Karl Roll

Transient Simulation of Electromagnetic Forming of Aluminium Tubes

The recent push to use more aluminium in automobiles has stimulated interest in understanding electromagnetic forming (EMF), which uses induced electromagnetic fields to generate high strain rates during the forming process. The high strain rates increase the formability of aluminum materials and might reduce elastic spring-back and wrinkling of the workpiece. Primary emphasis is placed on including of all relevant physical phenomena, which govern the process, as well as their numerical representation by means of simplified electrical equivalent circuits for the EMF machine and fully coupled field approach of the transient electromagnetic and mechanical phenomena. Moreover, the thermal effects due to Joule heating by eddy currents and plastic work are considered. The numerical model predicts the electromagnetic field, temperature, stress, and deformation properties that occur during the forming process. The numerical results of the tube deformation are compared with available experimental data.

Modelling and Analysis of Integrated Hotforming and Quenching Processes

In the automotive industry a general tendency to choose steels with enhanced strength for structural parts can be observed. This trend results from the increased lightweight design efforts to satisfy the fleet consumption restrictions. Hot forming and quenching of boron steel offers the possibility to improve the component strength and reduce the weight of structural parts. The main influences on the process are described and a method to model and simulate this process using the finite element method using LS-DYNA is presented. Experimental investigations of the contact heat transfer have been carried out to enhance the simulation accuracy. A prototyping tool of a structural part is used to examine the process under production conditions. Temperatures of the tool and the part are measured during the process. These temperatures are compared with the simulation results in order to reevaluate the results of the process simulation.

Modeling Laser Heating for Roller Hemming Applications

In order to improve fuel efficiency, the use of lightweight materials in the automotive industry is continuously increasing. The AA6XXX and AA7XXX aluminum alloys in particular are the subject of significant attention. The aim is to use them in the same way as other structural materials such as conventional steel. Unfortunately their formability presents major challenges. These alloys lack sufficient formability for bending/hemming operations in particular. To overcome this, forming can take place at elevated temperatures. In this study, a combined laser-assisted roller hemming process is set up. Therefore, a 4000 W Nd:YAG-laser with a wavelength of 1096 nm is used. The first step involves qualifying the process window for laser heating. Several parameters are defined and effects on temperature and surface quality for AA6014 are detected using the design of experiments. The second step involves setting up a finite element model of the heating process. Different modeling strategies for laser heating are compared and a new approach is presented. The laser heating source is modeled by a semi-circular Gaussian temperature distribution and is validated through comparison of the temperature with laser heating experiments, showing very good agreement in the range from 600-4000 W. Finally, the validated thermal model is coupled with the roller hemming simulation and investigations into the thermal process window are performed.

Simulation of Heat Assisted Forming of Commercial Aluminium Alloy AA5182

To improve the formability of commercial aluminium alloy AA5182, a new heat assisted forming method is used. The process sequence of this method is; cold forming (pre-forming, 90 to 95% of final shape), heat treatment and cooling it down to room temperature and final cold forming. AA5182 is a non-heat treatable alloy and hence heat treating a strain hardened non-heat treatable aluminium alloy leads to lose in strength and gain in plastic recovery. Therefore by heat treating a preformed part and then following it up by further forming stages, it not only gains the lost strength back but also shows increased formability. This behavior is particularly useful in forming more complex automotive interior body parts. To accurately simulate this method, modeling the effect of heat treatment is important. Initial investigations on tensile tests showed that the degree of pre-forming in combination with heat treatment is directly proportional to plastic recovery. Which means, the more the pre-strain is the more the recovery becomes viable. Based on this, a new algorithm has been developed and implemented in LS-DYNA to capture the effect of heat treatment. Finally experimental investigations were carried out on a cross die deep drawn cup to validate the developed simulation model.

Magnesium Sheet Metal Forming Considering its Specific Yield Behavior

In recent years very strong efforts have been undertaken to build light weight structures of car bodies in the automotive industry. Structural technologies like Space Frame, tailored blanks and relief-embossed panels are well-known and already in use. Beside that there is a large assortment of design materials with low density or high strength. Magnesium alloys are lighter by approximately 34 percent than aluminum alloys and are considered to be the lightest metallic design material. However forming processes of magnesium sheet metal are difficult due to its complex plasticity behavior. Strain rate sensitivity, asymmetric and softening yield behavior of magnesium are leading to a complex description of the forming process. Asymmetric yield behavior means different yield stress depending on tensile or compressive loading. It is well-known that elevated temperatures around 200°C improve the local flow behavior of magnesium. Experiments show that in this way the forming limit curves can be considerably increased. So far the simulation of the forming process including temperature, strain rates and plastic asymmetry is not state-of-the-art. Moreover, neither reliable material data nor standardized testing procedures are available. According to the great attractiveness of magnesium sheet metal parts there is a serious need for a reliable modeling of the virtual process chain including the specification of required mechanical properties. An existing series geometry which already can be made of magnesium at elevated temperatures is calculated using the finite element method. The results clarify the failings of standard calculation methods and show potentials of its improvement.

Deep Drawing Simulation Of High And Ultrahigh Strength Steels Under Consideration Of Anisotropic Hardening

In today’s sheet metal forming simulation, most attention is paid to yield loci functions, which describe the anisotropy of the material in yielding. The coefficients, defining the shape of the yield locus in these functions are usually fitted at a certain level of plastic work and are then valid for the whole range of plastic deformation. Modern high and ultrahigh strength steels, especially those with induced plasticity, may often exhibit only a very small anisotropy in yielding, but a severe anisotropy in work hardening for different loading conditions. This behavior can not be described by fitting the yield locus at a specific value of plastic deformation. An approach to take into account the anisotropic hardening of sheet metals is to provide different yield curves for several loading conditions and expand the yield locus dependent on the current form of load. By doing this, one can use a comparatively simple yield locus, like that of Hill from 1948, because all anisotropy is given by the different h...

Magnesium-Knetlegierungen fuer den Automobilbau

Wenngleich der Verwendung von Mg-Knetlegierungen im Automobilbau ein groses Potenzial zugesprochen wird, ist deren Einsatz bislang von geringer Bedeutung geblieben. Im Rahmen des vom BMBF-geforderten Verbundforschungsvorhabens „MIA — Magnesium im Automobilbau“ werden daher in Zusammenarbeit der Firmen DaimlerChrysler AG, ThyssenKrupp Umformtechnik GmbH, Otto Fuchs KG, Honsel GmbH & Co. KG, GP Innovation GmbH, OSK Kiefer GmbH sowie der BTU in Cottbus die Grundlagen fur die prototypische Fertigung von Bauteilen aus den Mg-Knetlegierungen des Typs AZ31 und AZ80 erarbeitet.

Extension of a Failure Criterion for Hemming Applications in the FEM Simulation

In the FEM calculation of sheet metal forming processes to determine the failure case, the forming limit diagram is basically used. To determine the failure case at bending condition, the forming limit diagram can not be used. This behaviour was shown by many authors. Bending tests with an aluminium material (AC170PX) have shown that a high deformation ratio can be achieved without failure. Based on the loading conditions and the previous strain path through the deep-drawing process, a resulting bendability at a certain point can be obtained. Depending on the pre-damage and the mentioned loading conditions of the material failure will be occurring during bending at different times. Current developments of failure criteria consider the failure as in ductile fracture or shear fracture, which must be considered separately in the simulation. To rule out a separate analysis of the mode of failure in the post-processing, an existing failure criterion is extended and will be presented in this work. For the applications flanging and hemming the following extension of a stress-based failure criterion is proposed. Based on the triaxiality and the equivalent plastic strain a monitoring of the stress ratio is implemented in the FEM simulation. During the forming simulation the monitoring system observe the stress ratio based on the principal stresses resulted from the integration (Gauss) point of the shell element. According to the evaluation of the stress ratio evolution, a relevant definition will take into account how the damage will be accumulated. If the critical value of damage in the integration point of the shell element is reached, failure will be occur based on the position of the sheet thickness.

Modelling of Strain Hardening Behaviour of Sheet Metals for Stochastic Simulations

In current forming simulations, Ghosh or Hockett-Sherby extrapolation functions are used to model strain hardening effects of sheet metals. When it comes to stochastic simulations, the respective parameters have to be recalculated according to the scattering of the mechanical material properties like yield or tensile strength. As present stochastic samplings for deep drawing simulations only consider yield strength and tensile strength, it is non-trivial securing the extrapolated area of strain hardening curves due to lack of data beyond uniform strain. It is current practice to improve the description of the flow curve beyond uniform elongation point by using the maximum force criterion, which takes into account the gradient of the yield curve at the last known point. The corresponding system of equations has to be solved numerically. We propose a method for adjusting parameters of the Ghosh or Hockett-Sherby extrapolation functions, which overcomes the need of numerical calculations and keeps flow curve information from the extrapolated interval, even if the stochastic sampling doesn’t incorporate any data regarding that area.

Influence of Geometrical Shapes and Sheet Thicknesses on the Dimensional Accuracy of Single and Assembled Parts

Based on elastic stress and strain states after forming and joining processes, single and assembled parts show deviations regarding their dimensional accuracy. Therefore an analysis of selected influencing factors and their influence on the dimensional accuracy of assembled parts is performed in this paper. In this article a novel approach is presented that characterizes the impact of three geometrical shapes (convex/concave/straight) and different sheet thicknesses on the dimensional accuracy along a linked forming and joining process chain. The process chain consists of a deep drawing and a clinching process. Depending on sheet thickness, material and geometrical shape, the dimensional accuracy of single parts and joined assemblies varies. For the single parts the geometry of the specimen S-rail is used. Several types of assemblies are used for the proposed approach combining this specimen with a plane sheet or a second S-rail. The FEM-tools LS-DYNA and Abaqus, are used to demonstrate this approach. Simulations and experiments with aluminum alloy 6014, mild steel CR3 and sheet thicknesses of 0.7, 1.0 and 2.0 mm are conducted for single and assembled parts. In summary, a significant improvement of the dimensional accuracy of an S-rail assembly is demonstrated using two non-dimensional accurate single parts. Future work will be to analyze frequently occurring part segmentations for the joining technologies and to optimize material mix and sheet thicknesses in order to improve deviations of the assembly to the nominal CAD geometry.