Study Of Influence Of Retrogression And reaging (RRA) Treatments On Microstructure Evolution And Mechanical Properties Of Aluminium Alloy 7075

Low , Wen Jie (2010) Study Of Influence Of Retrogression And reaging (RRA) Treatments On Microstructure Evolution And Mechanical Properties Of Aluminium Alloy 7075. Project Report. UTeM, Melaka, Malaysia. (Submitted)

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Abstract

Aluminium alloy 7075 has been widely used in aircraft structural application over many years. It is, however, prone to both stress corrosion cracking (SCC) and exfoliation corrosion. Overaging has been used to resolve these problems. However, a strength penalty of between 10% and 15% is thus incurred. Therefore, a process known as retrogression and reaging (RRA) treatment, which capable to restore peak strength of T6 temper while retaining resistance to SCC and exfoliation of T7 temper. This project was set up to study the influents of retrogression and reaging (RRA) treatment and overaging heat treatments on the mechanical properties and microstructure evolution of this aluminium alloy. The mechanical properties of aluminium alloy were determined by using three mechanical tests, which are tensile test, Rockwell hardness test, and Charpy impact test. From the review of microstructure evolution, the precipitation in RRA treated aluminium alloy is extremely fine and distributed homogeneously inside the grains. It is slightly denser and more stable than that resulting from the T6 temper; whilst the grain boundary precipitation is quite different from that resulting from T6 treatment, the particles being coarser, and much alike to the precipitation resulting from T7 temper. The retrogression temperature is the main property controlling factor. Higher retrogression temperature increases the dissolution degree and promotes the formation of stable precipitates in reaging stage. Aluminium alloy 7075-T6 and aluminium alloy 7075-RRA possess highest yield strength, ultimate tensile strength, and Rockwell hardness among those heat treated aluminium alloy 7075. However, Charpy impact strength of aluminium alloy 7075-T6 and aluminium alloy 7075-RRA are lowest among those heat treated aluminium alloy 7075. Aluminium alloy 7075 (solution hardened) possess lower yield strength, ultimate tensile strength, and Rockwell hardness compare with aluminium alloy 7075-T6 and aluminium alloy 7075-RRA. However, aluminium alloy 7075 (solution hardened) possess highest Charpy impact strength among all types of aluminium alloy 7075 in this project. Aluminium alloy 7075-T73 possesses lowest yield strength, ultimate tensile strength, and Rockwell hardness among all types of aluminium alloy 7075 in this project. However, aluminium alloy 7075-T73 possesses higher impact strength compare to aluminium alloy 7075-T6 and aluminium alloy 7075-RRA.

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