Accurate drawbead modeling in stamping simulations

An adaptive line bead model that continually updates according to the changing conditions during the forming process has been developed. In these calculations, the adaptive line bead's geometry is treated as a 3D object where relevant phenomena like hardening curve, yield surface, through thickness stress effects and contact description are incorporated. The effectiveness of the adaptive drawbead model will be illustrated by an industrial example

Hardening prediction of diverse materials using the Digital Image Correlation technique

In recent years, due to the introduction of higher resistance materials in the automotive sector, sheet metal-forming tool-makers have been forced to deal with more challenging process designs. Therefore, the optimisation of the manufacturing process has become a key factor in obtaining a part which fits the required tolerances, and the finite element method (FEM) is the most widely used technique to speed up that optimisation time. However, to obtain a numerical result as close as possible to those of industrial conditions, the FEM software inputs must be highly accurate. The present work is focused on the hardening extension of the currently available reduced-formability materials, as it is a key factor in the correct prediction of the stress state and hence, of the springback during a sheet metal-forming process. The objective in this work was the selection of the most appropriate hardening model to extend the flow curve beyond the necking limit for a wide variety of material families currently utilised in the industrial environment. To carry out that analysis, a digital image correlation (DIC) technique was utilised during conventional tensile tests to extend the experimental flow curves of the analysed materials. Commonly used hardening models were fitted to the experimental tensile flow curves with the aim of selecting the model that best predicts the hardening behaviour of each analysed material family. The results showed that the DIC technique was valid for the extension of the hardening curve of the analysed materials and for the final selection of the most suitable hardening model for each analysed material family

Comparison of the hardening behaviour of different steel families : from mild and stainless steel to advanced high strength steels

Although steel has been used in vehicles from the automotive industry's inception, different steel gradesare continually being developed in order to satisfy new fuel economy requirements. For example,advanced high strength steel grades (AHSS) are widely used due to their good strength/weight ratio.Because each steel grade has a different microstructure composition, they show different behaviourswhen they are subjected to different strain paths in forming processes. Materials with high yieldstrength tend to be influenced by phenomena of cyclic plasticity such as the Bauschinger Effect, whilelow yield strength materials tend to harden when they are subjected to cyclic loading.Different steel grades are used in different forming processes, which are usually optimised bynumerical tools such as Finite Element Models. This method requires proper hardening rules in order tocorrectly predict the real behaviour of the materials. For instance, AHSS are usually well modelled bymeans of mixed isotropic–kinematic hardening models.The methodology for developing a mixed hardening model to be implemented infinite elementcodes and simulate sheet forming processes requires three steps: (i) an appropriate experimental test toobtain stress–strain curves, (ii) a model able to predict accurately the behaviour of the material and (iii) aparameter identification method. Currently, there are few studies which analyse and model thehardening behaviour of different steel families following the same methodology. In this work, a widerange of steels from low to high yield strengths were characterised and their hardening behaviourmodelled with the same methodology so as to provide comparative data.In particular, the Chaboche and Lemaitre hardening model was successfullyfitted to the experi-mental stress–strain curves obtained from a tension–compression test. The test was performed at lowcyclic deformations (72%) due to the limitation of the test to achieve higher deformations during thecompression without buckling. Therefore, this modelization is useful for low deformation processes suchas the roll levelling process (Silvestre; 2013, Silvestre et al.Steel Res Int; 2012, 1295), in which themaximum deformations achieved are lower than 2%

Numerical study of advanced friction modelling for sheet metal forming: influence of the die local roughness

Numerical simulation of sheet metal forming processes has become indispensable in the last decades. Although the complexity of the frictional behaviour is identified as a key factor for the prediction accuracy, the industry commonly considers a constant friction coefficient for the whole tool. Furthermore, the influence of roughness distribution of the tool in the friction behaviour has not yet been addressed. In this study the influence of the die local roughness in an advanced friction model has been evaluated in three industrial automotive components. The newly implemented advanced friction model (TriboZone) is suggested for advance or mature process verifications

Simulation of Cold Forging Processes Using a Mixed Isotropic-Kinematik Hardening Model

Cold forging is a manufacturing process where a bar stock is inserted into a die and squeezed with a second closed die. It is one of the most widely used chipless forming processes, often requiring no machining or additional operations to get tight tolerances. Because materials to be formed are increasingly harder and the geometrical complexity is greater, the finite element simulation is becoming an essential tool for process design. This study proposes the use of the Chaboche hardening model for the cold forging simulation of a 42CrMoS4Al material industrial automotive ball pin. The material model has been fitted with experimental data obtained from cyclic torsion tests at different reversal plastic strains as well as monotonic torsion tests at different strain rates. Comparison between the classical isotropic hardening and the new mixed hardening model are presented for the different forging steps

New drawbead tester and numerical analysis of drawbeads closure force

Currently, a great deal of controversy exists regarding the real forces generated in drawbeads during sheet metal forming processes. The present work focuses on the analysis of the uplift force. First, a detailed literature review is carried out to analyse previous experimental procedures used to measure uplift forces. It is found that previous setups do not perfectly replicate the real geometry of industrial drawbeads. In order to obtain reliable forces, an experimental drawbead tester capable of adequately replicating industrial drawbeads is developed. Later, a variety of steels ranging from mild steels to 3rd-generation ultra-highstrength steels are tested and reliable uplift and also restraining force values are obtained. The main purpose of the work is to share with the research community reliable experimental data that allows precise evaluation of the accuracy of current drawbead models and that supports the generation of new numerical and equivalent drawbead models. In parallel to the experimental procedure, a step forward in the understanding of the drawbead closing phenomena is also achieved through a 2D numerical model. The main purpose of the model is to identify the variables that greatly affect uplift force. Going beyond previous studies, in which some variables were analysed, the present work covers, in a holistic manner, the impact that material properties, the geometry of drawbeads and contact behaviour between sheet and drawbead have on the uplift force. It is determined that surprisingly minor geometrical deviations in the drawbead nominal geometry have a large impact on the uplift force

Contact pressure, sliding velocity and viscosity dependent friction behavior of lubricants used in tube hydroforming processes

The final quality of sheet and tube metal formed components strongly depends of the tribology and friction conditions between the tools and the material to be formed. Furthermore, it has been recently demonstrated that friction is the numerical input parameter that has the biggest effect in the numerical models used for feasibility studies and process design. Industrial dedicated software packages have introduced friction laws which are dependent on sliding velocity, contact pressure and sometimes strain suffered by the sheet and currently, temperature dependency is being implemented as it has also major effect on friction. This last dependency on temperature is attributed to the viscosity change of the lubricant with temperature. In this work, three lubricant having different viscosity have been characterized using the tube sliding test. The final aim of the study is to obtain friction laws that are contact pressure and sliding velocity dependent for their use in tube hydroforming modelling. The tests, performed at various contact pressures and velocities, demonstrate that viscosity has a major effect on friction. As shown in the literature, the friction coefficient is also varying with the contact pressure and sliding velocity

Contact pressure and sliding velocity ranges in sheet metal forming simulations

In the last few years many efforts have been carried out in order to better understand what the real contact between material and tools is. Based on the better understanding new friction models have been developed which have allowed process designers to improve numerical results in terms of component viability and geometrical accuracy. The new models define the coefficient of friction depending on different process parameters such as the contact pressure, the sliding velocity, the material strain, and the tool temperature. Many examples of the improvements achieved, both at laboratory scale and at industrial scale, can be found in the recent literature. However, in each of the examples found in the literature, different ranges of the variables affecting the coefficient of friction are covered depending on the component analysed and the material used to produce such component. The present work statistically analyses the contact pressure and sliding velocity ranges achieved during numerical simulation (FEM) of sheet metal forming processes. Nineteen different industrial components representing a high variety of shapes have been studied to cover a wide range of casuistic. The contact pressure and sliding velocity corresponding to typical areas of the tooling have been analysed though numerical simulation in each case. This study identifies the ranges of contact-pressure and sliding velocities occurring in sheet metal forming aimed to set the characterization range for future friction studies

Zeharkako ebaketa prozesuaren parametroen eragina erresistentzia handiko altzairuzko xafla lodietan

Transversal cutting processes are usually employed for metal coil cutting or precut reduction operations. The final result of a cutting operation, in terms of sheared edge quality and process forces, is dependent on the process parameters and on the material’s behavior. In order to determine the effects of the process parameters, an hydraulic blanking prototype is employed to perform transversal cutting operations on a High Strength Steel plate with 10 mm of thickness. These results will help to understand the force requirements of the process and to meet customers’ sheared edge quality requirements.Zeharkako ebaketa prozesuak metalezko bobinak ebakitzeko edo aurre-formatuen murrizketetarako erabiltzen dira. Ebaketa eragiketaren emaitzak, ebaketa kalitateari eta prozesuaren indarrei dagokienez, prozesuaren parametroen eta materialaren portaeraren araberakoak dira. Prozesuaren parametroen ondorioak zehazteko, ebaketa prototipo hidrauliko bat erabiltzen da zeharkako ebakidura eragiketak eginez 10 mm-ko lodiera duen erresistentzia handiko altzairuzko xafla batean. Emaitza hauek prozesuaren indar eskakizunak ulertzen eta bezeroen ebaketa kalitatearen baldintzak betetzen lagunduko dute