casting porosity problem

[BobM3]

We have problems with an aluminum 356 permanent mold casting - porosity causes oil leakage from the gear box casting. When I assembled these gear boxes back in the 1970s I don't remember this being a problem. I think the foundry has either changed the process or the material. They claim they haven't. I don't believe the permanent mold could wear in a way that would cause these problems. Could it be the process (pouring too fast, cooling too fast, ??)?

 

[BillPSU]

Melting and pouring practices can cause your problem. The metal may not be properly degassed and also the mold could have been modified in the 30 year time span. Were the casting impregnated previously after pouring?

 

[swall]

In addition to improper degassing , porosity in aluminum castings can be caused by excessive metal temperature prior to pouring. Molten aluminum has a "kick over" point (around 1250F, I think, but don't quote me) to where the hydrogen solubility escalates. Below that temp, things can be managed with proper degassing. But, are you sure the problem is porosity and not shrinkage or hot tearing?

 

[mewhg]

Can you seal the internal areas of the housing to prevent leakage? I know its not the metallurgical answer. One product that works for this is Glyptal 1201 which was developed by GE.

 

[arunmrao]

Have you checked the venting in the die molds. If the air release paths are blocked it could result in blowholes. Radiograph the leaky casting to know if it is spongy or not. Suggestions given by others should also be looked into. For the present vaccuum impregnation will help you.

 

[israelkk]

To what spec did you request the casting?
For aerospace and military specs the specs define the quality of the casting and the testing methods for pours etc.

Look for casters that can meet aerospace and military specs.

 

[BobM3]

Great info guys! The foundry says the castings have never been impregnated. I can't say the problem isn't shrinkage or hot tearing. Is there a way to tell by looking at the bad areas? What look like holes (where the oil leaks out) look more like tiny cracks when looking under a magnifying glass. We do use a sealant from Microseal. I'll try the GE stuff. We don't have a spec on the part as far as integrity. I'll look at other casting houses to see what they offer.

 

[NickelMet]

Let me say that first of all, I've never worked in aluminum casting. But, I have experience in Ni-alloys, Fe-alloys, and steels.

If it is a degassing problem, when you look at the holes (or cracks as you say), does the internal surface look smooth or rough?

In some of the Ni-alloy castings of my career, degassing problems that were noticable as small cracks or open "worm-holes" were smooth on the inside. Whereas real cracks were rougher, like the metal is being torn apart.

Just my experience...trying to pass along the knowledge.

~NiM

 

[sbi]

All castings our porous, plus a permanet molds some time need to add gas to make the metal run properly

that is why we impregnaste and casting that will be in contact with any sort of petrolium product

we use a vacume /presure prossess that is Mil spec 276A

Bill L

SBI
Central Ne.,USA

 

[arunmrao]

sbi,
You need to change your vendor for castings supply. What is the alloy and where is the need to deliberately add gas to make it run? Please give some more details.

 

[sbi]

We pour a few 356.2 this walled castings that from a few decades ago we learned from Stall to monitor the gas content of the metal very closly, we have several of the vacume systems to help with this, once and awlile we get a bundle that is almost gas free from the supplier, this is great for sand cast , but P-mold will not pour untill we add a 2 lb potato to a 400lb molttant batch.
part of this problem left us when we bought our automatic molding machine and started to run the casting there, much better speed, helped keep cost down, good looking casting.

SBI
Central Ne.,USA

 

[swall]

The theory behind the potato is that the moisture induces hydrogen into the melt. Dendritic microporosity in aluminum castings is due to hydrogen and too much is going to give you porous castings. But, a compete absence of hydrogen microporosity can lead to surface shrinkage defects, on large, flat surfaces, like gas grill lids. I recall Stahl telling me years ago that they had to add hydrogen whenever they ran the gas grill tops, mainly in winter, when the ambient humidity was low. But, I never heard them advocate that practive across the board. They most definitely did not do it for our permanent mold master cylinder castings.