Aluminum and Copper Wire

by Marty Weiser

Introduction

This is a related article to the excellent article by Marty Weiser on Annealing Copper Wire.
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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 work.

The Short Answer

Properly 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

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.

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 as well.
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 break.

Wire is generally made by pulling a rod through progressively smaller 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 Aluminum

Aluminum 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.

Quenching Steel

Steel (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 Wire

Back 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 weight.

The copper wire is fairly pure and the melting temperature is fairly 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 Wire

Aluminum 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.

And finally

For more information on annealing copper, see Marty's companion article:
Annealing Copper Wire