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Maximizing Extrusion Ratio of High Temperature Superconducting Metallic Precursors (MP) Disclosure Number: IPCOM000028008D
Publication Date: 2004-Apr-19
Document File: 4 page(s) / 1M

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The key element of this superconducting metallic precurosr process involves the selection of composite billet preheat temperature such that at the desired reduction, the adiabatic heating induced by the work of deformation in the material as it passes into and through the die elevates its homologous temperature to 90%

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Maximizing Extrusion Ratio of High Temperature Superconducting Metallic Precursors (MP)


The stress-strain relationship, flow and fracture properties of most metals are dependent on temperature (figure 1). In general, strength (the stress at tensile failure) and flow stress decrease, while ductility (the strain to tensile failure) increases (figure 2) with increased temperature. Elevated temperatures can assist deformation via increased mobilities and reduced pinning of defects such as dislocations (reduced flow stresses) and structural changes. Therefore, low cost, high speed extrusion of most metals is typically done at elevated temperatures that allow the required level of ductility, with acceptably low flow stresses to avoid stalling of the extrusion press.

Extrusion is also a preferred step in the fabrication of miltifilamentary metal / sheath composite wires or tapes that are precursors to multifilamentary superconducting oxide / metal matrix conductors. Unlike the superconducting oxides, metal precursors (MP) to the superconductors can be deformation processed to large plastic strains (> 99%) without fracture or voiding. In the first step of this process, a metal can is packed with the precursor metal powder. The can is sealed and evacuated, followed by extrusion to a rod or tape form. This extrudate is then cut to length and rebundled into another can, that is again extruded, thereby forming the fine-filament composite precursor wire that can be further processed to form the superconducting oxide /metal sheath composite.

A key parameter in extrusion is the homologous temperature, Th, of the workpiece, where Th is defined as the ratio of material temperature (T) to it melting temperature (Tm) in degrees K

(i.e., Th = T/Tm). The extrusion constant Ke of a material defines the extrusion pressure (P) needed in the chamber to reduce the workpiece cross sectional area from Ao to Af via equation 1. (such that the reduction R = (Ao/Af)).

K3 - P/ln(R) equation 1.

Extrusion constants are typically determined empirically. The extrusion pressure required to extrude a composite material such as the precursor metal / sheath billet is determined by the flow stresses of the composite constituent materials, as well as the friction. If friction is constant, then the flow stress differences between the materials are directly related to the constituent material extrusion constants, and a rule of mixtures formalism (equation 2.) correlates the composite extrusion constant (Kc) to the constituent material extrusion constants (Ke1, ke2, ..., kei) where the length-invariant cross-sectional area fractions of the constituent materials are given by (a1, a2, ..., ai).

Kc = a1 Ke1 + a2Ke2 + ... +ai Kei equation 2.

Empirically, it has been found that the extrusion constant of the metallic precursor alloy to the superconducting oxides decreases with increased homologous temperature, Th, see figure 3. The composite environment then allows a uni...