(reposted from SED)
I want to try soldering some aluminum plate (0.032) onto each side
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
others. Ive 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.
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
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
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.
Well I'll glue it at room temp and then use it at room temp, so maybe
not as much of a problem.
Yeah I'm pretty much stuck with aluminum. (But I'm open to other
materials if there are any suggestions.) I just started these
acoustic experiments. I wanted to show impedance matching for
acoustics. (Like a 1/4 wavlength AR coating in optics.) I've got a
brass cylinder that's acting like an acoustic Fabry-Perot cavity.
There are transducers on each side (one source and the other
detector). The transducers have an acoustic impedance of about 3 (in
some units.) The brass has an impedcance of about 40 in the same
units. So to make an AR coating I need 1/4 wavelength of a material
with a mean value between the two. (sqrt(3*40)) ~11.
Aluminum has an impedance of ~17 which seems to be close enough. I
should also try some glass which has about the right impedance.
(10) ..Temp co wise it might be worse.
Grin.. yeah nice idea, but not for my application.
Here's a table of acoustic velocities and impedances.
I think acoustic impedances may be expressed in ohms as well. Although
it will depend on the system of units used.
Ahh. If the temperature range is small, epoxy will work, at least well
enough for experiments. One can get slightly flexible epoxy from
industrial suppliers like Master Bond and Loctite. They will know
what's best. The strongest materials require very good cleaning of
surfaces and cure at 250 F.
I have an old Bransonic ultrasonic cleaning unit with a stainless-steel
tank. The ceramic transducer is epoxied to an aluminum interplate which
is in turn epoxied directly the the stainless steel tank. This unit
still works after decades of use.
Is there any reason not to just drill and tap the brass and mechanically
attach the aluminum plate with a few steel screws, with some silicon
grease between brass and aluminum masses?
On Fri, 29 Jun 2012 15:29:46 -0400, Joseph Gwinn
I have not glued aluminum to brass but I have tried many adhesives for
gluing brass to other materials including other metals. I find that
most fail miserably.
The best for gluing steel to brass (which I do most often) is
Campbell River, BC
Ahh OK, I'll give it a try if the soldering doesn't work.
Say will the plumbers goop work if the surface is wet?
(I've got this Intex blue plastic pool for the kids.
I noticed a little pin hole leak in the side wall.
I stuck the every present duct tape on it, which has reduced it to a
On Fri, 29 Jun 2012 22:40:23 -0400, Joseph Gwinn
The glue manufacturers I asked did not have a dicky. Indeed brass is
the problem. It does not matter how you pre-treat it. Incantations and
saying Lord' prayer backwards does not work either.
Campbell River, BC
There are techniques that work as well, not just adhesives.
For instance, for aluminum...
The problem is that aluminum grows an oxide layer is just a few seconds.
So what you have to do is prep the oxide layer and bond to that.
Phosphate wash followed by two-part primer like EpiBond or Randoplate.
Now your glue can bond to the primer and get some adhesion.
Urethane glues will actually work with the primer, Looks like it
dissolves into the primer.
But for general bonding metal to metal, pick a Goop, any Goop,
and stick with it.
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