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Die Casting News

Die Casting Book Review "Gas Porosity, A Guide to Correcting the Problems"

Posted by Hill and Griffith Company on Jul 26, 2018 4:36:31 PM

North American Die Casting Association publication by William Walkington

(We found another die casting education gem available online. This excellent 25 page publication reviews in detail the causes and possible solutions to gas porosity. To illustrate the problem, the image below is linked to a 3D X-Ray video animation done by North Star Imaging.)

Die Casting Gas Porosity 4

Table of Contents:

Identification
Flow Porosity and Gas Porosity
Pressure and Gas Porosity
Sources of Gas
Trapped Air
Venting System and Vacuum System
Runner and Gate Design
Steam and Gas From Vaporized Lubricant
Gas From Hydrogen
Conclusion
Appendix (simulations: www.diecasting.org/publications/516)

GAS POROSITY - A GUIDE TO CORRECTING THE PROBLEMS IDENTIFICATION

The first step in any defect analysis is to identify the problem. There are several kinds of porosity that can look similar, so it is absolutely critical to identify the most likely cause before starting the troubleshooting process. A wrong judgment (for example, judging porosity to be shrink when it is really gas) can result in all the corrective efforts being totally wasted. Thus, it is extremely important to examine samples of the porosity carefully (using magnification if necessary) before corrective action starts. Gas porosity is probably the most common type of porosity in die casting. The primary identification of gas porosity comes from the appearance. Identification can be quick and relatively easy in 􀃳ny (but not all) cases. Identification always requires sectioning and examination of the porosity and it often requires some magnification. Usually 8-to-10 power is sufficient for most situations - every die cast shop should have a SO power stereo microscope available for quicker and more accurate defect identifications. (fhese microscopes are relatively inexpensive.) The user does not have to be a trained metallurgist for this kind of defect identification, a little experience will suffice very well. The easiest identification case is when the porosity appears as round bubbles, shown below. Not all cases will be this easy and there will be some mistakes made, but regardless, the best effort at identification should always be done before starting corrective actions.

 Die Casting Gas Porosity 1

Figure 1: Gas porosity is round with smooth walls
(sometimes shiny) and often grouped together (but not always). It is not sharp, jagged or crack-like in appearance.

The key to the identification is a round, smooth walled shape (think bubbles). Sometimes it is shiny, sometimes not - but it is a smooth wall. Gas porosity is often rather random in location, and can move around, but sometimes it is grouped together consistently in one spot. (This is a clue as to the source of the porosity, see the following descriptions.) The identification features noted above may not always be present - for example, there can be some trapped gas in shrink porosity, which is very jagged and irregularly shaped - but for the most part, if the primary cause of the porosity is trapped gas, then the porosity will have smooth walls and the bubble shape. See the following illustration.

 Die Casting Gas Porosity 3

Figure 2: Pure gas porosity. The high concentration of gas bubbles is probably caused by too much lubricant. or from water leaking Into the die. (J. Brevlck, XSO).

We will come back to the sources of the trapped gas, but first we will discuss some other kinds of porosity that either look like gas porosity or also contains trapped gas - and where gas porosity is a factor but not necessarily the primary cause.

FLOW POROSITY AND GAS POROSITY

Flow porosity is different from gas porosity, although it is frequently closely associated with gas porosity, and is mentioned here because it will often look very similar to gas porosity. Flow porosity is generated when two metal flows come together and some space is left between the flows. 1bis happens when there is an oxide skin (for aluminum or magnesium), or a skin formed by some solidifying metal on the edge of the metal flow front. This skin may prevent a complete mixing of £lows and will function to keep the flow surfaces from mixing homogeneously. This surface, or skin, can also help bridge over openings and maintain some separation between metal flows. These areas of porosity may have smooth surfaces like the trapped gas porosity, so the surface of the flow porosity hole may look the same as the smooth surface of a gas porosity hole. However, the flow porosity will have sharp comers where the flows come together, and the fact that the hole was formed from two skins coming together will be apparent under magnification. Probably a magnification of 8-to-10 power is sufficient. See Figure 3.

 Die Casting Gas Porosity 6

Figure 3: Typical flow porosity. This is usually formed by a gap between different metal flows. (J. Brevick, X50). 

There can be an excess of gas porosity developed at the leading edge of a metal flow path
(depending on the turbulence and speed of the metal) - this gas porosity may be associated with flow porosity, and may be in the same area. This type of gas porosity will usually appear to be concentrated towards the end of the flow path, wherever that is. (In some cases, this can be in a blind or dead end shape that may not be too far from the gate.) One way to visualize it is to think about a little bit of froth at the leading edge of the metal flow.

(To continue reading the resource go to, https://www.flow3d.com/wp-content/uploads/2014/08/Gas-Porosity-A-Guide-to-Correcting-the-Problems.pdf. You can also download it here.


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