A Dislocation based Constitutive Model for Warm Forming of Aluminum Sheet

The formability of aluminum sheet can be improved considerably by increasing the temperature.\ud At elevated temperatures, the mechanical response of the material becomes strain rate dependent.\ud To accurately simulate warm forming of aluminum sheet, a material model is required that\ud incorporates the temperature and strain rate dependency. In this paper, the dislocation based\ud Alflow hardening model is used. The model incorporates the influence of the temperature and\ud strain rate effect on the flow stress by means of the storage and dynamic recovery of dislocations.\ud It also includes the effects of solute level, particle fraction and grain size. Cylindrical cup deep\ud drawing simulations are presented using shell elements. The anisotropic behavior of the sheet is\ud described by using the Vegter yield locus. Experimental drawing test data is used to validate the\ud modeling approach, where the model parameters follow from tensile tests

Finite element simulation of aluminum sheet warm forming using alflow hardening model

In order to accurately model the plastic deformation of Aluminum sheet at elevated temperatures, a model is required that incorporate the temperature and strain rate dependency of the material. In this article, two physically based models are compared: Bergstr¨om and Alflow model. Although both models can be fit quite well to monotonic tensile tests of 5754-O alloy, large differences appear if strain rate jumps are applied. The Alflow model also represents the negative strain rate sensitivity behavior of Al-Mg alloys at temperatures below 125±C

Effect of temperature on anisotropy in forming simulations of aluminum alloys

A combined experimental and numerical study of the effect of temperature on anisotropy in warm forming of AA 6016-T4 aluminum was performed. The anisotropy coefficients of the Vegter yield function were calculated from crystal plasticity models with an adequate combination of extra slip systems. Curve fitting was used to fit the anisotropy coefficients calculated at discrete temperatures. This temperature dependent constitutive model was successfully applied to the coupled thermo-mechanical analysis of deep drawing of aluminum sheet and results were compared with experiments

Thermo-mechanical Forming of Al–Mg–Si Alloys: Modeling and Experiments

In an ongoing quest to realize lighter vehicles with improved fuel efficiency, deformation characteristics of the material AA 6016 is investigated. In the first part of this study, material behavior of Al–Mg–Si sheet alloy is investigated under different process (temperature and strain rate) and loading (uniaxial and biaxial) conditions experimentally. Later, warm cylindrical cup deep drawing experiments were performed to study the effect of various parameters on warm forming processes, such as the effect of punch velocity, holding time, temper and temperature on force-displacement response. The plastic anisotropy of the material which can be directly reflected by the earing behavior of the drawn cups has also been studied. Finite element simulations can be a powerful tool for the design of warm forming processes and tooling. Their accuracy will depend on the availability of material models that are capable of describing the influence of temperature and strain rate on the flow stresses. The physically based Nes model is used to describe the influence of temperature and strain rate and the Vegter yield criterion is used to describe the plastic anisotropy of the sheet. Experimental drawing test data are used to validate the modeling approaches

Simulation of stretch forming with intermediate heat treatments of aircraft skins - A physically based modeling approach

In the aerospace industry stretch forming is often used to produce skin parts. During stretch forming a sheet is clamped at two sides and stretched over a die, such that the sheet gets the shape of the die. However for complex shapes it is necessary to use expensive intermediate heat-treatments in between, in order to avoid Lüders lines and still achieve large deformations. To optimize this process FEM simulations are performed. The accuracy of finite element analysis depends largely on the material models that describe the work hardening during stretching and residual stresses and work hardening reduction during heat treatments due to recovery and particle coarsening. In this paper, a physically based material modeling approach used to simulate the stretch forming with intermediate heat treatments and its predictive capabilities is verified. The work hardening effect during stretching is calculated using the dislocation density based Nes model and the particle coarsening and static recovery effects are modeled with simple expressions based on physical observations. For comparison the simulations are also performed with a phenomenological approach of work hardening using a power law. The Vegter yield function is used to account for the anisotropic and biaxial behavior of the aluminum sheet. A leading edge skin part, made of AA 2024 has been chosen for the study. The strains in the part have been measured and are used for validation of the simulations. From the used FEM model and the experimental results, satisfactory results are obtained for the simulation of stretching of aircraft skins with intermediate heat treatments and it is concluded that the physics based material modeling gives better results

Thermo-mechanical forming of Al-Mg-Si Sheet

In warm forming of aluminum sheet, the temperature and strain rates vary considerably. In simulations, the material model must be capable to predict stresses within this wide range. Here, the physically based Nes model is used to describe the behavior of AA6061-T4 sheet material under warm forming conditions. A significant change of earing behavior is found between room temperature and 250 ºC. Crystal plasticity calculations showed a reasonable correspondence of changing r-values if extra slip systems are considered at high temperatures. Satisfactory results are obtained for simulation of tensile tests and cylindrical deep drawing