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  1. #1
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    May 2021
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    Default Brazing long(ish) T fillets in thin(ish) aluminum

    tl;dr - Setting up a T of 16 gauge aluminum for a fillet braze of 6 linear inches or more, I get end lifting before the metal gets hot enough to melt the brazing rod. What are my options?


    ts;wm - Not a professional metalworker, here, so I’m hoping someone will have seen / had my problem before and can offer a suggestion how to address it. I have a set of structures to be fabricated of 16 gauge sheet aluminum. They involve a set of parallel 4.5” seams on some pieces as a set of parallel 7” seams on others capped with a perpendicular piece at one end. Here’s a sample of the shorter seams.


    1 - 4 inch seam.png



    For those whose immediate instinct is to ask “Why don’t you weld this?” the short answer is, “It turns out I suck at welding and I don’t have the time needed to get good at enough at it.” My MIG steel welds are hideous, but serviceable enough to hold the machine I’m working on together in a low pressure application. To add the complexity of working with touchier material like aluminum plus learning to use the spool gun all in these tight corners is just a recipe for disaster. So to do this aluminum structure that only needs to sit inside a larger steel box and hold itself together (will only need to bear a couple kilos of weight) I decided to braze the seams instead. It’s much easier to get torch flame and brazing rod into the tight spaces and I’m not under the gun like I am with MIG welding to go slow enough, smoothly enough to deposit filler metal WHILE moving quickly enough to avoid blow throughs all in a corner I’m not entirely sure I can get the spool gun into while properly controlling it.


    So, the ugliness of my first assembly aside (nobody but me is ever going to see that side), it suits my needs of holding itself together and being the size and shape it needs to be (so I can fit it into the larger steel box, when I’m ready). This was done with a propane torch and Alumaloy rods which, as long as I was patient enough to let the base metal get hot enough to melt the rod and suck in the filler, works basically as advertised and produces a joint I can’t break with my bare hands, meeting my minimal strength needs.


    Trying to capitalize on the “success” of my first assembly (again, not a professional metal worker here, just trying to get this task accomplished), I used the same process which was to clamp the bottom piece to some ¾ stainless bar hanging off the edge of the table (trying to minimize the heat lost to the surface while I try to heat the metal enough to reach the working temp of the rod) and use a clamp to keep the top piece vertical (not intending to hold it down) while I very gently swipe the seam with the brazing rod after each few passes of the torch waiting for melt temp to be achieved. I checked my pieces first to make sure they’re relatively flat and while the seam interface isn’t perfect it didn’t need to be on the pieces with the shorter seams and all seems to fit the purpose of brazing (to suck material into that internal gap), anyway.


    2 - Flat fin.png
    3 - Clamped down wing.png
    4 - Placed T from front.png
    5 - Placed T from side.png




    But when I started heating the interface, I got lifting on the end that wasn’t clamped. It got about 1/4” high, but the photo doesn’t show it quite that bad because by the time I turned off the torch, dropped the gloves and grabbed the camera, it had already started to settle back to it’s original shape.



    6 - Lift of top piece after heating.png



    After this I tried all manner of things to compensate for this. I tried adding weight to the top piece to hold it down, but to no avail.



    7 - Elaborate attempt to hold the top piece down.png



    I also tried cutting notches in the top piece so I could clamp it down, but then when I heated the interface the aluminum got soft before I reached the brazing rod’s working temp and the top piece bent which doesn’t work for my needs. Wondering if there was a “Goldilocks zone” of pressure that was firm enough to keep the piece vertical and the ends down but light enough to prevent the squishing of the piece when I got near the brazing rod’s working temp I tried a few more times, but anything gentle enough to not bend the piece was not strong enough to keep the ends from lifting because the clamps inevitably would not stay in place.


    So, I thought I’d post this in a couple of the more popular metalworking forums on the interwebs to see if anyone had run into this problem before and what they did to get their assembly put together the way they wanted it. In the absence of that, can anyone explain the physics of what’s happening here? The bottom piece does not appear to be warping at all and the top piece appears to be dead straight the whole time, so where is my lift coming from? Is the top piece expanding along the whole surface turning that previously flat edge oblong? If so, why wouldn’t it do that on the smaller pieces I did earlier? Or is it a function of surface area and that smaller pieces DID do it, but not enough for me to notice?


    The metalworking guru at the makerspace I’m working at didn’t have any immediate ideas on how to do this as a brazing only process (he isn’t a huge fan of brazing, but seems to agree with me that it’s a viable process for my application; at least until now) and proposed I use MIG welding to tack weld the corners and then come back and braze in the hard to reach middle channels. I’m holding on to that as possibility but I’d like to see if there’s a brazing only method available to me.


    Any comments or suggestions would be appreciated.


    Thanks,


    Scott

  2. #2
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    HI sawozny

    Welcome to the MetalWork Forums.
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    Long story made short regarding the aluminium brazing process.The process does not lend itself well to the sheet metal aspect of alloy welding. The propane torch heat source while providing heat enough to braze the aluminium has far too much spread heat which causes the distortion of your sheet metal. .This process works better on the more substantial and solid forms of aluminium.

    This product bobs up at boat shows and the like and also on TV and the demonstrators make it seem easy as they have practiced to the point of perfection. To be honest I don't think there are many in our ranks who use it on a regular basis.A lot may have tried it but move on to MIG or TIG.

    Welcome again

    Grahame
    Moderator

  3. #3
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    The reason you are getting 'lift' is that the sheet is not expanding uniformly - the bottom edge is expanding but the top is not growing as much so the result is a bend, a bit like a bi-metallic strip. When it all cools it returns to the original shape.

    I don't do Al brazing but TIG Al. One of the things drummed into me is 'lots of tacks'. Trying to weld/ braze from one end is a disaster in the making. I would suggest tacking first - a tack (MIG, TIG or even braze) every couple of inches, and then filling in, although you might find that if there are enough tacks, it does not need the filling in for strength. If you have one I would also suggest a backing bar or two. One underneath and if you have one on the other side of the T, you should be able to get your tacks on one side well enough that tacking (or even brazing) on the other side does not move as much. Worth noting too that it is better to have the tacks on either side opposed to each other rather than staggered. The stresses they then put on the material are more balanced.

    Michael

  4. #4
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    If you have access to a metal folder, you can make the top section in 3 pieces rather than 5.
    This would avoid those corner joints. At the base, you can fold the metal into an "L" section which will be
    a lot stronger. If possible, use rivets or metal threads to fasten it. I would avoid T joints if you can.

    If you have to weld it, TIG is going to be the best process for this.

    Regards,
    Paul.

  5. #5
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    Hi Grahame,

    Thanks very much for the welcome.

    Quote Originally Posted by Grahame Collins View Post
    Long story made short regarding the aluminium brazing process.The process does not lend itself well to the sheet metal aspect of alloy welding. The propane torch heat source while providing heat enough to braze the aluminium has far too much spread heat which causes the distortion of your sheet metal. .This process works better on the more substantial and solid forms of aluminium.
    Yeah, I'm coming to the conclusion that thermal expansion is my enemy, here. Now it's just a matter of figuring out what to do about it.

    Thanks,

    Scott

  6. #6
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    Hi Michael,

    Quote Originally Posted by Michael G View Post
    The reason you are getting 'lift' is that the sheet is not expanding uniformly - the bottom edge is expanding but the top is not growing as much so the result is a bend, a bit like a bi-metallic strip. When it all cools it returns to the original shape.
    Yeah, I think the most significant source of my uneven expansion is that end of the interface that comes to a point. The lack of material at that end clearly can only expand so much compared to the much taller middle area and once I think about it in physics terms the bend (and return to normal) all make perfect sense. I just needed help from the good folks here to help me understand the physical nature of the issue.

    Quote Originally Posted by Michael G View Post
    I don't do Al brazing but TIG Al. One of the things drummed into me is 'lots of tacks'. Trying to weld/ braze from one end is a disaster in the making. I would suggest tacking first - a tack (MIG, TIG or even braze) every couple of inches, and then filling in, although you might find that if there are enough tacks, it does not need the filling in for strength. If you have one I would also suggest a backing bar or two. One underneath and if you have one on the other side of the T, you should be able to get your tacks on one side well enough that tacking (or even brazing) on the other side does not move as much. Worth noting too that it is better to have the tacks on either side opposed to each other rather than staggered. The stresses they then put on the material are more balanced.
    Yeah, from my welding work, I definitely understand the value of spreading out lots of tacks to manage the effects of thermal expansion as I lay down my seam in pieces spread out as much as I can. The challenge here is that just like there's no crying in baseball, there's no tacking in brazing. By the time you heat the material enough to add more filler rod, all that heat has spread so far that any brazed "tacks" placed have softened or liquefied and the piece previously being held up by those "tacks" falls.

    So now I have to look at whether or not I want to modify my pieces to avoid that point (and the related uneven thermal expansion that results in the lift I'm getting) or add a tack welding step into the mix which, while do-able with the gear available at the maker space I'm working at was something I was hoping to avoid.

    Regardless, I really appreciate your feedback.

    Thanks,

    Scott

  7. #7
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    Quote Originally Posted by paul33 View Post
    If you have access to a metal folder, you can make the top section in 3 pieces rather than 5.
    This would avoid those corner joints. At the base, you can fold the metal into an "L" section which will be
    a lot stronger. If possible, use rivets or metal threads to fasten it. I would avoid T joints if you can.

    If you have to weld it, TIG is going to be the best process for this.

    Regards,
    Paul.
    Hi Paul,

    If I had this to do over again, knowing what I know now, I'd look more seriously at bending and mechanical fastening, but for the moment, I'm kind of committed to this path.

    Also, regarding welding process, I definitely see how much nicer TIG welds are over MIG welds when dealing with fussy materials like aluminum, but I don't have the time to take on an entirely new skill set, so learning that will that will have to wait for another project. Even if they're not as pretty, MIG tacks will probably have to do if I can't find a brazing-only process that works.

    Thanks very much for your feedback.

    Scott

  8. #8
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    The following might help.
    Ok it is not welding per se ,but hey! all the basic principles apply.

    For as long as there has been welding ,there has been distortion.

    Its is a animated film clip made by Lincoln electric to guide the workforce welding for the war effort in the early forties.

    https://www.youtube.com/watch?v=2vuGlcbDwKY

    Hope it helps.

    Grahame

  9. #9
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    Quote Originally Posted by Grahame Collins View Post
    The following might help.
    Ok it is not welding per se ,but hey! all the basic principles apply.

    For as long as there has been welding ,there has been distortion.

    Its is a animated film clip made by Lincoln electric to guide the workforce welding for the war effort in the early forties.

    https://www.youtube.com/watch?v=2vuGlcbDwKY

    Hope it helps.

    Grahame
    Hi Grahame,

    That's a great video. It feels better to know that fabricators of all stripes have been in a battle with thermal expansion probably as far back as people have been working with metal.

    Thanks,

    Scott

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