Investigation of Bonding Generation during the Drawing of Multifilamentary Wires

Experimental shear tests and Finite Element Modeling (FEM) have been used to study the generation of bonding during the multi-pass drawing of multifilamentary wire, focusing on the experimental conditions and materials of interest to composite superconductor manufacture. The wires were of two kinds: (1) a simple cylindrical monocore arrangement consisting of a Nb7.5%Ta core inside a Cu can (or sleeve), (2) a six-around-one restack of the monocores. For the modeling a thermo-mechanical, elasto-plastic, finite-element model was employed. Two kinds of simulations were performed: (1) the initial state of the billet was assumed to be without bonding. The FEM simulation was then performed, and from this the normal pressure, deformation, and temperature at the core/sleeve interface were calculated; (2) a pressure welding model was used to estimate the bonding stress generated at the interface by the multi-pass drawing process. On the experimental side, the Cu-clad Nb monocore 'billet' was drawn to certain size, restacked into a Cu can to form a 7 restack multifilamentary billet, and then drawn to small size. High resolution scanning electron microscopy was used to observe the characteristics of the Nb/Cu and Cu/Cu interfaces and mechanisms for the observed interfacial bonding were proposed. Furthermore, specially designed shear tests were carried out to determine the interfacial shear load. The results of these led to calculations of the interfacial shear stress (i.e. the bonding stress at the interfaces), the results of which were compared with the FEM predictions in order to establish the validity of the FEM model