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Optimum Structure of Aircraft Aluminum Alloy 7A09

Update time:2019-03-22 17:38:12Number of clicks:0次
Among the aluminium materials used in Chinese aerospace vehicles, 7A09 alloy is one of the most preferred high strength alloys for structural parts.

West Aluminum-West Aluminum Industry Telecom: Among the aluminium materials used in Chinese aerospace vehicles, 7A09 alloy is one of the most preferred high strength alloys for structural parts. The semi-finished products available are sheet, strip, bar, profile, thick-walled tube, forging, etc. The chemical composition of the alloy is more reasonable than 7A04 alloy, so it has more superior comprehensive properties and becomes one of the main materials for designers. Its chemical composition (mass%) is 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 are 0.05, totaling 0.10, and the rest are Al.


7A09 alloy has good formability in annealing and solution treatment state, low formability after artificial aging and satisfactory fracture toughness in T6 state; although the strength under T73 overaging is lower than that under T6 state, it has good stress corrosion cracking resistance and high toughness. T76 material has high anti-stripping corrosion performance. T74 also has high strength and stress corrosion cracking resistance.


The tensile strength Rm of 7A09 alloy is higher than that of 2A12 and 2114 aluminium alloys, and the stress corrosion cracking resistance is also higher than that of them. Therefore, the use of 7A09 alloy to manufacture aircraft parts not only has greater weight loss effect, but also has higher safety. The fatigue strength of 7A09 alloy has not been improved synchronously. Therefore, it is necessary to seriously consider this problem in the design of parts mainly bearing fatigue load. When the temperature rises, the strength of 7A09 alloy decreases rapidly, so its working temperature should not exceed 125 C.


The incomplete annealing specifications of 7A09 alloy are 290 ~320 C, 2h~4h, air-cooled; the complete annealing specifications are (390 ~430 C)/(0.5h~1.5h), cooling to <200 C at a cooling rate of <30 C/h, and then air-cooled.


The solution treatment temperature of the alloy ranges from 460 ~475 C, but the treatment temperature of the aluminium clad sheet should be lower than 2 times, so as to avoid the penetration of the alloy elements into the aluminium clad layer and reduce the corrosion resistance of the material. The cooling medium should be room temperature, warm water or other suitable medium, and the transfer should not exceed 15s. The treatment temperature of T6 sheet is (135 +5)/(8h~16h), and that of other materials is (140 +5)/16h. The manual aging specifications of T73 sheet, extruded sheet and forgings are shown in table.


The smelting equipment of 7A09 alloy is the same as that of other deformed aluminium alloys. The smelting temperature is 710 750 and the casting temperature is 710 735 respectively. The smaller size of ingot is the lower casting temperature, and the melting temperature of 7A09 alloy is 477 638


7A09 alloy is an important stress structure aluminum alloy, which has been widely used in the manufacture of fighter, mid-range bomber, conveyor and trainer. It is used to manufacture front landing gear parts, wing front beam, girder, fuselage butt frame arm and support, partition, rib plate, main beam joint, flat tail upper and lower wall plate, hydraulic system parts, hydraulic oil tank piston rod and inner and outer cylinder. Spare parts.


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 its thermal conductivity is higher than that of T6 material. The specific heat capacity of 7A09-T6 alloy is 888 J/(kg.) and its room temperature electrical conductivity is 18.5 MS/m at 25 C. In addition to stress corrosion cracking, the general corrosion resistance of 7A09 alloy is comparable to that of 2112 alloy. The stress corrosion cracking resistance of 7A09 alloy in S-T direction is susceptible to alignment of stress corrosion. The threshold values of stress corrosion in L-T direction and L direction are more than 300N/mm2. Therefore, the stress corrosion cracking resistance in these two directions is sufficient to meet the application requirements. For higher requirements, T73 material can be used. The tensile strength Rm of this material is about 10% lower than that of T6 material, but the threshold value of stress corrosion cracking in L-T direction is much larger than that of 300N/mm2.


For parts requiring both strength and stress corrosion cracking properties, T74 state material should be used. The threshold value of stress corrosion cracking resistance of 7A09-T74 alloy forgings is 210N/mm2. Anti-corrosion measures of 7A09 alloy include anodic oxidation, chemical anti-corrosion treatment and coating.


The structure of 7A09 alloy consists of a-Al solid solution and second phase particles. The second phase consists of three types: the first type is intermetallic compounds formed during solidification, such as Al7FeCR, Al3Fe and Mg2Si, which are large in size, crushed under pressure and distributed in clusters with size ranging from 0.5 to 10 microns, insoluble in solid solution when heated, and reducing the toughness of the material; the second type is particles containing chromium. For example, Al2CrMg2, which is precipitated from the solid solution during the heating process before homogenization and processing, has a size of 0.05-0.5 um, which has a significant hindrance to the recrystallization process and grain growth of the material. The third type is aging strengthening phase, which is incorporated into the solid solution during solution treatment, and aging precipitates from the solid solution, which is an important factor affecting the properties of the material. The strengthening particles of T6 state materials are mainly in the GP region (< 4 nm). The main strengthening particles of T74 materials are transition phase_', which is 5-6 nm. The strengthening phases of T73 materials are transition phase_', which is 8-12 nm, and_phase, which is 20-80 nm.


The formability of 7A09-O material is similar to that of 2112-O alloy, and it has good formability at 180 ~370 ~C. The formability of new quenched material is roughly the same as that of 2A12 alloy. After quenching, the sheet still has good formability within 4 hours at room temperature. The time for freezing to keep formability is 24 hours at 0%, 3 days at - 7 ~18 ~C and 7 days at - 18 ~C.


The forging temperature of 7A09 alloy ranges from 320 to 440 C, and the open forging temperature should be less than 400 C. Excessive high temperature will lead to hot brittleness, especially when forging freely. 7A09 alloy is not easy to melt, even resistance welding is not as good as 2A12 alloy. Forgings can be quenched in hot water below 80 C. The quenched and aged 7A09 alloy has good machinability.


(Chongqing Sino-Aluminum Xinda Aluminum Industry Co., Ltd. Publicity Department collates contributions)


Source: Network

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