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Framebuilders Thinking about a custom frame? Lugged vs Fillet Brazed. Different Frame materials? Newvex or Pacenti Lugs? why get a custom Road, Mountain, or Track Frame? Got a question about framebuilding? Lets discuss framebuilding at it's finest.

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Old 09-22-10, 09:52 PM   #1
accordionfolder
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fillet brazing aluminum for a frame?

Just curious (and I do apologize if this is a redundant question, it's late...) if it is considered safe to fillet braze an aluminum frame?

Thanks in advance!
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Old 09-23-10, 01:47 PM   #2
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Depends what you mean. It is possible to braze aluminum, and the alloys to do it properly are not any longer generally sold or known. You can buy them relatively expensively from certain suppliers, all of whom seem to keep the flux and braze composition relatively tight to the chest. The process was used to do stuff like attach the cap and tube assembly to alloy gas tanks, during WWII. How strong it is I don't know. It was obviously very gap filling and vibe resistant. The tank seams were gas welded. Possibly they brazed the caps assemblies so the threads wouldn't need post welding remachining, as we do on BBs, etc... Even with steel there are certain brazing materials that perform quite differently, so I have no idea what the bench marks would be here.

Another question might be can I braze Al with the normal stuff bikes are brazed with, and the answer there is no, for all kinds of reasons, from incompatibility of the flux, to the fact the melting temps of those alloys is above Al.

Most commonly people are asking whether they can use that stuff that is sold on late night TV, and at fairs, and that is also discussed in 40s era PDFs of say Pop Mech. Probably one could since there is some size of fillet of maybe even chewing gum that is so massive it will stabilize a structure, but it isn't strong enough for regular looking joints.

Other options include, TIG welding, which is probably out or you wouldn't have asked about brazing, and gas welding, which is theoretically possible, and cheap, but not something I know anyone who has done it on a bike. I have a welding video, mostly shot around WWII that has picks of people welding with gas, pipes so large the guy was standing in them while they rotated.

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Old 09-23-10, 02:38 PM   #3
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Lay down the aluminum bead and then clean it up to be smooth with the usual hand tools ..
If there is a lot to be done there are thin belt air tools to use.
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Old 09-23-10, 07:39 PM   #4
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Wait.

So this is nearly impossible, or this is perfectly normal?

I'm confusled. This would be with a normal oxy acetylene set up.
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Old 09-23-10, 07:50 PM   #5
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I've wondered about doing it. I might try it sometime to see if it can take a bend test or something like that. There are all sorts of brazing rods that are formulated for aluminum, I remember it when I was looking at repairing an alu frame. If nothing else, I think those aluminum brazing rods would be good enough for repairing threads on alumnium bikes, or making new derailleur hangers. I'll try them out sometime and report back on my findings.
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Old 09-23-10, 08:08 PM   #6
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Anybody knows how the 50s Barra frames were made? Check pics here and here. No idea if they were brazed or gas welded (and for alu I don't even know what is the difference between welding and brazing). They probably were made with a torch, pretty awesome. I bet it takes GOOD skills.

Edit: Sablière also made those types of frames

Last edited by tuz; 09-23-10 at 08:27 PM.
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Old 09-27-10, 04:04 PM   #7
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If one follows the same line of reasoning for al, then brazing is joining with another metal, not the native metal, at lower temps than welding. There is the late night tv stuff that is zinc based, I believe. There is also the aero grade stuff, and there are several suppliers, but tinman tech is the one I know best. Strength would largely be a mater of being able to actually build the fillet size. The late night stuff is mostly for accumulation work like building up a surface with lumps of metal, or unloaded joining.

Last edited by NoReg; 09-27-10 at 04:09 PM.
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Old 09-27-10, 04:45 PM   #8
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I didnt know u were able to weld aluminum back in the day, that was the reason the aluminum bikes were glued or bolted all together, since when AL it can be welded??

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Old 09-27-10, 09:30 PM   #9
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You can definitely oxy-acetylene weld aluminum, it's not easy from what I've heard, and I imagine back in the day the metallurgy wasn't as strong. I'm sure that the tubes on the old aluminum bikes were much thicker than today's tubing and likely different alloys. I remember seeing a really old cast aluminum bike from the 1890s. I think the advent of TIG made it easier for production welding of thin-walled aluminum... just my random speculation on it.
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Old 09-28-10, 12:09 AM   #10
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Don't know the history of welding Al, other than that they were doing it widely in WWII. Stuff like all those now surplus canteens, etc... Aircraft parts. They were using a wide range of methods, gas, stick, carbon arc.

Here is a tinman vid on aluminum brazing

http://www.youtube.com/watch?v=RRWmpSE-hXk
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Old 09-28-10, 08:54 AM   #11
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I've done some alu brazing recently during a O/A course I took. It is freaking difficult (for me), and I believe you would want to heat treat afterwards, but it does work, and seemed really strong - we were t joining 1/16" alu plates. I think we were using harris alu brazing rods and flux.

I bet you could do a bike out of this stuff. Someone should braze up some alu joints, then braze them. If the tube breaks first.....
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Old 09-30-10, 03:32 AM   #12
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No way man, not for a structural application. Any weldable/heat treatable aluminum (short list anyway) used in a structure MUST be hardened through a solution heat treating process that raises the temp to over 900 degrees. On the other hand, if heated over 400 the hardened condition is lost.

If your next question is "can aluminum be gas welded" the answer is the same. Filler material used in gas welding (1100 series) does not respond to heat treating.

If your next question is "what about 7005"?. That material is heat treated before fabrication and the whole frame responds to a low temp age BUT no filler that works for gas welding responds to aging either or is anything but "butter soft"

If your next question is "can a lugged frame be built from aluminum and brazed" The answer is "I think so but it would be very expensive, and not by a small-timer and at the end of the day it would still be an aluminum frame but heavier and still either suffer from fatigue issues or be too stiff.

I have built more than 500 custom aluminum frames.

gas weld/braze is perfect for many aircraft/automotive applications as a properly fabricated item is so soft it's nearly plastic and unaffected by vibration. In fact, 3003H14 is formulated for extreme stretching and 5052 is engineered to work harden during fabrication.

Last edited by ftwelder; 09-30-10 at 03:36 AM. Reason: more info
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Old 09-30-10, 12:16 PM   #13
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Hey ftwelder, with your experience, could you venture in guessing how those French frames were made?
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Old 10-01-10, 10:03 PM   #14
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What about Durafix? I have no idea about this stuff, just saw on Youtube http://www.youtube.com/watch?v=jijW310xvp4
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Old 10-06-10, 02:50 AM   #15
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Quote:
Originally Posted by tuz View Post
Hey ftwelder, with your experience, could you venture in guessing how those French frames were made?
TIG was invented in 1940 but it could have done with other process as mentioned. Once a product like durafix is used to repair a frame, the cost to do a lasting repair will triple. My guess on that French frame would be TIG welded and ground down like cannondale. It could have a shape pressed on the end of the tube that resembles a lug adding enough surface to make some type of solder work, . Aluminum transfers heat very quickly and any temp over 500 will begin to soften material. Here is an example of a fully finished TIG weld on aluminum.

You certainly could use other methods for bonding. The shape of the mitre lends a lot of strength to a joint but it's just so risky when TIG welding is so effective.

IMG_3676 by frankthewelder, on Flickr

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Old 10-26-10, 04:21 AM   #16
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Quote:
Originally Posted by velosprinter View Post
What about Durafix? I have no idea about this stuff, just saw on Youtube http://www.youtube.com/watch?v=jijW310xvp4
Most of these brazing rods are of the same construction, mainly zinc and aluminium. The zinc lowers the melting-point of the rods to less than the base metal. I've used these before on beefy aluminium parts like auto transmission housings, alternator mounting-ears, etc. The joint is actually pretty strong at about 35-40kpsi, close to 6061-T6. But definitely not as strong as the more exotic 7000-series. Either way, you won't be able to get any more strength out of the joint via heat-treatment due to the filler-composition. And you may even lose a lot of strength in the tubing by heating it up too much.

You can definitely get a concave fillet like with brass-brazing. Just takes careful prep and brushing of the surface to break up the oxide layer. Then heating up the joint sufficiently hot to get the rod flowing, but not so hot you melt the underlying metal. One good trick is to lay down a layer of soot with just acetylene flame first. Then just as it disappears under oxy-acetylene flame, it's about the correct temp and you'll want to brush it quickly and flow the rod. Definitely more involved than brass brazing, but not terribly difficult.

But yeah, I prefer TIG for aluminium, gives the best results with thin tubing. Practice on beer cans first before doing bike frames.
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Old 10-26-10, 12:52 PM   #17
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Nice joint ftwelder... really looks brazed.
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