Simple shear model of twist extrusion and its deviations

Twist extrusion (TE) is a severe plastic deformation method with a potential for commercialization. Advancing TE toward industrial use requires in-depth understanding of deformation during the process and its dependence on processing factors. The helical flow model introduced with the concept of TE provides for a concise description of deformation in the process. To date, however, it was unclear under which conditions the helical flow model yields accurate predictions of deformation in TE. This paper presents a systematic finite-element study performed to identify effects of some key process and material factors on deformation in TE and its departure from the ideal deformation described by the helical flow model. It was found that high strain-hardening rate and friction lead to violations of the assumptions of the helical flow model and that these violations result in departure from the ideal deformation. Deviations from the ideal deformation tend to increase on decreasing the length of the twist channel. Friction effects appear especially critical to be considered for accurate prediction of deformation in TE. Finite-element simulations taking friction into account show good qualitative agreement with earlier marker-insert experiments. The results of the present finite-element study allowed for defining the simple shear model of TE.