Thermo-mechanical finite element analysis of the AA5086 alloy under warm forming conditions

Abstract

Warm forming processes have been successfully applied at laboratory level to overcome some important drawbacks of the Al−Mg alloys, such as poor formability and large springback.However, the numerical simulation of these processes requires the adoption of coupled thermomechanical finite element analysis, using temperature-dependent material models. The numerical description of the thermo-mechanical behaviour can require a large set of experimental tests. These experimental tests should be performed under conditions identical to the ones observed in the forming process. In this study, the warm deep drawing of a cylindrical cup is analysed, including the split-ring test to assess the temperature effect on the springback.Based on the analysis of the forming process conditions, the thermo-mechanical behaviour of the AA5086 aluminium alloy is described by a rate-independent thermo-elasto-plastic material model. The hardening law adopted is temperature-dependent while the yield function is temperature-independent. Nevertheless, the yield criterion parameters are selected based on the temperature of the heated tools. In fact, the model assumes that the temperature of the tools is uniform and constant, adopting a variable interfacial heat transfer coefficient. The accuracy of 2 the proposed finite element model is assessed by comparing numerical and experimental results.The predicted punch force, thickness distribution and earing profile are in very good agreement with the experimental measurements, when the anisotropic behaviour of the blank is accurately described. However, this does not guarantee a correct springback prediction, which is strongly influenced by the elastic properties, namely the Young's modulus.

Formability prediction using bifurcation criteria and GTN damage model

“…In order to validate the numerical implementation of the undamaged anisotropic material behavior, the results predicted for uniaxial flow properties, i.e., normalized flow stresses and r-values as functions of the tensile angle orientation with respect to the rolling direction, are shown in Fig. 3, along with the reference results reported by Neto et al [85]. These types of numerical results are usually used to identify the anisotropy coefficients of the material.…”

“…Hill'48 anisotropy coefficients used in the simulations (Neto et al [85]). In addition to the above results, validations in terms of yield locus and stress-strain curves for uniaxial tension tests at o 0 and o 90 from the rolling direction are also presented Fig.…”

The punch speed influence on warm forming and springback of two Al-Mg-Si alloys

“…The correlation between the punch speed and the strain rate imposed on the material was studied using finite element analysis by Palumbo and Tricarico [14] and Neto et al [8] in the warm forming of the cylindrical cup test. Both studies analyzed a non-heat-treatable Al-Mg alloy under non-isothermal conditions, with tool geometry identical to the one used in this study.…”

“…Both studies analyzed a non-heat-treatable Al-Mg alloy under non-isothermal conditions, with tool geometry identical to the one used in this study. Neto et al [8] analyzed three different temperatures (RT, 150 and 240 °C) and concluded that the strain rate in a point initially located in the flange (5 mm from the perimeter) is independent of the test temperature. For a punch speed of 5 mm•s -1 , a point sliding over the die (flange region) suffers a plastic strain rate increase from about 10 −2 to 10 −1 s −1 .…”

“…A warm temperature, below the recrystallization temperature of the material, increases the ability of the sheet metal to be formed into a desired shape without necking. The research on this topic has a high relevance to industry and has been focused on the: design of warm forming tools [2], development of alternative warm forming technologies [3], improvement of thermo-mechanical finite element analysis algorithms [4,5] and characterization and modeling of the mechanical behavior at warm temperatures [6][7][8]. Bolt et al [9] compared the warm forming behavior of a non-heat-treatable (Al-Mg) with a heat-treatable alloy (Al-Mg-Si).…”