7A04 Alloy – Yield Strength Close To Tensile Strength
7A04 alloy is an Al-Zn-Mg-Cu series heat-treatable aerospace super-hard aluminum alloy, which can be heat-treated and strengthened. Its composition has been listed in GB/T3190-2008, and it is compatible with the B95 alloy of the Soviet Union and Russia and the AlZnMgCu1 of Germany. .5.3.4365 alloy is equivalent, because in 1944 the Soviet Union’s 95th factory successfully trial-produced this alloy semi-finished product, hence the name B95 alloy. In 1957, China Northeast Light Alloy Co., Ltd. (then Harbin Aluminum Processing Plant) produced this alloy with the help of Soviet experts. Alloy plates and extruded materials.
The main feature of 7A04 alloy is that its yield strength Rpo.2 is close to the tensile strength Rm, its plasticity is low, and it is sensitive to stress concentration, especially when subjected to vibration loads and repeated static loads. For the design, manufacturing and assembly of parts, stress concentration and additional stress factors should be avoided as much as possible. 7A04 alloy is not heat resistant and will soften sharply if the working temperature is higher than 125℃. The stress corrosion cracking resistance of thick 7A04 alloy workpieces in the short transverse direction (S-T) is low.
The chemical composition of 7A04 alloy (mass%): 0.50Si, 0.50Fe, (1.4—2.0) Cu, (0.20—0.6) Mn, (1.8—2.8) Mg, (0.1—0.25) Cr, (5.0 ——7.0) Zn, 0.10Ti, other impurities are individually 0.05, the total is 0.10, and the rest is Al. Specification for incomplete annealing of 7A04 alloy semi-finished and intermediate product plates, strips, profiles, bars and pipes (290—320)℃/(2—4)h, air cooling; specification for complete annealing (390—430)℃/ (0.5-1.5) h, furnace cooling to ≤ 200 ℃ at a temperature reduction rate of ≤ 30 ℃/h, air cooling out of the furnace, or annealing at (320-380) ℃ for (1-2) h. In order to improve the efficiency of the metallurgical furnace, the factory often anneals at 400°C-420°C for 2 hours, cools it to <150°C at a cooling rate of 30°C/h, and then air-cools the furnace.
The solution treatment temperature of this alloy is 465℃—475℃, quenched in water at room temperature, and the transfer time is shorter than 25s. Aluminum processing plants often adopt highly targeted solution treatment and aging treatment specifications according to different products and products of different specifications.
The smelting and casting equipment and fuel of 7A04 alloy are the same as those of other aluminum alloys. However, due to its large melting temperature range and large differences in the density of the main alloying elements, segregation is prone to occur during smelting, and the ingot is prone to cracks, looseness, and solution. It is easy to get air, and more oxidized inclusions will be formed inside the ingot. In order to reduce these defects, in addition to strictly controlling the composition of the alloy, attention must be paid to the control of the smelting and casting process.
Round ingot casting temperature is 720℃—745℃, air ingot casting temperature is 725℃—740℃; slab casting temperature is 685℃—745℃; larger size ingots, hollow ingots and slabs should be laid before casting , The gate part should be tempered after casting.
7A04 alloy is mainly used to manufacture aircraft stressed structural parts: beams, stringers, bulkheads, skins, ribs, joints, landing gear parts, etc. The semi-finished products that can be supplied are: plates, strips, profiles, strips, bars, pipes, wall panels, free forging and die forgings, etc.
The alloy has high notch sensitivity and low axial tensile fatigue strength. The application must strictly select the shape of the design structure, select the shape with the lowest stress concentration, and the transition of the part section and all the transition parts should be smooth to avoid For sudden changes and reduction of eccentricity, all fillet radii should be ≥2mm. Avoid scratches, abrasions and severe dents when processing parts.
For large semi-finished products, especially large complex forgings and profiles, the transverse performance should be strictly inspected, and various necessary non-destructive testing should be carried out to ensure that the defects are within the allowable range. The corrosion resistance of 7A04 alloy in the T4 state is unqualified, so artificial aging is required. The corrosion resistance of the artificially aged aluminum-clad 7A04 alloy sheet is equivalent to that of the aluminum-clad 2A12 alloy sheet. The corrosion resistance of extruded products and small cross-section parts is equivalent to that of unclad aluminum duralumin. The graded aging treatment of 7A04 alloy has reliable stress corrosion cracking resistance. Anodizing and surface painting treatment can effectively protect the extruded material. Corroded.
7A04-T6 alloy sheet has a positive elastic modulus of 66GN/mm2 at 20°C, an extruded material of 71GN/mm2, and a plane strain fracture toughness Kc of 50.64MN/(m3/2). The structure of semi-continuous casting 7A04 alloy ingot includes α-Al solid solution, T (AlCuMgZn) phase and S (Al2CuMg) phase, as well as a small amount of Mg2Si, AlFeMnSi and Al6 (FeMn) phases. The structure after solution treatment is α S After aging at 100 ℃-140 ℃, the structure becomes α S MgZn2 T phase.
7A04 alloy can be deformed by cold or heat. The hot upsetting deformation rate of the ingot is less than or equal to 60%, and the maximum deformation rate of the deformed material can reach 80%. The forgings with complex shapes can be manufactured, and the forging temperature is 380°C-430°C. When performing thermal correction on parts or semi-finished products, the mold temperature is 130℃±15℃, and the temperature of the part itself is 130℃±10℃ or 150℃±10℃. The holding time at 130℃±10℃ is 10h—12h. The holding time for heating at 150℃±10℃ is 7h.
The supply state of the extruded wall panel is T6, it can be corrected in this state, and it must be heated in the new quenched state. The single-stage aging specification is 140℃/16h; two-stage aging at 120℃/3h 160℃/3h can obtain good comprehensive performance. 7A04 alloy has good spot welding performance and machinability.
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