7000 high-strength aluminum has high specific strength and good toughness, and is widely used in the aerospace field. However, high-strength aluminum alloys are prone to local corrosion such as pitting corrosion, intergranular corrosion, and exfoliation corrosion,etc.
Due to the large stress that the material bears, stress corrosion cracking is prone to occur, which brings hidden dangers to aviation safety. Therefore, the stress corrosion research of 7000 series aluminum alloy has attracted much attention. Appropriate heat treatment conditions can be selected to improve the stress corrosion resistance of 7000 aluminum alloy.
The characteristic of aluminum alloy stress corrosion is intergranular fracture, which is dominated by the precipitation structure of aluminum alloy grain boundaries. This type of corrosion requires the grain boundary to meet a condition, that is, the grain boundary is an anode relative to the structure around the grain boundary, so that the corrosion can selectively extend along the grain boundary.
For 7000 aluminum alloy, the anode zone is η phase (MgZn2). The stress corrosion performance of 7000 can be improved by heat treatment. It is to adjust the morphology and distribution of η phase on the grain boundary to achieve their goals.
For example, the η phase of the 7150 aircraft aluminum alloy in the T6 Temper is continuously distributed in the grain boundary, so the intergranular corrosion is very easy to develop in depth, thereby causing stress corrosion cracking.
The grain boundary of 7150 aluminum plate with T77 temper treated with regression and re-aging is intermittently distributed with coarse η phase, which will inhibit the further development of corrosion, thereby improving the alloy's stress corrosion resistance.
For the microstructure of 7000 aluminum alloy, the intragranular precipitation structure dominates the strengthening, and the grain boundary precipitation structure dominates the corrosion. The 7150 aluminum plate in the T6, T77, T76 and T76+T6 has different corrosive properties.
Original Source:https://www.aircraftaluminium.com/a/7150-aircraft-aluminums-corrosion-resistance-performance.html
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