Effect of non-isothermal retrogression and re-ageing on through-thickness homogeneity of microstructure and properties of Al-8Zn-2Mg-2Cu alloy thick plate

In order to improve the through-thickness homogeneity and properties of aviation aluminum alloy thick plate. The effect of heating-cooling retrogression and re-ageing on the performance of Al-8Zn-2Mg-2Cu alloy thick plate was investigated by hardness tests, electrical conductivity tests and transmission electron microscopy (TEM) observation. Results revealed that, during retrogression heating, the fine pre-precipitates in surface layer dissolve more and the undissolved η′or η phases are more coarsened than that of center layer. During slow cooling after retrogression, precipitates continue coarsening but with a lower rate and the secondary precipitation occurs in both layers. Finer precipitates resulting from the secondary precipitation are more in surface. However, the coarsening and secondary precipitation behaviors are restrained in both layers under quick cooling condition. The electrical conductivity and through-thickness homogeneity of precipitates increases while the hardness decreases with cooling rate decreasing. After the optimized non-isothermal retrogression and re-ageing (NRRA) including air-cooling retrogression, the through-thickness homogeneity which is evaluated by integrated retrogression effects has been improved to 94%. The tensile strength, fracture toughness and exfoliation corrosion grade of Al-8Zn-2Mg-2Cu alloy plate is 619 MPa, 24.7 MPa·m1/2 and EB, respectively, which indicates that the non-isothermal retrogression and re-aging (NRRA) could improve the mechanical properties and corrosion resistance with higher through-thickness homogeneity.