A New Method to Characterize and Model Stress-Relaxation Aging Behavior of Aluminum Alloys Under Age Forming Conditions

Abstract

A new method that utilizes theories of thermally activated deformation and repeated transient stress-relaxation tests has been proposed and validated in this study for the characterization and modeling of the stress-relaxation aging (SRA) behavior of aluminum alloys and its dependence on stress and temperature. Using the new method, key deformation-related variables, i.e., stress components, activation volume, and activation energy, of the aerospace grade heat-treatable aluminum alloy AA7B04 have been obtained as a function of aging temperature (388 K, 413 K and 438 K), stress (both elastic and plastic), and SRA time (up to 4 hours). It has been found that the apparent activation energy Qa of the material remains constant in the elastic region but decreases with the increase in strain in the plastic region, and also decreases with the increase in temperature for all initial loading stresses. These characteristics contribute to a much higher degree of stress relaxation in the plastic region and at higher temperatures than in the elastic region and/or at lower temperatures. The obtained changing activation volume V and Qa indicate that the deformation rate is controlled by forest dislocation interactions in the elastic region (V decreases from over 200b3 to less than 100b3), and by a cross-slip mechanism at high stress levels in the plastic region (V decreases to a few tens of b3). Based on these theories and results, a novel and simple constitutive model has been proposed, with which the stress-relaxation behavior of AA7B04 at different aging temperatures (388 K to 448 K), preloaded from elastic to plastic regions for up to 16 hours has been successfully predicted. The proposed model eliminates the limitations of conventional SRA models which mainly deals with elastic loading and isothermal conditions, and provides a foundation to effectively predict the springback after advanced non-isothermal SRA forming of aluminum alloy structures in the aerospace industry.