The material presented in this video are some highlights from NADCA’s online webinar - Die Casting Defects - presented by Dr. Morris Murray of M. Murray & Associates. Morris discusses gas porosity and its major sources, and goes into some detail on each source.
Transcript from Die Casting Defects
There are three main sources of gas porosity: trapped air, steam, and gas from lubricant. In the 1970s, you didn't worry so much about the gas from lubricants. Nowadays, it is one of the major areas to control to reduce gas porosity.
Gas porosity is round and relatively smooth, but can be flattened to some extend due to the pressure we apply. The actions to reduce gas porosity, in general, are not the same as the actions for reducing shrink porosity. That is an important statement. How we reduce gas porosity is different than how we reduce shrinkage porosity.
Gas porosity can appear as fairly large bubbles. It can be distorted or round.
Remember gas porosity shrinks when the pressure increases at the end of a cavity fill or during intensification. It also expands during solidification. The gas pore often does not contract equally as some of the metal may be virtually solidified. If you have half of a gas pore that is starting to freeze and the other half is still liquid and as the pressure comes on, it will form a sausage shape.
You can see in this example that you have a gas pore that started off as round but as the pressure was applied it was pushed in on one side and that forms a sausage shape gas pore. This example is near the surface and it is contracted on one side. This other example shows contraction on both sides. So, as I've said it can be round, it can be a number of different shapes. You have to remember this is a dynamic process.
Any turbulence in the metal movement that allows some air bubbles to be trapped in the metal will mean that these bubbles will be there when the casting solidifies. If you have air coming in through the shot sleeve due to the runner gating system or to the die cavity flow that air will be trapped in the casting.
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