Influence of tool plunge depth and multiple passes on the microstructure, texture evolution and hardness of age-hardenable aluminium 7075 alloy

In this study, the surface properties of AA7075 were modified through the solid-state friction stir processing technique using a hexagonal-shaped H13 pin. Single pass friction stir processing was conducted at two different shoulder plunge depths at constant tool rotation and linear speeds with the resulting cross-sections being examined using optical macroscopy. Six friction stir processing passes were then carried out intermittently at optimal shoulder plunge depth and the corresponding microstructural evolution along the nugget was investigated using advanced microstructural characterization techniques such as electron backscatter diffraction and high-resolution transmission electron microscopy. Bulk texture examination was conducted using the neutron diffraction method, while hardness measurements were taken using Vicker's hardness testing machine. Single pass friction stir processing conducted at a shoulder plunge depth of 0.25 mm produced defect-free material consolidation with a refined grain structure along the nugget. Minimum grain size averaging 3.78 μm was observed at the top, while slightly increased grain sizes were noticed at the middle and bottom regions of the nugget. After 6 friction stir processing passes, grain size was reduced significantly averaging 1.84 μm at the top, while size at the pin-driven zone exhibited an increasing trend due to heat input and strain rate variations. Both, single-pass and 6 pass specimens exhibited a weaker texture along the nugget due to randomness in grain orientation and the effect of particle-stimulated nucleation mechanism. In the single-pass specimen, hardness decreased along the nugget, with a maximum of 85.2 HV at the top, while 6-pass specimen exhibited an improved hardness of 133.5 HV due to the strengthening caused by grain refinement, precipitates and dislocations.