hello, I have a question for all of you with knowledge on
soldering aluminium. What kind of solder do I need.?
I have always been told to NEVER solder aluminum . . . I don’t
know why? Perhaps, others can fill in the details.
Aluminum cannot be soldered in the same way as silver, gold,
copper etc. It’s an extremely reactive metal, in that it very,
very, much wants to form oxides, and will almost instantly form
an oxide surface. That transparent colorless oxide film turns
out to be quite tough (in another structure, it’s chemically the
same as sapphire, so this isn’t such a surprise) and firmly
adhered to the aluminum, so once that oxide film forms, it
protects the metal from further oxidation. Aluminum is often
anodized, a process which simply increases the thickness of that
oxide film, while also allowing it to absorb a dye color if
desired. The increased oxide thickness greatly increases the
surface durability. Titanium and Niobium do much the same
Anyway, getting a metallic solder to work means getting a metal
to metal bond, hopefully allowing some diffusion between the
parent metal and the solder, rather than a metal to oxide bond,
which is little more than a poor glue which will quickly fail
even from thermal expansion and contraction as the joint cools.
The problem is that most fluxes have little effect on the oxide
layer on aluminum, so getting a clean surface to solder to is
almost impossible with our normal soldering fluxes.
However, there HAVE been several solder formulations developed
for aluminum, each with it’s own specific type of flux. These
are not normally within the domain of jewelers experience, as
mostly we don’t make a lot of aluminum jewelry that would need
soldering. The solders have more in common with the "soft"
solders, tin/lead, tin/silver, etc, that we use with
copper,silver,gold types of alloys (usually as last resorts)
instead of the silver and gold solders (which are more properly
called brazing alloys, though we jewelers seldom bother to use
that word. I can’t tell you much about the mechanical
properties (strength, etc) of those solders, but your local
welding supply dealer, where you get your gas tanks filled,
should have all the info you need. My understanding is that
these solders are usually applied with a torch flame, but I’m
not sure. The one I saw required the user to “scrub” the joint
with the solder wire to assist the flux in getting through the
oxide. But other than such stuff, I don’t expect it would take
any sort of special equipment. Aluminum is a wonderful heat
conductor, like silver, so you’ll need to be able to get the
whole thing warm, rather than soldering just a tiny spot. But my
recollection of these solders is that they don’t require
especially high temperatures, so almost anything should work.
Other joining methods you should keep in mind are MIG and TIG
welding, adhesives, rivets, and where possible, things like
bolts… Two heavier thickness items in aluminum can be drilled,
tapped, and joined with a fairly small steel threaded rod,
forming a double end screw, if you will. (Assumes you can rotate
one piece against the other to attach them. The result is a
completely hidden fastener. The steel is sufficiently higher in
strength than the aluminum that your overall item is quite as
strong as the aluminum itself, if you plan it right. Couple this
with a proper adhesive (super glue or epoxy, for example) to both
back up the finished joint, and lock the fastener in place, and
you’ve a quite strong and permanent joint.
Hope this helps.