Marine Weldable High Strength 7A33 Alloy

- Jul 09, 2019-

7A33 alloy is an aluminum-zinc-magnesium-copper heat treatable corrosion-resistant high-strength structural aluminum alloy, composition (% by mass): 0.20Si, 0.30Fe, (0.25~0.35) Cu, 0.05Mn, (2.2~ 2.7) Mg, (0.10~0.20) Cr, (4.6~5.4) Zn, 0.05Ti, other impurities 0.05, total 0.10, and the rest are Al. It has high seawater and marine atmospheric corrosion resistance, no intergranular corrosion, stress corrosion and erosive tendency; tensile strength Rm is comparable to 2Al2 alloy, and has relatively high fracture toughness, process plasticity and low notch sensitivity; Can be spot welded and seam welded; suitable for the manufacture of seaplanes, amphibious aircraft, helicopters used in ships and coastal areas, aircraft skins and structural parts, can replace 2Al2 alloy.

7A33 alloy sheet can be made of 7A01 alloy as aluminum cladding layer, its composition (% by mass): 0.30Si, 0.30Fe, 0.01Cu, (0.9~1.3) Zn, 0.45 (Si+Fe), other impurities alone 0.03, the rest is Al .

Complete annealing specification of 7A33 alloy material: (390~420) °C/1h, with the cooling rate of ≤30 °C/h, the furnace is cooled to 200 °C and then air-cooled. The intermediate annealing specification (320~350) °C/(1~2) )h, the air is cold. Solution treatment specification: (460 ± 5) ° C / (20 ~ 40) min, quenching water temperature ≤ 40 ° C; two-stage artificial aging: first grade (70 ~ 90) ° C / (10 ~ 20) h, secondary (150 ±5) °C / (10 ~ 20) h, the time between solution treatment and artificial aging is not limited. The heat treatment time of repeated solution treatment is reduced by 50%, the artificial aging specification is unchanged, and the number of repeated solution treatments is limited to two times.

The main supply of 7A33 alloy is a thin plate with the state of O and T6. The melting temperature of the alloy is 606 ° C ~ 645 ° C. The slab can be produced by conventional smelting and casting equipment. 7A33 alloy has been successfully applied in the manufacture of seaplane skins. In order to ensure that the alloy has good corrosion resistance in the marine environment, the chemical and semi-finished products of the alloy should be strictly controlled.

7A33 alloy has a thermal conductivity of 109W/(m·°C) at 100°C, an average specific heat capacity of 840J/(kg·°C) at 20°C to 100°C, and a linear expansion coefficient of 22.7×10-6/°C at 20°C to 100°C. Like other deformed aluminum alloys, 7A33 alloy has good oxidation resistance. The electrical conductivity of the aluminum-free T6 state sheet is 23.2MS/m and the resistivity is 44.7nΩ·m. The alloy has good corrosion resistance in various environments. Performance, not sensitive to intergranular corrosion, stress corrosion cracking and exfoliation corrosion.

Corrosion tests conducted by the relevant units in Qingdao with a thickness of 1.5 mm without any anti-corrosion show that the corrosion resistance of 7A33-T6 alloy is much higher than that of 2A12-T4 alloy.

The main phase of the 7A33 alloy is MgZn'2, that is, the η' phase, which is in the form of flakes. In addition, there is a chromium-containing dispersed phase E (Al18Cr2Mg3); the impurity phase is Al13Fe4, Al-Fe-Si, and the intergranular precipitate phase η.

7A33 alloy has good hot and cold forming properties and pressure processing properties. The thin plate has good process forming performance, and all plastic indexes are equivalent to those of 2A12 alloy, which can be made into various parts of the aircraft. The newly quenched state plate has a long incubation period and small deformation resistance, which is beneficial to the forming of parts.

This alloy has good spot welding and seam welding properties. The welding crack tends to be much smaller than that of 2A12 alloy, and the spot welding strength is comparable to that of hard aluminum alloy. The surface treatment of 7A33 alloy is the same as that of general aluminum alloy, and has excellent grindability and machinability.