The Preferred Structure Of The Aircraft Is Aluminum Alloy 7A09

- Apr 22, 2019-

Among the aluminum materials used in China's aerospace vehicles, 7A09 alloy is one of the preferred high-strength alloys for the main structural members. The semi-finished products that can be supplied include plates, strips, bars, profiles, thick-walled tubes, forgings, etc. The chemical composition is more reasonable than the 7A04 alloy, so it has superior overall performance. It has become one of the main materials of the designer. Its chemical composition (% by mass): 0.5Si, 0.5Fe, (1.2~2.0) Cu, 0.15Mn , (2.0 ~ 3.0) Mg, (0.16 ~ 0.30) Cr, (5.1 ~ 6.1) Zn, 0.10Ti, other impurities a single 0.05, a total of 0.10, the rest is Al.

      7A09 alloy has good forming properties in the state of annealing and solution treatment, and the forming property is low after artificial aging, and has satisfactory fracture toughness in the T6 state; although the strength under T73 overaging is lower than that in the T6 state, It has good resistance to stress corrosion cracking and has high toughness. T76 material has high resistance to flaking corrosion. T74 has both high strength and resistance to stress corrosion cracking.

      The tensile strength Rm of 7A09 alloy is higher than that of 2A12 and 2A14 aluminum alloys, and the corrosion corrosion cracking resistance is higher than that of them. Therefore, it is used to manufacture aircraft parts, which not only has greater weight reduction effect, but also higher safety. However, the fatigue strength has not been increased correspondingly, so it is necessary to seriously consider this problem when designing parts that are mainly subjected to fatigue-resistant loads. When the temperature rises, the strength of the 7A09 alloy decreases rapidly, so the working temperature should not exceed 125 °C.

      Incomplete annealing specification for 7A09 alloy: 290°C~320°C, 2h~4h, air cooling; complete annealing specification: (390°C~430°C)/(0.5h~1.5h), cooling cooling rate ≤30°C/h It is cooled to ≤200 °C and then air-cooled.

      The solution treatment temperature of the alloy is 460 ° C ~ 475 ° C, but the processing temperature of the aluminum coated sheet should be lower than the lower limit, not more than 2 times, so as to prevent the alloy element from penetrating the aluminum coating layer, reducing the corrosion resistance of the material, the cooling medium is room temperature. , warm water or other suitable medium, the transfer should not be >15s. The processing temperature of T6 sheet (135 °C ± 5 °C) / (8h ~ 16h), other materials (140 ° C ± 5 ° C) / 16h, T73 sheet, extruded materials and forgings manual aging specifications are shown in the table.

      The melting equipment of 7A09 alloy is the same as that of other deformed aluminum alloys. The melting temperature is 710°C~750°C, the casting temperature is 710°C~735°C, the casting size is smaller, the casting temperature is lower, and the melting temperature of the alloy is 477°C~638. °C.

      7A09 alloy is an important structural aluminum alloy. It has been widely used in the manufacture of fighters, medium-range bombers, transporters and trainers. It is used to manufacture front landing gear parts, wing front beams, girders and fuselage docking. Key components such as frame arm and pillar, partition, rib, main beam joint, flat tail upper and lower wall, hydraulic system parts, hydraulic tank piston rod and inner and outer cylinder.

      The fracture toughness (Kc, N/mm2,) of 7A09-T73 alloy is higher than that of T6 material, and the crack growth rate is lower than that of T6 material. Its fatigue strength is also better than that of T6 material, and the thermal conductivity is also better than that of T6. It is higher, 156W/(m.°C) at 25°C; the specific heat capacity of 7A09-T6 alloy at 50°C is 888J/(kg.°C), and the room temperature conductivity is 18.5MS/m. In addition to the stress corrosion cracking performance, the general corrosion resistance of the 7A09 alloy is comparable to that of the 2A12 alloy. The 7A09 alloy has stress corrosion resistance in the S-T direction, and the stress corrosion threshold value in the L-T and L directions is greater than 300N/mm2, so the stress corrosion cracking resistance in these two directions is sufficient to meet the application requirements. If the requirements are higher, T73 material can be used. The tensile strength Rm of this state material is about 10% lower than that of T6 material, but the stress corrosion cracking threshold value of L-T direction is much larger than 300N/mm2.

      For parts that require both strength and stress corrosion cracking, T74 materials are preferred. The corrosion-resistant cracking threshold of the 7A09-T74 alloy die forgings is 210N/mm2. The anti-corrosion measures of the 7A09 alloy include anodizing, chemical anti-corrosion treatment and coating coating.

      The structure of 7A09 alloy consists of α-Al solid solution and second phase particle. There are three types of second phase. The first type is intermetallic compounds such as Al7FeCR, Al3Fe, Mg2Si formed during solidification of the alloy. It is crushed, in the form of a block, distributed in a string, with a size of 0.5μm~10μm. It is insoluble in solid solution when heated and reduces the toughness of the material. The second type is a chromium-containing particle such as Al2CrMg2, which is the homogenization and processing of the ingot. In the pre-heating process, the precipitation from the solid solution is 0.05μm~0.5μm, which has obvious hindrance to the recrystallization process and grain growth of the material; the third type is the aging strengthening phase, and the solid solution is dissolved during the solution treatment. In the aging, precipitation from solid solution is an important factor affecting the properties of the material. The T6 state material strengthening particle is mainly ≤4nm GP region, the main strengthening particle of T74 material is 5nm~6nm transition phase η', the strengthening phase of T73 material is 8nm~12nm transition phase η' and 20nm~80nm η phase Particles.

     The forming properties of 7A09-O materials are comparable to those of 2A12-O alloys, and have good formability at 180 ° C ~ 370 ° C; the formability of the new quenched materials is roughly equivalent to that of 2A12 alloys, and the plates are quenched at room temperature for 4 h. There is still good formability, and the time for freezing to maintain formability: 24 h at 0 ° C, 3 d at -7 ° C, and 7 d at -18 ° C.

      The forging temperature of 7A09 alloy is 320°C~440°C, and the forging temperature should be ≤400°C. If it is too high, it will produce hot brittleness, especially in free forging. 7A09 alloy is not easy to weld, even if resistance welding is not as good as 2A12 alloy. Forgings can be quenched in hot water ≤80 °C. The 7A09 alloy after quenching and aging has good machinability.

      Transfer from aluminum road network