The material presented in this video are some highlights from NADCA’s online webinar - Porosity Basics: Introduction to the Causes of Porosity.
To successfully manage and reduce porosity, one must first understand what causes porosity. This course will cover the physical causes of porosity. In particular, the sources shrink porosity formation and gas porosity will be explained. The course will conclude with traditional concepts for reducing porosity formation.
For information on purchasing a downloadable copy of this webinar in its entirety, please visit: diecasting.org/store/detail.aspx?id=WEB057
Transcript from the video highlight, "As we've been saying, there are really two sources for porosity in die casting. Solidification shrinkage, shrinkage when we go from a liquid to a solid for all the commonly cast die casting metals. Aluminum, magnesium, zinc. The liquid takes up more space than the solid, so when we get this shrinkage, when we go from liquid to solid, we're gonna get a hole somewhere. And if we don't do something about it, then we'll get large holes in our casting.
That's solidification shrinkage. And then gas, as we talked about, gas can come from different sources. We can have air in the cavity, we can have lubes in the cavity, maybe get water in the cavity, maybe we get hydraulic oil dripping in the cavity. There's all sorts of different origins, and we'll talk a little bit about that as we get later on into today's presentation.
But, often we'll see a difference in the shape of the pores. So on the left, at the bottom, we're seeing a very smooth surface on a spherical pore. And often what people say is, if it's a smooth pore like the one on the left, it's a gas pore. And then often we see jagged pores such as the one shown on the right, at the bottom right. And people often say if it's a jagged pore, that's a shrinkage pore. Well, in some ways that kind of makes sense. But, as we'll see, basically things are not quite that simple.
With shrinkage, we get this volumetric contraction in the metal as it solidifies. And often shrinkage occurs towards the end of solidification, and if the grains have a jagged shape and if we don't have enough material to fill the remaining holes between the jagged grains, then we expect to see the jagged surface of the grains in a shrinkage pore.
And gas ... For example, if we leave out a glass of water or if we have some kind of fizzy drink, the bubbles are forming in that water or in that drink and often those bubbles are spherical. And therefore, it makes sense to call a spherical hole a gas pore.
But as I said, this is not quite accurate. And actually, what happens is that smooth pores such as the one shown on the bottom left actually tend to form very early during the solidification process while the jagged pores shown on the right tend to form late during the solidification process.
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."
Die Casting Porosity Causes and Solutions
Technical Article for Die Casting Engineer Magazine by Tim Cowel, Technical Director, Hill and Griffith Company
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