Brazing a structural component - thinkable?

[trainguy]

Hi all.

We are looking to reinforce a stainless steel stressed skin structure. The stainless steel used is 301LN 1/2 Hd. Someone suggested exploring the addition of a brazed stainless steel flat bar, because it may be easier than welding. Existing skin is spot-welded to stiffeners, but seems too thin with insufficient spot welds.

Naturally, I have many questions.

Static shear strength of brazed joint
Fatigue strength (or any data)
Any long-term corrosion issues?

Any references would help.

tg

 

[chicopee]

Brazing of common steel plates with copper alloy rods is done all the time and you would see that in auto body shops. Brazed joints of copper alloy object with silver solder is also a regular practice. With stainless steel, you may want to check compatibility issues with brazing rods. As far as I am concerned brazed joints are very strong, however, I am not familiar with the practice that you are suggesting and perhaps other responders are. Try it on a couple of specimen, document the procedure used and the stress calculations, and test the specimen by an independent lab before going full production; inform your clients of what you have done to improve your product.

 

[Duwe6]

try one of the BAg-7 fillers. The rules are in the back half of ASME Sect IX.

 

[tbuelna]

Brazing a stiffening bar to a sheet metal panel surface would seem like a good application for brazing. Lots of surface area for the joint. The common silver based brazing alloys used with 300 series cres would have a tensile strength of 30ksi or more. The main things to consider are the temperatures (>1200degF) used for the brazing process and fixturing needed to position the parts during brazing. As for corrosion issues, here is a link that discusses the problem.

 

[EdStainless]

I'll second the BAg-7.
You will need to check and see what degree of stress relief you get on the 301LN after the 1400F braze cycle.

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P.E. Metallurgy, Plymouth Tube

 

[Tmoose]

OP said - "but seems too thin with insufficient spot welds."

I'm unclear what is "too thin" and why it "seems" that way.
Poor product performance, or visual based concerns with weld quality or the stiffener itself, or problems with manufacturing?


Are you leaning toward brazing the existing stiffener, or brazing an entirely new style of stiffener ?

Are you making the panels in-house, or buying them? Regardless, if this panel creation is a significant part of the product cost I'm thinking spot welding is likely to be the most economical method of attaching ribs or stiffeners, by a long shot.

Perhaps it is a matter of re-designing the stiffeners a bit, and adding a few strategically spaced spot welds.

Spot welding is good enough for Porsche much of the time. (and other OEMs as well)

https://www.youtube.com/watch?v=des-xY65nMs

 

[racookpe1978]

What are the implications of a structural fire on the joint? Would the loss of strength as the braze heats up up (melts out ?) drop the structure on the fire fighters/rest of building?

 

[tbuelna]

trainguy-

There was quite a bit of basic research conducted by the US aerospace industry regarding furnace brazing of lightweight structures using high-temp metals (like stainless steel) back in the 1960's. So you should be able to find plenty of technical references on the NTRS and DTIC websites. Here's one fairly recent paper on analyzing brazed joints.

 

[EdStainless]

There are a lot of panels made with PH stainless honeycomb internal and 3xx stainless skins. These are all brazed with self fluxing alloys (since these use B you cannot braze to high N grades like 301LN).

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P.E. Metallurgy, Plymouth Tube