Influence of hot rolling and shot peening on the microstructure and corrosion behavior of friction stir processed AA7075 alloy

The current study investigates the effect of friction stir processing (FSP) and subsequent thermomechanical treatments on the microstructural evolution and corrosion behavior of AA7075 alloy. Four conditions were examined: base AA7075 alloy (BM), friction stir processed AA7075 (FSP), FSP followed by hot rolling (FSP-HR), and FSP followed by shot peening (FSP-SP). Optical microscopy results showed significant grain refinement from 52 ± 8 µm in the base alloy to 5.2 ± 1.1 µm after FSP, with further reductions to 3.1 ± 0.7 µm and 1.6 ± 0.4 µm after hot rolling and shot peening, respectively. X-ray diffraction confirmed the presence of a dominant α-Al matrix along with MgZn ² , Al ² CuMg, and Al ₁₃ Fe ₄ phases, with no formation of new phases after processing. Electrochemical studies demonstrated considerable enhancement in corrosion resistance after thermomechanical treatments. Tafel polarization results exhibited a decrease in corrosion current density from 4.1 µA/cm ² for the base alloy to 2.6 µA/cm ² for FSP, 1.9 µA/cm ² for FSP-HR, and 1.3 µA/cm ² for FSP-SP. Correspondingly, the corrosion rate decreased from 1.70 mpy for the base alloy to 0.54 mpy for the shot-peened sample. Electrochemical impedance spectroscopy revealed a significant increase in charge-transfer resistance from 950 Ω·cm ² in the base alloy to 5200 Ω·cm ² under FSP-SP conditions, indicating enhanced corrosion resistance. SEM analysis of corroded surfaces confirmed reduced pitting and more compact corrosion products in processed samples. Overall, the combination of FSP with hot rolling and shot peening effectively enhances microstructural refinement and corrosion resistance of AA7075 alloy.