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

Abstract

A new method that utilises theories of thermally activated deformation and repeated transient stress-relaxation tests has been proposed and validated in this study for the characterisation and modelling of the stress-relaxation ageing (SRA) behaviour of aluminium alloys and its dependence on stress and temperature. Using the new method, key deformation-related variables, i.e. stress components, activation volume and energy, of the aerospace grade heattreatable aluminium alloy AA7B04 have been obtained as a function of ageing temperature (388, 413 and 438 K), stress (both elastic and plastic) and SRA time (up to 4 h). It has been found that the apparent activation energy Qa of the material remains constant in the elastic region but decreases with increasing strain in the plastic region, and also decreases with increasing 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 200b 3 to less than 100b 3 ), and by a cross-slip mechanism at high stress levels in the plastic region (V decreases to a few tens of b 3 ). Based on these theories and results, a novel and simple constitutive model has been proposed, with which the stressrelaxation behaviour of AA7B04 at different ageing temperatures (388 to 448 K), pre-loaded from elastic to plastic regions for up to 16 h has been successfully predicted. The proposed model eliminates the limitations of conventional SRA models which mainly deal with elastic loading and isothermal conditions, and provides a foundation to effectively predict the springback after advanced non-isothermal SRA forming of aluminium alloy structures in the aerospace industry.

A Unified Constitutive Model of Stress Relaxation of Ti-6Al-4V Alloy with Different Temperatures from Elastic to Plastic Loading

“…The apparent activation energy Q a is the important variable required for the creep strain rate modeling as a function of temperature in the SR stage. Li et al [32] have found that the Q a in the elastic region remains almost constant during the RSR experiments of AA7B04 aluminum alloy. The activation energy Q in elastic region is described as follows:…”

“…These data can be used to characterize and analyze the deformation behavior and mechanism of materials during SR. The creep strain rate .ε c of titanium alloy can be modeled using the following equation[32]:…”

Effect of Deep Cryogenic Treatment on the Artificial Ageing Behavior of SiCp–AA2009 Composite

“…SiCp are evenly distributed in the composites (Figure 6a,b). The shape and size of grains in the composites remain during ageing, as the ageing temperature is not high enough to enable the apparent change of grains [29]. Meanwhile, it can be observed that, after ageing for 20 h, apparent precipitates can be observed in the large scale image of SEM in Figure 6b.…”

Stress relaxation behavior of low carbon steel at different temperatures *

“…Guo [11,12] describes the stress relaxation behavior of copper wires with different diameters by the thermal activation theory, realizes the connection between the physical mechanism and the deformation process in the stress relaxation process, and analyzes and confirms that the interaction between dislocation and grain boundary is the dominant mechanism of stress relaxation. Based on the thermal activation theory and repeated stress relaxation experiments, Li [13] proposed a new constitutive model to predict the stress relaxation behavior of AA7B04 under different temperature and loading conditions.…”

“…Figure7shows the relationship between strain rate and stress in stress release. According to equation(13), linear fitting is performed for  e s ln ln , p and the strain rate sensitivity coefficient m is the slope of the equation. As shown in the figure, the value range of strain rate sensitivity coefficient is: 0.04845 ∼ 0.3063.…”