aluminum solder?

That's almost like asking "what is glue?". There are a number of materials that can be used to solder aluminum. Some will work well on other metals too. Most require a flux designed to work with them on aluminum. Bond strengths vary quite a bit.

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Tin solders Al just fine. A good flux must be used to keep the oxide off it is often placed on and then through it, scrape a section through the Al and then the solder will mix. Often you can find silver boxes in hardware stores - small rolls of it with the special flux.

Martin Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH, NRA Life NRA Second Amendment Task Force Charter Founder

brassbend wrote:

Depends on what it is...

Aluminum solder is anything that wets aluminum. This includes most metals on the periodic table with a melting point less than aluminum's. Zinc, tin, gallium, mercury and some of the alkaline metals (maybe just lithium). The heavy white metals thallium, lead, indium and bismuth probably don't stick too well. Gallium and mercury amalgamate (aluminum amalgam is used in various chemical reactions), but they just weaken and oxidize it at room temperature. Cadmium is similar to zinc so may wet it well, but it is more of a "heavy metal" than zinc, so may not.

Of the "good" metals, zinc and tin are the cheapest and thus most practical. Coincidentially, they happen to be the most used. Pure tin, maybe with some copper or silver to strengthen it, would work well enough. Lead-tin may work, the tin holding it to the aluminum. I've done it before, but due to the poor joint (I don't have the fluxes), it's hard to say how well it works.

As for zinc, it alloys very well with aluminum so is more like brazing, and as a matter of fact its melting point is somewhere around 800°F so it's pretty much brazing by definition anyways.

The "magic solder" stuff that comes with a flux (probably based on corrosive fluorides) and melts at a low temperature is probably tin and zinc, possibly with cadmium and lead. The "miracle rod" product that "doesn't" require a flux (because you rub the rod against the work to break the oxide) is either pure zinc or an alloy with up to 12% aluminum.

Tim

-- Deep Fryer: a very philosophical monk. Website:

What is glue?

Here are some thoughts:

Tin the aluminum under oil, then solder as normal.

Aluminum normally won't accept regular solder. That's because of the oxide coating on its surface. By covering the surface with oil, you can scrape away the oxide coating while preventing oxygen getting to the surface and reforming it. You can then tin the surface with an iron, through the oil coating, and get a proper bond. Then you can clean the oil off and solder as usual.

This is a home shop technique. There are probably better solutions in a production environment.

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Thanks for the information, I was under the impression that "Aluminum solder" contained Aluminum and was made solely for use with Aluminum.

LB

Aluminum solders are indeed made specifically soldering aluminum to aluminum, and in some cases aluminum to other metals. Materials that contain aluminum, as Allstate #31 and others, are higher-temperature brazing materials.

The Kester "aluminum solder", found at a hardware store in the silver box, is tin.

Harris Staybrite is 96% tin, 4% silver. They say it'll stick to aluminum with a special flux, but my welding store doesn't carry the flux so I can't vouch for that. It will also stick to brass, copper, steel and stainless.

Allstate "Strongset 509", a zinc- cadmium alloy, will join aluminum to aluminum, also to copper, brass, steel and stainless steel. It's quite strong at 29,000 PSI. It's pricey. The flux is organic and burns easily. This stuff works very nicely if one can avoid burning the flux. If the flux burns, it's a mess: clean up, start over. Dang, I just learned tonight that they recommend applying the flux to the solder then applying the solder to workpiece that is hot enough to melt it. Geez, I never thoughta that! I'll be trying that tomorrow.

Allstate 107, a "silver bearing alloy" (they don't say with what) will also join aluminum to aluminum, copper, brass, steel and stainless -- but it isn't as strong at 20,000 PSI. "Commonly used to join aluminum wires to copper lugs." It sounds a lot like Harris Staybrite so it's probably tin-silver.

offers a couple of aluminum solders and companion fluxes. He doesn't say what's in them. Everything I've tried from him has worked well.

There are others, but all that I know of are similar to one of those mentioned above.

Use of the recommended flux is very important with any of these materials. Wrong flux = lousy results.

I have several rolls of "alu-sol" multicore solder. Says "solders aluminum, most aluminum alloys, and other metals. No additional flux required" Data sheet says tin/lead alloy with tin/zinc amine flouride complex - whatever the heck that means. It works.