Enhancing mechanical properties and corrosion resistance of a high strength 7A99 Al alloy by introducing pre-rolling in solution and aging treatments

Abstract The balance between the mechanical properties and corrosion resistance of high strength 7xxx Al alloys is an important topic. This study introduced pre-rolling into the solution and aging treatments for a recently developed 7A99 Al. The cold rolling before solution was considered as the first group, and the cold rolling after solution but before aging was the second group. For the first group, a great deal of strain energy was stored in the alloy, which contributed the occurrence of recrystallization during subsequent solution process and a finer grain structure. For the second group, the dislocations induced by cold rolling before aging accelerated the coarsening of η' and η precipitates. Moreover, T76 aging also caused the larger size of precipitates due to its higher temperature than T6 aging. The pitting corrosion of 7A99 Al tended to initiate at Al7Cu2Fe intermetallic, and the dissolution of MgZn2 phase along grain boundaries facilitated the intergranular corrosion. The cold rolling after solution process and T76 treatment both improved the resistance on stress corrosion cracking, since the grain boundary precipitates had large size, discontinuous distribution and high Cu content. The combining of cold rolling before solution and T6 treatment contributed to the enhancement of strength due to the fine grain strengthening, solution strengthening and precipitates strengthening, and the corresponding yield strength and ultimate tensile strength reached as high as 628 and 665 MPa. Compared to T6 treatment, the strength of T76 treated samples was reduced only by 3%-5%, which is an advantage of the proposed method.