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
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