On Sat, 30 Apr 2016 10:24:51 -0700, "Bob La Londe"
wrote:

wrote in message
.. .
On Sat, 30 Apr 2016 10:25:24 -0400, Joe Gwinn
wrote:

In article ,
wrote:

All yooze soldering type folks,
I want to solder a steel tube into an aluminum one. I'm thinking that
if I tin the steel with the mostly tin lead free plumbing solder and
the aluminum with the mainly zinc aluminum solder I should then be
able to join them together with the lead free plumbing solder. I know,
because I have done it more than once, that I can get the lead/tin
solder and lead free plumbing solders to wet aluminum but it is not
nearly as easy as using the zinc based stuff. I know there are other
ways to do this but I am looking for a fairly simple, low tech, strong
and cheap, way to join the one tube into the other with a reliable
method using widely available materials that won't be affected by
solvents like gasoline, alcohol, acetone, benzene, and the other
typical components of pump gas. This is not just for me. I also want
to be able to show other folks how to do this, folks who have limited
resources and education.

It's easy enough to solder steel to aluminum, as others have explained.
But there are some more fundamental problems:

1. The difference in temperature coefficients of linear expansion,
which will cause such assemblies to try to tear themselves apart
mechanically. If the contact area is small enough, the mushiness of
solder will handle the mismatch, but the joint will eventually fatigue
and break.

2. This is a built in galvanic couple, and if not dry and isolated,
the assembly will rapidly corrode. The aluminum will corrode first,
protecting the steel.

What are you trying to build? There are ways to avoid these problems.

Joe Gwinn
Gas tanks for bicycle motors. I can see how corrosion might be a
problem if the joint was wet a good deal of the time but this joint
will be dry most of the time. The joint is a steel filler neck
soldered into an aluminum tube. With the diameters involved, and a 70
degree F rise the steel would expand .0008 and the aluminum .0015, so
basically twice as much. Do you think the joint would fatigue
quickly? I'm trying to picture this joint in my mind, the stresses
involved. When soldering the aluminum will expand quite a bit so when
the joint cools to room temp. the steel will be in compression. As the
joint warms the steel will just be under slightly less compression.
What would you do in this case in order to meet the requirements in my
original post?
Eric

Mounting flange for the filler neck and fuel filler hose connecting the two?


I suppose I could go that route, but it would look cleaner if the
steel is soldered into the aluminum tube. There are lots of folks
putting little two and four stroke motors on bicycles. The kits come
with a gas tank. Some like the tanks and some others either fab their
own or buy a custom one. Lots of the people mounting these kits are
doing so on a tight budget and get really creative. I thought it would
be cool if I hammer form an aluminum tank and post the process on
youtube or a forum. My goal is to show how this can be done with
minimal tools, skill, and cost. That's why the tank halves need to be
soldered instead of welded. And the filler neck needs to be sawn off
of the stock tank and soldered into the fabbed up neck on the new
tank.
Eric