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How to solder aluminum
NOTE: SOLDER IS NOT NEARLY AS STRONG AS ORIGINAL ALUMINUM PART. FOR EXAMPLE, A SOLDER HOLE PLUG ON A CAR A/C RADIATOR WILL FAIL DUE TO PRESSURE, VENTING THE EXPENSIVE COOLANT. ON A RADIATOR, AN ENGINE OVERHEAT CAN ALSO POSSIBLY BREAK A SOLDER REPAIR. IF YOU CANNOT AFFORD TO BUY A REPLACEMENT PART, ALWAYS SOLDER A PATCH PLATE (BRASS FOR EXAMPLE) OVER THE HOLE FILLED WITH SOLDER. SOLDER ALONE WITHOUT A PLATE OVER IT WILL NOT HOLD UNDER PRESSURE.
To solder aluminum you need two things:
1) A special flux for Aluminum. Search around on the internet for the best deal and cheapest cost (when shipping is included). I have bought
2) Plenty of heat.
This example will discuss soldering a car radiator tube.
Find out where the leak is. There might be more than one leak. If it is a large hole, it is always best to replace the radiator for safety. A new radiator is always cheaper than a head gasket and piston rings replacement when your engine overheats after the poorly made repair fails.
I was repairing an A/C radiator, so engine safety was not an issue.
Soldering aluminum requires LOTS of heat. Even a 300W gun-type electric soldering iron is sometimes not good enough. A small precise propane torch is much better. There are special torches for precision soldering like
Clean the surface well. Do not apply flux at this point. Doing so will only cause it to burn off and char later.
Decide where to apply heat. This should never be directly at the area to be soldered as this will burn the flux. But it cannot be too far away as aluminum transfers heat away very quickly.
Apply a torch next to the area to be soldered. Apply heat. Dip the solder wire into aluminum flux. When you think the aluminum is warm enough for soldering (this is impossible to tell visually), have the flux drop on the aluminum, then press the solder wire to the surface. If solder immediately melts and wets the surface, you are good. If it does not, then do not force it to. You will only get a cold solder joint without any wetting if the surface is not hot enough. When aluminum is hot enough, applying solder and flux is about as easy with other easy to solder metals. Do not keep the heat too close or too hot so that the flux does not burn off or turns black.
Leave a good bump (hill) or solder over the problem area for safety.
If repairing a larger hole, a patch of something like copper, bronze, etc might be required.
Click here to download a video of me repairing the radiator. The torch is a little too big and hot (this is the only thing I had at that point, my small precision torch broke down, and this torch does not adjust the size of the flame). But you can see the solder flowing easily when aluminum is hot enough. Also, I had to reapply heat because the surface was not clean enough and I did not cover some parts with flux in one step as I could.
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