Die design for axisymmetric hot extrusion

Abstract

A comprehensive finite element model is presented to obtain the temperature distribution in the workpiece as well as in the tooling in hot and warm extrusion processes. Thermo-mechanical analysis of hot extrusion process is carried out by combining the comprehensive finite element thermal model with the deformation models [upper bound and rigid-plastic finite element models presented earlier (N. Venkata Reddy, P. M. Dixit and G. K. Lal, J. Mater. Process. Technol. 55, 331 (1995); N. Venkata Reddy, P. M. Dixit and G. K. Lal, ASME J. Engng Ind. 118 (1996)) [1, 2]. The predictions of the combined thermo-mechanical finite element method (TMFEM) are first compared with experimental results to validate the method. Then it is shown that the temperature distribution and the extrusion power obtained by the combined upper bound/finite element method (UBFEM) are in good agreement with those of TMFEM. Since UBFEM takes significantly less computational time than TMFEM, it is used to obtain the optimal die profile at various process conditions by minimizing the extrusion power. A simple fracture criterion proposed by Venkata Reddy et al. [ ASME J. Engng Ind. 118 (1996)] [2] based on the concept of the hydrostatic stress component in the deformation zone falling to zero is used along with TMFEM to predict the die lengths at which the initiation of internal defects takes place. Finally, it is shown that the optimal die profiles satisfy the conditions for prevention of internal defects.