THEORETICAL STUDIES ON THE MATERIAL GRAIN DEVELOPMENT DURING SEVERE PLASTIC DEFORMATION (SPD) PROCESS

Abstract

Ultra fine grained (UFG) materials have promising mechanical properties such as combination of high strength and high ductility, improved fatigue behaviour and high dynamic strength. UFG metals allow design and manufacture of structural components that use less material and require fewer i manufacturing steps. This provides energy and manufacturing cost savings. Several techniques Mr producing ultrafine grained materials are currently being practiced. These techniques are limited in their ability to produce the size and quantities of material needed for commercial use. There are several reported severe plastic deformation (SPD) processes which are capable of producing UFG materials. Among others, equal channel angular pressing (ECAP) is the most investigated SPD process. During ECAP, a rod shaped material is pressed through a die which has two or more channel angles with s i ie cross section intersecting in a certain angle. Since the cross section of the channels is the same, the processed rod has theoretically the same geometry as before. After several passes through the same tool, ultra fine grains are induced in material. However, this process is limited mainly to shorter path lengths due to high press forces. High magnitude of pree forces are resulted by the high frictional forces and by the induced high strain. There have been many attempts to develop continuous SPD process. Nevertheless, the reported methods are incapable of producing cross-sections larger than a few square millimeter. Therefore, these process are uneconomical and the industrial utilisation of these materials is still limited with a few applications. This calls for development of a new continuous process which is required for the commercialization of SPD processes. In this work a new severe plastic deformation process is developed to produce bulk UFG materials by combining the conventional equal channel angular pressing with the incremental bulk forming method rotary swaging. The developed tool system consists of two forming rotary swaging tools which contain four forming sections in a single forming pass. Round bars made of copper gypu.ritY 99.9%) with a diameter of 20mm are used in the first set of investigations. It was possible to reprocess the samples with the developed process repeatedly. The developed process is capable of inducing a signif ant grain refinement through subgrain development. With the help of commercial finite element code Abaqus/explicit the simulations were carried out. The simulated results were in good agreement with experimental results. The loads which were acting on tool system are very low when compared with other SPD process. Therefore, equal channel angular swaging (ECAS) has promising features for a continuous severe plastic deformation method.