Aluminum alloy modifier to reduce leaks on permanent mold castings

[customcastings]

Hi there.

Would anybody in this forum know of a alloy modifier that will modify aluminum alloys (in our case, 356) so that the castings will have less chances to leak under pressure? We are developing a casting that needs to resist up to 70 psi pressures.

emilio

 

[etch]

Well there are three ways to modify the metal,


one, degass thouroughly
two, modify with suitable grain refiner. Ti-B. this will give smaler grains that should help pressure tightness
three, modify with sodium or strontium, this can be hit or miss., but cn have spectacular results.

 

[greybert]

Palmer Foundry in Palmer, MA does leak proof Al castings for the semiconductor industry. We are in the process of switching casting over to them to eliminate a void issue on Al 356. Check with them. 413 283-2976, Bob logan owner.

Bob Pajak

 

[BillPSU]

I am aware of Stahl Specialty in Kingsville, MO who produce master brake cylinders which resist substantially greater pressures than 70 psi. You might give them a call. They have produced their own standard for the reduced pressure test.

 

[GaryBoone]

I would agree with the previous post, you can improve the quality (leaker reduction) by the additions of a grain refiner (Al-Ti-B) and a modifier (Sr-Al). I suggest contacting AFS for information on suppliers.

In addition to the above, there may be other issues related to melt cleanliness, i.e., inclusions and excessive hydrogen gas. Again, AFS should be able to help. Here is a link to their site:

http://www.afsinc.org/Directory.htm

 

[unclesyd]

If the temperature isn't too high they are vacuum impregnation/infiltration processes that will make your casting pressure tight. It was used for years on transmission housings and others. It was used big time in military applications. This is used to avoid big changes or licenses for process.

Here is one website among many

http://www.magnatechmfg.com/mt_vacuum.asp

 

[customcastings]

We are trying to reduce the leakers by degasing the metal and adding Tibor, but I am not sure about treating the alloy with a modifier (Strontium). I heard that by adding a modifier to the bath, the resulting alloy will have a structure more suitable to the formation of paths which will lead to leaks. Would this information be correct?
The results with the current runs will be out soon, and I will post them right away.

Emilio

 

[GaryBoone]

Additions of a strontium modifier will improve feeding during solidification, thus minimizing shrinkage porosity. If you don't get the results you need with the Tibor grain refiner alone, you may want to consider a trial with both Tibor and Strontium. If so, I would suggest a strontium addition of 0.015 wt.% Sr.

You may call me at 610-693-5822 for discussion if you wish.
Gary Boone

 

[customcastings]

Well, the results of our castings were pretty much the same, with the addition of strontium to the alloy. However we did notice a considerable improvement when TiBor was added to the alloy. The degasing didnt do much either, surprinsingly.

Emilio

 

[CoryPad]

Can you quantify any of this? For example, did the degassing actually decrease the casting's gas content? Did strontium modification allow better fluidity (any test results)?

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[customcastings]

We separated the batches by
- degassed castings
- Tibor grain refined
- Strontium moodified
and a combination of degassed+tibor refined

Our scrap rate after pressure test was about 10% in all cases, except for that without the grain refiner, which was out worst case scenario (0ver 20%).

With regards to fluidity, we didnt notice that much of a diference on this specific die with which we ran the experiment.

Emilio

 

[etch]

if your still unhappy with results i would try sodium unstead of strontium, i have never recieved good results with strontium and found sodium gives better results.

Strontium seems good for turning macro shrinkage into micro shrinkage, whereas sodium can have effect of eliminating it completely.

 

[trytrue]

What process do you use? sand or permanent mold?
What type of porosity you have in the leaks? microshrink, gas porosity, Al oxides, inclusions?
Here you have some facts about refiner and modifier (AFS has several technical informations about these facts).
Ti-B is an excellent grain refiner (give smaller DAS secondary dendritic arm spacing) helps to reduce shrink and microshrink. Improve mechanical properties and soundness.
Al-Sr (better than Na) is a modifier of silicon (Si). Al-Sr modified Si from large plates acicular particles to fine fibrous Si particles (improve mechanical properties). But it increases the size of the microshrink and gas porosity, so be careful to add Al-Sr. Al-Sr has not effect in fluidity.
The RPT (reduce pressure test) specific gravity needs to be equal or more than 2.59 to eliminate gas porosity due to hydrogen.
In permanent mold when you have 12 mm or less of thickness you will reduce dramatically the microshrink.
If you want I can help to determine the anomaly type that you have in your leakers (free of charge). Just let me know and I will send you my address.
I used to work in gravity permanent mold making master cylinder (60,000 pieces per week) and we reached 300 ppm rejections (included leakers and other anomalies).