Aluminum Soldering

(reposted from SED) I want to try soldering some aluminum plate (0.032”) 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. I’ve 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.
Thanks
George H.
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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
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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_for_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?
George H.
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<http://en.wikipedia.org/wiki/Thermal_expansion#Thermal_expansion_coefficients_for_various_materials
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
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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.
http://ndtsystems.com/Reference/Velocity_Table/velocity_table.html
George H.

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But the acoustic loss may be high.

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?
Joe Gwinn
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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 Household Goop.
Michael Koblic, Campbell River, BC
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On Jun 29, 8:15 pm, snipped-for-privacy@gmail.com wrote:

Hmm, Well in the deep dark past of grad school, we used stycast 1266 to glue Teflon 'washers' into TeCu plates. Brass can't be that much different.
And there may be better epoxies by now.
George H.
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On 6/29/2012 9:24 PM, George Herold wrote:

Nope.
Goop, pick your favorite flavor.
Personally I prefer Plumbers Goop for this job - thinner and easier to spread evenly.
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wrote:

Yes. The difference between theory and practice. I will have a look at Plumbers' Goop. Never tried it.
Michael Koblic, Campbell River, BC
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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 trickle, but...)
George H.
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On Mon, 2 Jul 2012 07:13:19 -0700 (PDT), George Herold

"Seal All" is your friend for that application. From Ecclectic Products - same company that sells ShoeGoo AND goop- A SUBSIDIARY OF WHILLAMETTE VALLEY CORP
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snipped-for-privacy@gmail.com wrote:

I'm pretty sure it's a chemistry issue. For such things, one asks the glue manufacturer. If I recall, brass is the problem.
Joe Gwinn
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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.
Just Goop.
Michael Koblic, Campbell River, BC
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On Sat, 30 Jun 2012 13:53:45 -0700, snipped-for-privacy@gmail.com wrote:

You saying BRASS is difficult to solder?????
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snipped-for-privacy@snyder.on.ca wrote:

No, glue. I think the zinc reacts badly with some kinds of adhesives. Aluminum has this problem too. Anyway, there are glues that work, and glues that don't work.
Joe Gwinn
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On 7/1/2012 11:46 AM, Joseph Gwinn wrote:

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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How does this work for people to metal?
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On 7/1/2012 3:00 PM, Joseph Gwinn wrote:

Cyanoacrylate. Glues people to almost anything.
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Yes, but Goop?
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