The manufacturing process of high-temperature alloys plays a decisive role in their microstructure and performance in extreme environments. From raw material melting to final forming, multiple precise control processes are required to ensure component uniformity and avoid the occurrence of defects, in order to meet the stringent demands of aerospace, energy, and power industries. Among them, melting and forming, as the core steps, directly lay the foundation for the alloy’s performance, and their progressiveness is a key indicator to measure the manufacturing level of high-temperature alloys.
The core objective of the melting process is to achieve precise composition control and purity enhancement. The mainstream processes can be divided into two major categories: vacuum induction melting and vacuum arc remelting. Vacuum induction melting utilizes the principle of electromagnetic induction heating to dissolve raw materials in a vacuum environment, effectively reducing gas impurities and losses of easily oxidizable elements. By utilizing stirring functions to achieve composition homogenization, it is suitable for the preliminary preparation of alloy ingots. To further enhance purity, vacuum arc remelting is often used as a subsequent process. Using the initial melted ingot as an electrode, secondary melting is carried out under high temperature in a vacuum arc, which can remove defects such as shrinkage cavities and inclusions, refine the grain structure, and increase the density of the ingot to over 99.9%, providing high-quality billets for subsequent forming.
For certain high-performance high-temperature alloys, electron beam melting (EBM) has become the preferred process. This process utilizes high-energy electron beams to bombard raw materials, achieving melting temperatures of over 2000°C. It efficiently volatilizes low-melting-point impurities, making it particularly suitable for alloys containing active elements such as titanium and aluminum. This process can significantly reduce oxygen and nitrogen content and prevent the precipitation of harmful phases. For example, nickel-based single crystal superalloys can undergo EBM pretreatment to effectively enhance the purity of the ingots, creating favorable conditions for subsequent single crystal formation.
In the forming process, an appropriate process route needs to be selected based on the alloy’s application and performance requirements, which mainly falls into two categories: casting forming and plastic forming. Casting forming is suitable for components with complex shapes, where directional solidification and single crystal casting technologies are crucial. Directional solidification eliminates transverse grain boundaries by precisely controlling the cooling direction, allowing grains to grow along the stress direction, thereby significantly enhancing the alloy’s creep resistance. Single crystal casting, on the other hand, obtains grain-boundary-free single crystal grains through seed crystal guidance, and is applied to core components such as aeroengine turbine blades. Its high-temperature fracture life is several times longer than that of ordinary cast alloys.
Plastic forming changes the morphology of alloys through pressure processing while optimizing their microstructure. Common processes include forging, rolling, extrusion, etc. Forging can break down coarse grains in ingots, forming a dense fibrous structure, and enhancing the strength and toughness of alloys. For example, after isothermal forging, the yield strength of Inconel 718 alloy turbine disks can be increased by more than 30%. Rolling and extrusion are suitable for the preparation of profiles such as plates and tubes. By controlling the rolling temperature and reduction ratio, the dimensional accuracy and performance uniformity of the profiles can be ensured. After forming, auxiliary processes such as sizing and surface treatment are required to eliminate residual stresses and avoid surface defects, ultimately ensuring that high-temperature alloy components meet the actual service requirements.
