Aluminum and Copper Wire
by Marty Weiser
This is a related article to the excellent article by Marty Weiser on Annealing Copper Wire.
A couple of questions came up (on the IBC) regarding copper (Cu) and
aluminum (Al) wire that were sent my direction. So here are both the
short and the long answers after discussing with a couple of folks at
The Short AnswerProperly
annealed copper wire can become harder during storage due to
precipitation hardening. This effect will increase with increasing
temperature - i.e. in the sun versus a cool spot. Aluminum wire that is
too hard can be softened by annealing since it will remove the effects
of work and precipitation hardening, although it is not that common for
bonsai wire. In addition, the aluminum must normally be cooled rapidly
after annealing to maintain the soft state.
The Long Answer
There are three major ways to harden a metal or alloy.
- The first is to work harden it by deforming the metal. This is
what makes copper wire become stiffer when you apply it to the branch
and is why you can break a paperclip by bending it back and forth.
- The second is precipitation hardening where you heat the
alloy to allow the different elements to internally react and form a
second reinforcing phase in the material. This is used for many
aluminum alloys and was discovered when some parts were left in the
attic of a barn over the summer.
- The third is quenching which involves heating selected
alloys and then cooling them to lock in an unstable crystal structure
which is generally very hard. This is used for steel cutting tools. In
all three cases it is common to have some effects from the other
methods going on.
I have used a some terms above (and others that will be used) that
should be defined:
- Alloy - a mixture of two or more different atoms. Generally
applied to metals, but can be applied to ceramics and polymers as well.
Normally the alloying is intentional to improve the properties (steel
is iron with carbon and other metals), but it can be due to impurities
- Crystal structure - an orderly arrangement of the atoms.
- Phase - region of the same crystal structure and chemical composition.
- Dislocation - a type of disruption that allows different parts
of the crystal to move relative to each other. It is like a bump in a
rug that can be pushed across the room to move the rug without having
to move the entire rug at once.
- Hardening - the process of making that alloy stronger. It is
generally accompanied by an increase in the resistance to penetration
which is what we generally call hardness.
Work Hardening in Copper
Copper and its alloys are generally hardened by work hardening since
this induces dislocations to form. There are several different
directions in the copper crystal for the dislocations to move just like
you can start two or more bumps in a rug. However, in copper the
dislocations moving in different directions have a great deal of
difficulty moving around each other - they become tangled just like a
case of gridlock when the traffic lights go out at rush hour. As a
result, the dislocations are no longer free to move when you bend the
wire so the wire is harder and stiffer. This also occurs in other
metals but to different degrees. The dislocations in aluminum can move
past each other fairly easily so it does not work harden a great deal.
The dislocations in the steel paperclip lock up so badly that they
convert into micro-cracks that eventually link up and cause the clip to
Wire is generally made by pulling a rod through progressively
dies until it reaches the correct diameter. This work hardens the alloy
and can make it too hard and stiff to use. Most wire is annealed after
drawing to soften it and make it more usable. Annealing is done at a
temperature of around 70 - 80% of the melting temperature on a scale
that starts at absolute zero (-273.15 degrees Celsius or 0 Kelvin).
Annealing allows the atoms to rearrange themselves into a lower energy
state. For a work hardened material the dislocations can sort
themselves out and even disappear as new, much more perfect crystals
arise. Copper work hardens so much that it is normally annealed one or
more times between drawing steps. For electrical wire it is generally
drawn to final size after the last annealing - this gives a smooth,
shiny surface and a wire that has some stiffness, but not too much.
Precipitation Hardening in AluminumAluminum
alloys are normally hardened by precipitation hardening. The part is
annealed and quenched (cooled rapidly) so that all of the
different elements are mixed together on an atomic scale - this process
is known as solution heat treating. This mixing is just like heating
water to allow you to dissolve more sugar in the syrup. At low
temperature the lowest energy state is composed of multiple phases
where the different elements form different crystal structures with
different chemical compositions. This process occurs very slowly at
room temperature in aluminum, but heating to 100 - 200 degrees C will
allow it to occur in minutes to hours and is called tempering. Copper,
silicon, and magnesium are common alloying elements in aluminum and
they form compounds like Al2Cu that precipitate from the matrix. These
precipitates act like stop signs to the dislocations - they have to
stop, but can move around the precipitate under the right conditions.
As a result the alloy will become stronger since dislocation motion is
slowed. Different combinations of time and temperature will result in
different precipitation distributions - high temperature favors a few
large ones that the dislocations cannot move around (but rarely
encounter) while lower temperatures give many smaller precipitates that
the dislocations that are more like yield signs, but are everywhere.
(an alloy of iron (Fe) and a small amount of carbon that often has
other elements added to improve the properties) is normally hardened by
quenching from high temperature. Iron has a face centered cubic (FCC)
crystal structure at room temperature and a body centered cubic (BCC)
crystal at high temperature. Carbon dissolves better in the BCC
structure and it takes time to rearrange the atoms from the FCC to the
BCC and vice-versa. Quenching high temperature BCC structure from red
hot (700 - 900C) to room temperature in water or a similar fluid
freezes in the BCC structure. However, at room temperature the iron has
shrunk a little and the carbon no longer fits so the crystal structure
distorts to form a body centered tetragonal (BCT) structure. It is
very, very difficult to move dislocations in the BCT structure so the
steel is now very hard and brittle. Tempering the steel at a moderate
temperature (300C or so) allows the iron and carbon to find a more
stable structure composed of FCC iron and fine precipitates of Fe3C (a
ceramic). This structure is still very hard, but will bend a little and
makes a fine cutting tool.
Effect of Purity on Copper WireBack
to our copper and aluminum bonsai wire. Bonsai consumes a very,
very small amount of copper and aluminum wire compared to electrical
uses so the bonsai wire makers buy electrical wire and convert it to
bonsai wire. The electrical conductivity of a metal is best when the
metal is fairly pure - an alloy almost always has a lower conductivity.
However, higher purity is more expensive, particularly as you go beyond
99.99% purity (100 ppm impurities) so a compromise is used. Copper is
fairly strong so most electrical wire is commercially pure 99.95% or
better to get high conductivity. Aluminum is not as strong so the wire
is normally alloyed a little copper and tempered to increase the
strength so that the wire does not stretch and sag under its own
The copper wire is fairly pure and the melting temperature is
high (1083C) so any precipitation hardening processes will be slow at
room temperature. Since bonsai wire is normally annealed under less
than ideal conditions (there is normally a fair bit of oxide on the
surface) we can be assured that there is oxygen dissolved into the
copper during annealing. A commercially important grade of copper is
oxygen Free Hard Copper (OFHC) which recognizes how easily oxygen
dissolves into copper. It is probably copper oxide that precipitates
and makes the wire harder after a year or two. Reannealing the wire
will dissolve the precipitates and make the wire softer, but unless the
atmosphere is closely controlled it will probably dissolve even more
oxygen into the copper so it will precipitation harden even faster.
Annealing Aluminum WireAluminum
and its alloys melt at a much lower temperature (450 - 560C) so
precipitation hardening can occur at room temperature and temperatures
that are only a little bit above room temperature. It is entirely
possible that commercial aluminum wire has seen high enough temperature
to become a fair bit harder, so annealing may be desireable. However,
reannealing is tricky and will destroy the anodization (an organic dye
in a porous aluminum oxide film). Aluminum wire should be annealed at
about 300 - 350C (570 - 660F and well below red hot) for 10 -20 minutes
and quenched in water. Getting the wire too hot will result in a molten
mass of aluminum which is apt to react strongly with a wide variety of
materials - possibly resulting in a fire that will be very, very
difficult to put out. There are few such dangers with copper since the
reaction with oxygen gives off far less heat and copper is a bit hard
to melt with the heat sources available to most of us.
For more information on annealing copper, see Marty's companion article:
Annealing Copper Wire
copyright 2002 all rights reserved