Preparation and extrusion of multifilamentary NbTi conductor billets

In a cooperative program with industrial superconducting wire manufacturers for the purpose of studying billet preparation and extrusion, a total of twenty billets were assembled. Fourteen of these contained no superconductor but were ''dummy'' billets to evaluate methods of stacking to achieve the best packing density. These billets were not extruded but were sectioned transversely and longitudinally for evaluation of billet preparation techniques. The remaining six billets were assembled with NbTi stacking elements in a copper/copper nickel matrix using a standard conductor configuration. Two of these billets were instrumented with thermocouples and used to evaluate preheating techniques. The other four were extruded and drawn to finished conductor size for critical current measurement. This study showed that hexagonal stacking elements, contoured billet jackets and hot isostatic pressing resulted in high density uniform billets for extrusion. Rotary forging was found to be unacceptable for billet compacting. Fast billet heating was achieved in both the salt bath and by induction heating, but the induction method gave a more uniform temperature distribution during the heating cycle. Asymmetry from poor packing density in the billet resulted in nonuniform filament size distributions and poor critical current performance. Billet yield was increased as much as 16 percent by contouring the stacking element positions within the billet