Improving the mechanical and corrosion properties of aerospace-grade AA7075 through microstructural modification using low-temperature friction stir processing

In this study, the microstructure of aerospace-grade aluminium 7075 alloy was modified using multi-pass friction stirring, and its mechanical and corrosion properties were investigated. Macroscopic observations indicated the formation of defect-free stir zones with better material consolidation. Electron backscatter diffraction investigation revealed significant grain refinement on the specimen subjected to 4 friction stir processing passes with grain size averaging 2.6 mu m. Transmission electron microscopy observations confirmed the phenomenon of grain boundary pinning by second-phase MgZn2 precipitates, which favoured the formation of a fine-grained structure due to dynamic recrystallization. X-ray diffraction analysis confirmed the occurrence of lattice distortion in the matrix due to variations in the coefficient of thermal expansion between the matrix and the precipitate. Bulk texture results exhibited a maximum texture index of 2.0800 in the 4-pass friction stir processed specimen with copper and S being major components. The average microhardness and tensile strength increased to 122.97 HV and 470 +/- 15 MPa after 4 friction stir processing passes due to finer grains, dense dislocations and finely dispersed precipitates in the microstructure. Potentiodynamic polarization analysis displayed enhanced corrosion resistance in the friction stir processed specimens compared to the base alloy. Maximum resistance to corrosion with a corrosion potential of -1.0852 V was exhibited by the 4-pass specimen due to grain refinement and the formation of a stable passive film.