Spring Steel Wire
In the D3D Universal 3D printer extruder, we are using 1.25 mm spring steel - or 0.05 inch wire.
Welding wire is 0.035" - close to the diameter - but much lower strength - at 70ksi.
Spring steel wire is 260 ksi - about 4x stronger! 
Kevlar is about 520 ksi. (3620 MPa)
- McMaster -