Co-deformation and bonding of multi-component billets with application to Nb-Sn based superconductor processing
The standard procedure of the fabrication of low temperature superconductor precursors is the co-extrusion of composite materials followed by co-drawing. Defects including wire breakage and poor bonding between core and sleeve as well as between individual filaments are the main manufacturing problems. Understanding of interfacial bonding during the co-extrusion and co-drawing of the composites including the distributions of deformation, stress and temperature, and the generation process of interfacial bonding under different conditions will be helpful for the selection of parameters in the manufacture. First, the bonding created during the extrusion of subelements was examined and characterized using a focused ion beam (FIB) technique together with scanning transmission electronic microscopic (STEM) and the results confirmed that perfectly bonded subelements could be obtained by proper assembly, HIP, and subsequent extrusion. The second part of the work was aimed at the bonding generation during co-drawing process. Finite Element Method (FEM) was used to simulate the co-deformation process in order to investigate the effects of die angle, area reduction, core ratio, and the variation of bonding between components on the deformed geometry, and stress distribution in the product. The FEM simulation incorporated with a Pressure Bonding Model to study the generation process of interfacial bonding between components during the drawing process. This work starts with the multiple-pass drawing of a simple cylindrical monocore arrangement consisting of a Nb7.5%Ta core inside a Cu sleeve and a six-around-one restack of the monocores. The effect of the drawing pass, area reductions, die angles and core ratio on the deformation and bonding generation was investigated. On the experimental side, Cu-clad Nb7.5%Ta 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 Nb7.5%Ta/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.
School:The Ohio State University
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:interfacial bonding co drawing fem superconductor fabrication
Date of Publication:01/01/2005