Kinematics of metal flow during twist extrusion investigated with a new experimental method

We propose an experimental method for investigating the kinematics of metal flow in twist extrusion (TE). The method is based on a theoretical model of the velocity field. The parameters of this model are determined to satisfy the observed flow-line for characteristic points of the specimen. The model incorporates two physical constraints: (1) metal flow is limited by the surface of the die; (2) metal volume remains constant. The advantage of this method is that it takes into account the actual rheology of the metal and friction conditions. We show that TE forms a vortex-like flow that stretches metal particles. The stretching increases with subsequent TE passes. The equivalent strain increases from 0.3 to 0.5 in the paraxial zone of the specimen to 2.0-2.5 at the periphery. We analyze different sources of inaccuracy of the method and show that its error in estimating the equivalent strain is of the order of 0.1, which is perfectly acceptable when investigating severe plastic deformations. (C) 2008 Elsevier B.V. All rights reserved.