Localized severe plastic deformation by plane stress local torsion

The process of plane stress local torsion (PSLT) is proposed as a localised severe plastic deformation for the enhancement of holes. The PSLT process is capable of creating large shear strains and consequently substantial grain refinement in the vicinity of the hole with a gradient in the radial direction. It is of great interest and importance to investigate the effect of the PSLT process on a range of materials commonly used in the industry. The specimen design is a prerequisite for any experimental investigation of the local torsion process. A number of potential designs which enable the implementation of the PSLT process are investigated in this research work. The potential designs of the specimen are compared for their abilities to perform the PSLT and to produce the shearing strain within an annular zone around the hole. The computer-aided specimen design recommends a feasible design for successful implementation of the process. Then, the deformation of the specimen in the vicinity of its hole surface is studied by introducing a parametric kinematically admissible displacement field (PKADF) within the deformation affected zone (DAZ). Calibration of the parameters in relation to the deformation of the material during the process is done by an elastic-plastic finite element solution which was performed in ABAQUS. Subsequently, the calibrated model is used in an upper bound solution of the problem to estimate the torque-twist response of the specimen during the PSLT process. Mechanical behaviour and microstructure of the ultra-fine grained Al6061 and interstitial free (IF) steel specimens processed by the PSLT are also investigated in this work. The PSLT process is simulated using the finite element method. Microstructural changes and the extent of grain refinement are examined by means of optical microscopy, SEM and EBSD. It is shown that a considerable mechanical properties improvement can be achieved adjacent to the surface of the hole by the PSLT processing. The micro shear punch and microhardness tests reveal the degree of enhancement in ultimate and yield strengths of the materials processed by the PSLT.