Stress Corrosion Cracking of AA7075 Aluminum Alloy Processed via Additive Friction Stir Deposition

High-performance structures are needed for many structural applications, including land, air, and marine-based transportation systems. To that end, additive friction stir deposition (AFSD), a solid-state manufacturing process that operates on the principles of friction stir processing, is quite a promising additive manufacturing tool with the potential to create metallic materials with desired properties. This study focuses on understanding the stress corrosion cracking (SCC) susceptibility of AA7075 aluminum alloy processed via AFSD. The SCC was evaluated by carrying out a slow strain rate tensile (SSRT) testing in 3.5 wt.% NaCl solution at room temperature in as-processed and post-processing heat treatment (T73-type temper) conditions. The tensile testing of the alloy in as-processed was done at initial strain rates (constant crosshead velocity) of 10-4, 10-5, and 10-6 s-1, whereas the alloy in T73 condition was subjected to SSRT testing under similar conditions at 10-6 s-1. Scanning electron microscopy, electron backscattered diffraction, and transmission electron microscopy were used for microstructural characterization. The grain size obtained after deposition was 3.81 +/- 2 mu m. The SSRT testing results show that the as-processed AA7075 aluminum alloy exhibited a high susceptibility to SCC at 10-6 s-1. The T73 heat treatment caused reduced susceptibility of the AFSD-processed AA7075. The TEM results showed the secondary phases were rich in Cu in the as-processed case. The level of Cu enrichment in coarse secondary phases was significantly smaller after T73 heat treatment. The high potential difference between the matrix and the precipitate due to Cu depletion in the matrix might have made the as-deposited alloy more susceptible to SCC. The T73 heat treatment was found to be effective in improving the SCC resistance of the AFSD-processed AA7075 alloy.