Annealing Copper Wire
by Marty Weiser
Marty Weiser is a regular contributor to the Internet Bonsai Club. He
describes himself as "Marty - actually a ceramist who is working as a
metallurgist and has moved on to marketing". The following article is
the best description of the process and the physics of annealing copper
wire that I have come across.
Copper has a cubic crystal structure. It is the particular variation
(face centered cubic) that gives it it's extreme ductility so that you can bend a wire into really sharp
curves. Aluminum, silver, and gold among others have the same crystal structure.
Bending a copper wire work hardens it which introduces defects known as
dislocations into the structure. These defects interfere with further deformation and
make the copper hard and strong so it is not easily rebent. This is why copper is so good for bonsai -
it bends easily the first time, but then holds its shape. Aluminum work hardens less than copper
while gold barely work hardens at all.
Annealing the copper eliminates the dislocations so that the copper is
once again composed of nice perfect crystals. This allows the copper wire to be reused since it
is now soft and easily bent. Generally annealing is done at greater than 1/2 of the melting point
on the absolute temperature scale. Copper melts at 1083C = 1356K so the annealing is done
at greater than 678K = 405C = 761F. However, it will take a fairly long time at the lower
end of the range so it is more common to anneal at about 700 to 800C. The copper can be worked and
annealled many, many times for the purposes of bonsai since we really don't need extremely
tightly controlled properties. The properties will degrade with repeated cycles for various
reasons - oxidation being the most obvious.
The copper will maintain it's soft crystal structure after annealing at
any realistic cooling rate (from very slow like letting fire die down to fast like throwing it in a
bucket of water). Generally, I would suggest water cooling to prevent excessive oxidation of the surface.
Steel (iron + carbon) on the other hand will change it's properties dramatically upon rapid
cooling . However, it is possible to cool copper fast enough to make it into a brittle material. This normally
involves cooling rates of greater than 10 million degrees C per second which can only be obtained by
spraying a very thin film on to a very cold surface (this equipment is very expensive).
For bonsai wire it is generally not necessary to take strong
measures to prevent oxidation
unlike jewelry or electronics where a precise part is being made. So go
ahead and heat it with
whatever safe form of heat you have to a dull red glow. The woodstove,
the barbecue, a torch, or the gas stove will work just fine.
Just remember that the copper will be hot enough to ignite most
flammable objects if you set it
down on them and it will give you a nasty burn if you are not careful.
In addition, copper is an
excellent heat conductor so heating one end of the wire to red hot will
quickly result in the other
end getting hot too.
Increase In Work Hardening Strength
Fully annealed copper wire will increase in strength by a factor of about
two as it work hardens. In addition, it takes relatively little deformation
to bring about a fair bit of this strengthening. I am hoping to run some
good solid tests later this summer.
I estimate that the act of wiring a branch and then bending it once into
some reasonable shape will increase the strength of the wire by a about
50%. I don't have any strength data for copper versus aluminum here
with me, but if memory serves me right the strength of fully annealed
copper is 50 to 100% higher than fully annealed aluminum. Given that
aluminum does not work harden as much as copper this means that once
wrapped around the branch and bent a given diameter of copper will
have about two to three times strength (and whence holding power) of
Large Gauge Wire
A further comment is that copper work hardens enough that merely
taking a large diameter wire off of a coil will increase its strength
appreciably. This can be good in the hands of one who really knows
what they are doing since the stronger wire will then hold more.
However, if the wiring skills are not as advanced then the wiring will be
that much more difficult since they are starting with a really stiff chunk
of wire (almost rod). It is therefore probably a good idea to anneal
really large diameter copper wire in a straight piece rather than on a coil.
Of course the other alternative is to use two or more pieces of smaller
later posted a followup article to the IBC explaining in more detail
the process of work hardening and precipitation hardening in copper and
aluminum wire. You can find this information in the related article:
Copper and Aluminum Wire
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