Annealing Copper Wire

by Marty Weiser

Introduction

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. BW

Work Hardening

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

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 aluminum wire.

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 diameter wire.

And finally

Marty 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