Microstructure, texture and mechanical properties of aluminum processed by high-pressure tube twisting

The new severe plastic deformation technique known as high-pressure tube twisting (HPTT) is a continuous process to obtain grain refinement in bulk metallic materials of tubular geometry. HPTT was applied to commercially pure aluminum up to a shear strain of 24. The mechanical properties were enhanced markedly and were characterized by compression and ring-hoop tensile tests. It was shown that the axial force during HPTT does not contribute to the plastic flow of the material; it is only controlled by the shear component of the applied stress. The microstructure and the crystallographic texture were examined in detail by electron backscatter diffraction, transmission electron microscopy, and X-ray diffraction, and these showed the existence of a gradient within the cross-section of the tube. The next-neighbor grain misorientation distribution showed a bimodal nature, of which the small-angle peak was attributed to the “internal” new grains originating from the interior of the initial grains and the second one from the new grains situated at the grain boundaries of the initial grains. The measured textures were typical shear textures with specific tilts in the sense of the shear direction at large strains, which can be attributed to the effect of strain rate sensitivity of slip. The main component of the textures was the C component, which continuously strengthened with the shear. The results obtained suggest that HPTT is an efficient processing way to transform the microstructure of Al tubes into ultra-fine-grained structures in one single operation.