Brazing of tungsten

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Scott.L.Hanson-at-seagate-dot-com>

Terry -

Both straight tungsten and tungsten carbide braze very nicely.

Getting enough heat into the parts will be a problem if there is a large
copper or brass heat sink involved. Industrial practice for cutting tool
inserts is to use induction heating, but a good propane torch with a
needle-point burner can sometimes be used for small parts. Anything with
mass will require a MAPP gas torch or better yet an oxy-acetylene unit. A
large factor is the melting point of the brazing alloy. The high-silver
content (~45%) alloys have the lowest fusion temperatures and the best
fluidity. The best solution is to use a brazing paste, which is a blend of
powdered alloy and the appropriate flux. Clean both surfaces, apply a very
thin film of the brazing paste, position the tungsten, and apply heat. When
both parts are up to temperature, the paste will "reflow", capillary action
will pull everything together and result in a perfectly uniform paper-thin
joint. If it is a cylindrical joint (section of tungsten rod in a brass
bushing, etc) leave several thousandths of diametral clearance so the joint
can "fill up" with the brazing alloy without any "dry spots". This will
result in the greatest strength and best thermal transfer. (All very much
like a surface-mount component and how it behaves under a hot-air reflow
process.)

I have considered using an industrial "heat pipe" as a cooling element in a
static gap, but have not played with it yet. A heat pipe will have hundreds
of times better heat transfer ability compared to a solid copper rod of
equivalent size. At one time these were novel and quite expensive, but are
now very commonly used in many industrial applications. I sometimes
comeacross them as surplus.

Scott