A new severe plastic deformation technique of combined extrusion and torsion to prepare bulk ultrafine grained copper

Reducing grain size is a well-established method for strengthening metals. In this study, a novel severe plastic deformation technique—combined extrusion and torsion (CET) with composite strain—was developed to fabricate bulk ultrafine grained metals. A single pass of CET treatment (with a rotation velocity of 1 r/s and extrusion speed of 3 mm/s) refined coarse-grained copper from 54 μm to 450 nm at room temperature, resulting a significant increase in hardness from 0.55 GPa to 1.3 GPa. The CET technique addresses the limitations of conventional extrusion-processed copper (without torsion), which suffers from gradient microstructure and hardness distribution. It provides enhanced strain accumulation under the same extrusion ratio conditions. The more homogeneous microstructures and properties of CET-processed copper rods are attributed to the reduced strain gradient due to torsion. Additionally, targeted finite element analysis indicated that the CET technology requires 37 % less extrusion load and offers at least 30 % more strain compared to conventional extrusion methods. Compared with other severe plastic deformation methods, such as equal channel angular pressing and high-pressure torsion, which involve simpler deformation processes, the CET technique shows considerable promise for large-scale manufacturing of ultrafine-grained metals.