To address the complex components required in fields such as aerospace, powder casting effectively solves technical challenges inherent in precision casting, such as shrinkage cavities, porosity, and compositional segregation, as well as numerous problems associated with welding forgings. Currently, in the production of Ti2AlNb alloys using powder metallurgy, we focus on the hot isostatic pressing (HIP) process parameters (time, temperature, pressure), subsequent heat treatment and hot deformation processes, and the study of the microstructure and performance of powder alloys and the dimensions of formed components. Our company has prepared Ti2AlNb pre-alloyed powder using the plasma rotating electrode process (PREP) and then produced Ti2AlNb powder alloys through the powder hot isostatic pressing process.
Preparation method of Ti2AlNb powder alloy: Ti2AlNb pre-alloyed powder with a particle size below 250 μm is loaded into a cylindrical low-carbon steel sheath in an atmospheric environment. After compaction, spot welding, vacuum degassing, and sealing welding, hot isostatic pressing (HIP) is completed in an RD(Z)-1-850 type hot isostatic pressing furnace of Steel Research Institute Haopu Technology Co., Ltd., to obtain powder metallurgy billets. Hot isostatic pressing process: The furnace temperature is raised to 1030 ℃, the pressure is greater than 140 MPa, the holding time is 2–4 h, and then the furnace is cooled. Heat treatment process: Solution heat treatment conditions are 980 ℃ for 2 h, followed by furnace cooling, with a time to room temperature of not less than 1 h. Aging heat treatment conditions are 890 ℃ for 4 h, followed by furnace cooling, with a time to room temperature of not less than 1 h. After hot isostatic pressing, the outer sheath and core are removed by machining and chemical milling to obtain titanium alloy closed impeller parts. The figure below shows the assembly relationship and physical image of the experimental closed impeller sleeve and core.
Fig. Assembly relationship and physical diagram of shrouded impeller sleeve core.
Tensile properties were tested at 650 ℃ using a CMT5305 electronic universal testing machine, according to GB/T 228.2-2015 "Metallic materials, tensile testing-Part 2: High temperature test method"; creep life was tested at 650 ℃/360 MPa using an RD-100 micro-controlled electronic creep testing machine, according to GB/T 2039-2012 "Metallic materials, uniaxial tensile creep test method"; the specimen dimensions for both tensile and creep tests were 5 mm in diameter and 25 mm in gauge length, with at least two parallel specimens. The Ti2AlNb powder alloy samples, after mounting and polishing, were etched with Kroll reagent (3% HF + 6% HNO3 + 91% H2O, volume fraction), and SEM images were obtained using a TESCAN MIRA4 field emission scanning electron microscope. EBSD analysis was performed on a Thermo Quattro S-type SEM equipped with an electron backscatter diffraction (EBSD) probe, and the data processing software was HKL Channel 5. The porosity of the hot isostatically pressed Ti2AlNb alloy was characterized using a VersaXRM-500 X-ray microcomputed tomography (Micro-CT) system. The analysis area of the powder compact was approximately Φ1.9 mm × 2.2 mm.
In the fabrication of powder metallurgy components, the design of the encapsulation structure is crucial. A reasonable encapsulation structure design needs to ensure that all parts of the powder compact are densified and achieve near-net-shape forming. As the container for hot isostatic pressing (HIP) of powder metallurgy alloys, the encapsulation directly affects the metallurgical quality and dimensional appearance of the finished product. In practice, variations in process parameters during HIP densification can lead to differences in the densification process. Especially when fabricating large, complex components, the encapsulation/mold structure is usually relatively complex, and the differences in densification process from the surface to the interior of the powder compact are more pronounced, even exhibiting a phenomenon of "non-uniform densification." This phenomenon leads to inconsistent densification levels and shrinkage deformation in different parts of the component, thus affecting the component's mechanical properties. Using EIGA powder, various types of Ti2AlNb powder metallurgy components were successfully fabricated under a HIP regime of 1030 ℃/140 MPa/3 h. Testing showed that the dimensional deviation of key components was less than 2%, and the surface quality was superior to that of cast alloys. By adopting a finite element-assisted encapsulation structure design, the densification process tends to be uniform throughout the entire densification forming process, effectively avoiding the phenomenon of "non-uniform densification" and helping to ensure the consistency of the overall performance of the component.
1) Compared with the PREP method, the Ti2AlNb pre-alloyed powder prepared by the EIGA method has a wider particle size distribution, a small number of satellite spheres, and a higher tap density.
(2) Compared with the Ti2AlNb alloy formed by hot isostatic pressing of PREP powder, the high-temperature strength and creep life of the alloy corresponding to EIGA powder are slightly reduced, but the elongation is better due to the higher uniformity of the microstructure. In the component preparation process, the finite element simulation results show that the Ti2AlNb impeller prepared by EIGA powder has a smaller shrinkage and is more suitable for hot isostatic pressing of complex components.
(3) Using finite element simulation to assist in the design of the encapsulation, complex Ti2AlNb powder metallurgy components were successfully prepared using EIGA pre-alloyed powder. The components have high dimensional and surface accuracy and no metallurgical defects.
