The material presented in this video are some highlights from NADCA’s online webinar - Porosity Basics: Control Porosity with Processing Parameters
Many die casting process variables affect the final porosity level in a casting. This course will discuss the effect of these different parameters on porosity. It will connect these processing parameters back to the underlying porosity formation mechanisms covered in the first porosity course. Start solving porosity by optimizing your process parameters for reduced porosity. For information on purchasing a downloadable copy of this webinar in its entirety, please visit: http://www.diecasting.org/store/detai...
From the transcription of the video, "Okay, so some of the background, we talked about yesterday as we talked. There's two general sources of porosity in our die castings. First one's solidification shrinkage. This is the volumetric shrinkage that occurs as we go from liquid to solid. There's absolutely nothing we can do to prevent that from occurring, but what we can do, as we talked about yesterday, is as the casting is solidify, feed more liquid into the cavity to make up for the reduction in volume that's going on with the castings.
And then the second source of porosity of course is gas entrapment or gases from various sources. We talked yesterday about the various sources, and we'll talk a little more about this in today's webinar.
So, going back to the volumetric contraction we talked about yesterday with aluminum alloys, somewhere between four to six percent of volumetric contraction with zinc, maybe three to four percent depending on the alloy, and then magnesium alloys is somewhere in the middle, somewhere around four and a half volumetric percent contraction.
But when we look in the literature about the level of porosity in die castings, pretty much we never see a casting with more than three percent porosity or so and certainly often less than that. So, what this is implying is that we can't discount one source of porosity over the other. We're going to get air entrapment or gas entrapment. We're going to get shrinkage porosity, and therefore when we're addressing porosity, we have to think about both of them. We can't just focus on air entrapment or gas entrapment. We can't just focus on shrinkage porosity.
And then yesterday we also discussed the two shapes of porosity. The picture on the left here at the bottom is the smooth pores and on the right the rough, jagged pores. And we talked about the physics of pore formation, that the smooth pores, the one on the left, these tend to be associated with porosity formation. When there's a lot of liquid at high liquid fractions very early in the casting process or in the solidification process, because of that, because of the way gases tend to be entrapped, therefore the smooth, spherical porosity tends to be associated with entrapment of gases.
The jagged pores, the one on the right, tend to form lower liquid fractions. At the end of the solidification when there's a large amount of solid present and only a small amount of liquid and therefore the pore has to conform to the shape of the surrounding grades. And, again, normally but not always that's associated therefore with shrinkage 'cause shrinkage tends to occur right at the end of 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."
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