Effect of Dual Annealing on Microstructure and Properties of BT25 Titanium Alloy
Titanium alloy is one of the important structural materials for modern advanced aircraft and aero engines. To effectively increase the thrust-to-weight ratio of aero engines and rockets, high specific strength heat-resistant titanium alloys must be used. At present, the highest operating temperature of titanium alloys such as TC4, TC6, TC11, etc., which are commonly used in China, generally does not exceed 450°C. The high-temperature performance of such titanium alloys has been difficult to meet the current engine requirements.
BT25 titanium alloy belongs to Ti-Al-Zr-Sn-Mo-W-Si series martensitic α+β type deformed hot-strength titanium alloy. Because the alloy element contains high melting point tungsten, the heat strength of the titanium alloy is greatly improved Therefore, it effectively improves the service temperature (up to 550℃) and working life of the alloy. According to reports, the working time of BT25 alloy can reach 6000h at 500℃ and 3000h at 550℃. Therefore BT25 is an ideal material for heat-strength titanium alloys for engines. Semi-finished products made of BT25 alloy include forgings, die forgings, bars, etc. At present, most public reports on BT25 titanium alloy in China focus on BT25 smelting and bar preparation. Researchers have studied the effect of annealing temperature on the structure and properties of BT25 titanium alloy, which provides a reference for obtaining good comprehensive properties.
The annealing process is studied using rolled bars. The material specification is Φ16.5mm, and the chemical composition (wt%) is 6.2～7.2Al, 1.5～2.5Mo, 0.8～2.5Zr, 0.8～2.5Sn, 0.5～1.5W, 0.10～0.25Si, and the balance is Ti. The α+β/β transition temperature of this alloy is measured by metallographic method to be 1010～1020℃, and two-phase and single-phase annealing are performed respectively near the transition temperature. Use 4XB-TV type metallurgical microscope to observe the microstructure of the alloy before and after heat treatment, and use WDW-50 microcomputer-controlled electronic universal testing machine to test the tensile properties.
BT25 titanium alloy can be single-annealed or double-annealed. Single annealing is to heat the alloy to an appropriate temperature for a certain period of time and then cool; while double annealing is to first heat the alloy to an appropriate temperature (referred to as the "first annealing temperature") for a certain period of time and then reheat to a certain temperature (referred to as (“Second annealing temperature”) Keep it warm for a certain period of time and then cool down.
The research results are as follows:
(1) Double annealing has a certain strengthening effect, and the room temperature and high temperature strength are higher than single annealing, but the plasticity is slightly worse.
(2) During double annealing, with the increase of the first annealing temperature, the primary equiaxed α phase particles gradually increase, the content decreases, and the secondary α phase increases. The strength of the alloy decreases, and the plasticity and toughness increase.
(3) During double annealing, the equiaxed α phase size increases slightly with the increase of the second annealing temperature, the degree of spheroidization is further improved, and the secondary α sheet layer is also coarser. The strength, plasticity and toughness of the alloy have little change, and the high temperature performance is stable.
(4) BT25 titanium alloy adopts 940～980℃×1h, air cooling +530～560℃×6h, air cooling double annealing process can obtain better microstructure and good overall performance, and can give full play to the thermal strength of the alloy Sexual potential.