Bonding Al to Al, to braze or to epoxy weld is the question.

[Anonymous]

I have most of the supplies i need to start the framed backpack project.  I’m trying to figure out the best way to secure the tubes to each other.  The frame will be made up of Easton 7075 Al tent poles with their 90 degree elbows that fit into the straight tubes (and not as snug as i thought they would).

I’m looking at epoxy, or using Al alloy brazing.  Either way, i know i have to prepare the surface being bonded well, by cleaning very well, abrading, and acid etching some since these are hard anodized which lowers the surface energy of the substrate and makes for a poor bond.

I’m not sure what is better, epoxy or brazing?   I’m thinking of possibly doing a combo, epoxy for the inner and then brazing on the outer.   Normally when brazing, you want to heat up the material more than the brazing rod to get a nice clean application, but since i will have to do the epoxy first, i will have to heat the brazing rod directly to apply to the cold Al tube as to not damage the epoxy bond–it probably won’t look as clean, but as long as it’s functional i’m not too concerned.

Anyways, what is a good epoxy to use (i’m seeing lots of different and at times conflicting info online about this)?  And what’s a good brazing brand/material to use?

Thank you much for any help

[Erik G]

I would go with epoxy.

Brazing could be pretty tough since the elbows fit loosely on the tubes – it can be a real challenge to fill large gaps by brazing. We design very small gaps for tube-and-socket braze joints, like less than .010 inches for a 5/8 OD tube for example. Filling the gaps would be quite easy with epoxy. I think both methods will be more than strong enough. Personally, I think brazing would be cooler though (for no good reason I can think of).

As for which epoxy to use – no clue there.

Good luck on your pack, I’m looking forward to seeing some pics!

[Anonymous]

Hi Erik,

Thank you for the feedback

[James Marco]

Well, I do not think I would braze/solder it. Any heat will remove the temper from your tubing making it rather soft in these areas.

3M dp270 works pretty well. Make sure to clean all the parts, scuffing them. Then wipe down with acetone. It as OK fill characteristics and can be used to back-fill any loose areas. Apply a bit and gently heat it with a hair dryer. It flows easily at first, then the heat will set it.

 

 

 

[Anonymous]

Hi James, would that still happen if you don’t heat the tube at all, but just the brazing rod till it melts?  I thought this might get by that issue (i know this is contrary to the usual method)?

Thank you for the epoxy suggestion–i will look into it.  Unfortunately, since these tubes are hard anodized, i have to do a lot more than just clean and scruff to bond them well–i’ll have to get through the thick oxide layer through a combo of sanding and acid etching.

[DancingBear]

The heat from the globs of molten brazing rod would still be enough to ruin the temper of the tubing – plus it wouldn’t give a good bond.

[Anonymous]

Ok, thank you Dancing Bear for the feedback.  I’m pretty new to the concept of bonding aluminum via brazing.  A lot of the aluminum specific brazing stuff melts at around 700F vs aluminum at 1200F and so i thought it might not get the Al hot enough to mess with the temper (especially since it would cool rapidly on the cool and very conductive Al), but if you’re speaking from experience on this, that’s enough for me.

[Andy Stow]

James and DancingBear are right. If you don’t get the aluminum hot enough, braze or solder blobs will just fall off. If you do get it hot enough, you’ll destroy the temper on the tubes. Annealed 7075 is not what you want for a frame. Use good epoxy.

[Anonymous]

Any suggestions for a good epoxy for this purpose Andrew?

[Andy Stow]

JB Weld will probably work well, in all seriousness. Or PC-7. Follow the instructions for JB weld… clean with degreaser (not alcohol) and sand surfaces.

[Anonymous]

This fellow’s testing doesn’t quite hold up to rigorous scientific testing, but it perhaps helps one to discriminate epoxies a bit:

His limited testing suggests that Loctite Metal epoxy is a good epoxy.  I wish there was more testing done like this comparing different products, but with better protocol.

[James Marco]

Yeah, as was said, soldering/brazing rely on getting both metals hot enough to allow the rod to actually penetrate the first few molecular layers of the tubing forming the bond. It it doesn’t flow into the join, pretty much on it’s own, then it will not stick. Anything that hot will ruin the tempered aluminum. I suppose it could be heat treated after, but this can be tricky process in a home oven. Often, you disturb the molecular balance beyond repair with soldering/brazing, though. Some metals will boil off (aluminum being one) and destroy the tightly controlled alloy composition that makes tempering possible.

Good epoxy is your best bet. Yes, the LocTite was mentioned as being good, but insure you get the one designated for aluminum. Some epoxies will not bond to it very well.

Anodizing, some molecular reduction of the surface, can often be bonded over. They use it to bond parts together with glue. It is OK to leave it on. Just make sure to clean it with a clean paper towel and acetone. You might have to sand it if it was waxed or varnished. Try a couple lengths to see.

[Roger Caffin]

Heat will destroy the 7000-series temper. Any flame would be too much.

I have used Loctite Black Max on my pack frames built out of Easton tubing. It works very well. See 1st pic at
http://bushwalkingnsw.org.au/clubsites/FAQ/DIY_RNCPacks.htm
for an example.

Now, how you curve the tubing – that’s black magic. That tube will snap.

Cheers

[Anonymous]

Thank you Andrew, James, and Roger.

Roger, i plan to put play sand in the tube and very gently bend it.  It won’t need much curve, as dyneema type cord will be strung vertically and horizontally across the frame, helping to keep it off my back some naturally.

[Clifford Deakyne]

Justin,

Hysol 9394 is an Al filled rt curing epoxy that adhers well to Al.  Be sure the metal is clean and well abraded. sandblasting works best îf available.

Cliff

[Anonymous]

Thank you for the tip Clifford.  That’s one of the ones that came up in recent research so far–nice to have some confirmation.

[Bill Giles]

If you take a look at how the old military ALICE frames are put together, you will find that there is another option, rivets.

[Eric Blumensaadt]

JB WELD epoxy is very strong. (And very gray. ;o)

[Anonymous]

Thank you Bill and Eric for the suggestions.  I have some stuff picked out.  I’m a little less worried than i was before, about the method and exact product i’m going to use, because i may also use FiberFix on the joints for extra reinforcing.  I’m just waiting on the company that markets same, to get back to me on whether or not FiberFix uses carbon fiber at all, because if it does have some in it, it may corrode the Al poles/tubes i’m using.

 

[Roger Caffin]

whether or not FiberFix uses carbon fiber at all, because if it does have some in it, it may corrode the Al poles/tubes i’m using.

Um – corrode the anodised aluminium surface??? Can you give any references for that? It seems a bit odd, to me.

Cheers

 

[Sam Farrington]

Justin,
I tried a lot of epoxies, but could still pull the tubes apart by rotation in a vice.
Then one of the regulars here suggested the JB Weld, sold at Walmart. It worked very well, producing a joint that I could not loosen in a vice.

For the bending, use a fine sand from Lowes or Home Depot. Pack it repeatedly and tightly in the tube with plastic caps to seal the ends, available at most hardware stores, or Goodwinds, a kite site.
I bend the tube over a kitty litter bucket, near the base, which is large radius soft plastic.
It is a workout, and only tried it in warm weather.
A tube bender will crack the tube (unless you use 6061-T6 from Online Metals with a lower temper. Warmed the tubes for 24 hours to room temp even with that.
Wearing gloves helps, sitting with the bucket between my feet, and moving the tube a little with each downward press. If careful, you can get an even bend that will not have weak spots. Roger built a device to get tighter bends – that is why he is a little smug, hah-hah.
But he helped me a lot with some bends that needed to be tighter.
Good luck.

[James Marco]

Roger, I thought that was fairly well known. Since the Carbon is electrically conductive, it can and will corrode most metals, Al being commonly known.

Check this out for a quick overview:

https://www.corrosionpedia.com/2/1556/corrosion/galvanic-corrosion-of-metals-connected-to-carbon-fiber-reinforced-polymers

Note for that used in epoxies, usually just some fibers, you can go ahead and use it because it is electrically insulated by the epoxy.

Also of note is aluminum will corrode almost instantly when removed from the cleaning bath or from sanding. It reacts with O2 in the air as quick as you sand it off. A smooth metalic surface shows less corrosion than a sanded area. In any case, it is always present unless you use an inert atmosphere. This makes epoxies not stick well to aluminum. Try this, us some thinned epoxy on some sand paper. sand the piece with this while the epoxy is still wet. The slury will be embedded into the matrix and not bother the joint. And it keeps the corrosion from happening before you can place it in the joint.

 

[Anonymous]

Roger, it may not corrode the anodized part, but it will corrode the areas i treated for the epoxy.  Since i would want the FiberFix to bond well with it as well, i also treated areas for that application (the FiberFix).  Hence why i’m concerned about potential carbon fiber–but maybe James is right, the epoxy may help to keep it insulated from that interaction/reaction.

[Anonymous]

Thank you Sam, very good information there!   Perhaps the sand i was using, wasn’t fine enough.  Also, while i had packed it fairly well the first time before bending, i didn’t bother to re-pack it down the next day when i tried bending it some more.

[Anonymous]

James, thank you for the suggestions.  I was hoping that the quick dunk in a mildy acidic solution would slow down that reaction.  I’m not well versed in chemistry, but i thought i remember reading that mildly acidic conditions tend to slow down oxidation reactions, which is why things like citric acid, ascorbic acid, etc tend to be used a lot as non toxic preservatives in various foods.  I was hoping similar might apply to the Al by letting the solution dry on same.

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