Work hardening cu wire?

We manufacture rectangular magnet wire (ETP) via the "Conform" process for use in transformer windings. The material is approximately 1/4 hard as extruded. Is there a way to predict the increase in yield strength attainable by using in-line bending rolls, ie. "work hardening"? My goal is to predict or design a suitable process to achieve higher yield strength material without altering dimensions (reduction of cross-section). Thanks- GBR

Hello,

I am not aware of any mechanical process that would increase the yield strength without changing the dimensions of the rectangular copper wire. Please remember these definitions:

Yielding
Any increase in stress beyond the yield point will cause the material to be deformed permanently. In this yielding region, the deformation will be relatively large for small, almost negligible increases in the stress.

Strain Hardening
When loading is carried beyond the yielding region above, the load needs to increase for additional strain to occur. This effect is called strain hardening, and it is associated with an increased resistance to slip deformation at the microscale (for polycrystalline materials). Eventually, the stress-strain curve reaches a maximum at the point of ultimate stress and fracture occurs.

Sincerely,
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com

Peter,

I'm not sure why it would be necessary to increase the yield strength of magnet wire. Unless you are doing something like we discussed earlier where you ended up with a higher yield and better ductility, however that is not the case here.

Shape magnet wire is different than round wire. You have corners to contend with plus you have to make edgewise bends. If you flex the wire through rollers and increase the yield and make it less ductile you are going to have problems forming the coils. Also since magnet wire coils are usually supported either on a bobbin, coil form or self formed or bonded, I don't see where you would ever need "stronger" wire. Especially since most magnet wire is dead soft or nearly so after enameling and this includes sizes that vary from nearly invisible (OK so I'm getting old and it is more difficult to see 50+ AWG wire any more)to very large cross sections.

GE used to use a turks head mounted between the pay off and the coil winder and would "change" the dimensions of the enameled wire slightly to meet specific needs. But again the dimensional change was only several thousands of an inch decrease in width and thickness. The elasticity of the insulation was adequate enough that the coil was still good. It got stuck into an oil filled transformer so that also masked any surface damages.

Spectre/richard