Troubleshooting Aluminum Brazing: Tips and Resources

[Shawnzyoo]

Not sure if this should be here or in materials...

So I am attempting to braze 2 flat pieces of aluminum
I apply a copious amount of flux
I believe I am heating teh metals thoroughly
my braze is not flowing into any of the joints ever
i have tried numerous times
once I even ended up melting the aluminum piece
so far all that I have really burned is my pride and my hand
I have searched the internet for advice and a few sections from books
any suggestions or places to go for help?
thank you
--Shawn

 

[willib]

Not sure if this should be here or in materials...

So I am attempting to braze 2 flat pieces of aluminum
I apply a copious amount of flux
I believe I am heating teh metals thoroughly
my braze is not flowing into any of the joints ever
i have tried numerous times
once I even ended up melting the aluminum piece
so far all that I have really burned is my pride and my hand
I have searched the internet for advice and a few sections from books
any suggestions or places to go for help?
thank you
--Shawn

Shawn what type of braze ar you using..?? If its copper based , it isn't going to work..
i believe with aluminum you must use mig or tig welding..
http://www.lincolnelectric.com/knowledge/articles/content/alum.asp

i guess you can braze aluminum..but as the pdf states aluminum is harder to braze because the melting temp. of the filler is very close to the meltin point of the base metal ..and you have to know what kind of Aluminum you are trying to braze..
http://www.physics.purdue.edu/cmsfpix/mechanics/panel_support/materials/Aluminum-Brazing-Alloys-and-Fillers.pdf

 

[Shawnzyoo]

I am using an Aluminum and Silicon brazing wire
that says it is supposed to work with all Aluminum Alloys
melting point of 1070F
i keep thinking i am not getting it hot enough...
or that temperature range is too small for my oh so inaccurate propane torch
i have been researching welders and had planned on going and getting one in the next week, to speed things up, and make it more accurate
--Shawn

 

[willib]

Try cleaning both surfaces really well..

 

[willib]

the problem with brazing aluminium is oxidation from the propane torch..

 

[Danger]

Try cleaning both surfaces really well..

Although it's difficult to realize, aluminum oxides almost immediately upon contact with air. Pure aluminum looks almost like chrome. For proper bonding, you should sand or grind down the surface until it's shiny and then immediatly apply the flux.
Another problem with aluminum is that it's a terrfic thermal conductor. Trying to heat the metal enough for it to subsequently melt the solder or braze material is almost impossible. The stuff just 'wicks' the heat away.

 

[brewnog]

I'd agree with everything which has been said.

It's always hard joining aluminium, and I would expect brazing to be even harder than welding, because it's dependent on achieving good wetting of the filler material, and as Danger said, it's rather hard to get a perfect surface finish because an oxide layer forms extremely quickly, and the bulk of the parent metal will just soak up all the heat you put in. Have you tried an oxy-acetylane torch rather than just a propane one?

 

[Shawnzyoo]

if I sand and/or grind down the aluminum
doesn't the rough surface finish prohibit the braze from flowing?
by using a propane torch is this accelerating the oxidation?

also, I have not tried an oxy-acetylene torch since I do not have one...
but I am getting a MIG welder real soon, which has me quite excited

 

[faust9]

Don't sand the surface. Get a stainless Steel brush http://www.sjdiscounttools.com/kdt2309.html and scrube the surface. The SS brush will clean the joint without leaving silicate particles embedded into the aluminum. Wash the freshly scrubbed surface with distilled H2O (DI would be a little better) and weld the joint as soon as humanly possible after drying the joint.

The problem with welding Al is the formation of aluminum oxide which has a melting temperature of around 3500 degrees F---much much greater than the melting point of Al alloys. You need to completely remove the Al2O3 before attempting a weld otherwise your weld will fail ether during the welding process(as you have found) or later on due to a weak mechanical joint. It can't be stressed enough how cleanliness is next to godliness when welding Al. Do not touch the freshly cleaned surfaces with your bare hands BTW. Wear Latex gloves or fresh cotton gloves if possible to avoid joint contamination---at least until you get the area fluxed.

When you do the weld you mush move quickly because the heat from welding accelerates Al2O3 formation. Also, make sure the Al alloy you are attempting to weld is weldable. Some alloys are not.

[edit]You can never 'completely' remove all Al2O3. Remove as much as humanly possible though.

 

[brewnog]

faust, what about brazing?

 

[faust9]

faust, what about brazing?

The same process holds for brazing and welding. Clean, clean, clean. Brazing isn't any more difficult than tig welding with one little hitch---when you braze Al you have to move quickly. You can't overheat the joint unless your intent is to produce a weak brittle weld. You have to move as quickly as possible while ensuring proper filler flow. A contact pyrometer on the backside of the joint may help BTW if you have one.

 

[Danger]

Thanks for the additional info, faust. Lot's there that I didn't know.