| Equal channel angular extrusion (ECAE) is an innovative forming process application for reaching a heavy and uniform plastic deformation inside bulk material without substantial change in its geometric shape. Due to its unique advantages, ECAE has been practically used to process a wide variety of materials for different applications. However, in order to apply ECAE successfully to produce the solid products, it is essential to obtain a thorough understanding of ECAE deformation mechanisms based on a theoretical analysis. In this dissertation, a new self-consistent velocity field is suggested to characterize the deformation region. With the aid of the velocity vector diagram, the effective strain obtained for each ECAE pass is determined. An analytical form of the punch pressure is derived from the energy balance law. A unified mathematical description for four fundamentally different ECAE routes (A, B, C and C′ ) is proposed. To predict the material flow pattern for any route and number of the extrusion passes, a 3-dimensional geometry based computational model is established and a visualization program is developed. To further obtain important information such as the stress and strain distributions during the extrusion process, a numerical model for ECAE, based on the finite element method, is established. In this model, the mixed finite element flow formulation is adopted and the corresponding variational functional is established. The perturbed Lagrange method is suggested to introduce the constant volume constraint. A 4-node bilinear mixed type finite element is constructed by following the “one A one B” guideline. By taking into account the peculiarities of ECAE deformation, the diagonalization of the H-matrix in the mixed finite element formulation is realized. Other issues such as the contact-friction algorithm and the treatment of the elastic zone are also elaborated. In the numerical examples, the sensitivities of both the geometric and material parameters to the ECAE deformation process are studied. The punch pressure and load distributions on the die walls are investigated. The capability of the developed finite element program in predicting the friction coefficient and punch load for realistic ECAE extrusions is also demonstrated. |
