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

Mechanical and forming properties of AA6xxx sheet from room to warm temperatures

The influence of temperature on the mechanical behaviour of the heat treatable Aluminium alloy EN AW-6061 has been investigated with a series of tensile tests. It is found that temperature has an effect on both the storage of dislocations and dynamic recovery. The results have been used to fit the dislocation based Nes work-hardening model. Simulations show that the model captures properly the dependence of yield stress and work-hardening rate with temperature and temper. The work-hardening model has been implemented into the Dieka FEM to simulate the warm deep drawing of cylindrical cups. Comparison of the simulated and experimental punch force and cup thickness reveals a good correspondence and validates the proposed modelling approach

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

Formation of hot tear under controlled solidification conditions

Aluminum alloy 7050 is known for its superior mechanical properties, and thus finds its application in aerospace industry. Vertical direct-chill (DC) casting process is typically employed for producing such an alloy. Despite its advantages, AA7050 is considered as a "hard-to-cast" alloy because of its propensity to cold cracking. This type of cracks occurs catastrophically and is difficult to predict. Previous research suggested that such a crack could be initiated by undeveloped hot tears (microscopic hot tear) formed during the DC casting process if they reach a certain critical size. However, validation of such a hypothesis has not been done yet. Therefore, a method to produce a hot tear with a controlled size is needed as part of the verification studies. In the current study, we demonstrate a method that has a potential to control the size of the created hot tear in a small-scale solidification process. We found that by changing two variables, cooling rate and displacement compensation rate, the size of the hot tear during solidification can be modified in a controlled way. An X-ray microtomography characterization technique is utilized to quantify the created hot tear. We suggest that feeding and strain rate during DC casting are more important compared with the exerted force on the sample for the formation of a hot tear. In addition, we show that there are four different domains of hot-tear development in the explored experimental window-compression, microscopic hot tear, macroscopic hot tear, and failure. The samples produced in the current study will be used for subsequent experiments that simulate cold-cracking conditions to confirm the earlier proposed model.This research was carried out within the Materials innovation institute (www.m2i.nl) research framework, project no. M42.5.09340

An age hardening model for interrupted ageing of the alloy AA6061

Physically based age hardening and work hardening models have been developed to simulate the effect of interrupted ageing on the yield strength and hardening behavior of the Aluminum alloy 6061. Experimental results show that, depending on the interruption temperature, two secondary precipitation strengthening regimes exist during re-ageing: below 50 degrees C displaying almost no influence on the peak hardness of the alloy, and above 50 degrees C where a significant increase in hardness is observed. This behavior is modeled using the Kampmann-Wagner numerical (KWN) method. The developed age-hardening model is then combined with the Kocks-Mecking work-hardening model. The results indicate that although secondary precipitation above 50 degrees C could improve the yield strength, hardly does it have any effect on the work hardening behavior of the alloy. The experimental results show that the main effect of secondary precipitates is on the post-necking regime, where the higher volume fraction of secondary precipitates increases the strain after necking. An explanation is proposed based on the fact that secondary precipitates can act as slip distribution centers and therefore they may increase the homogeneity of slip distribution and the strain-to-fracture

Recrystallization texture development under various thermo-mechanical conditions in aluminum alloys

The texture development during recrystallization annealing is affected by the thermo-mechanical history. A variety of hot and cold rolling parameters account for various recrystallization textures both qualitatively and quantitatively. Asymmetric rolling by a differential circumferential velocity of the top and bottom rolls is applied to the investigated aluminum alloy. The resulting shear deformation gives rise to a non-conventional texture evolution in the hot band. The influence of hot band textures on the development of the deformation and recrystallization textures is discussed based on experimental data and results of simulations with crystal plasticity models