expansion cannot be compensated for, and a mass conservation error can result.

Note

Simulations that use parallel processing can have minor mass distribution errors. These errors are introduced by the multiple partitions of the liquid zone: the phase replacement at each time-step is performed entirely in one partition (when the amount of melt that is replaced on the partition is sufficient to compensate for the shrinkage of the liquid zone).

Phase replacement is performed in the partition that contains the cell with the global maximum value of pore probability. If two or more partitions contain cells with the same maximum value, the cell that has the lowest volume fraction of void determines the partition in which the phase replacement is performed. If cells with the same maximum value of pore probability and the same volume fraction of void exist in two or more partitions, the partition with the lowest index is chosen for phase replacement. In these cases, the phase replacement is performed in a single partition rather than being spread evenly across all of the appropriate partitions. The melt mass is conserved perfectly in every case, but the mass distribution can be uneven in parallel simulations.

The two Macro Porosity models are alternatives; you cannot use both in the same simulation.

To set up shrinkage-related defect detection:

Prerequisite: a VOF Multiphase simulation with the appropriate models selected. An Energy model must be selected. The Macro Porosity (pure thermal) model also requires the Pure Thermal model selected.

See Modeling Casting and Macro Porosity Model Reference.

In the simulation, under the Eulerian Multiphase > Eulerian Phases node, create a gas phase for the pores/void.
The gas phase must be a single-component gas. For the Macro Porosity (fully coupled) model only, the gas must be compressible. The Macro Porosity (pure thermal) model has no constraints on the equation of state of the void phase.
Set the material properties of the gaseous void phase and ensure that the following requirements are met:
- $d ρ / d p$ must be larger than zero.
- This void phase must have a much lower density than the liquid phase.
  As the ratio of liquid to void phase density gets closer to 1, more fluid is required to be converted from liquid to void phase to achieve a certain pressure increase in a cell.
Define the macro porosity model:
1. Create a phase interaction and activate the VOF-VOF Phase Interaction model.
2. Select the VOF-VOF Phase Interaction node and set the Primary Phase to the liquid melt phase and set the Secondary Phase to the gaseous void phase.
3. Reopen the Phase Interaction Model Selection dialog and, in the Optional Models group box, select the appropriate model: Macro Porosity (fully coupled) or Macro Porosity (pure thermal).
4. To set the macro porosity properties, edit the [Phase Interaction] node:
  - For the Macro Porosity (fully coupled) model, set the Void Pressure and Scaling Factor.
    See Macro Porosity (fully coupled) Model Properties.
  - For the Macro Porosity (pure thermal) model, set the Solid Cell Indicator and Pore Probability Indicator.
    Pores can develop only in the liquid melt, not the solidified melt. The Solid Cell Indicator gives information about which cells are filled with liquid and which are filled with solid. The Pore Probability Indicator tells you how likely it is that pores form.
    
    See Macro Porosity (pure thermal) Child Nodes.

Post-Processing

The Macro Porosity (pure thermal) model keeps the mass of the primary phase constant. However, if you create a report accumulating the mass, the report shows an error in mass conservation at the end of each time-step. This error is because the mass is corrected before the first iteration of each time-step. The temperature change during the time-step can alter the density, and therefore change the phase mass within the time-step. To check the result of phase replacement directly, you need to run the simulation for one time-step with the energy solver frozen. This keeps the temperature field unchanged from the time when the phase replacement is performed until the end of the time-step when the report is evaluated.