7A04 Alloy With Yield Strength Close To Tensile Strength

- May 13, 2019-

7A04 alloy is an Al-Zn-Mg-Cu heat treatable aerospace superhard aluminum alloy, which can be heat-treated and strengthened. Its composition has been listed in GB/T3190-2008, with the Soviet Union and Russia's B95 alloy and Germany's AlZnMgCu1. .5.3.4365 alloy is equivalent, because the Soviet Union's No. 95 factory in 1944 successfully produced the alloy semi-finished product, hence the name B95 alloy. In 1957, with the help of Soviet experts, Northeast Light Alloy Co., Ltd. (then known as Harbin Aluminum Processing Factory) produced the Alloy sheet and extruded material.

The main feature of the 7A04 alloy is that its yield strength Rpo.2 is close to the tensile strength Rm, and its plasticity is low, which is sensitive to stress concentration, especially in the case of subjected to shock loads and repeated static loads. For the design, manufacture and assembly of components, stress concentration and long-term additional stress factors should be avoided. 7A04 alloy is not heat resistant, such as operating temperature higher than 125 ° C will sharply soften. The stress corrosion cracking resistance of the thick 7A04 alloy workpiece in the short transverse direction (S-T) is low.

Chemical composition (% by mass) of 7A04 alloy: 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, 0.10 Ti, other impurities a single 0.05, a total of 0.10, the rest is Al. 7A04 alloy semi-finished products and intermediate products, strips, profiles, bars, tubes, incomplete annealing specifications (290 - 320) ° C / (2 - 4) h, air cooling; full annealing specifications (390 - 430) ° C / (0.5-1.5)h, furnace cooling to ≤200 °C at a cooling rate of ≤30 °C / h, air cooling, or annealing (1 - 2) h at (320 - 380) °C. In order to improve the efficiency of the metallurgical furnace, the factory is often annealed at 400 ° C - 420 ° C for 2 h, cooled to < 150 ° C at a cooling rate of 30 ° C / h, and air-cooled.

The alloy has a solution treatment temperature of 465 ° C - 475 ° C, quenching in room temperature water, and the transfer time is shorter than 25 s. Aluminum processing plants often use highly targeted solution treatment and aging treatment specifications for different products and different specifications.

The smelting casting equipment and fuel of 7A04 alloy are the same as those of other aluminum alloys. However, due to its large melting temperature range, the density of main alloying elements is very different, so segregation is easy to occur during smelting, and the ingot is prone to cracks and looseness. Easy to inhale, more oxidized inclusions are 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 720 ° C - 745 ° C, air round ingot casting temperature 725 ° C - 740 ° C; slab casting temperature 685 ° C - 745 ° C; larger specifications round ingots, hollow round ingots and flat ingots should be laid before casting After casting, the gate part should be tempered.

7A04 alloy is mainly used to manufacture structural members of aircraft: girders, purlins, partitions, skins, ribs, joints, landing gear parts, etc. Semi-finished products are available: sheet, strip, profile, row, bar, pipe, siding, free forging and die forging.

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 low stress shape of the stress concentration, and the transition of the part cross section and all the transfer parts should be smooth, avoiding Mutation and reduction of eccentricity, all fillet radii should be ≥ 2mm. Avoid scratches, scratches, and severe dents when processing parts.

For large semi-finished products, especially large complex forgings and profiles, the lateral performance should be strictly checked, and all necessary non-destructive testing should be carried out to ensure that the defects are within the allowable range. The corrosion resistance of the 7A04 alloy in the T4 state is unacceptable, so the artificial aging is required, and 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-section parts is equivalent to that of aluminum-free hard aluminum. The graded aging 7A04 alloy has reliable resistance to stress corrosion cracking. Anodizing and surface painting can effectively protect the extruded material. Corroded.

The positive elastic modulus of the 7A04-T6 alloy sheet at 66 °C is 66 GN/mm2, the extruded material is 71 GN/mm2, and the plane strain fracture toughness Kc is 50.64 MN/(m3/2). Semi-continuous casting of 7A04 alloy ingots in addition to α-Al solid solution, T (AlCuMgZn) phase and S (Al2CuMg) phase, there are also a small amount of Mg2Si, AlFeMnSi and Al6 (FeMn) phases, and the solution is α S after solution treatment. Phase, after aging at 100 ° C - 140 ° C, the structure is α S MgZn2 T phase.

7A04 alloy can be cold and hot deformed. The hot and coarse deformation rate of the ingot is ≤60%, and the large deformation rate of the deformed material can reach 80%. The forgings with complicated shapes can be manufactured, and the forging temperature is 380 ° C - 430 ° C. For hot orthodontic parts or semi-finished products, the mold temperature is 130 °C ± 15 °C, while the temperature of the part itself is 130 °C ± 10 °C or 150 °C ± 10 °C, and the holding time at 130 °C ± 10 °C is 10h - 12h, while The holding time was 7 h when heated at 150 ° C ± 10 ° C.

The supply state of the extruded wall plate is T6, and it can be orthopedic in this state, and it must be heated in the newly quenched state. The single-stage aging specification is 140 ° C / 16 h; two-stage aging at 120 ° C / 3 h 160 ° C / 3 h can obtain good overall performance. 7A04 alloy has good spot welding performance and machinability.

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