Post-forming, electro-plastic effect internal stress reduction in AA5754 aluminium alloy

Aluminium alloys are one of the most efficient materials for weight reduction in the car industry. However, the favourable performance of alloys conflicts with the difficulties of transforming these materials through plastic deformation processes such as stamping. Among the different issues, they are characterised by a high springback effect. Numerous authors have explored the use of the electro-plastic effect (EPE) to mitigate internal stresses and the resultant springback. However, up-scaling existing laboratory solutions to an industrial framework is a critical challenge. Therefore, in this work, the post-forming electro-plastic effect (PFEPE) is explored in AA5754 material. Stress-relaxation + PFEPE experiments were conducted using different electric pulse charge passing though the sample, and, apart from the impact on residual stresses, the potential occurrence of recrystallisation was evaluated. The results indicate that a range of pulses exists (>2000 A·ms/mm2 and <4500 A ms/mm2) in which a 10–30% reduction in stresses can be achieved without critically impacting the material's mechanical performance

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

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

Monitoring of a Hammer Forging Testing Machine for High-Speed Material Characterization

Dynamic testing of materials is necessary to model high-speed forming processes (i.e. hammer forging, blanking) and crash/impact behavior of structures, among others. The most common machines to perform medium to high-speed tests are the servo-hydraulic high-speed tensile and compression machines and the Hopkinson bars. The paper analyses the possibility to use a laboratory forging hammer for the characterization of materials at medium and high strain rates. For this, an automatized forging hammer has been constructed which is accelerated with a pneumatic cylinder and is able to speed up the upper anvil up to 5 m/s. This forging testing machine can be employed to perform a variety of material characterization tests, such as, uniaxial upsetting tests, plane strain compression tests as well as crash tests. To ensure the correctness of the experimental results obtained from tests performed in this home-developed laboratory facility, it is essential to verify and validate the acquired data. With this aim, copper cylindrical specimens have been deformed at different speeds. A high-speed camera has been employed to monitor real specimen strains using DIC and a load cell has been also utilized to measure the force applied during deformation. In order to obtain valid material rheological results, force data obtained from the load cell has been combined with DIC strain data to draw reference flow curves. Analogous stress-strain values have been calculated analytically using both techniques independently, solely high-speed camera data, on the one hand, and only load-cell data, on the other hand. A comparison of results has been performed and discussed in order to select the best monitoring technique to implement in the laboratory forging hammer

On the accurate characterization of the drawbead up-lift forces

The competitiveness of the automotive sector has led to a high demand of accuracy and reduction in lead-time of the deep drawing tool making process. In that regard, the numerical simulation of the deep drawing process has become a key method for the correct die design. Even though the accuracy of these simulations reached some high quality levels in terms of formability and defects, the material holding force remains an open issue among the die maker companies. This inaccuracy is related with the inability of shell elements to correctly reproduce the behavior of the material around the drawbeads. In order to overcome this problem, commercial stamping software used an analytical model to predict the drawbead holding forces. Nevertheless, most of these models are based on an experimental methodology developed in the 70's that do not exactly represent the industrial drawbead configuration. In order to be able to experimentally analyze the necessary up-lift force of each drawbead, in this work a new experimental procedure is presented. A wide range of automotive sector materials, ranging from mild steels up to high strength steels, have been tested and new values, compared with previous experiments, have been found. In that regard, the force distribution on the drawbead is also studied stressing the importance of the flat surfaces around the drawbead more than the drawbead punch itself

Drawbead uplift force analytical model for deep drawing operations

Drawbead uplift force calculation has been an open issue among the deep drawing tool maker and software developers in the last years. Starting from the original work of Stoughton (1988) many have been the models presented in order to improve the predictions. However, still nowadays, the main deep drawing software are not able to accurately predict the uplift force and clear example of that are the intensive effort of the software developers in that topic as well as the conversion factors used by the main OEM when acquiring a new tool. In this work, a new semi-analytic model of drawbead closing force calculation is presented. The model is not only able to predict the uplift force for different steps of the closing (very useful for the set-up process) but it has been validated when using a high strength steel (DP780) for different drawbead configuration

The influence of the kinematic hardening on the FEM simulation of Tension Levelling Process

Tension levelling is used in the steel industry to remove shape defects present in cold rolled strip. This technology is increasingly being employed by steel makers to level AHSS steels, because conventional roll levelers are not able to correct explicit local errors like wavy edges and central buckles. In this study, the influence the hardening law has on the simulation of tension levelling processes using FEM is studied. Tension-compression tests have been performed in a DP1000 steel using several reversal cycles and a mixed nonlinear kinematic hardening law has been fitted to the experimental data using different amount of backstress tensors. It is observed that numerical results are influenced by the introduced hardening law and thus is an important input when simulating the tension levelling process

Friction Modelling for Tube Hydroforming Processes&mdash;A Numerical and Experimental Study with Different Viscosity Lubricants

The final quality of sheet and tube metal&ndash;formed components strongly depends on 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. For these reasons, 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 a major effect on friction. In this work, three lubricants having different viscosity have been characterized using the tube-sliding test. The final aim of the study is to fit friction laws that are contact pressure and sliding velocity dependent for their use in tube hydroforming modeling. The tests performed at various contact pressures and velocities have demonstrated that viscosity has a major effect on friction. Experimental hydroforming tests using the three different lubricants have corroborated the importance of the lubricant in the final forming of a triangular shape. The measurement of the axial forces and the final principal strains of the formed tubes have shown the importance of using advanced friction laws to properly model the hydroforming process using the finite element modeling