TA7 alloy is a medium-strength α-type titanium alloy (Ti-5Al-2.5Sn) that cannot be strengthened by heat treatment. It is usually used in the annealed state and has good fracture toughness at room temperature and high temperature. The alloy has good welding properties and is suitable for tungsten electrode and metal electrode welding processes under inert gas protection. It can be used to manufacture parts such as receiver casings and wall panels. The alloy’s long-term working temperature can reach 500°C, and its short-term working temperature can reach 800°C. TA7ELI alloys with low interstitial impurity elements are suitable for use at low temperatures. The semi-finished products produced include plates, bars, cakes and rings, etc., which can also be used to produce castings.
Chemical composition of titanium alloy TA7 (%)
TA7 titanium alloy heat treatment system
1) Annealing: Annealing treatment is carried out according to different semi-finished products. The specific parameters are as follows:
Plate: Temperature 700 – 800°C, time 10-120 minutes, air cooling.
Bars and forgings: Temperature 750 – 850°C, time 0.5-4 hours, air cooling.
Casting: temperature 600 – 700°C, time 1-2 hours, air cooling or furnace cooling.
2) Stress relief annealing: To eliminate residual stress, the parameters are as follows:
Temperature is 540-650°C, time is 15 – 360 minutes, air cooling or furnace cooling.
3) Hot isostatic pressing treatment of castings: used to improve the comprehensive performance of castings. The specific parameters are as follows:
Under the argon pressure of 100 – 140MPa, the temperature is 910°C ± 10°C, the time is 2 – 2.5 hours, and the furnace is cooled to below 300°C.
Smelting and casting process of TA7 titanium alloy
The smelting and casting process of TA7 titanium alloy includes the following steps:
1) Smelting: The ingot requires more than two vacuum consumable electrode arc furnace smelting. During the smelting process, the alloying elements aluminum (Al) and tin (Sn) are added in the form of AI-Sn master alloy. When welding consumable electrodes, use argon shielded plasma welding method, and tungsten argon arc welding is strictly prohibited.
2) Casting: Castings can be cast using different processes, including the following:
Graphite processing type: Use graphite material as a mold and cast it in a vacuum consumable electrode arc condensation furnace.
Graphite tamping type: The graphite material is also used as the mold and cast in a vacuum consumable electrode arc condensation furnace, but a tamping process is added to the process to improve the density and structural uniformity of the casting.
Metal type: Use metal materials as molds and cast them in a vacuum consumable electrode arc condensation furnace.
Investment casting mold: Using investment casting technology, molten TA7 titanium alloy is injected into the mold and solidified and cooled.
Through the above process steps, the smelting and casting of TA7 titanium alloy can be realized.
Corrosion resistance of TA7 titanium alloy
TA7 titanium alloy has good corrosion resistance under normal atmospheric and seawater conditions. However, compared with TC4 titanium alloy, TA7 alloy is more sensitive to thermal salt stress corrosion.
In an environment covered with artificial sea salt, stress corrosion may occur when TA7 alloy is exposed to a temperature of 316°C and a stress of 207MPa for 100 hours. This implies that under such high temperature and high stress conditions, the TA7 alloy may be corroded and suffer a certain degree of damage.
Therefore, when designing and applying TA7 alloy, it is necessary to consider factors such as temperature, stress and chemical media of the environment, especially in high temperature, high stress and salt corrosion environments, and take appropriate corrosion prevention measures to ensure the alloy’s durability. Long term stability and reliability.
Weldability of TA7 titanium alloy
TA7 titanium alloy has good welding properties. The use of tungsten arc welding (TIG) or metal arc welding (MIG) under inert gas protection can make the alloy show higher toughness at low temperatures.
However, it should be noted that titanium alloys are prone to brittleness problems related to intermetallic compounds during the brazing process, so brazing is generally less used. In comparison, welding methods such as argon arc welding, spot welding and seam welding are more common.
Argon arc welding joints have a high strength coefficient at room temperature, high temperature and low temperature. The seam welded joint has a higher strength coefficient at room temperature and high temperature, but a relatively lower strength coefficient at low temperature.
Application overview of TA7 titanium alloy
1) Die forgings and ring parts: TA7 alloy is often used to manufacture engine adapters, front casings, sealing ring casings and other parts. These parts require high strength and high temperature resistance, and TA7 alloy can meet these requirements.
2) Plate hot press forming parts: TA7 alloy plates can be made into lining plates, bracket seats, wall panels and other parts through the hot press forming process. These parts are usually used in structural supports and bulkheads of aircraft, and require high strength and lightweight requirements.
However, it should be noted that TA7 alloy is sensitive to the content of interstitial elements and impurities, especially when used under ultra-low temperature conditions. Therefore, the purity of alloy components needs to be strictly controlled during the manufacturing process to avoid brittleness problems caused by impurities.
In general, TA7 titanium alloy is widely used in many fields in the aviation industry due to its high strength, high temperature resistance and lightweight characteristics.