In article
,
George Herold wrote:

On Jun 29, 11:23*am, Joseph Gwinn wrote:
In article
,
*George Herold wrote:

(reposted from SED)
I want to try soldering some aluminum plate (0.032²) onto each side
of
a brass cylinder. *When trying to solder aluminum in the past I
failed. *I think I heard that some Al alloys are easier to solder
than
others. *I¹ve got a choice (From McMaster-C) of 6061, 2024. 7075, and
1100. * Any idea of which is better?

I was also planning on getting some aluminum flux and some Zn/tin
solder from McM-C. *Other suggestions welcome.

If the temperature coefficients of linear expansion of the brass and the
aluminum don't match to within something like 1%, the joint will surely
tear itself apart, no matter how well soldered the joint is.

Joe Gwinn

Hmm, OK thanks. I was thinking of trying a thin layer of epoxy too.
But soldering would be better.
The linear expansion coef. of brass and Al differ by ~ 4E-6/C
http://en.wikipedia.org/wiki/Thermal_expansion#Thermal_expansion_coefficients_f or_various_materials

So if soldering happens at 300C (?) then the thin aluminum plate has
to shrink by a part per thousand (or so). Maybe 2 mils for a 1.5 inch
diameter cylinder. I have no idea if a thin Al plate can accomidate
such strain. (I'll find out.)

Would a nice soft alloy like 1100 be best?

If it's thin enough that it will yield.

I have the impression that the area of the plate is large, and 1.5"
diameter is in the range. A few mils may not sound like much, but when
metals fight, the stresses are large, and solder is weak.

The issue is that below the solidus temperature (where the solder
becomes solid) as the two metals cool they shrink differentially,
putting shear stress on the joint. If the solder gives enough (which
depends on the max dimension between far points that are soldered), then
no problem. Unless we have temperature cycling, which will fatigue the
solder and break the joint.

Rigid epoxy will also fail. What can work is a silicon rubber adhesive,
so long as the adhesive layer is thick enough to accommodate the shear
strain without tearing.

War story: In the 1970s, I and a partner made lab instruments for
measuring how much drugged rats run around (yes, this is useful to
know). The instrument consisted of a big sheet steel base about 12"
wide by about 24" long by maybe 4" high, with a milky plexiglass cover
glued to the top. Light diffused through the plexiglass to light
receptors within the base. The problem was that the glue joint between
plexiglass cover and steel base was breaking, despite use of the best of
adhesives (a silicon rubber adhesive caulk), the covers were coming
loose, and customers were unhappy.

Plexiglass has something like five times the temperature coefficient of
aluminum, never mind steel, so normal variation in temperature in the
customer labs was enough to tear the joints apart, even in labs with
only a few degrees of diurnal variation. We measured the strength of
the joints - it was something like 50 or 100 pounds per inch in a lap
joint, and yet the joints broke within a year.

The solution was simple enough: Put 1/16" thick spacers in the joints
so there was always enough rubber that it didn't tear as temperature
variation caused the cover to get larger and smaller compared to the
steel base.


Anyway, is there any reason the aluminum sheet cannot be brass instead?

The other alternative is an interlayer having intermediate tempco, to
spread the strain out. For instance, stainless steel has a tempco
intermediate between aluminum and brass.


Joe Gwinn