At 09:31 AM 9/13/2005, Pat Barthelow wrote:
>In an earlier discussion, someone said:
>
> >Remember that when you look at a shiny piece of aluminum you are still
> >looking at a layer of oxide. Aluminum Oxide forms almost instantly and is
> >a
> >means of protection.
>
>AA6EG, Pat needs confirmation:
>
>I have been told that if you need to solder to alumium, using ordinary
>solder, and rosin flux, that it is possible to take an aluminum object, lay
>a small squeeze of motor oil on it, keeping the surface away from oxygen,
>and aggressively burnish the aluminum with a scraper, file, or other
>scraping tool...through the oil, keeping the newly burnished surface from
>developing a micro oxide coating. Then, taking a large soldering iron and
>some solder, heating the aluminum through the oil, and simultaneously
>applying your solder through the oil (which now, I am guessing is probably
>smoking (:-> )to the aluminum surface, and the solder will 'tin' the
>alumunum. Once tinned you can then solder to in conventionally.
>
>Can anyone confirm this technique?
Yes, that's how you do it. Easier described than done. A bit of practice
helps.
You can also get fluxes that are designed for soldering aluminum. And,
brazing rod designed for aluminum.
However, flux and keeping the oxygen away isn't the real problem. It's
thermal conductivity. I faced this recently trying to solder 50 SMA
connectors onto a 36"x36" sheet of copper (and gave up). You need to get
enough heat into the aluminum to melt the solder (but not melt the
aluminum), which is quite challenging, because the high thermal
conductivity sucks the heat away. For a small aluminum part, you can just
wait til the whole darn thing is hot enough to solder, but I'd hate to have
to solder to a big sheet. Maybe a good sized torch with a rosebud tip?
(clearly you don't want an oxidizing flame!) You're really going to want to
get the entire aluminum thing almost to solder melting temperature, so all
you have to do is get that little increment hot enough.
Most solders melt around 217 which is well below the Aluminum melting point
around 660C. Recommended tip temperatures are in the 260-370C 500-700F
range. However, soldering temperatures are certainly hot enough to destroy
the temper or heat treatment of the aluminum.
There's also a whole raft of Tin Bismuth and Tin Indium alloys that melt at
just over boiling water temperatures (118C for SNIn52)
http://www.aws.org/wj/2004/02/046/ is a writeup from the American Welding
Society. They comment that 5XXX and 6XXX alloys are hard to solder. 6061
is solderable.
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