Porosity in Al Cast 1

[Salvatio]

I would really need some help here about porosity.

I've recently joined a company which manufactures brake/clutch masters for automobiles which use AL alloy and through gravity die cast. since that it is a relatively med-small type of factory, we only use fluxing and grain refinement for the melt, and the cast products have always maintain quite a high efficiency, 80%. But for the past few month, efficiency has dropped to 60-40%, and that is way too low. Most of it is cause by porosity, both at internal and on the surface of the cast.

Will there be an alternative other than de-gassing to solve this?
Could the shrinkage of the cast which causes pressure change/variation be the cause of porosity?
Should we reduce the pouring/mould temp?

The pouring temp is about 700 Deg C, and mould temp 500 Deg C. The Alloy is grade AC2B.

Thanks.

 

[arunmrao]

Have you changed the furnace charge in recent times? Also are you using any degasser before pouring the metal? Is the porosity that is observed a shrinkage or a gas porosity? Is the temperature measuring instrument recently calibrated?

Please also check the period or shift when the maximum rejection is occurring. There can be an error by the operator.

If you are close to Foseco representative, he can be helpful too.

 

[Salvatio]

We're currently using 2 sets of electric arc furnace and thermo-couples for the temp control. While changing to another furnace, the measuring intrument will be calibrated. The problem comes in two, for most, gas porosity, and also internal shrinkage.

There is a time-sampling inspection, but they usually don't get those with defects. It is only during subsequent mechining process could be found by visual inspection.

No degasser is used before pouring.

 

[arunmrao]

It is rather unusual to hear of arc furnaces being used for melting aluminium alloys. Is it an indirect type furnace, where an arc is struck between 2 electrodes and the charge below melts. In what kind of a furnace are you holding the liquid metal after melting?

Perhaps your source of problem is the melting equipment itself. Please take adequate steps to degass the metal and pour.
You could build a small furnace using LPG as fuel and understand the benefits.

Can I ask as to where your foundry is located?

 

[COKA]

You really should degass your metal before pouring. I worked for an aerospace aluminum foundry, and we used to take a sample before pouring the metal. We would do metallurgical analysis prior to casting which was always representative of the melt. For example, if the sample was too porous, we would make sure to degass the melt more. I personnaly think that degassing is the key for porosity. You should start degassing and you should as well develop a test method where you would take a sample after say each 15 minutes of degassing and compare the porosity until it becomes negligible. Trust me, you do not need to be a big company to use a degasser, and it would solve a lot of your problems!
As for the shrinkage, it is probably due to a temperature difference between your mould and the melt or between two melt flows. A problem could be with your runners and gates. If they are not positioned at a proper location, there may be a temperature and flow difference between the metal that travels farther, ie. through useless runners or gates where it becomes "cold", and the "hot" metal that meets it. I had that problem, and all I had to do was move a runner about 0.5" to avoid shrinkage.

Coka

 

[etch]

i tend to agree with the problem being gas, if defect appeared in last two months then in all likelyhood it is down to the ambient weather conditions outside. Degassing cannot be more critical to a components and i have found the through extensive tests that degassing with a flux,gives good results but the metal has to stand for 5 minutes to allow all gas to rise to surface, from then on gas will slowly be picked up throughout the day.

is suggest the following
1. ensure that the tools are kept warm and dry
2. ensure ambient conditions are warm, no doors open and no cold air coming in.
3. check gas levels prior to casting, we done this by machine, but a simple sample under vacuum will show if excess gas is present.
4. make sure returns being charged are warmed up, prior to charging into a pot in use (if done this way)