Solder/Brazing Aluminum Heat Sink
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  1. #1
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    Default Solder/Brazing Aluminum Heat Sink

    We are having an issue at my shop on a hot job with brazing a tube into a plate. The plate has a groove machined into it for the tube to fit into. The and channel have a 90° bend with a 2.0 radius. The issue we are having is the tube melting before we can get the filler rod to flow. Especially around the radius where the wall is thinner from forming. We are using a torch to heat the assembly from underneath. the plate is 0.5 thick and tube is 0.375 od x .035 wall. The filler rod we used is Hobart Brazing Rods, Model# 770206 (this has a 700°F melting temp). Our welder suggested getting a filler material in a sheet rather than a rod but I am not having any luck tracking some down. Any suggestions are welcome.

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    Aluminum can be soldered, but it sucks. When I've done it, it required a lot of mechanical rubbing of the filler rod to break the oxide and get it to wet. I've never seen it just flow like regular soldering or brazing.

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    While no brand names come to mind I know there are pastes for brazing aluminum that contain both brazing alloy and flux.

    One thing that comes immediately to mind is that although you are using the torch from below, the curved part of the tubing may be reaching higher temperatures if it protrudes past the edge of the heat sink. If so, perhaps some baffling is needed or even something like a copper heat sink during brazing.

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    Try Lucas Milhaupt. Are you brazing or soldering?

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    How about gently blowing air through the tube until the block comes up to temp..

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    How about a graphite plug.

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    After many problems and inconsistent results soldering or brazing aluminum tubes (and in fact other projects involving joining aluminum parts without welding) I now have both parts copper plated and soft soldering using regular soft solder. Copper plating of aluminum is pretty straightforward and 5-10 micron thickness is all that is needed. Did hundreds of joints this way without a single failure.

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    We are brazing.

  11. #9
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    True, but any alloy that I can melt on a soldering iron is pretty low temp stuff. Basically you've got a few hundred degrees difference between the melting point of the rod and the melting point of the tube. You need good control of the heat and shielding or plugging to make sure the tube doesn't get hotter than needed.

    I do some things with an oxy-propane bench torch, and sometimes it's just too much heat. I'll go back to a regular air-propane torch to get better control.

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    Get a smaller torch tip and aim the heat more at the plate than the tube?

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    shiapeng123, your company doesn't even have a US sales office. Why are you listing your location as "New Mexico"?


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