Theoretical And Numerical Investigation On Warm Formability Of AA5086 Sheets And Its Experimental Verification

With an increasingly severe effect of global warming, automotive vehicles with low energy consumption, low pollution and high fuel economy are becoming more and more popular. Lightweight has become the development theme of word auto-industry. Due to its small density and high specific strength, aluminum alloys have attracted lots of attention. However, the low formability at room temperature greatly limits their application. In the context of no change in alloying constituent, warm forming can improve effectively the formability of aluminum alloy sheet. Based on theoretical calculation and numerical simulation, the effects of temperature and strain rate on the warm formability of 5086 aluminum alloy sheet are studied and the results are verified by experiments.Uniaxial tension tests at different temperatures (20-200 ℃) and strain rates (0.02-2s-1) are carried out to investigate the flow behavior of AA5086. Three constitutive models (modified Voce, Ludwick and KHL model) are adopted to describe well the deformation of AA5086. Based on Marciniak-Kuczynski model, the M-K algorithm is developed by combing above three constitutive models and Hill48 anisotropic yield criterion. Using the calculated results of M-K algorithm, the effects of necking criterion, initial groove angle, constitutive model, yield criterion and initial imperfection factor on forming limit curves (FLCs) are investigated. It is found that the initial groove angle has distinguishing influences on limit strains under different strain paths. When the strain path is in the range from 0 to 0.4, forming limits are always achieved with a zero groove angle. While when the strain path is not in the range from 0 to 0.4, limit strains depend greatly on the initial groove angle. The limit strains obtained with a zero groove angle in the literatures overestimate clearly its true sheet formability.To validate the reliability of the developed M-K algorithm, Marciniak tests at different temperatures (20-200℃) and strain rates (0.02-2s-1) are executed to obtain the forming limits of AA5086. The comparison between experimental FLCs and calculated ones of M-K algorithm shows that in the range of temperature and strain rate covered in this work, the predicted FLCs with modified Ludwick and KHL constitutive models are generally in good agreement with experimental ones. However, due to the saturation of the modified Voce constitutive model affects the evolution of strain and stress around the necking localization, there is great discrepancy between the predicted FLCs with the modified Voce constitutive model and experimental ones.Then the offsetting interaction of forming speed and temperature at high strain rate is investigated by numerical simulation. To construct an accurate constitutive model of AA5086 under high strain rate conditions, a dynamic tensile device is used to study the flow behavior of AA5086. Based on the obtained experimental data at high strain rate, a modified Voce constitutive model (named Lin-Voce model), in which the strain hardening shows a linear relationship with temperature, is proposed and its material parameters are identified by inverse analysis technique. Then, this equation is implemented to FE-Marciniak mode to predict forming limits of AA5086 under high temperatures and strain rates. Results show that for aluminum alloys, the warm formability depends on the offsetting interaction of temperature and strain rate. At low strain rate, temperature softening plays a prominent role and the sheet formability is enhanced greatly at elevated temperatures. But under high strain rates, thermal softening maybe offset or even exceeded by strain hardening, leading to the decrease of formability. Therefore, the blank must be heated to a certain temperature above 300℃ if one wants to improve the formability of AA5086 under high strain rates conditions.