FEM simulation study on the equal channel angular extrusion processing of CP-Ti

Abstract

Pure titanium is more favorable as implant material, other than widely used titanium alloys, regarding to the biological compatibility. However, coarse-grained commercial-purity titanium (CP-Ti) has much lower mechanical properties than titanium alloys, which restricts the bio-medical application of CP-Ti. In this research work, CP-Ti billets are processed using equal channel angular extrusion (ECAE) to produce ultrafine-grained bulk materials to improve the tensile strength of CP-Ti without much lose in the ductility. ECAE is the extrusion of a billet through a die with two channels of equal cross-section intersecting at a predetermined angle. Ideally, the material is simple sheared within a thin layer in the intersection of die channels to impose a large uniform strain in the billet, without concomitant change in the billet dimension. However, the strain uniformity is not guaranteed in the extrusion of most metals, due to the strain hardening or flow softening property of the metal, the die design or processing parameters. Even more, the highly localized shear deformation raises concerns about the workability of the ECAE billet. Unfortunately, titanium is a well-known difficult-to-work material. It’s mechanical behavior changes from strain hardening to flow softening behavior, when the deformation rate increases or the processing temperature decreases. Strain nonuniformities ranging from moderate flow localization to severe shear banding and even shear fracture have been observed during ECAE of CP-Ti. Therefore, the optimization of ECAE processing conditions on the deformation uniformity and the workability of CP-Ti becomes critical. This thesis studied the plastic deformation behavior of CP-Ti billets using finite element (FE) simulations. The efforts are made on two aspects. First, the detailed plastic deformation behavior of Ti billets from a single pass to multiple passes along certain deformation routes is predicted. It is found that uniformi...