The article introduces the preparation process of TC18 titanium alloy forging cake for aviation, and studies the effect of two different double annealing processes on its structure and properties. The results show that: using 820 ℃ × 2h, FC to 750 ℃ × 2h, AC; 580 ℃ × 3h, AC double annealing process, TC18 titanium alloy forging cake has excellent room temperature mechanical properties, and can obtain ideal strength and toughness matching .
Keywords: TC18 titanium alloy; double annealing; mechanical properties; toughness
Chinese Library Classification: TG146 Document Identification Code: A Article Number: 1006-8937 (2012) 29-0035-02
TC18 titanium alloy is a high alloying, deep hardenability transitional α + β titanium alloy, which has the characteristics of high strength, high toughness, excellent plasticity and weldability. The Russian brand is BT22. BHAM) was successfully developed in 1974 with a nominal composition of Ti-5Al-5Mo-5V-1Cr-1Fe.
TC18 titanium alloy is generally used in the annealed state, and the strength level is equivalent to the strength of the solid solution aging state of TC4, TC6 and other titanium alloys. It can also be strengthened by solution aging. Its semi-finished products include plates, bars, extruded profiles and forgings. It has been widely used abroad to prepare main load-bearing structural parts such as mainframe landing beams, fuselage butt frames, etc. In the aircraft structure, TC18 titanium alloy is used to replace high-strength steel or TC4 titanium alloy, which can reduce weight by 15% to 20%.
This article introduces the preparation process of TC18 titanium alloy forging cake for aviation. It focuses on the effect of using different first-level high-temperature annealing temperature on the structure and properties of TC18 titanium alloy during double annealing, so as to provide a basis for the determination of TC18 forging heat treatment process.
1 Experimental method
- 1.1 Melting of alloy
The raw materials are grade 0 sponge titanium, AlMo alloy, AlV alloy and pure Fe and pure Cr. The Φ440mm × 1800mm ingots are obtained by vacuum melting furnace three times. The chemical composition of the ingots is shown in Table 1.
- 1.2 Forging of cake material
The ingot is forged in the β-phase area to form Φ230mm, and then cut into a Φ230mm × 310mm cake blank with a sawing machine; in the β-phase area, the cake blank is upset to Φ320mm × 160mm, and then reversed and drawn to Φ230mm × 310mm, repeat Three times; the forging temperature of the finished product is in the α + β two-phase area, which is upset and rounded, and finally forged into Φ400mm × 90mm cake material.
- 1.3 Heat treatment of forging cake
For TC18 titanium alloy forgings, the recommended heat treatment system is double annealing, that is, the first stage of high temperature annealing: 820 ~ 850 ℃, heat preservation 1-3 h, furnace cooling to 740 ~ 760 ℃, heat preservation 1-3 h, air cooling; second Low temperature annealing: heating to 500 ～ 650 ℃, holding for 2 ～ 6 h, air cooling.
In order to study the effect of the first-stage high-temperature annealing temperature on the structure and performance of the double annealing, the double annealing test was carried out using test a and test b respectively.
From the forged Φ400mm × 90mm cake material, cut longitudinal and horizontal samples. After the sample is heat-treated, the microstructure is observed with an optical microscope, and the mechanical properties at room temperature are tested.
2 Experimental results and analysis
- 2.1 Microstructure of double annealed TC18 titanium alloy
The double annealed structure of test a consists of the primary αp phase and β matrix (see Figure 1a). The primary αp phase is evenly distributed and is equiaxed. The secondary αs phase is small and difficult to distinguish. All β grain boundaries have been fully broken . A large number of flaky secondary αs phases appeared in the double-annealed structure of experiment b (see Figure 2b), the primary equiaxed αp phase thickened, and the proportion decreased.
Studies have shown that during the double annealing of TC18 titanium alloy, the first stage of higher temperature solution annealing is to retain a certain number of metastable phases, and at the same time, the alloy deformed by forging is recrystallized. To prepare for the subsequent second-stage aging annealing.
The primary αp phase is formed by annealing at a higher temperature in the first stage, and is retained during the air cooling process. The secondary αs phase is obtained by the transformation of the β phase at high temperature during the air cooling process. The higher the high-temperature annealing temperature of the first stage, the lower the content of the primary αp phase and the more the content of β phase, and the more the secondary αs phase obtained from its transformation. Therefore, the first-stage annealing at 820 ℃ and 850 ℃ Compared with the first-stage annealing of the latter, the primary αp phase in the latter’s double-annealed structure decreases and the secondary αs phase increases.
- 2.2 Mechanical properties of TC18 titanium alloy double annealing
After two double annealing tests, the mechanical properties of TC18 titanium alloy are shown in Table 2.
It can be seen that as the first-stage high-temperature annealing temperature is increased from 820 ° C to 850 ° C, the tensile strength is increased to a certain extent, and the plasticity is reduced. This is because with the increase of the first-stage high-temperature annealing temperature, the primary αp phase decreases and the number of flaky secondary αs phases increases, and the interface between the two phases increases, resulting in the enhancement of the strengthening effect of the second phase and the strength of the alloy Rise. The strip-shaped interface hinders the slip of dislocations, and the dislocations do not easily bypass the sheet-like αp phase, causing deformation difficulties, and thus the plasticity decreases.
At the same time, as the first-stage high-temperature annealing temperature increased from 820 ℃ to 850 ℃, impact toughness and fracture toughness decreased. This is because the content of the equiaxed αp phase decreases, the number of flaky secondary αs phases increases, the tortuosity of the crack propagation path decreases, and the energy required for fracture is reduced, so the impact toughness and fracture toughness decrease.
Comparing test a and test b, the former double annealed TC18 alloy has excellent mechanical properties at room temperature, and it has an ideal strength and toughness match. Its structure and performance fully comply with the specifications of titanium and titanium alloy forgings for aviation (GJBT2744A-2007) Requirements.
① Increase the first-stage high-temperature annealing temperature. After double annealing, the primary equiaxed αp phase decreases and the secondary flaky αs phase content increases. At the same time, the tensile strength increases to a certain extent, the plasticity decreases, and the impact toughness and fracture toughness also decrease.
②Using the double annealing process of 820 ℃ × 2h, FC to 750 ℃ × 2h, AC; 580 ℃ × 3h, AC, TC18 titanium alloy forging cake has excellent room temperature mechanical properties, and can get ideal strength and toughness matching.