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Gas defects in an Al diecasting

2002

Hi there,

Gas porosity is a common problem in Al diecastings. It may be caused by Hydrogen pick-up from the atmosphere, air entrapment during injection, die and plunger coating/ lubricant, water vapour, etc. I am working on a water pump body which has gas porosity problem: 1) Is there any way to tell them apart based on the colour, surface characteristics or composition, morphology, position in the casting, etc. 2) How can I make a small impregnation unit and what type of resins should I use?

Any comments or references is highly appreciated.

Regards,

Behzad Niroumand
IUT - Isfahan, Iran

My opinion on gas porosity,

Gas may be solvable by working on the process settings or the design of the gating system. Shrinkage porosity can not be changed without changing the design of the part or by finding a way to keep more pressure on the casting to feed the shrink during solidification. Finding a way to keep more pressure on the casting during solidification may also make the gar porosity smaller and less noticeable, but it won't changes the amount of gas. The term gas or trapped gas means any kind of gas found in the casting sometimes other castings disciplines will use the term trapped gas to mean only hydrogen gas and not mean include any type of gas.

The usual types of trapped gas in a casting will be air entrained during the metal handling or in the injection process, moisture or steam from the air heating or the burning of the libricants. Gas porosity voids tend to be spherical, although occasionally some get squeezed almost flat or tear shaped in high pressure phase and have a smooth and maybe shiny inside surfaces. Most of the gas porosity bubbles do have a dark color so the color alone in not sure indication of the sorce of the bubble. However if the bubble with an area where there is lubricant burning and mixing with the aluminum is the most likely sorces of this from the plunger lube.

John Kinart
Citation Lake Zuirch - Lake Zurich, Illinios
2003

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July 1, 2008

I'm encountering a aluminium die-casting panel gas problem right now. As John said the gas porosity bubbles could be trapped inside the solid and so we tried to do some "make up" on the bubbles and then polish the surface back to smooth as much as possible. It is also because I found this bubbles occurs mostly insides the solid other than on the surfaces.

Unfortunately, we have to use CNC drill down into the solid for the design purpose. At that time, the bubbles can be seen easily as I stated last paragraph that the bubbles mostly occur insides the solid. More importantly, we can not apply the "make up" and polishing process on that surfaces because those area is not easy to reach by the worker's hands.

May I know is there any good suggestion on those improvement? As John said that it is not feasible to change the amount of gas and it seems the bubbles will randomly occur.

Victor Leung
audio manufacturing - Hong Kong