On Sat, 30 Apr 2016 09:15:55 -0700, wrote:

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

This is not from personal experience, but when making solder and
brazed joints between dissimilar materials (particularly those with
widely different TCoE, like aluminum and steel), industry practice is
to design as much overlap in the joint at possible. Two pieces joined
along a narrow seam line are considered to be the least desirable
situation.

You want as much shear area as you can get. This is just a general
design guideline, though. Different circumstances can be different in
joining.

--
Ed Huntress