Effect of Ageing Treatment on Mechanical and Corrosion Properties of 7075 Aluminum Alloy

摘要

The SCC behavior and the strength of 7075 aluminum alloy in various ageing treatments (T6, T7351, RRA) have been tested using slow strain rate tension (SSRT) technique. The effect of the ageing time and the ageing temperature on the strength and SCC of the aluminum alloy have been studied. The experimental results show that the strength of 7075T6 aluminum alloy at a strain rate of 9.68 x 10~(-7) s~(-1) is maximal among 7075T6, 7075T7351 and 7075RRA,and that 7075T6 aluminum alloy is most susceptible to SCC. The fracture stress, the time to fracture, the reduction of area and the total fracture energy of 7075T6 aluminum alloy have markedly decreased in the solution of 3.5%NaCl. The SCC resistance of the alloy can be well improved in T7351 state, but at the expense of a strength loss of 20 percent compared with that in peak ageing (T6) state. The results also indicate that retrogression and re-ageing (RRA) may increase the SCC resistance of the 7075 aluminum alloy without sacrificing its strength. The electrochemical noise data revealed the changes on the specimen surfaces from passivity to passive-active condition. It is the propagation of pits instead of the pitting nucleation that plays main role during SCC. The SCC susceptibility and the propagation rate of pits are extremely high after T6 ageing treatment, Which is due to the difference of microstructure of 7075 aluminum alloy in various ageing state.