Brazing Aluminum

Where do you stand on one of the eternal questions of metalwork: brazing, or welding? As your Hackaday writer, and the daughter of a blacksmith, it’s very much on the welding side here. Brazed joints can come apart too easily, which is why in the territory this is being written in at least, they are not permitted for the yearly vehicle roadworthiness test. If you’ve ever had to remove a brazed-on patch with an angle grinder, you’ll know which one you’d trust in a crisis.

What if the metal in question is aluminum? [George Graves] sends us a link to a forum discussion on the subject from a few years ago, and to a YouTube video which we’ve embedded below the break. Miracle brazing rods claim astounding toughness, but the world divides into those who favour TIG’s strength versus those who point to brazing’s penetration far between the surfaces of the metal to be joined. Having experimented with them a while back, we’ll admit that it’s true that aluminum brazing rods join broken parts impressively well. But yet again you won’t see this Hackaday writer riding a bike that wasn’t welded with the trusty TIG torch.

Take a look at the video, and see what you think. Even if it’s not a joint you’d stake your life on it’s still a technique that’s a useful addition to your workshop arsenal.

Whichever side you stand on brazing, please feel free to duke it out in the comments. Meanwhile there remains the important question of whether this al-oo-min-um stuff they’re talking about is as good as al-you-minnie-um.

73 thoughts on “Brazing Aluminum

  1. while welding is normally a much better way to ensure strength, Brazing is handy I gave it a go and it worked for me as I don’t own a welder. I found it too produce joints that were strong enough for my purposes. I would not recommend it for things where structural integrity is important for example a go-kart chassis but things like furniture it would be great.

    1. I have to disagree, before TIG/GTAW (or Heli-Arc for the old school) was readily avalible and we’ll characterized it was very common to see brazed race car frames and roll cages. It has been shown that a mild steel joint if properly brazed with Aluminium Bronze or Silicon Bronze that the mild steel tubing will fail before the braze joint. Also to this day Hooker Headers still brazes their high end headers because there is less residual stress in the joint and this lowers the possibility of stress cracking.

      Brazed joint require a little more thought than welded joint and possibly more skill to pull off but when done right you can trust your life to them.

      1. FWIW steel bike frames are brazed in order to not de-temper the tubing; and you can put together silicon bronze boat fittings easily, but you have to be sure of a good fit of the parts first.

        I’ve used the aluminum brazing rods for years and they’re absolutely wonderful for small fittings & repair.

          1. @Stripeytype The part about lugged and brazed frames being much lighter than welded is misinformation and bollocks.

            The reason lugged frames exist is due to the process by which they were historically built nothing to do with lightness.

            Prior to being built using torches they were hearth brazed. The frames were held into alignment using lugs and pins then heated in a hearth until a whole section of the frame was cherry red. then brazing rod was jammed into the lugs and pulled in using capillary action. The lugs held the tubes in place during this process.

            The metallurgical games you reference DO have a basis in fact as you don’t want a thin metal tube to get into a temperature where it changes phase to avoid embrittling the joints. HOWEVER this has been somewhat mitigated by the move from mang-moly (531 etc.) to chromoly tubing.
            but again, nothing to do with lightness.

            @thinkerer, modern steel bike frames are mostly not brazed. Bicycle tubing is butted (thinner wall thicknesses in the centre) and the only parts that are brazed are in the very thin sections of the tubing (bottle cage mounts, cable guides etc.) I’d like to add “in the olden days bike frames WERE …” to your comment

            Source: I’m a framebuilder.

      2. Yeah I mean I am not skilled by any means, So I can imagine if I really knew what I was doing it would be a lot stronger. The joints I was getting were pretty strong but nothing that could take a sharp impact and I wouldn’t trust my life too them. I was talking from a try things out for the sake of doing so DIYer. The joints do look nicer than welds I have too say, I might give it another go and see if I can make some stronger joints. It is all about practice and technique after all.

      3. A brazed joint is more vibration resistant, which is why old british motorcycles used to have a lugged/brazed construction to the frames.
        The chassis on my drag bike (funnybike, although it races in a class called comp bike over here) is brazed (bronze welded) construction, with a 1428cc twincharged engine with (not quite) “enough” power, its a pretty severe test, but this isnt the first incarnation of the chassis either, Ive got timeslips from a previous owner in the 70’s from the 90’s and it saw action before that. Ive seen some new age funnybikes of late with bronze welded chassis too, so its not a age thing, it must have still have some clear advantages to the builder.
        I do tig weld aluminium though.

        1. I have only ever brazed Aluminium and I think that is why I said about the structural integrity as a DIYer I wasn’t making the best joints and Aluminium isn’t particularly strong for a metal. I am going to give it another go as many people on here have been saying they can be strong joints.

          1. With brazing, surface area is you friend and distances between the host materials is you enemy. The filler itself is always going to be weaker that the host material so if you fill a large gap between two host materials then the filler will break first under stress. As strength is measured in PSI it relates to the surface area of the braze. Even though the filler is weaker you can have enough of surface area filled between the hosts to equal the host strength in the joint so that under stress the host will break first.

  2. A serious concern with both welding and brazing aluminum is how it affects the heat treating of the material. Welding of course wrecks the heat treat throughout the heat affected zone but brazing is also above the annealing temperature, which means that the base metal will lose temper around the joint unless the whole thing is re heat-treated afterwards.

  3. And then there is folks like me who have dc tigged ally using flux (not solder flux lol flux for gas welding ally) and stick alu rods(flux coated) its messy but done clean enough produces a strong weld on a poverty budget. I must say tho when well prepped some of the aluminium brasing rods are very impressive.

    1. If you are referring to true DC Heli-Arc, I applaud you as it is becoming a lost art. As welding machines have become more advanced with AC balance, HF start, and built in pulsers (which remove the skill of operating the pedal), DC GTAW is only seen on aluminium in very specific instances where you are looking for maximum penetration in thick sections.

      On a similar note gone are the days of calling the local welding shop for custom gas mixes. There was a time that many shops could make “special” mixes for maximum penetration or shielding/cleaning or whatever you needed. THESE days all they want to do is sell what’s in the lot because Praxair, BOC Gases, or Air Liquide actually owns them and unless you want 500 cylinders it’s not worth it to them.

  4. I’ve used them before – if all you have is a propane torch, they’re not bad.

    The downside is that they’re mostly zinc, I believe. If you braze a joint, then later decide to weld it instead, make sure to grind off all the brazing first, because you absolutely don’t want to weld zinc (metal fume fever).

    1. I don’t think about the zinc in my other posts…because of the low melting point it is very easy to vaporize the zinc. As you mentioned causing metal fume fever, or Monday night fever as I know it.

      When I first tried this stuff I tried to GTAW over it…even with grinding, good luck cleaning it up enough to lay a proper weld over the mess it leaves behind. It’s like trying to braze over a solder joint, just isn’t gonna happen.

  5. Brazing and welding, properly done, are both plenty strong for almost every (certainly every hobbyist use) application you could use them in. A brazed joint with the right rod has a minimum strength of 40k psi which is over the strength of structural steel. Sure in a chrome-moly roll cage you might want that extra 10k psi that welding may give you, but I think the heat exposure of the process plays more of a role than the method of joining. In either case, if you don’t have a proper fit before the braze/weld it won’t matter which method you choose.

    Another process worth mentioning is MIG aluminum wire. There’s a youtube build log out there of a guy [SV Seeker] welding a whole boat superstructure from aluminum box tube using MIG with shielding gas, outside.

          1. I looked this up when more awake, and realized that there are a number of steels with a yield strength under 70K PSI. I have been using the assumption that 70K PSI was the minimum for quite a while.

          2. A36 had a minimum yield of 36k psi, but I suspect most of them are more towards the 70k psi range you bring up. Which is also the upper range of a good braze.

          3. You can get steel rated this high but most applications use A992 with a yield strength (Fy) of 50ksi and ultimate (Fu) of 65ksi. Almost all connections are made with plate steel which is made with A36 Fy 36ksi and Fu 58ksi. Structural steel is typically welded or bolted so IDK off the top of my head any specs for brazing. Generally the weld strength is 70ksi but derated based on the angle of force applied. Weld thickness and total length of welding is adjusted to accomplish the desired strength. It isn’t as simple as this A36 is weaker than the 70ksi weld, you have to remember that these strengths are all based on cross-sectional area.

            Here is a page from a cursory search about some brazed joint design: http://www.lucasmilhaupt.com/en-US/brazingfundamentals/jointdesign/

  6. I’ve used this stuff before…it was disappointing. Also it is not brazing it is still soldering, brazing by definition occurs at over 450c, most of these filler alloys (zinc and aluminum) melt around 390c. While close it is just fancy solder.

    I can say from experience that if you have multiple pieces to aluminum solder together, you are hosed becuase the entire part will hit the solder’s melting very quickly thanks to aluminum’s exellent thermal properties. So what happens is that as you get things hot enough your entire assembly falls apart; been there done that.

    If you don’t have access to equipment for welding aluminium, save your self the frustration and stick with mechanical fasteners…in aerospace if it’s not welded it’s riveted.

    All that said a proper braze in other materials with higher melting points is nothing to scoff at…in the shop I braze almost everything unless welding is necessary. In my experience brazing when possible prevents warping in thinner sections and lower internal stresses lowering stress cracking. Also you can’t deny the convenience of jointing different metal with a high-silver (>30%) fillers.

    Tl;dr
    Aluminium brazing = marketing BS, it is solder.

    1. By your own admission the “distinction” between brazing and soldering is so trivial it’s completely unnecessary, though. You could call it *all* soldering and not have any ambiguities.

        1. The French have a limited, set in stone, vocabulary of about a quarter or less the size of English and are highly resistant to adopting words, so I guess they have to abuse the ones they’ve got.

      1. The temperature range may be trivial but the way they work could not be more different.
        Soldering is purely a mechanical joining of materials, like glue. Brazing melts the two materials, not in the manner of welding, but by forming an alloy of the filler rod and the base material.

          1. Yes & No.
            “It is sometimes found, particularly with butt joints
            stressed in tension, that joints with extremely narrow
            joint gaps have a tensile strength greatly exceeding
            the U.T.S. of the brazing alloy in the as-cast condition.
            This effect used to be attributed largely to the change
            in the composition of the brazing alloy, caused by
            alloying with the brazed components, it being held that
            the effect of inter-alloying would be more pronounced
            in joints with narrow joint gaps, owing to the relatively
            higher concentration of the parent metals or their
            constituents dissolved in the brazing alloy”
            — p9, second column, http://www.jm-metaljoining.com/french/pdfs-uploaded/Design%20and%20Strength.pdf

            Sloboda goes on to cite Bredzs (1954) that joint geometry also plays a role; that a tight joint can prevent the braze filler from deforming plastically only failing in a brittle failure. The alloy isn’t formed by melting but by the liquid braze filler dissolving the host metal.

        1. No, soldering alloys the metals involved as well. Neither brazing nor soldering melt the host materials, but you don’t need to reach melting temperature for them to dissolve. Do sugar or salt need to be raised to their melting temperatures to dissolve in water? Of course not. Same mechanism is involved here.

          Relative strengths of the different joints are *purely* due to the mechanical properties of the materials used, not because of a process difference.

    2. There are high-Al content rods for actual brazing on the market, it’s just that the closer you get to the melting temperature of the parent metal, the harder it is to use a torch…

  7. Seems like a useful option and getting clean geometry with aluminium is much easier than other metals if you are working with hand tools. But what is that rod alloyed with and what stops it migrating into the aluminium of the tube thereby softening it?

    1. These are typically Zinc/Aluminium alloys, and because of the low melting point (so low that it is actually soldering not brazing) there are very low migration worries. However since most of this is done with a propane torch there is a huge heat affected zone which severely anneals the aluminum turning it into “bubblegum” as a machinist may describe it. But to be fair welding does the same thing just in a more controlled area; when something must meet engineering calcs you would typically make the welds and then have the assembly heat treaded to regain it’s initial hardness and strength; trying this with aluminium solder would result in the assembly melting down I the oven and the heat treater would never take another job from you.

    2. I’m not sure if this answers your question, but in brazing, the whole point is that the filler rod dissolves some of the host metal forming an alloy of the rod and original metal. So a brazed joint is some mix of whatever filler rod you use and the surrounding metal.
      If you play your cards right and the stars are aligned in your favor you can get 70k psi out of a braze, well more than the steel around it will hold.

      1. This happens in normal soldering also, perhaps to a lesser depth. So in German we call “weichlöten” and “hartlöten” which translates to “soft soldering” and “hard soldering”, the difference is the used temperature range.

  8. I just tacked several 0.020″ 316 stainless steel sheets together with a cadmium-free brazing alloy and used my spot welder tips as a heat source.

    It is insanely easy!

    Put some flux on, lay in a section of solid brazing rod or sheet, and then apply the welding tips to either side of the length you want brazed.

    It lets me join much thicker sheets than my hacked-together MOT spot welder will handle, and doesn’t require the clamping pressure that a good spot weld needs.

    I will definitely use this in the future– it’s good for stitch welding (or in this case brazing) two pieces together and it’s cheap as sin.

    If anybody has questions about this, reply here. I’m very happy with the results and want to share. It’s a cheap and quick way to get industrial tacking from junky, homemade tools.

    1. Also if a bunch of parts are prepped for welding, you probably don’t want to change mind at last min and braze… because of any bevels or loose tolerances left to allow easier penetration.

  9. Does anyone know if the aluminum brazing/soldering rods discussed here can be used to join an aluminum part to a copper one? I’m interested in terms of creating a low-resistance electrical bond between these two types of materials as opposed to concerns about structural integrity.

    1. I do believe that there are fillers for aluminium to copper and aluminum to steel. They are typically used in HVAC applications. BUT be warned you ate actually soldering so the joint needs to be isolated from vibrations for best results.

      1. Harris makes an aluminum solder kit that can be used with a normal soldering iron. It will solder iron to aluminum which is often used I’m LiPo battery packs. The kit comes with the special solder and some flux. Works pretty well.

    2. Electrical solder will stick to the braze material, so if you create a spot of braze on the aluminum you can then solder your wire to it. You can also sand the brazed area to clean it up before soldering.

      The biggest difficulty in all this is that it takes a lot of heat; you need a good torch!

  10. I have used aluminum brazing to modify a V8 intake manifold and the mod has held for the best part of 20 years. I needed a very big propane torch to raise the temperature of the parts enough to braze. I now use AC TIG to weld aluminum as the equipment has become significantly cheaper.

  11. One data point WRT welding vs brazing in automotive applications: the Rolls-Royce monocoque is brazed. Welding occurs at higher temperatures, which leads to more thermal expansion, which leads to more problems with internal stress in panel structures.

    SIDE NOTE TO THE HACK-A-DAY WRITERS: At your next meeting, would you all please line up and FIFO-slap whoever keeps saying, “push those polarizing statements.. flamewars generate comments!” Manipulation only works when the marks can’t see it coming, and you haven’t been executing well. The “X is the future,” “let’s reignite the Mac-vs-PC wars,” and “One True Foo” structures have become predictable.

    Trust the subject matter. Hacks are interesting, and have been published successfully under various names — techniques, tips, tricks, kinks, wrinkles, and receipts — for more than a hundred years.

    1. I completely agree, open discussion is great but click bait titles hurt us all.

      Do a little research into the process of which you are writing about and present an honest article. Googling the soldering vs brazing spells this whole thing out clearly without all the misinformation of the manufacturerer’s BS.

    2. I won’t give you much hacker credit for the ‘FIFO-slap’.

      What you need to do is grab them by the ears and bang their head on the floor in Morse Code to get the message across !!!

      </sarcasm>

      But more seriously, you can’t ask google questions and while it may be plain to you, it may not be obvious to others. Even the definition of what brazing is changes depending on what country is under your feet.

      In the video you can see the packet of whatever rods have different languages and just as ℉ changes to ℃, the term brazing rod changes to soldering rod.

      *This* process is called brazing, soldering or cold welding. If you youtube for copper braising you will also find some videos of copper soldering and in some they are using the normal copper/silver filler and in others they’re using tin/lead (50/50%) filler.

      Some of the other points for the average hacker is the gas used. In this case aluminium (aluminum) brazing rods are a low enough temperature to use cheap disposable gas cylinders and the MAP Pro (MAPP replacement) is probably just slightly better than the Propane as long as the heat sink your brazing is not too large.

      Using propane for copper brazing will be quite a frustration and you will need around 25% silver for it to even work. MAP Pro is only slightly better as it has a better heat transfer so it faster to heat even though it’s roughly the same temp as Propane.

      I use a 4.5kg bottle of LPG or LNG or BBQ gas, filler rods that are lower temp and a lot of patience. I bought lots of attachments for it for the fame end.

      *NOTE: When using a BBQ gas cylinder without a regulator fitted directly to the gas cylinder you MUST use gas hoses that are rated for the full non-regulated pressure or they will explode.

  12. I have some of those rods but I have never tried them. Watching this video may get me motivated to give it a shot. I also have the spool gun for my mig, and I have yet to try that. I thought I would be playing with more aluminum but as it turns out I play with a lot more sheet metal.

    1. These rods produce a really weak joint. Get a bottle of argon and get your spool gun going if you want to weld aluminum for real. It does take practice and buy a bunch of spare tips for the gun. One tip, if the wire burns back to the tip and welds to the end, just toss the tip. Do not try to clean it out and reuse it. It just does not work. Once the molten aluminum wets the tip its all over with.

    1. *sigh* iron oxide(s), not just iron and aluminum/aluminium. Chemistry bro!…

      The oxygen in the rust reacts with the Al. I’ve seen thermite being used to repair a railroad, I’ve never actually used it nor want to. Too dangerous without serious safety protocols in place. Also just about the worst thing to watch without proper eyeware…

      1. Also even with the proper mix igniting thermite is hard, very hard. You will not do it with a normal torch without an intermediate such as magnesium (thin material like a ribbon – solid magnesium is hard to ignite too).

        1. It’s probably even harder in small quantities, loses too much heat to the work or surroundings to be able to sustain reaction.

          Come to think of it, for demo in chem lab they used a half grapefruit sized crucible full… where you’d think they’d use the smallest amount possible for safety.

    1. Further to this – yes, up to the 80s, the aspirational bike frame was special double-butted (thicker wall at the ends, thinner wall in the middle) chromoly tubing, lugged together and brazed. I have one nice lugged frame.

      Early welded frames were viewed as crude and were problematic especially in aluminum, but now that it’s possible with automation to produce consistently great welds, welded frames now dominate.

  13. I’d like to see you take a TIG welder to a piece of 3/8″ soft copper tube and end up with a working air conditioner.

    I’d never knock welding or welders, it’s a great skill to possess. Some metals and applications just can’t handle the heat.

    There was this time I took a job fixing the air conditioning system on a tug boat in dry dock. The boat was filled with mechanics, welders and electricians and they all took a break to watch me braze a couple of 1″ copper pipes. They were fascinated by the idea of connecting joints with such (relatively) low temperatures.

    1. TIG brazing can be done, but it is not easy. The heat is concentrated in too small of a space and temperature control is really hard. I have done some of it for filling holes and gouges in steel prior to powdercoat. If I had an oxyfuel rig, I would definitely use it instead.

  14. I went down to Home Despot, picked up a couple of these brazing rods and tried this. It works pretty well. Definitely not structural but if you’re wanting to attach a mount point or something similar it’s a good option.

    Thanks for the tip! Always good to have more tools in the quiver.

  15. I used aluminum brazing rod to build a VR mount for my Porsche 924 project when I converted to MegaSquirt EFI controlling Ford EDIS distributorless ignition:

    The first prototype was a little rough, but subsequent development improved the product and it has been reliably working for many years now on a vehicle that saw pretty heavy duty use as an Autocross/Daily Driver.

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