Titanium alloy is one of the important structural materials of modern advanced aircraft and aero engine. To effectively improve the thrust-to-weight ratio of aero engine and rocket, high specific strength heat-resistant titanium alloy must be used. At present, the most widely used titanium alloys such as TC4, TC6, TC11 and other titanium alloys in China generally do not exceed 450 ℃. The high temperature performance of such titanium alloys has been difficult to meet the current engine requirements.
BT25 titanium alloy belongs to the Ti-Al-Zr-Sn-Mo-W-Si system martensite α+β type deformed heat-strength titanium alloy. Since the alloy elements contain high melting point tungsten, the heat strength of the titanium alloy is greatly improved It can effectively improve 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 thermally strong titanium alloys for engines. The semi-finished products made of BT25 alloy include forgings, die forgings and bars. At present, most of the public reports on BT25 titanium alloy in China focus on the smelting and bar preparation of BT25. Researchers have studied the effect of annealing temperature on the structure and properties of BT25 titanium alloy to provide a reference for obtaining good comprehensive performance.
The rolled bar is used to study the annealing process. The material specification is Φ16.5mm, 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 was measured by metallographic method to be 1010~1020℃. Two-phase and single-phase annealing were carried out near the transition temperature. Observe the microstructure of the alloy before and after heat treatment with a 4XB-TV metallographic microscope, and use the WDW-50 microcomputer to control the electronic universal testing machine to test the tensile properties.
BT25 titanium alloy can be used for single annealing or double annealing. 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 after cooling and then reheat to a certain temperature (abbreviated “The second annealing temperature”) After holding for a certain period of time and cooling. The research results are as follows:
(1) Double annealing has a certain strengthening effect. The strength at room temperature and high temperature is higher than that of single annealing, but the plasticity is slightly worse.
(2) During double annealing, as the first annealing temperature increases, 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, as the second annealing temperature increases, the size of the equiaxed α phase increases slightly, the degree of spheroidization further increases, and the secondary α layer becomes coarser. The alloy’s strength, plasticity and toughness 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 comprehensive performance, and can give full play to the heat strength of the alloy Sexual potential.