The material presented in this video are some highlights from NADCA’s online webinar - Porosity in Die Casting.
Four types of diecasting gas porosity problem characterizations:
1) Round pores - Precipitation in the liquid melt or in the beginning of solidification, unrestricted bubble growth, high H2 contraction required
2) Long, broad pores - Bubble formation with still high liquid factor, arrangement between growing bubbles and dendrites, high to medium H2 concentration
3) Long, fissured pores - Bubble formation during formation of the dendrite network, bubble expansion limited by still open melt channels, medium to low H2 concentration
4) Small, fissured pores - Precipitation shortly before the end of solidification, shape and size of pores determined by closed interdentritic spaces, low H2 concentration
Learn more about diecasting porosity from this earlier post, "Gas Porosity And Surface Die Casting Defects"
Transcription from the video highlight,
"Here's again a little bit of background here, to reinforce the idea that the shape of the pore depends upon where we are in the solidification process. Again, that kind of helps you think through about how to minimize porosity in the casting. This is data actually from an AFS, American Foundry Society, paper back in 1994. What they were looking at is the formation of gas porosity in aluminum alloy. All these pores that they were talking about here were actually formed from hydrogen. They looked at four different mechanisms here. They talked about round pores, long, broad pores, long fissured pores, and small fissured pores. The one on the left are occurring very early in the solidification process. The one on the right, very, very late in the solidification process. But all of these pores were coming from hydrogen.
Let's just go through this quickly. On the left it says round pores. You can see there's a little bit of solid. This is kind of a diagrammatic representation of the solid. Here are the pores. Pores are a long way from the solid, so then their shape is not really affected by the solid. The liquid, again, is not putting any constraints on the shape of the porosity. The bubble wants to minimize its surface area, surface energy, so it forms a sphere. Hydrogen porosity very early in the process, the solidification process, forms spherical pores.
Now, at number two we're a little bit later in the process here, now the solid has started to have a constraint on the shape of the porosity. The pore has to form kind of between the grains, and so we start to get these long, broad pores. Maybe there's 50% solid formed in here. Again, still gas porosity, but it's starting to form more irregular shape.
Now, in step three, we're quite late in the solidification process, maybe 90% solid. You can see in this case now the pores have to form between the areas where there is still liquid, and so again, they'd like to be spherical, but they're constrained by the grains that are surrounding it, and so now we get these long, fissured pores.
Then finally, right at the end of the solidification process, if any hydrogen comes out of solution at that point, we get these small fissured pores. It's basically hardly any liquid left at all, so we kind of get these triple point type pores. Again, it's all gas porosity. Sometimes it's jagged, sometimes it's spherical, but it's basically when it's occurring during the solidification process, and that's what's controlling the shape of the pores.
To purchase access to this course, visit the NADCA marketplace, at www.diecasting.org/store. Purchasing a course through the online education system grants you one year of access to the recording, presentation, support material, and test, when applicable."
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