Microstructure, Texture and Mechanical Behaviour of Friction Stir Back Extruded (FSBE) AA6061-T6

Based on the friction stir phenomenon and hybridised with conventional bulk forming, a novel thermo-mechanical processing method, friction stir back extrusion (FSBE), was recently proposed as a means to produce tubular lightweight alloy products with refined microstructures and enhanced mechanical properties. A detailed investigation on the effects of FSBE on the mechanical properties and microstructural evolution in the 6061-T6 aluminium alloy is presented in the current work. Microstructural observations indicate a significantly refined grain structure within the processing zone near the inner surface of the tube due to the large shear deformation and frictional heat generated by the process. Non-homogeneity is noted across the tube, where grain refinement effect is less profound closer to the outer tube surface because of the reduced shear deformation and thus less driving force for recrystallisation. EBSD microtexture analysis is performed at several locations along and across sections of the processed tubes, revealing a very strong fibre-type shear texture within the processing zone. Uniaxial tensile testing showed a drop in the yield strength for the processed material compared to as-received one, which is attributed to the dissolution and coarsening of precipitates of the T6 tempered specimens. FSBE, nevertheless, is shown to have great and favourable impact on the tensile strength, tensile ductility and hardening behaviour of the processed material. Post processing heat treatment is also applied, and its effects on the microstructure and mechanical behaviour are discussed.