Soldering copper to aluminum

[Evan]

I am making some 1.2 ghz yagi antennas for radio astronomy, in order to detect the emissions from bursts emitted by solar flares. I wish to make the antenna elements from 4032 aluminum welding rod as it is corrosion resistant, stiff, cheap and easy to bend. The only issue is making good connections to the driven element that won't corrode quickly. I will be using 300 ohm twinlead for the phasing harness and soldering is the most secure connection possible. However, it is impossible to solder copper to aluminum directly with regular electronics solders.

I dropped in the local Canadian Tire store and checked out the welding supplies. They carry a product from Bernzomatic that is advertised for soldering aluminum to aluminum. Checking out the label it is an alloy of copper, zinc and magnesium and doesn't require flux.

I was skeptical but bought some to try. Surprisingly, it works well to solder 4032 to 4032 but unfortunately it will not flow to copper. Then it occurred to me that perhaps it would act as an intermediary for regular lead solder. Yes it will.

By "tinning" the bare aluminum with the Bernzomatic rod and then tinning that with the tin/lead solder it is easy to solder copper wire to the aluminum part. The absence of any highly corrosive flux insures the connection will not corrode. It requires only a propane torch to reach soldering temperature of the special rod.

[lakeside53]

Good tip. Could be useful.

I find you can usually "tin" solder onto aluminum with standard flux core solder if you use the iron to scrape the aluminum surface (to get though the oxide) as you apply solder. I've also used a stainless brush though a puddle of solder. Once it's "tinned" the rest is easy.

The benzomatic rod makes torch soldering viable.

I've also used HTS- 2000 rod for aluminum brazing. http://www.aluminumrepair.com/index.asp

I wonder of the benzomatic product is similar?

[darryl]

Interesting. I'll keep that in mind.

[Evan]

I have never been able to get an actual bond to aluminum with any ordinary solder. You can get it to stick to the aluminum but it isn't flowed to the metal. It is not a true solder connection where the metals mix at the interface to form an alloy.

[MichaelP]

Interesting. Thank you.

It is not a true solder connection where the metals mix at the interface to form an alloy.

I have to respectfully, but strongly disagree. Metals (parent and the soldering alloy) do not mix and form alloy as a result of soldering or brazing. Parent metal does not melt. This is the difference between those two processes and welding.

[Your Old Dog]

Thanks for the post. It could be of some use to Ham radio operators.

[jnissen]

True standard solder will just ball up and not stick to aluminum. The two step approach is a good tip. We ran into this about a two years ago. My son was building a fuel cell and used an aluminum perforated plate in place of traditional graphite electrodes. Anyway he punted at the time and used a brass contact to make a mechanical connection to the aluminum. Not as reliable but it worked for the test at the time. This would have allowed him to directly make a good soldered connection.

[lakeside53]

If you work the surface as you heat and apply lead solder (you will need a decent iron to get the aluminum up to temp), it will stick nicely. if you don't, it will "ball up" like you say. I once soldered hundreds of wires to aluminum (way back). I suspect they are still there

Pre-tining with benzomatic or other modern materials is an attractive alternative.

I re-read the HRS-2000 site. They say that rod can be used to braze to both copper and aluminum (and others). I have a bunch of that rod - I'll go try it as a "pre-tin".

[beanbag]

Interesting. Thank you.


I have to respectfully, but strongly disagree. Metals (parent and the soldering alloy) do not mix and form alloy as a result of soldering or brazing. Parent metal does not melt. This is the difference between those two processes and welding.

Then what do you call it when a gold wire dissolves into a puddle of tin/lead solder, even though it is below the melting point of gold?

[The Artful Bodger]

The old technique for soldering to aluminium was to make the joint under a puddle of oil.

Apply the oil and rub the oil covered surface with a big soldering iron (hot of course) then apply the solder. The requirement is for the soldering iron to clear the oxide off the aluminium and to complete the join without air getting onto the metal.

[Evan]

I have to respectfully, but strongly disagree. Metals (parent and the soldering alloy) do not mix and form alloy as a result of soldering or brazing. Parent metal does not melt. This is the difference between those two processes and welding.

The metals form a eutectic at the boundary. Solders act as a solvent for the metals with which they are compatible. The metals form a eutectic at the boundary. The result is an alloy at the interface. It is not the same as welding. In particular the welding process refers to joining the same metals together by melting both parts at the junction.

[Boucher]

There used to be a company that specialized in joining difficult materials. They had eutectic in their name. Their products were a little pricey but they really did work.

[Tony]

Good tip!

Just curious though, how does this soldered connection compare (electrically)
to a mechanical connection? meaning using a small screw.. or perhaps drilling
in the end of the aluminum rod, inserting the wire, and crimping it?

Tony

[Weekend_Scientist]

Hey Evan,

That's a great tip. Thanks for the details.

I'm going to have to make up some aluminum-copper connections just so I can her my other builder/tinkerer friends say "How'd you do that!?!?"

Speaking of difficult to solder materials, does anyone know a material that can be used to solder to tungsten? I'm talking tungsten metal not tungsten carbide.

[Tony]

Weekend.. steel will work just fine.. put some tungsten in a tig torch and
try to weld some steel without touching it.. in no time flat you'll have a very
strong tungsten-steel bond!