Multistage aging treatment influenced precipitate characteristics improve mechanical and corrosion properties in powder hot-extruded 7055 Al alloy

The precipitation evolution, mechanical properties and corrosion behavior of powder hot-extruded 7055 Al alloy subjected to T6, RRA, T76 and T74 aging treatments were investigated. The TEM and DSC results reveal that the dominant intragranular precipitates under those conditions are GP (I, II) + eta' phase, GP (II) + eta' phase, eta' phase and eta' + eta phases, respectively. The quantitative characteristics (size, interspacing and volume fraction) show that the small volume fraction of intragranular precipitates limits the yield strength of the T6 alloy, while the enlarging of interspacing caused by the coarsening of intragranular precipitates is the main reason for the reduction of yield strength of over-aged alloys. The continuously distributed grain boundary precipitates (GBPs) with low Cu content in the T6 alloy are responsible for higher corrosion susceptibility, while the separated and coarsened GBPs with high Cu content formed in the deepening aging process contribute to the better corrosion resistance. At the same time, the drop in corrosion potential of Al matrix also reduces the corrosion driving force of GBPs with the aging degree deepening. The optimal comprehensive performance of the 7055 Al alloy can be obtained after RRA treatment, namely the highest tensile strength of 739.7 MPa and the acceptable corrosion resistance comparable to that of the T76 state.