Analysis of the billet deformation behaviour in equal channel angular extrusion

Equal channel angular extrusion is a relatively novel method for deforming materials to very high plastic strains, with no net change in the billet's shape. However, before the technique can be exploited it is important to understand the deformation behaviour within the die and its relationship to the tooling configuration and friction conditions. Billets containing scribed grids, simple finite element analysis, and microstructural evidence have been used to investigate this issue. It has been found that the strain achieved is sensitive to the die angle, friction conditions, and the application of a back-pressure, all of which can have a large effect on the microstructure and strain inhomogeneity within the processed billet. The distribution of strain is most uniform, and approximates most closely to a simple shear, if the deformation zone is constrained to be as narrow as possible. The best processing conditions appear to be obtained with a sharp die corner, low friction, and a constraining back-pressure. Embedded marker wire experiments have shown that on repeatedly extruding a billet with a constant strain path rotation of material occurs around the billet's ends. This results in the sheared billet wrapping around on itself during the process and thus maintaining the billet's shape, despite the increasing shear strain in each extrusion cycle.