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New Model for Predicting Micro-Porosity

Microporosity in a diesel engine part
Figure 1: Micro-porosity in
an A380 diesel engine part.
Courtesy Albany-Chicago

Simulation of microporosity in a diesel engine
Figure 2: FLOW-3D correctly
predicted the increase in
micro-porosity in this section
of an A380 diesel engine part.
Courtesy Albany-Chicago.

This article highlights a new modeling capability included in FLOW-3D, Version 9.0.

Cast metal parts are sometimes unusable because they have internal gas pockets, or bubbles, which develop when the metal shrinks during solidification. The term typically used to describe such bubbles or voids is "porosity."

When these bubbles are relatively large and localized the porosity is called macro-porosity. Prediction of macro-porosity in the interior of cast parts is a capability of most software packages currently used for the modeling of metal casting processes. FLOW-3D is no exception.

Another type of porosity, characterized by a more uniform distribution of small bubbles with a total average volume fraction on the order of one percent, is referred to as micro-porosity. This type of porosity is also caused by metal shrinkage during solidification, but its character is different from macro-porosity because it develops at a later stage in the solidification process. This distinction in types of porosity is important because each type requires a different modeling approach, one not present in any current software package.

In this note, Flow Science announces the introduction of a new model that has been implemented in FLOW-3D for predicting the occurrence of micro-porosity. The model is simple, requires only basic material property data, and adds virtually no noticeable CPU time to a solidification simulation. Best of all, the model is complimentary to FLOW-3D’s existing macro-porosity model and may be used in conjunction with either a complete hydrodynamic shrinkage simulation that includes fluid flow, or with a simpler heat-transfer and shrinkage simulation having no fluid flow.

The new micro-porosity model is founded upon the fact that, after metal has cooled enough for its solid fraction to exceed the point of rigidity, there can be no (or very little) additional liquid flow to compensate for shrinkage. In aluminum A356, for instance, the solid fraction for rigidity is about 63% see Solidification Characteristics of Aluminum Alloys, Vol.3: Dendrite Coherency, Arnberg and Bäckerud, American Foundrymen’s Society, Inc., 1996). This means that there is still 37% of the metal remaining to be solidified. In developing the new micro-porosity model, we assumed it is this last stage of solidification that accounts for micro-porosity.

The model is passive, meaning it does not affect other models in the program. Very little data is needed to use the model, but the computed results do depend on the physical data specified for the metal. Most important are the metal densities at the liquidus and solidus temperatures and the critical solid fraction (solid fraction for rigidity). This data controls the maximum amount of micro-porosity that can form.

The model has been tested against three experimental data sets, and gave good qualitative distributions and very reasonable quantitative results. In addition, one of Flow Science’s beta testers in the high-pressure die-casting community has used the model on a production part, with very good results.

Alex Reikher of Albany-Chicago contributed the images shown on this page. He tells us the part his company was producing had to be re-gated because of problems with the existing die. Albany-Chicago turned the part 180 degrees in an effort to produce a better quality casting (which, overall, did occur), but certain sections ended up with more porosity and less tensile strength (though the resulting mechanical properties were still within the required range). Figure 1 shows a comparison for one section in the part, while Figure 2 shows comparisons for another section.