What's the main cause for incomplete silver brazing on stainless steels?

[rollingcloud]

I have never done any brazing. Our vendor in America is having issues brazing a 321 SS half sleeve onto a 321 SS tube. The tube OD is about .375''. They have a large fallout due to incomplete brazing. The gap tolerance between the sleeves and the tube is .001 - .014''. The sleeve ID is .190 - .200'', it's a standard part. The filler material is Silver Alloy (54Ag - 40Cu - 5.0Zn - 1.0Ni). From the photo, is the gap too large for the capillary action to move the molten filler metal through the joint?

Also, the machining upper tolerance limit of .014'' appear to be too large, I believe .005'' is ideal and .010'' should be the max? Can someone confirm?

 

[EdStainless]

Gap could be a big part of this, how the flux and braze are applied could also have an impact.
And the heating as well.

 

[rollingcloud]

Gap could be a big part of this, how the flux and braze are applied could also have an impact.
And the heating as well.

Do you mean you can still achieve a complete brazed joint if the gap is greater than 0.010'', with the proper amount of flux and the appropriate filler wire size?
The joint has no load bearing requirement; the vibration is the main concern. But I am aiming for 100% complete brazed joint edge.

 

[mfgenggear]

OP

its been a while. I got involved with intricate aircraft engine fram assemblies. but like I said its been a while.
first off contact and obtain first hand information of the required brazing fact from the manufacture of braze.

my first thoughts is yes gap is too large, but again verify with manufacture.

the assembly should be tack weld in place in a weld fixt, then brazed.

 

[EdStainless]

No, what I meant is that you can still fail even with the correct gap if you get other things wrong.
We used to make tubing that went into brazed assemblies, and these usually had special dimensional requirements.

 

[mfgenggear]

OP
TIP #1 the gap has to be at the temperature of the oven, thermal expansion of both details has to result in a .005 gap.
tip#2 there is a procedure for prep and clean, and application of braze paste. at the require oven temperature braze will flow, oven brazing in my opinion more consistent that torch brazing but both are done often.
Tip#3 it is important to get an even gap , and detail be precise as possible. xray will detect if there was good braze flow and penetration. one possible cause of lack is contaminations or improper application of braze paste.

 

[rollingcloud]

Thanks guys, I have asked the vendor to provide more details, hopefully it's just a gap issue.

 

[3DDave]

In oven braze keeping the gap small is even more important than torch as in oven braze the brazing material is put into contact with the joint and it's difficult to add more if the joint is too large. In torch one can just keep feeding, with the opposite problem - too much and it starts leaking out the other side.

 

[rollingcloud]

In oven braze keeping the gap small is even more important than torch as in oven braze the brazing material is put into contact with the joint and it's difficult to add more if the joint is too large. In torch one can just keep feeding, with the opposite problem - too much and it starts leaking out the other side.

The supplier used 140-mesh powder. Vacuum furnace was most likely used. Would a 60-mesh work better? Or it would not change the outcome given the large gap? Is powder the worst form of a filler material for large gaps?

 

[mfgenggear]

OP
method used was a paste applied with a tube. Like it is done with rtv sealant. I should have mentioned this earlier, those pictures you took look terrible. Those are a mess. Oven brazing should be very clean with capillary action sucks all paste between the details. Details need to be a close fit, very similar to soldering.
When the solder flows it is sucked in
Between copper fittings.
Those parts in the pictures are a mess.
Google images of what oven brazed parts. They are very clean. Start having conversation with other suppliers. Who pay more attention to your needs and gives good brazing.

 

[rollingcloud]

OP
method used was a paste applied with a tube. Like it is done with rtv sealant. I should have mentioned this earlier, those pictures you took look terrible. Those are a mess. Oven brazing should be very clean with capillary action sucks all paste between the details. Details need to be a close fit, very similar to soldering.
When the solder flows it is sucked in
Between copper fittings.
Those parts in the pictures are a mess.
Google images of what oven brazed parts. They are very clean. Start having conversation with other suppliers. Who pay more attention to your needs and gives good brazing.

It's actually supplier's vendor, they used to have a different brazing vendor which was doing fine for the most parts, see attached photo for a good brazing case. I am guessing they just need to match the mating parts before brazing to ensure the gap is small.

 

[TugboatEng]

The 45% silver alloys have the best gap filling capability but their maximum gap rating is still only 0.006".