Characterization of microstructure, crystallographic texture, corrosion behavior and mechanical properties of severely deformed AA7075 alloy

Abstract

• Increased ECAP passes improve strain and stress uniformity in AA7075. • Annealed grains are fibrous; post-ECAP shows finer grains and mixed orientations. • 4Bc route refines grains and texture significantly; 4A route shows highest strain. • ECAP processing enhances both strength and corrosion resistance. In this study, we investigate the microstructure, phase composition, and their mechanical and corrosion evolution for AA7075 aluminum alloy before and after equal channel angular pressing (ECAP) under different passes and routes. Samples were subjected to the ECAP process using a 90° die, up to 4 passes via routes Bc, A, and C. The results reveal significant changes in grain/subgrain crystal orientation post-ECAP compared to the as annealed material, indicative of micro-strain accumulation and grain refinement. Notably, a substantial grain refinement was observed after the 4Bc condition, with an average grain size of 1.6 µm, indicating a nearly homogeneous grain structure. Conversely, grain size increased slightly to 2.4 µm and 3.24 µm for the 4C and 4A conditions, respectively, accompanied by a higher density of low-angle boundaries. The high density of dislocations acts as pathways for aggressive ions, facilitating deeper penetration and potentially leading to higher corrosion rates. Additionally, the uniform refined structure under 4Bc promotes the formation of a thicker and more protective passive film on the surface, significantly enhancing corrosion resistance compared to the 4A and 4C conditions. Mechanically, significant improvements in yield and ultimate strength were noted, with increases by factors of 1.25 and 1.35, respectively, compared to the as-annealed sample. These findings are supported by finite element analysis to verify stress and strain uniformity. Subsequent increases in the number of passes resulted in more heterogeneous stress distribution, with strain hardening dominating over strain softening. Further escalation in the number of passes up to 4Bc yielded a subsequent rise in maximum stress. These findings highlight the synergistic effects of ECAP passes and routes, providing valuable insights into the development of competitive aluminum alloys with high mechanical properties and reduced corrosion rates, essential for applications requiring combined properties.