Constitutive analysis of tensile deformation behavior for AA1100 aluminum subjected to multi-axial incremental forging and shearing

Abstract

The evolution of flow stress with imposed strain for an aluminum alloy (AA1100) processed by severe plastic deformation (SPD) has been described by utilizing three different models. The applied constitutive laws include a generalized three-dimensional dislocation density based model, a formulation based on the combination of the Estrin and Mecking model with an Avrami-type equation (EMA), and finally a relationship based upon the dimensional analysis using Pi-theorem. To verify the predicted flow stress, newly developed SPD process called multi-axial incremental forging and shearing (MAIFS) were performed from one to eight passes on the aluminum alloy workpieces. It was shown that the predictions of the EMA and Pi-theorem based models agreed well with the experimental trend of flow stress variations over a wide range of strain, and these models were able to address both hardening and softening observed at high strains. But, some modifications should be carried out on the dislocation density based model to give results with a reasonable agreement with the experimental data in the range of strain softening. The correct true stress–strain curves of each of eight passes after tensile test flow localization were firstly calculated according to a neck evolution model proposed by Segal et al. (2006). Then, a general descriptive constitutive equation for tensile flow behavior of the material after each pass of SPD process was developed based on an extension of the model originated from Pi-theorem. Furthermore, the severely deformed specimens kept fairly large total tensile elongation and for proving ductility, scanning electron microscopy observations of fracture surfaces were carried out. These observations indicated that the fracture surfaces were characterized by a dimple-like structure, which implied the existence of a ductile failure mode.