Investigation into the shear effect of torsion extrusion process

Abstract

Torsion extrusion (TE) is a severe plastic deformation (SPD) technique with the potential to be used for the preparation of large bulk materials with enhanced mechanical properties. Shear deformation is the key factor for SPD to enhance the materials properties. However, it is not clear by far that what shear strains are dominant in torsion extrusion process and how they distribute in the extruded materials. These issues are crucial to the working parameters design of the torsion extrusion process for maximizing the shear effect. To this end, a plasticine billet with two colors was applied as experimental material, and the material flow pattern was revealed by measuring the twisted bicolor interfaces. The torsion extrusion was implemented using a die with a wavy cross-sectional bearing segment (W-TE), which is able to prevent the circumferential sliding of the materials against the die. Flow velocities were re-built according to the bicolor interfaces at each section in a cylindrical system. Strain rates and shear strains induced by torsion deformation along the radius of the material were formulated based on the velocity field and Eulerian variable derivatives, from which the dominant shear strain can be recognized. A shear effect index (SEI) was proposed to evaluate the contribution of the shear strains induced by torsion, and the distribution of SEI was analyzed. Furthermore, the W-TE experiments on the as-cast TiB₂/2026 Al composite were carried out to demonstrate the positive correlation between the shear effect and the microstructural improvement, and finite element simulation for such experiments showed the consistency of the simulated deformation patterns with the plasticine measured ones. The evaluation method and the experimental results deepened the understanding to the shear effect of torsion extrusion process.