The vacuum melting of C103 niobium alloy is a crucial step in the production of this high-performance material. The key lies in achieving high purity and uniform composition of the ingot through a high vacuum environment and multiple melting processes, which lays the foundation for its subsequent processing and excellent performance. It enhances its strength through solid solution strengthening (Hf, Ti, Zr are incorporated into the Nb matrix) and dispersion strengthening (compounds are formed by Hf and C, O). It also possesses excellent high-temperature properties, good plasticity and weldability, making it an ideal material for manufacturing aerospace high-temperature components such as rocket engine nozzles and thrust chambers.
The melting of C103 alloy mainly adopts vacuum self-dissolution arc melting (VAR) or electron beam melting (EBM). Due to the presence of high-melting-point niobium and low-melting-point titanium and zirconium in the alloy, ensuring uniform composition is the core difficulty of the process. Preparation of electrodes: This is the preparatory stage before melting. One of the core methods is the self-dissolution electrode method, which involves first using powder metallurgy or welding methods to form a "self-dissolution electrode" with initially uniform composition from the alloy elements. For example, one process is to first sinter W, Ta, and Nb powders into an intermediate alloy, then weld Hf, Ti, and Zr plates on its surface, and finally form the self-dissolution electrode.
Vacuum melting involves placing the prepared electrodes into a vacuum furnace for melting. A high vacuum environment is crucial as it can effectively remove gases (such as O, N, H) and volatile impurities from the metal, preventing the alloy from oxidizing at high temperatures. Melting usually requires two to three cycles (i.e., secondary melting or tertiary melting), and each remelting process can further improve the chemical composition uniformity of the ingot and reduce segregation and defects. The ingots obtained from melting cannot be used directly. They need to undergo high-temperature homogenization treatment to eliminate casting stresses and further uniform the structure. Then, through plastic processing methods such as forging, extrusion, and rolling, the ingots are processed into the required plates, rods, tubes, or forgings. During the processing, vacuum annealing is also interspersed to eliminate work hardening and restore the plasticity of the material.
Key control points for vacuum degree throughout the process: ≤5×10⁻³ Pa (VAR), ≤1×10⁻⁴ Pa (EBM), to prevent Hf/Ti from oxidizing and forming hard and brittle inclusions. Uniform composition, precise electrode ratio, stable melting rate, multiple melts, avoiding Hf/Ti segregation. Molten pool control: stable arc/electron beam, appropriate molten pool depth, preventing component stratification and porosity. Cooling speed: strong cooling of water-cooled copper crucible, refining grains, reducing columnar crystals, improving subsequent processing performance. Impurity control: high-purity raw materials, clean furnace chamber, no graphite contamination (C103 avoids brittle carbon).
The post-casting treatment involves uniform annealing at 1600°C for 3 hours, under vacuum/argon protection, with furnace cooling, to eliminate dendritic segregation and improve plasticity.
The hot processing involves extrusion/forge at temperatures above 1200°C, and rolling/drawing at temperatures below 500°C, resulting in finished products (plates, rods, wires). Before entering the vacuum furnace, preparing a high-quality self-consuming electrode is half the success. Due to the significant difference in melting points between high-melting-point metals such as niobium, tungsten, and tantalum in C103 alloy and low-melting-point metals such as hafnium, titanium, and zirconium (up to 800°C or more), direct mixing and melting is prone to cause uneven composition. Therefore, a very ingenious "stepwise method" emerged:
To prepare the intermediate alloy, first, mix the high-melting-point tungsten (W), tantalum (Ta) powders with niobium (Nb) powders. Then, through the powder metallurgy method (pressing followed by vacuum sintering at a high temperature ranging from 1700°C to 1900°C for 10 to 15 hours), a uniformly composed Nb-Ta-W intermediate alloy is produced. This step ingeniously solves the problem of uniformity for high-melting-point metals.
The self-consuming electrode will be assembled, and the intermediate alloy ingot obtained will undergo surface anti-oxidation treatment (such as coating with glass powder) and be hot forged. Then, using tungsten inert gas welding, the Hf plate, Ti plate and Zr plate will be precisely and parallelly welded onto the four sides of the ingot. Thus, a "self-consuming electrode" is fabricated, which has the correct proportion of each element on a macroscopic scale and the high-melting-point elements have been pre-alloyed on a microscopic level.
