The core idea of preparing NbTi/Cu superconducting wires by powder metallurgy is to directly composite alloy powder with copper, by passing the complex alloy casting and barrier layer assembly steps of traditional processes. The overall process can be divided into four stages: raw material mixing, encapsulation, densification, and plastic processing.
Raw Material Mixing: NbTi alloy powder (particle size approximately 10-50 μm) is mixed with pure Ti powder or pure Nb powder (particle size approximately 1-5 μm). The core proportion of pure Ti or pure Nb powder is 20%-30% by weight. These will form a dispersed α-Ti precipitate phase during subsequent heat treatment, serving as flux pinning centers and effectively improving the current carrying capacity of the wire.
Copper Matrix: The mixed powder is filled into an oxygen-free copper ingot with multiple channels. Copper, as a stable matrix, provides good thermal and electrical conductivity. Protective Encapsulation: The copper ingot containing the powder is encapsulated in a stainless steel encapsulation. This outer sheath protects the internal material and prevents oxidation during subsequent high-pressure processing. The degassing seal involves heating and evacuating the entire system to remove gas from the powder gaps and within the sheath; this is crucial for ensuring the material is dense and defect-free.
Hot Isostatic Pressing (HIP) involves placing the sealed sheath into a HIP furnace and sintering it under high temperature and immense pressure in all directions. This results in diffusion and bonding between powder particles, directly forming a completely dense NbTi/Cu multicore composite ingot. The outer steel sheath is then machined off using a lathe. Plastic Extrusion: After heating and holding the composite ingot at room temperature, it is extruded in one pass to obtain a multicore extruded bar with a smaller diameter and uniform microstructure. This step effectively welds the internal interfaces and refines the grain size.
Cold Drawing: At room temperature, the extruded bar is drawn through a series of dies with progressively smaller dimensions in multiple passes. The deformation must be controlled in each pass to ultimately obtain a superconducting wire with the desired diameter (down to millimeters or even micrometers). The core advantages of this powder metallurgy method lie in its simplified process and reduced costs. It cleverly bypasses the complex smelting and casting processes of traditional methods, as well as the barrier layer necessary to prevent copper-titanium reactions, significantly shortening the process flow.
After obtaining the basic wire, performance can be further optimized through the following methods: Microstructure control: By precisely controlling the number, temperature, and time of aging heat treatment, the morphology and distribution of the α-Ti precipitate phase can be controlled, thereby significantly improving the critical current density of the wire under high magnetic fields. Core wire refinement and loss control: Continuing to increase the number of drawing passes can refine the core wire diameter to the micrometer or even nanometer scale. This not only effectively reduces superconducting AC losses but may also further enhance performance at specific scales using the "surface pinning" effect. Artificial pinning centers: In addition to the α-Ti precipitate phase, researchers are also exploring the use of alloy systems such as NbTiTa to optimize performance by introducing a more uniform and controllable second phase as "artificial pinning centers."
