Tailoring surface properties of aerospace-grade aluminium alloy through solid-state friction stir processing

Abstract

Aerospace grade AA7075-T6 alloy was friction stir processed at different tool rotation speeds and traverse speeds with a square profiled pin and the resulting microstructural evolution, texture formation and mechanical properties were analysed in this study. Low-magnification optical macroscopy, electron back-scattered diffraction, and bulk-texture analysis were conducted to probe material consolidation, microstructural grain refinement and crystallographic orientation of the grains inside the stir zone. Mechanical properties such as microhardness and tensile strength analysis were further conducted and the results correlated. Macrostructural observations show better consolidation in the specimens processed at 1050 r/min with 30 and 60 mm/min traverse speeds, and 1200 r/min with 30 mm/min as traverse speed. Electron back-scattered diffraction results confirm significant grain refinement in the sample processed at 1050 r/min, 60 mm/min and having a minimum grain size of 3.49 μm. Bulk-texture analysis indicated the presence of randomized grain orientation with copper and S as major texture components. Maximum hardness and tensile strength of 147.1 HV and 432 MPa were exhibited by the specimen processed at 1050 r/min and 60 mm/min due to grain refinement, increased dislocation density and finely distributed strengthening precipitates. Unprocessed base material displayed bimodal ductile and brittle mode of failure, while ductile fracture behaviour was exhibited by the friction stir processed specimen.