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PJ
 
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Default Soldering aluminum tubing to steel sheet metal

Racer X typed:
I've got a tank made of mild steel sheet metal, and I want to
permanently attach some aluminum tubing to the bottom surface of the
tank. The tubing is for some hot fluid (200F-250F) that will be used
to heat the contents of the tank. The aluminum tubing is only 3/8"
diameter, with a wall thickness of about 0.060" (maybe 1/16"). I
estimate that the sheet steel that I'm attaching it to is about 16
guage, maybe 18 guage (0.050" to 0.060").

What's the best way to do this and the best tools to use for the job
for a DIY type person like myself. I'm not a professional, but I do
have some experience welding and soldering. I have both an
oxy-acetylene torch setup and a cheap Propane torch that mounts on a
small hand held propane cylinder.

I've done a bit of reading and research, and I think I should
probably use some Cadmium/Zinc solder, probably 30% or 40% Zinc.
I'm not sure exactly what flux to use on the steel, or on the
aluminum.

I'm kind of guessing that the propane torch would be a better choice
for this job because I'll need more general heating of the materials
and indirect heat. I've got some extra material of both types to
practice on as well, so I won't start out ruining my main project.

I'm just asking for advice or confirmation that I'm on the right
track here. Also, would a normal welding supply place have the
appropriate solder? What kinds of flux would I need for this? I
plan on calling them tomorrow and asking, so I guess I'll find out.

One resource I checked said not to use flux on the aluminum, but to
apply the solder with steel wool to scrape away the surface
oxidation on the aluminum. The procedure described was to heat the
aluminum with the torch, drop a few drops of melted solder on the
aluminum, then use steel wool to work the surface of the aluminum
while continuing to apply heat to maintain the temperature slightly
above the melting point of the solder. As the oxides are scraped
clean, the solder will wet and tin the aluminum. I'm probably going
to be soldering down 12 to 18 feet of this tubing, so that sounds
like a lot of work with the steel wool. Would there be a better
method using some kind of chemical treatment or flux to remove the
surface oxides from the aluminum?

I plan to tin both surfaces to be joined with the solder, then sweat
them together and apply additional solder to fill any gaps and build
up a little under the tubing. I want as much heat transfer as
possible from the aluminum to the tank in the finished product. I'm
assuming that I'll need 30% to 40% zinc in order to have some
decent paste range to work the extra solder into the gaps and build
up around the bottom half of the tubing.

Also, this surface will be exposed to weather (after painting), so
corrosion resitance is fairly important, as well as strength.

So, am I on the right track? Or am I about to make a huge mistake?


The task you are trying to accomplish is very possible. You are
dependent on your choice of solder and flux's. I would recommend the
Harris brand. Harris Stay-Brite 8 silver solder would be my choice for
the solder. It will bond both metals.
http://www.jwharris.com/jwprod/solderalloys/#SBSolder

My choice for the flux on the steel would be Harris Stay-Clean flux.
It is available in liquid and paste forms.
http://www.jwharris.com/jwprod/braze...uxes/#soldflux

Additionally, Harris makes an aluminum flux for soldering using the
same silver solder mentioned above, Stay-Clean Aluminum. It is also on
the flux page I posted above.

The solder has very high strength and is food safe. The web sites I
posted tells it all.

I would "tin" the pieces with solder - using the appropriate flux -
and then join them together as a unit by adding additional solder. A
propane or Mapp torch should be more than adequate. I have tremendous
success with this solder and flux combination. I do a bit of work in
stainless steel, copper, brass and aluminum. Most heating/air
suppliers that I'm familiar with carry this brand.

I hope this helps.
PJ