Brazing or Soldering Aluminum to Stainless Steel

[moya034]

A friend of mine has a project he's working on where some aluminum has to get joined to stainless steel. I read on the AWS website that this is possible.

I did a bit of google searching, but had a hard time finding what techniques, fluxes, and filler metals I need for the job.

I'd prefer brazing over soldering, but I'm interested in either.

Any help would greatly be appreciated :D

[usmcpop]

Just some food for thought:

http://www.aws.org/wj/supplement/may99/ROULIN.pdf

Here's one way rocket scientists do it :D

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19680013309_1968013309.pdf

[moya034]

Here's one way rocket scientists do it :D
That NASA method should work perfectly... all you need is a fully stocked laboratory and a degree in chemistry :D

[toolaholic]

Myoa , Swing by My cave I'll let You have My pop rivet gun :p

[Aerometalworker]

Guys,
Brazing aluminum to steel or stainless ( not soldering ) has been done for almost 90 years....I do it a few times a year to put steel filler necks on aluminum fuel tanks. Its nothing new.......but its not something commonly done.
-Aaron

[moya034]

I do it a few times a year to put steel filler necks on aluminum fuel tanks.
What fluxes and filler metal do I need to use for that operation?

[Aerometalworker]

What fluxes and filler metal do I need to use for that operation?

If it was that easy....everyone would do it :)
Seriously though, its a bit of a technical problem that involves much more then a flux and filler. Heres a hint......Silicon....and Silver....Think on that for a while and let me know what you come up with.
-Aaron

[moya034]

Heres a hint......Silicon....and Silver....Think on that for a while and let me know what you come up with.
Hmm... let me guess... the stainless steel is "tinned" with a silver brazing alloy, and then the aluminum is brazed to that using an aluminum-silicon alloy?

If my research is correct, the aluminum-silicon alloy has a lower melting point then silver based brazing alloy.

[Aerometalworker]

Hmm... let me guess... the stainless steel is "tinned" with a silver brazing alloy, and then the aluminum is brazed to that using an aluminum-silicon alloy?

If my research is correct, the aluminum-silicon alloy has a lower melting point then silver based brazing alloy.

Ding Ding Ding,
Yep you got it. Now the tricky part is keeping everything clean between opperations. Sometimes I pickle the silvered part before brazing.
Use a good aluminum BRAZING flux and a 5% silicon brazing filler, be careful of overheating the silver.
-Aaron

[moya034]

Awesomeness!!!

I already have silver brazing alloy and flux lying around, so as soon as I get some of the aluminum-silicon alloy and related flux I will give this a shot and post the results.

What do I use for the pickle solution?

[obewan]

You really need to be careful about the CTE stresses if it is destined for any kind of high temp environment.

We had an al-ss electrical connector for an aircraft turbine that turned into a real nightmare.

First, we went with friction welding. We had good tensile results, but in service and during subsequent TIG welding on an adjacent area, the CTE stresses between the two dissimilar alloys shattered the friction welds. Our yields were sometimes as low as 25%.

Next, we looked at brazing. My research found the same NASA report posted in this thread, but the titanium layer was not feasible. I found a few reports on brazing and/or soldering straight up, but most warned that brittle intermetallic compounds are possible. We went to every aluminum aerospace brazing shop in the Thomas Register, and they all no-quoted our application and told us to explosion weld it.

Next, we went to explosion welding, and had better results than the friction weld process, but yields were still only 85% at best, which killed us because the part is around $350 pop and runs in large numbers.

Now we have a redesign with a threaded connector with a solder seal. That we expect to work, but in the future, we plan to avoid at all cost tube weld joints between aluminum and stainless.

[Aerometalworker]

You really need to be careful about the CTE stresses if it is destined for any kind of high temp environment.

We had an al-ss electrical connector for an aircraft turbine that turned into a real nightmare.

First, we went with friction welding. We had good tensile results, but in service and during subsequent TIG welding on an adjacent area, the CTE stresses between the two dissimilar alloys shattered the friction welds. Our yields were sometimes as low as 25%.

Next, we looked at brazing. My research found the same NASA report posted in this thread, but the titanium layer was not feasible. I found a few reports on brazing and/or soldering straight up, but most warned that brittle intermetallic compounds are possible. We went to every aluminum aerospace brazing shop in the Thomas Register, and they all no-quoted our application and told us to explosion weld it.

Next, we went to explosion welding, and had better results than the friction weld process, but yields were still only 85% at best, which killed us because the part is around $350 pop and runs in large numbers.

Now we have a redesign with a threaded connector with a solder seal. That we expect to work, but in the future, we plan to avoid at all cost tube weld joints between aluminum and stainless.

Good Point,
Most every brazed part I see runs at normal ambient temperature, and avoids differences in thermal expansion. Most turbine parts I have seen have gone away from bonding to mechanical attachment. Also one always wants to be aware of corrosion issues due to the galvanic properties of the 2 metals now electically bonded. Like I said, 99% of the dissimmiliar metal brazing I do is for things like fuel tanks, induction manifolds and other "cool" running parts that dont see much of a temperature gradient durring opperation.
-Aaron

[wldsmke]

git up on it boys!:D

[B_C]

I have a HARD time soldering a wire....:D