How to (help): copper wire spring

[taylaron]

Greetings,
I need to make a spring out of pure copper wire. I'm using 30 awg off a spool but I'm struggling to make the copper retain its original shape after being stretched.

I don't know much about material science except that when metal is brought to a high temperature and then quickly submersed in a cooling agent the metal becomes very strong, as the molecular structure is "frozen" into place, but makes the metal brittle. Alternatively, a piece of metal brought to a high temperature and then let naturally cool tends to increase its spring characteristics.

I was wondering if anyone had suggestions on how I can make a decent copper spring. Specifically, what temperature I should heat the 30 awg wire to during the second process above and for how long. Heating would be done in a household oven.

I've done youtube searches and such, but I would like some opinions or suggestions.

Thanks,
-Taylaron

 

[Unrest]

One way is to do "work hardening". Just bend it a few times and it becomes stiffer. I'm not sure if electrical wire is already in a very stiff state or not. If you heat it and let it cool, it softens up again. The effect of heating and the sequence and timings vary greatly between different metals and can go in different directions. There are all these diagrams you can use to work out what phases a specific alloy will contain after some heating/cooling process.

It's not clear what you mean by "spring". Obviously wire straight of the spool is already a spring. Do you want it to store more spring energy per volume of copper? Do you have some constraint that prevents you just using more wire? Do you want greater deflection without yielding? Do you want greater load without yielding? In some cases a brittle material may be the best spring, or a ductile material. It depends where the yield point is and what the elastic modulus is.

 

[AlephZero]

Bending copper just once will work harden it significantly. Straightening out the wire from the spool is enough to do that, even if the wire was annealed after it had been spooled.

If you try to bend it twice to get the final shape you want, you will have problems because the second bend will tend to "spring back" after the first bend work-hardens the material.

To anneal copper you need to heat it till it just starts to glow red hot for a short time (for thin wire, a few seconds is enough) and then let it cool naturally in the air. A household oven will not be hot enough. You need a temperature of about 700 C. Get a small propane/butane torch or something similar.

You can anneal copper repeatedly without accumulating much permanent damage to the material structure, provided you don't over-heat it and oxidize the surface.