Bill;
There's a wealth of mis-information floating around about "fillet brazing"
(also called "braze welding") so I'll try to help straighten out some of
this confusion.
First, fillet brazing is distinctly different from the more common type of
brazing-- both in filler rod material, technique, and joint temperature.
Both types of brazing are best done with an oxy-acetylene torch although
other gasses are sometimes used with O2 (MAPP, natural gas, or propane),
both use a filler rod, and both do not involve melting the base metal, only
the filler rod. Both types of brazing also require clean, well-fitting
joints and the use of a flux.
Ordinary brass brazing is done at a fairly high temperature with an
oxy-acetylene torch and the liquidus (melted filler rod) is very
thin-flowing (runny). Capillary action makes it capable of joining tubing
that is telescoped together but it has very poor strength in butt joints.
Low strength together with low ductility make it unsuitable for structural
joints. This type of brazing is what most weldors have been exposed to and I
think this resulting bad experience with low-strength joints has soured the
reputation of all types of brazing.
Fillet brazing is another story. The filler rod is an alloy that melts at a
lower temperature and is capable of forming a "bead" that (other than its
color) looks like a nice fusion-welded joint. Good fillet brazing rods are
available with strengths of around 80k psi to 95k psi and they have
excellent toughness. These alloys use nickel and silver together with a few
others in a copper base, giving a good combination of wetting (silver),
ductility & toughness (nickel) and strength.
Fillet brazing isn't difficult. The important things to remember to produce
a strong joint is to make the fit of the tubing (or whatever) VERY close.
You cannot expect to use a braze fillet to cover up gaps-- although it can--
and achieve a strong joint. A strong fillet brazed joint depends on having a
close fit! Another critical point is to have the joint CLEAN. Both pieces of
the joint should show shiny bare metal and be covered with a coat of flux.
You cannot fillet braze something that has paint, rust, or plating still on
it. The third important point is to not overheat the joint. Fillet brazing
takes place at a red heat. Overheating the joint can result in intermetallic
formation and these produce weak, brittle joints.
It's fairly easy to see what is happening in the joint while heating it with
the torch if you discard your normal dark welding goggles and use a much
lighter shade. "Cobalt blue" goggles are usually recommended for fillet
brazing but I usually just use dark sunglasses. Since we are using a far
lower temperature torch, the eye-damaging UV radiation isn't a concern like
it would be with a TIG or MIG arc.
Since the base metals are not melted together as in a fusion welded joint,
fillet brazing can join a surprisingly different combination of metals. This
is why 1020 and 4130 can be fillet brazed together. Forget aluminum, but
some stainless alloys can be brazed.
There are a number of fillet brazing alloys available from welding products
vendors such as:
Harris-Welco Welco #14:
http://www.jwharris.com/welprod/cubasedalloys/
ESAB Allstate #13
http://www.esabna.com/html/esabna01.html
Eutectic Eutectic #16 http://www.eutectic-na.com/
(a perfectly worthless website!)
There may be others, but I've used these rods, usually in 3/32" or 1/8"
sizes. I find it easier to buy the bare rod and apply a paste flux to the
joint with a brush. This prevents joint oxidation as it is heated by the
torch better than the flux- coated rods.
To assure ourselves of the strength & toughness of fillet-brazed joints, we
prepared a number of right-angle tubing joints and brazed them together with
different brazing alloys as well as a gas fusion-welded joint. Each was put
into a big vise and beat on viciously with a big hammer. We bent the hell
out of the tubing but none of the joints that were fillet brazed with one of
the recommended alloys or the fusion welded joint failed. Conventional
brazed joints were easily broken.
To satisfy yourself that you are doing things right, try the same
experiment. Beat on it until it breaks; you'll either satisfy yourself that
it's a good technique or not. A friend's car went over the side of a
mountain for a ways during the old Clifton Hillclimb but neither he or his
fillet brazed space frame chassis was damaged!
Many high-dollar custom bike frames are fillet brazed, as were some
motorcycles (Rickman, for one), and race car chassis. It seems as if fillet
brazing was far more widely used in the UK than in the USA.
I hope this may help.
Regards, Neil
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