aluminium, tin-lead solder, soldering, metalurgical bonds, soldering
copper to aluminium, no longer impossible ,electrical bonds breaching aluminium oxide spontaneous passivation layer.abstract:
conventional wisdom states that it is impossible to solder to
aluminium. A method is presented to permit the easy soldering of
copper directly to an aluminium substrate that is readily
reproducible be persons with basic metalworking skills using only
a propane torch, using "Alumalloy Braze" and conventional resin flux
60/40 Pb/Sn solder.
by Ralph Klimek VK3ZZC February 2011, copyleft
interest in this technique stems from my Ham Radio interests. I
have often wanted to make good electrical bonds to aluminium metal
antenna elements and have been thwarted by the uncompromising chemistry
of the elements and the metal. A pressure bond up to now has been only
real possibility and those with experience learn the hard way that a
pressure bond lasts for only a couple of weeks when exposed to the
weather. An antenna that performed well just after it left the workshop
becomes increasingly unsatisfactory after only a few weeks exposure.
And this despite weather shielding the connections and even passivating
the junctions with silicone resin. A simple pressure bond may
conduct DC current but the RF AC conductance can be very low.
antenna designs are often limited by the difficulty of performing a
metal joining operation where the RF impedance is low, for
example at the centre junction of groundplane antenna or at the centre
of a dipole element. There then follows the very real difficulty
in terminating the copper coaxial conductors onto an aluminium driven
element. Those with experience know too well the corrosive effect
of two dissimilar metals exposed to weather. The feed point
impedance of a driven element in a multi element yagi array is of
the order of five ohms or less and I suspect that many of my failed
antennas were merely defective only at this feedpoint.
RF conductivity, the only true options are all copper elements
permitting well soldered , low impedance joints; at the cost of heavy
weight and monetary cost. My most successfull yagi antennas all had
copper driven elements. If only it were possible to make a true
metalurgical bond to aluminium at moderate temperature that would
be compatible with copper.
It is possible, sometimes, and with
exotic alloy solders and exotic fluxes. Aluminium soldering is nothing
new, however, manufactures keep their methods to themselves and makers
of the solders will not release usage notes. Here I present a
highly reproducible method that a competent Radio Ham can replicate
using only a simple and inexpensive propane gas torch.
method requires the use of a now commonly sold aluminium brazing rod.
This rod is made under the trade name Alumalloy and sold in the United
States under the name Durafix. It , I believe, is a ternary alloy made
from aluminium, copper and magnesium with a melting point of 430
degrees C. It has been available under various trade names in
Australia for a number of years, it is known here in Oz as " aluminium
rubbing solder". There is absolutely no application information
published about it. (conspiracy theories welcomed here!) I have
recently learned how to apply this remarkable alloy to make aluminium
to aluminium brazed joints, after watching some Youtube videos. Search
Youtube for the term "Alumalloy" and see for yourself. I have used it
with success to make some antenna elements with it. Only the next step
remained....bonding copper conductors to my aluminium antenna elements.
understand the method, you should be familiar with the surface
chemistry of Aluminuim. Element 13 is one of the most
electropositive of the metallic elements. The reaction between Al and
Oxygen is one of the most exothermic reactions known and forms the
chemical basis of the Thermite method of welding steel and powers the
Space Shuttle solid rocket boosters which burn Al powder and Ammonium
Perchlorate. What prevents your Al kitchen utensils and Al foil from
spontaneously bursting into flame ? ( and incinerating your dinner )
bare Al is exposed to the atmosphere , it immediately reacts with
Oxygen and forms an oxide layer on the surface. This oxide layer is
normally only a few atoms thick. The oxide is very tough, and binds
very strongly to the metal surface. Aluminiun Oxide is the chemical
basis of hard abrasives, Ruby, Saphire and integrated circuit
substrates. Thicker layers of the oxide can be artificially grown in a
process called anodising. The oxide layer totally passivates the metal
surface from further oxidative attack under normal atmospheric
conditions. The oxide is soluble in most mineral acids and strong
alkalis. It is this oxide monolayer that prevents the wetting of
solder and conventional brazes. This monolayer can be
mechanically breached and a direct metal-metal bond formed provided
that oxygen can be briefly excluded or actively scanvenged.
pressure bond to the surface of Al metal is really an oxide
sandwich. The oxide is very thin so a moderate amount of pressure
can get the electrode close enough to the Al base metal so that
electron tunneling occurs through the oxide monolayer. Perhaps when the
pressure bond is new, the oxide is breached and this permits a good
electrical connection. After a while, oxygen must penetrate into the
bond and slowly seperate the electrode from the base metal. This
is why , as myself and countless radio amatuears have discovered, their
newly minted yagi array quickly deteriorates once put out of reach up
on your tower.The technique.
Alumalloy braze melts at about 430 degress C, pure Al and its common
alloys at about 700 degress. 400 degrees is well within the
power of a propane torch, but utterly beyond the upper range of a
Heat the base metal from below. Touch the
brazing rod to the base metal. Do not heat the brazing rod directly
with the torch...it will just melt and oxidise.
When the metal is at
the right temperature the braze will begin to melt. As it melts rub the
base metal with the rod. This breaches the oxide monolayer and permits
an instant metal-metal bond to form under the molten surface. The
Oxide monolayer is unstable on the brazed surface and liquid braze will
literally burrow underneath it. Rub the molten braze bead with a
stainless steel knife and "tin" the surface of the base metal. The
purpose of rubbing with the steel blade is to breach large areas of the
oxide layer under the braze melt. Continous heating is required while
you are doing this. The initial bead of molten braze will not wet the
Alumium surface untill that surface is scratched UNDER the bead. The
molten bead temporarily excludes atmospheric oxygen and only then will
it bond with the base metal.
Wipe the layer of oxidised dross
with the knife blade away from the brazed surface and allow to cool.
Reheat from below. Apply conventional 60/40 lead-tin resin fluxed
solder to the brazed surface and do not overheat or permit the resin
flux to burn. A perfectly formed solder bead will form ! Allow a
large bead to form on the surface and cool. Your copper conducter
can now be reflow soldered to this surface. At this point a very
heavy 100W iron may have enough power , gas is better because of the
very high thermal conductivity of Aluminum metal. A perfect
copper to aluminum solder bond is thus made.
should be prepared by filing to bare metal with a very fine bastard
file to produce the smoothest surface possible. Polish with a
FINE wire brush, a suede fabric brush is what is really needed here. If
the surface has been anodised, this must be completely abraded away to
bare metal.Why does this work and how does Alumalloy braze work ?
does not require a flux ! It took me a while to appreciate this
fact as it flies in the face of common sense and convention. The braze
is an alloy of aluminum, copper and magnesium and maybe other
undisclosed metals. I beleive that the purpose of the magnesium
in the braze performs the function of oxygen scavanger. Under this
melt, a breached Al surface will bond directly and the magnesium
prevents the dynamic formation of fresh Al Oxide monolayer by competing
with Al for dissolved Oxygen in the melt. The magnesium is the flux
! I do not know this for a fact, it is only my educated guess.
However, it is true that Magnesium has great affinity for oxygen and
unlike aluminium, the magnesium oxide is not strongly bound to the
metalic surface. Maybe a high copper component is what makes this stuff
solderable, I do not know. I do not have the ways and means of
chemically analysing it, maybe someone out there does.
flabbergasted that standard electrical Pb-Sn solder is compatible with
the brazed surface , this was not expected and I found this only by
experiment.Do not just take my word for it, try it yourself.
is available in Australia, retail. I bought mine at Bunnings
where it is sold as Bernzomatic Aluminium Brazing rods. It is very
expensive, 2 little rods cost AUS $10. It comes with absolutely no
application advice. Maybe you have bought some and discarded it in
disgust because it " does not work "? Believe me ,it does work
but only with the right technique as described. The observation that
the braze is compatible with standard electrical solder comes to me ,
and I suspect the rest of the Ham world, as a complete and welcome
The continued quest to find a good way to electrically bond to weather exposed aluminium elements
recently found a sheet of light gauge stainless sheet during some
recent dumpster diving. I am interested in answering the question as to
whether aluminium and stainless steel would form a bimetalic
couple when exposed to the ambient weather. Could I fabricate
some fittings from stainless steel that would be sensibly
corrosion resistant.? The image below shows some of my results.
stainless steel can be cut, drilled and formed with some difficulty but
the good results make it well worth ones time to master the art of
working stainless steel. There is no sign (yet) of white
powder forming in the aluminium to stainless steel joint. This
has the potential for long term application on my antennas out of reach
on my high sticks. Now then, how to attach a copper conductor ?
It is not generally possible to soft solder stainless steel.
There are some generally unobtainable soft solders that will bond to
stainless steel. There is no force on earth or simple flux that will
allow standard 60/40 SnPb solder to bond. Stainless steel can be
easily brazed with 2% ( 5% is better) Silver brazing and
moderate heat. The braze IS compatible with 60/40 SnPb
solder. Problem solved. Even better then, braze a bit of 1/4inch
hydraulic copper pipe and use it to form a very solderable electrode
cup for your coax connections. The copper pots are held down with some copper wire scraps during the brazing process.
stainless steel with angle grinder and purpose made
stainless steel cutting disks. These will carry the designation
INOX. You can also use a very heavy tin snips with some care and brute
stainless steel , first with a new 1/8 inch cobalt
drill pilot hole, then enlarge. The special secret method of
drilling stainless steel is
to use moderate speed and moderate
to heavy drill pressure. Keep things going, never hesitate and keep
things cool. Do not drill free hand, IT JUST DOES NOT WORK ! Use a
drill press. Provide an aluminium backing plate to
the work piece. This helps the drill to completely penetrate and
prevents the stainless steel from deforming and work hardening
around the rotating drill bit.
Drill bits in stainless steel
do not last very long. You have to consider them consumeables or
you become skilled in the fine art of re-sharpening them. A
high speed steel new drill bit will have a lifetime of only 3
holes in stainless steel ! Cobalt steel may last up to 12 holes . .
. . Deal with it.
stainless steel , over a mandrel that is a little thinner than the
aluminium tube. This can be done with a heavy duty vice and standard
drill bits. You will have to experiment a bit to find the right
mandrel. Dont use a 12mm mandrel for 12mm tubing, your fitting will not
The next imperial size down is about right.
you know that is was possible to make poor quality standard 60/40
solder bonds directly to Aluminium ? Melt solder on Al while scratching
under the solder bead. Some bonding occurs; do take too long, oxygen
dissolved in the molten solder will quickly react and the solder bead
will no longer bind. It just barely works and I would not rely on
a bond like this. The bead will spontaneously fall off the base metal
after a few weeks, even though initially it appears to be strongly
bound. Why ? I believe the instability of this joint is
due to atmospheric oxygen that dissolves into the moltern lead
tin alloy. After about 2 weeks this oxygen diffuses through the the
solid solder to the aluminium layer where it reacts directly and
this reaction releases the solder bead. I wonder if a ternary
alloy of lead-tin-zinc might work. The zinc would act as an
oxygen scanvenger in the solidified bead. Remember
that metals are not totally impervious to gas. A chemically reactive
gas will diffuse through solid metal, albeit, very slowly
at room temperature.
to try. Will enough Magnesium dissolve in Pb/Sn solder to behave as an
oxygen scanvenger and permit direct solder to al base metal junctions ?
Discuss. How about Titanium ?
braze will also bind
well to the metal of die cast boxes and other die cast objects. Hams,
solder directly to your die cast box projects! Die cast metal or "pot
metal" is a variable alloy made from shop-floor-sweepings, zinc,
magnesium, aluminium and a hint of copper.
is available in the United States for " six dollars a pound ",
according to the Youtube infomercial. Someone should be importing this
by the shipping container load into Australia.
page created Tue Feb 1 17:36:45 EST 2011