Extrusion of a hole for tin core insertion in the subelement billet for internal tin superconductor

A process has been developed for extrusion of a hollow Nb/Cu subelement billet for internal-tin Nb₃Sn superconductor. The development is part of a comprehensive strategy for cost reduction in fabrication of high-performance Nb₃Sn wire. A hollow-core billet was extruded using a tapered-mandrel die so that the hollow core was preserved through the extrusion process. The extrusion went well, but a mistake in our preparation of the rods that filled the billet caused the resulting extrusion to be unusable for drawing. A small admixture of Cu-clad NbTi rods was interspersed among pure Nb rods in the billet stack. The initial cold drawing of the NbTi rods caused them to harden dramatically compared to the Nb rods. When the billet was extruded the NbTi rods did not reduce in dimension and instead arranged themselves end-on in the final extrusion. Notwithstanding this disappointment, the inner and outer Cu boundaries extruded without deformation and the center hole has a clean, smooth surface that would be suitable for insertion of a Sn core rod after pickling.