Pavel Hora

Simulation of the Press Hardening Process and Prediction of the Final Mechanical Material Properties

Press hardening is a well-established production process in the automotive industry today. The actual trend of this process technology points towards the manufacturing of parts with tailored properties. Since the knowledge of the mechanical properties of a structural part after forming and quenching is essential for the evaluation of for example the crash performance, an accurate as possible virtual assessment of the production process is more than ever necessary. In order to achieve this, the definition of reliable input parameters and boundary conditions for the thermo-mechanically coupled simulation of the process steps is required. One of the most important input parameters, especially regarding the final properties of the quenched material, is the contact heat transfer coefficient (IHTC). The CHTC depends on the effective pressure or the gap distance between part and tool. The CHTC at different contact pressures and gap distances is determined through inverse parameter identification. Furthermore a simulation strategy for the subsequent steps of the press hardening process as well as adequate modeling approaches for part and tools are discussed. For the prediction of the yield curves of the material after press hardening a phenomenological model is presented. This model requires the knowledge of the microstructure within the part. By post processing the nodal temperature history with a CCT diagram the quantitative distribution of the phase fractions martensite, bainite, ferrite and pearlite after press hardening is determined. The model itself is based on a Hockett-Sherby approach with the Hockett-Sherby parameters being defined in function of the phase fractions and a characteristic cooling rate.

Modeling for the FE-Simulation of Warm Metal Forming Processes

Better formability, less forming force and satisfactory quality are the most important characteristics of warm forming processes. However, the material models for either cold forming or hot forming cannot be directly adopted for the numerical simulation of warm forming processes. Supplement and modification are necessary. Based on the Zener‐Hollomon formulation, additional terms are proposed in the presented work to describe the softening effect observed during warm forming processes as well as the strain hardening effect. The numerical simulation provides detailed information about the history and distribution of both deformation and temperature, the phase transformation can then also be evaluated, provided the experimental data are available.

Hydro-Mechanical Deep-Drawing

An analysis, based on the energetic stability criterion combined with numerical calculations, for the hydraulic drawing of tapered cylindrical cups is developed in this paper. As the calculation shows, the limiting drawing ratio strongly depends on the pressure during hydraulic drawing. The theoretical and experimental results agree wall.

Experimental Investigations of Friction Carried out with the Tribo-Torsion-Test and Frictional Modelling

Complex frictional effects occur during the extrusion process between the extrusion die and the extruded material. The recently developed Tribo-Torsion-Test is used to measure friction under thermo-mechanical conditions similar to the extrusion process. Investigations have not been carried out only with nitrided but also with two different chemical vapour deposited CVD coated samples. In the context of this study the Tribo-Torsion-Test is introduced and laboratory results are presented.

Zero Failure Production Methods Based on a Process Integrated Virtual Control

Although the virtual methods are nowadays fully established as a widely used tool in the planning and optimization of forming processes they are still completely omitted for a direct, “intelligent” process control in the later production of the parts. The paper presents a proposal for a Process-Integrated-Virtual-Control (PIVC) considering the real process perturbations induced by deviations of material properties as well as by further time dependent process parameters like the tool temperature. For the detection of the material deviations an in-line eddy-current measurement method and the appropriate evaluation method for the definition of the stochastic yield curves will be presented. The paper closes with a virtually taught control system modifying the blank-holder forces in dependency of thermal conditions and material deviations. The goal of this PIVC coupled to in-line process controls is to achieve a Zero Failure production even under alternative time dependent process conditions.

A new cone-friction test for evaluating friction phenomena in extrusion processes

Friction is one of the most influential parameters in extrusion processes. The flow distribution is controlled using the effect of friction lengths on the extrusion dies. A comparison between multi-hole die aluminum extrusion experiments and simulations [1] necessitated studies on numerical and physical modeling of friction during aluminum extrusion. In order to measure the friction under conditions similar to extrusion processes a new experimental setup is proposed.

Forming Limit Prediction of Metastable Materials with Temperature and Strain Induced Martensite Transformation

Stainless steels as well as TRIP and TWIP steels show a hardening behavior, which can be described only in dependency on the deformation and temperature history during the real forming process. Because the hardening behavior is the determinate factor for the necking phenomenon, the prediction of rupture becomes also deformation path and temperature dependent. As a consequence, the common FLC‐method, using a single curve for the prediction of the failure state is not accurate enough. In this paper, a temperature dependent Forming Limit Surface (FLS) is presented.

An Improved Modeling of Friction for Extrusion Simulations

Realistic representation of friction is important in extrusion simulations. Purposefully designed multi‐hole die aluminum extrusion experiments showed that the conventional friction models, like the Coulomb and the shear friction models, are deficient to represent the boundary phenomena that occur during aluminum extrusion. Based on the observations, phenomenological and implementational improvements are made in the friction modeling.

Evaluation of Experimental Forming Limit Curves and Investigation of Strain Rate Sensitivity for the Start of Local Necking

The failure prediction in sheet metal forming is typically realized by evaluating the so called forming limit curves (FLC). Up to now, the FLC determination was performed with failed specimens of Nakajima or Marciniak test setups. Standard methods determine the failure by considering the occurrence of cracking and do not consider the possibility of time continuous recording of the Nakajima test. Consequently forming limit curves which have been evaluated in such way are often “laboratory dependent” and deviate for identical materials significantly. This contribution presents an algorithm for a fully automatic and time-dependent determination of the beginning plastic instability based on physical effects. The algorithm is based on the evaluation of the strain distribution based on the displacement field which is evaluated by optical measurement and treated as a mesh of a finite element calculation. The critical deformation states are then defined by 2D-consideration of the strain distribution and their time derivatives using a numerical evaluation procedure for detecting the beginning of the localization. The effectiveness of the proposed algorithm will be presented for different materials used for the Numisheet’08 Benchmark-1 with Nakajima test. Additionally the effect of strain rate sensitivity on the beginning instability of local necking is discussed. It can be shown that the strain rate sensitivity is from major importance and should not be neglected for the forming simulation of sheet metal materials.

An ALE Based FE Formulation for the 3D Numerical Simulation of Fineblanking Processes

Fineblanking is a manufacturing process which allows the mass production of blanked products with superior surface quality. The 3D numerical simulation of this particularly precise process is however challenging. This is because quality-critical tool features such as the die clearance and the shape of the cutting edges have dimensions up to two orders of magnitude smaller than the average part dimensions. If conventional Updated Lagrange codes are used, a very high FE mesh resolution becomes a must in order to accurately represent the surface evolution along the edge, which in turn makes the computation unfeasible. The methodology presented in this paper makes use of the Arbitrary Lagrangian Eulerian FE Formulation in order to keep control over the mesh region in contact with the tools. This way an optimal FE mesh can be guaranteed throughout the computation. This not only reduces the computational cost considerably, but also avoids mesh distortion along the cutting edge, allowing an accurate representation of the tool features. This approach will be used in conjunction to the stress limit criterion delineated in order to predict material failure in fine blanked products. Numerical results will be validated against the experiments carried out with a specially designed fineblanking tool in use at our institute.