Galling of Stainless with Aluminum?

[wchowe]

Hi All,
I had a failure Yesterday, and becasue it's a new product I don't have any history and would like to get some concensus on whether or not this is a problem.
I have a large aluminum casting (356 T77) which has a tapped hole. Into the hole a stainless steel (316) pressure switch is mounted. As I said, I had a failure, one of the guys removed the switch to replace it. When he unthreaded it, most of the aluminum threads came with it. The unit had been assembled for two weeks.
Although these two materials are not suitable for contact becasue of galvanic corrosion, they were only in (dry)contact for 2 weeks. I doubt that was a contributor. Galling? I would suspect that more if the two metals were the same grade of stainless. Besides, there was no automated or high speed assembly to build heat. I am thinking that its either a poor tap in the aluminum, weak aluminum, or excessive torque?
I am assuming that the switch is passivated. The casting is not anodized. Anodizing would be helpful with the corrosion, but due to the size I doubt I can do it. I can't put anything on the threads that would leak into the casting (lubricants, etc). The casting is under vacuum.
Is there anything I am missing here? Should I just dismiss this as an overtorqued condition?
Thanks
Bill

 

[israelkk]

To my experience galling between stainless steels and aluminum is a fact. We had similar probelm with 15-5PH and aluminum.

 

[CoryPad]

It could be low torsional resistance of the internal threads (due to low shear strength or defects like porosity or cracks), it could be poor geometry (insufficient axial or radial thread engagement, off-axis threads), or it could be binding/galling. I think any of these are possibilities, and I wouldn't exclude galling for any reason.

If you learn that galling is a fundamental problem, can you use thread masking (e.g. fluoropolymer tape)? Since this is a solid, it would avoid liquid contamination.

Regards,

Cory

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

 

[wchowe]

I couldn't find anything online to support Stainless and Aluminum Galling. In fact, I found one study that supported the opposite. Do you have a good websites that discusses galling of aluminum and stainless?
Thanks
Bill

 

[CoryPad]

Threaded fastener galling is one of those problems that is not documented that well. I can't think of a published source for this specific combination.

The variables involved are numerous and can have large ranges. While the CalTech document is a good data point, you can't be sure of the properties of their testing. Which alloys, any coatings or surface treatments, what surface finish, what installation procedure (e.g. angular velocity)?

Use your data to move you in the right direction. If you identify that galling is the problem, research what surface treatment(s)/coating(s)/lubricant(s) you can use. In addition to fluoropolymer tape, there are Dry Film Lubricants (DFL) that could work for you.

Regards,

Cory

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

 

[vc66]

Any particular reason you can't use stainless steel Heli-coil inserts?

V

 

[unclesyd]

I believe that you will have to used an anti-seize to resolve your problem. 356 Al can be tough to get good threads in.

Look down at the zinc based anti-seize for Al

http://www.jetlube.com/NewFiles/aseaz.html

This one is used around the boat yard

http://www.sealingdevices.com/loctite/c5a.html

Here is another one that has no Cu, Ni or Zn in it.
A little expensive but very good.

http://www.superior-industries.com/...uct_231.html?gclid=CLO93N_hv5MCFRUdsgodlBS3Dg

Here is Teflon based product that is used around boat. It pretty much does what is says.

http://www.sailorssolutions.com/index.asp?page=ProductDetails&Item=TG01

You can also check any large auto supply store, especially speed shops for an anti-seize compatible with Al. You normally find these as spark plug lubricants. Make sure it says OK for Aluminum.

 

[unclesyd]

Forgot one trick we used for fasteners in Al, especially 356. In a case such as yours we would not go for tightening on the first pass. We would installed a lubricated fastener to the point of hand tight. If we saw any measurable resistance that couldn't be overcome by hand+ we would back off and look at the threads on both parts. We might chase either or both threads and repeat the tightening process. We essentially didn't go to full tight until we had free running threads.

 

[pierdesign]

Yes, a casting, or other machined component, with threaded fittings under vacuum implies some sort of instrument or application for analytical purpose. Just about all thread lubricants or coatings are not recommended due to leaching into and causing contamination to other components of the vacuum based assemblies' performance, or even breaking vacuum.

All vacuum chamber applications I have seen and worked on employ helicoil inserts, as first mentioned above. Also, all aluminum components were nickel plated.