Re: [VAC] aluminum sheet repair

["Dr. Gerald N. Johnson" <geraldj@xxxxxxxx>]

The last aluminum "welding rod" that I used decades ago melted just
under the melting point of aluminum, but it did stick after a fashion.
Its color was different from the aluminum that I used it on. Generally
with aluminum you have to get the base metal hot, then scrub through the
oxide with the patch material. Then you can get a connection some times.
The rod alloy is quite a bit different from the aluminum and may lead to
local corrosion when wet.

There is a flux called Salmet (I think the vendor has a web page) that
allows using ordinary electrical solder or plumbing (at least the
lead-tin variety of old) on aluminum. I've seen it applied and have a
tube, but I've not yet used in on anything. Soldering to aluminum with a
soldering iron is very difficult because only big blocks of copper are
better at dispersing the heat from the work place.

There have been other techniques of getting lead-tin solder to stick
aluminum usually involving wire brushing under oil or molten solder to
get through the oxide. When used outdoors (such as on radio antennas)
these techniques have not stood up to a damp environment where there
would electrolysis from the dissimilar metals in contact with each other
and with a weak electrolyte such as water (often spiked with a bit of
the flux).

The 2024-T3 reported to have been used by Airstream on these pages
recently is heat treated and may not react well to being heated to
nearly its melting point. Its an alloy that hardens with age. I have
some pieces in my shop and while they cut superbly, they break instead
of bending to sharp radii.

Gerald J.