TiHabanero

I am going to give aluminum brazing a whirl and my understanding is there is no way to visually anticipate the temperature of the metals to be joined before adding filler rod. When brazing steel it is easy to see by the color of the heated joint when it is ready for filler, but aluminum doesn't give any cues as to temp of the metal. Is the only way to judge the temp of the metal is to add the filler, and if it melts there is enough heat? Overheating will be obvious by the metal melting!

unterhausen

Is this something that doesn't need any strength? I don't think there is anything on a bike where aluminum brazing is strong enough. I saw a link to a youtube video where someone did it to a bike frame, but who knows how long it will last? There is a real incentive to make youtube videos where the technique is cheap, but the thing may not work for more than a month.

If you're really doing this to a bike, you should figure out a way to practice on something you don't care about.

Andrew R Stewart 

I've watched someone do it in person (and the usual on line vids) and what I took away re: getting to the right temp was that he was "scratching" the piece surface with the "brazing" rod during the heat up. At some point the filler melted then wetted out on the Al a moment later. It seemed that the scratching was needed to cut through the oxide layer heating Al produces. Andy

spclark 

Exactly the issue. Aluminum's so reactive when exposed to Oxygen when heated it takes a patient hand that's well-experienced to know when things are going OK...why TIG welders command such a high premium. Stuff's really good too conducting the heat you're applying away from where you want it to do it's work. Not at all like steel.

You can buy aluminum 'solder' that supposedly works with propane or MAPP gas, what ARS describes is the suggested technique for use to keep the oxide layer away from the surfaces to be joined. I've tried it, it's not easy, and results I got I wouldn't trust to carry significant stress for very long without doing some practice pieces that then get destructively tested for durability. Tried wire-feed MIG too on Aluminum, gave it up after wasting half a bottle of Argon gas with little to show for it.

dsaul

What parts do you plan to braze?

There is no way that I would braze a frame together. I thought about trying it for cable guides on the aluminum frame that I built, after having some struggles with TIG welding the bottle bosses. I ended up using JB weld to hold the cable gudes on the chainstays and they are still on there after over 2 years. These are guides, not stops, since I like to use full length cable housing on my off road bikes. I'm not sure the shear strength is good enough for cable stops(although they claim 5000 lbs).

If you want to build an aluminum frame, I suggest getting an AC/DC TIG machine and learning to weld. I found that welding aluminum was easier than steel, because everything happens slower and you have more time to react to what's happening. I would like to build more aluminum frames, but the available parts in 7005 are too limiting and 7005 raw material is almost non-existant in the US.

spclark 

Be a good career choice as well (for those still young enough to have a choice) and a lot cheaper than a 4-year college degree.

TiHabanero

I'm not building a frame, just wanted to play with it to see just how strong a joint can be made. In a utube video the comment was that the joint will be just as strong as a tig joint, and I wanted to find out if that was true. Today I talked with a custom car builder that I work with and he said not to bother with it as the brazed joint doesn't rival a tig joint and that he had very poor results when he tried it. At that point he offered to give me the remaining aluminum filler rod he has in his garage!

spclark 

I wuz gonna offer you what I have laying about, thought mebbe it'd come back to bite me.

Offer's open, just PM me your USPS mailing address & it's yours.

You may be able to get a 'strong as' joint using the aluminum 'brazing' rod stuff than a proper TIG bead but you'll likely have to have a thicker, more pronounced joint seam to do it.

It's some kind of copper / aluminum alloy rod (link's to the MSDS, says aluminum, zinc, copper); TIG'd be the esteemed 'stack of dimes' done with skill and care, the brazed joint will need some post-braze smoothing to look decent.

repechage

Decades ago there was a material branded as Alumaweld. I thought it intriguing. Neighbor repaired a motorcycle engine case, at the time he noted it would keep the oil in but would not trust it for a bike frame.

spclark 

I had a friend back in the early '0's who told me he'd used JB Weld to 'repair' the head seals (basically o-rings) on a VW that lasted for years.

That's a filled epoxy formulation, may behave fine in that particular application even under extremes of heat. But I wouldn't trust it at all for 'welding' a bike frame together.


Heck there are bike frames made of wood and bamboo these days, command a nice price. Would I ride one? Not likely, they're well beyond what I can afford to pay for a leisure activity device.

TiHabanero

I should have taken pictures, but I didn't. We replaced an aluminum heat exchanger at work and I noticed hit had brazed fittings. I took it and was able to break the fittings off, by bending them back and forth a ton of times. If the fittings had been tig welded, there is no question that the alloy would have cracked at the welds. Not sure what to make of this, but I do believe I will go ahead and do some samples to see what it takes to produce a quality joint.

Again, this is not for a bike frame, but for a rear rack. Much different loads and forces it will have to support when compared to a frame.

spclark 

Good idea.

Destructive testing, when possible, in search of answers about what works and what won't, is about the only way to get comfortable with something you've never done before.

TiHabanero

Now where do I source the correct aluminum for the rack? Not sure what type of alum to use. Suggestions requested, please.

dsaul

That is another problem with home building things from aluminum, where you don't have access to heat treatment facilities. Heating the aluminum will anneal it, making it softer and more flexible. 6061 needs heat treatment to return it to the original hardness. 7005 can achiveve most of its original hardness after about a month of aging at room temperature, so it is usually the choice for home and small batch builders. The problem with that is almost no US suppliers carry 7005 tubing or plate.

You can still build it from 6061 and go with a larger tube and wall thickness to account for the lost strength/stiffness after heating it. I've built several non-bicycle structures from 6061 and have not seen any isssues from not heat treating them after welding.

spclark 

https://www.thefabricator.com/thewel...ation-for-6061

985°F isn't something most home-builders can achieve w/o significant investment in proper facilities or access to a commercial processor.

If you read that linked article you'll learn there's another path to hardness at temps achievable in most residential ovens. An hour at 400°F isn't any more expensive than roasting a 14# turkey for a holiday dinner. Time + temperature are the two important factors, alongside the technique used to do the joining. "Brazing" aluminum with the relatively low-temp rods like those I've given links to in previous posts may not hold up well if subjected to treatments given to welded joints; the alloys are different, so the chemistry's different.

From this one can see 6061 is offered in a variety of shapes for many of the purposes most fabricators might need. 7075's included among the various products at that link, but for a price well above that of same-size stock in 6061 alloy. Look around, there are other outfits than can supply 7075 if that's what you think will serve your intended purpose best.

vintage cellar

If you really want to know how to braze Aluminum properly you need to ignore just about everything about welding on Youtube, etc. and get yourself over to the Tinman Tech website.

He has the proper lenses to actually see whats happening to the base metal, along with some videos, flux, and filler.

His stuff is not cheap. He's a small operation, but I doubt he is getting rich on any of his stuff. Small quantity development and fabrication / selling of most "niche" stuff is just usually (unfortunately for hobbyists) expensive.
Apologies for my run-on sentences.

If you ever see actual vintage brazed aluminum frames from the pre-tig days (think Sabliere and Barra), you aren't seeing standard 6061 - they were using some sort of different alloy back then that preceeded 7005 by decades.
Maybe some of those old frames held up pretty well for a smooth-pedalling and careful rider. I kinda doubt it.

Brazing aluminum is supposedly quite tricky. Sometimes you hear people say something like "why they taught many thousands of people (many women) to braze aluminum in WW2" in order to make it seem more "do-able".
I always laugh at that kind of rose-colored glasses quasi-truth. Same people saying that stuff don't mention who exactly paid for all the considerable learning / large number of scrap pieces / and training that went into THAT effort, LOL.

Good luck and keep us posted if you go down this particular welding/brazing rabbit-hole.

TiHabanero

vintage celler, there is a wealth of info on that site. Thank you for the tip!

Got to thinking that the pedestrian aluminum rear racks one can buy off the shelf at a bike shop may have the legs brazed and not tiged. Makes sense when I look at the joints and the way some of them look. All I need is flux, filler and aluminum rods for the structure. This might prove interesting.

spclark 

That there is! I particularly enjoyed reading this essay for details about how different techniques will affect the final outcome.