New Cryogenic Processing for the Development of High Strength Copper Wire for Magnet Applications

A comprehensive study of the effects of 77 K deformation on the mechanical properties and electrical conductivity of copper is presented. A cryogenic wire drawing process has been developed that was expected to produce higher strength at 295 K than that can be achieved with conventional processing methods. This should be possible because of the reduction of dynamic recovery of copper relative to that observed for the room temperature process. With this cryogenic method, drawn pure copper wire achieved a strength level of 580 MPa and a conductivity of more than 92% IACS at room temperature. This strength level is about 50 % higher than that obtained by an equivalent room temperature copper drawn. The decrease in the conductivity was relatively low and its value was higher than the majority of the high strength composite conductors. Although cryogenic drawing is effective in delaying the dynamic recovery of copper, it can not suppress it. A decrease in tensile strength and an increase in electrical conductivity were detected for deformations larger that e = 2.3. This behavior was accompanied by an interesting crystallographic texture change, which can explain the properties recovery described. In addition, the processing methodology has the potential to link the development of new approaches to materials selection and production to specific needs in a variety of magnet designs. The potential for cryogenic deformation in the development of high strength conductors of pure copper is also discussed in this work.