An investigation of axisymmetric forward spiral extrusion

The process of axisymmetric forward spiral extrusion (AFSE) is proposed as a new severe plastic deformation technique. The AFSE is capable of imposing shear deformation through rotation of sample inside the die which has spiral engraved grooves. The deformation of the sample inside the AFSE die is studied and the kinematically admissible velocity field is proposed. The proposed velocity field is confirmed through a dedicated experimental procedure. Based on the derived velocity field, the upper bound analysis is performed to study the effect of process parameters on the required forming load. Comparison of the measured AFSE extrusion load with that of upper bound prediction showed a good agreement. According to the derived upper bound, minimising the friction will improve the efficiency of the process while does not have any adverse effect on deformation. Furthermore, the extrusion load and the reaction torque developing during AFSE increase nonlinearly with helix angle. To study the effect of grooves inside the AFSE die on strain homogeneity inside sample, Finite Element Analysis (FEA) is utilised. The 3D model is developed in ABAQUS software and is used to clarify the effect of groove geometry on deformation. Based on FEA results, the localised deformation of the sample at the groove caused strain heterogeneity and formation of an axisymmetric core and non-axisymmetric shell within the sample. Furthermore, both groove width and depth can affect the strain heterogeneity while the effect of former is greater than that of the latter. Mechanical properties and micro-structure evolution of Ti-IF steel that was processed by AFSE were studied in this project. The mechanical tests proved the positive effect of AFSE on enhancement of material mechanical properties. Besides, the micro-structural changes were investigated by Scanning Electron Microscopy and Electron Back Scattered Diffraction (EBSD). It is shown that considerable grain refinement has been achieved after couple of AFSE passes in Ti-IF steel samples. Hardness test results and EBSD observations indicate that multi-pass AFSE can help to homogenize the microstructure of the material. Besides, the texture analysis suggests that despite the formation of texture during AFSE, the subsequent mechanical properties are not considerably affected.