Heat treatment of nickel-bearing cast titanium aluminum alloy
2018-12-10
Many titanium-aluminum alloy aviation and automotive engine parts are molded using precision casting technology. Heat treatment is one of the key technologies to improve the microstructure of cast titanium aluminum alloy.
The original as-cast microstructure of cast titanium aluminum alloy is generally γ-TiAl/α2-Ti3Al layer structure. The lamellae are coarse and the distribution of the lamella size and orientation is not uniform. The high-temperature heat treatment in the alpha single-phase zone can achieve homogenization of its structure. However, due to the coarseness of the original structure and the difficulty in controlling the growth of the α-phase grains at high temperatures, the fully-laminated fully-titanium-aluminum (FL) laminates usually still have thick lamellar groups. In order to improve the room-temperature tensile plasticity of cast titanium aluminum alloy, a thin full-thickness microstructure of cast titanium alloy was successfully obtained through a multiple heat treatment process [5,6]. The process includes homogenization of 1 alpha single phase zone, thermal cycling from 2900 to 1150 °C, isothermal treatment at 31150 °C, and 4 reheating to a short time isothermal treatment slightly above the temperature of Τα. However, this heat treatment process is more complicated and the processing cycle is longer, which is not conducive to engineering applications.
In this paper, the heat treatment process of nickel-microalloyed cast Ti-46.5Al-2.5V-1.0Cr (atomic percent, the same below) alloys was studied, and the micro-alloying of nickel simplified the homogenization and refinement of the cast titanium-aluminum alloy. The mechanism of the heat treatment process and the mechanism of the formation of fine full-thickness titanium-aluminum alloy sheets were analyzed and discussed.
1 The test material is nickel-containing (0.2-0.5)% (atomic percent, the same below) cast TiAl alloy Ti-46.5Al-2.5V-1.0Cr (%), smelted using a cold crucible vacuum induction furnace, remelted 3 After pouring into a copper mold, an ingot of φ40 mm was obtained. The 30° heat-treated specimen was cut from the ingot by a wire cutting method.
The heat treatment test was performed under a vacuum of 0.133 Pa. The heat treatment system of equiaxed near gamma-NG and fine fully lamellar-FFL was obtained with reference to a cast Ti-46.5Al-2.5V-1.0Cr alloy [5,6]. The time is taken as 1150° C.×(48-168) h and 1370° C.×(5-10 min), respectively.
Tissue observations were performed under ordinary optics and light microscopes with polarized light. The metallurgical sample was etched with (volume percentage) 1% HF+10% HNO3 + 89% H2O solution.
2 Observe that the (99.8~99.5)%(Ti-46.5Al-2.5V-1.0Cr)+(0.2~0.5)% Ni alloy is a fully-ply full-grained structure with a certain preferred orientation. About 500 to 1500 μm. After the 1150°C×72h of the alloy, the obvious continuous segmental coarsening phenomenon occurred. After 144 hours of isothermal treatment, the original coarse and inhomogeneous as-cast microstructure was transformed into a small, almost uniform, isometric NG structure. Its average grain size is about 30 μm.
(99.8~99.5)(Ti-46.5Al-2.5V-1.0Cr)-(0.2~0.5)Ni(%) alloy original as-cast microstructure
Casting (99.8~99.5)(Ti-46.5Al-2.5V-1.0Cr)-(0.2~0.5)Ni(%) alloys were isothermally treated at 1150°C for 72h isothermal, and the layers were continuously sectioned for 144h isothermal. Comprehensive isometric organization.
Studies have found that nickel has the role of expanding the γ single-phase region of titanium alloy, adding (atomic fraction) more than 0.5% of nickel can make Ti-48Al alloy into a single-phase γ structure. Observed by rotating the stage 360° under a 100x polarized light microscope, a small amount of equiaxed α2 grains in the NG structure obtained in this study appeared obvious 4 bright extinction phenomena. Qualitative observations of the NG microstructure without nickel-titanium-aluminum alloy obtained from the literature show that the amount of α2 phase in the nickel-containing alloy NG is significantly less. Therefore, qualitatively speaking, the addition of 0.2% to 0.5% of nickel can increase the driving force for α2 (or α)→γ phase transformation of titanium aluminum alloy at 1150°C, which increases the enhancement of the energy fluctuation in the lamellar structure. It promotes the occurrence of discontinuity caused by the layer disturbance in the ply structure. These relatively large amounts of time produce a greater number of in-slice endpoints that effectively promote segmented continuous roughening of the lamellae, thereby allowing nickel-containing titanium aluminum alloys to be homogenized in a relatively simple heat treatment process. Refinement.
The experiment found that the titanium alloy thin-layer full-thickness sheet has the best comprehensive mechanical properties. Therefore, the obtained NG structure was reheated to 1370°C for 5 to 10 minutes and then cooled to obtain a fine equiaxed full-layer sheet structure (). The average size of the laminations was about 50 μm, slightly smaller than that of the cast Ti-46.5. Al-2.5V-1.0Cr alloy obtained in the same process FFL tissue layer sheet. According to the fact that the layer group is equiaxed and slightly larger than the γ grain size of the matrix NG structure, the formation mechanism of the FLI structure of the nickel-containing cast titanium aluminum alloy is different from that of the titanium-aluminum alloy FFL structure without nickel. High-temperature α-equiaxed grains are formed on the γ-phase matrix and grow slightly. Then they are cooled during the cooling process to the α + γ phase, and the γ-phase precipitates in the α-grain. A gamma/alpha lamellar structure is formed and then converted to a gamma/alpha2 lamellar structure during cooling to room temperature.
Cast (99.8 ~ 99.5) (Ti-46.5Al-2.5V-1.0Cr)-(0.2 ~ 0.5) Ni (%) alloy fine full-layer sheet microstructure morphology.
in conclusion
(1) Casting Ti-46.5Al-2.5V-1.0Cr alloy containing 0.2% to 0.5% (atomic fraction) of nickel can be treated by a simpler 1150°C×144h isothermal treatment to make the as-cast coarse non-uniform lamellar structure. Turns into a fine, uniform, equiaxed near gamma organization.
(2) The obtained near gamma structure was reheated to 1370°C for 5 to 10 minutes and then cooled to obtain fine equiaxed full-ply sheet structures.
