Papers by Author: Matthias Kleiner

Abstract: Sheet Metal Spinning is a flexible manufacturing process for axially-symmetric hollow components. While the process itself is already known for centuries, process planning is still based on undocumented expertise, thus requiring specialized craftsmen for new process layouts. Current process descriptions indicate a vast scope of different dynamic influences while the underlying mechanical model uses a simple static approach. Thus, a 3D Finite Element Model of the process has been set up at IUL in order to analyze the process in detail, providing online as well as cross sectional data of the specimen formed. Within the scope of this article, the results of the above mentioned Finite Element Analysis (FEA) are presented and discussed with respect to the qualitative stress distributions introduced in the existing theoretical models. Main emphasis of this paper is set on a discussion of the qualitative stress distribution, which is, to the current state, only known in theory.

Abstract: In recent years a steadily growing interest in applying lightweight construction concepts could be observed. This development is accompanied by an increasing demand for innovative forming strategies suitable for extending the forming limits of the typical lightweight materials. Deep drawing combined with an integrated electromagnetic calibration step is an example of such a technology. The feasibility and potential of this process combination is analyzed on the basis of a demonstrator part from the automotive industry. Thereby, aspects related to the practicability of the electromagnetic forming process itself are regarded as well as points related to the deep drawn preform. The concept of a 3D-coil insert, integrated into a deep drawing punch in order to realize the calibration in the deep drawing process, is introduced and based on the experimental results, conclusions regarding the applicability of the process combination are drawn.

Abstract: The further development of innovative forming processes like sheet metal hydroforming is only possible with the help of detailed knowledge about the workpiece properties and their formation depending on the process strategy. Up to now, the knowledge about the formation of macroscopic residual stresses in high-pressure sheet metal forming (HBU), regarding the influence of the sheet material properties, is still insufficient. The characteristics of the specific forming procedure HBU lead to specific stress and strain gradients in the sheet cross-section, and therefore lead to a characteristic distribution of the induced macroscopic residual stresses, particularly in the workpiece bottom zone. This paper decribes the investigations on the influence of the sheet material flow curve on the macroscopic residual stress distribution in the workpiece bottom.

Abstract: In order to manufacture a workpiece fulfilling specified requirements with the lowest possible weight, it is crucial to be able to work with a variety of materials and to combine them accordingly. The production of profiles based on hybrid materials demonstrates such an approach. The continuous and selective reinforcement of aluminum profiles with metallic elements like steel wire and steel wire ropes by composite extrusion is being investigated within the scope of research of the Transregional Collaborative Research Center (SFB/TR10). A stable production process for composite profiles with embedded continuous reinforcing elements was developed during the research work. In this paper, the process principle is shown and an overview of the special tools is given. Furthermore, the temperature and the strand speed as influencing factors on the final state of the composite are analyzed, based on real size experiments using a 2.5MN and a 10 MN extrusion press.

Abstract: This paper presents and discusses the latest achievements in the manufacture of threedimensionally curved aluminum profiles using an integrated bar extrusion process. While the conventional process chain lacks applicability and quality especially in low volume production, the new technology is promising, but still needs research to understand the correlation between the product to be manufactured and the contour accuracy to be achieved. This paper shows first results on the manufacture using a robot-guided second guiding tool and direct air quenching. The results will have an impact on further processing steps

Abstract: This paper presents the focus of research and the structure of the Collaborative Research Centre SFB/TR10 „Integration of Forming, Cutting, and Joining for the Flexible Production of Lightweight Structures” funded in 2003 by the German Research Foundation (DFG). Driven by the demands of low volume production. The aim of the Research Centre is to investigate the scientific basis and methods to design integrated process chains for the flexible manufacture of space frame structures.

Abstract: The production of continuously reinforced profiles by use of aluminium as base material and a reinforcement made of steel or carbon offers a great potential for modern lightweight constructions. Within this scope, they present the potential for an increase in usage of space frame constructions in automotive or aerospace engineering. But with the insertion of reinforcement in the material flow of the extrusion process some problems can occur that are negligible in thee conventional extrusion processes: in the composite development area a significant local perturbation of the material flow is induced that can lead to the induction of high tensile stresses into the reinforcement. Due to this, failures like cracking of the reinforcement elements during the extrusion process has been detected in experimental investigations. A second problem occurring is the necessity of prediction of the seam weld position and prediction of the seam weld quality. The reinforcement can only be induced by bridge dies between two strands and due to this it is always positioned in a seam weld. While in conventional extrusion the seam weld positions is often only an aesthetical problem, now this position mainly influences the extruded profiles properties like moment of inertia. This paper deals with the problem of determination of seam weld position on the example of a double-t-profile extrusion. By use of a coupled thermo-mechanical finite element simulation with the commercial FE code HyperXtrude from Altair the velocity fields of an extrusion process with and without reinforcement were calculated and the resulting material flow was analysed. The numerical results went along with experimental investigations to verify the calculated results.

Abstract: Joining of lightweight frame structures in small quantities is subject to specific conditions, which are exemplarily determined for joining by forming processes. Experimental investigations have been carried out to evaluate both feasibility and capability of joining by forming processes. Joining has been accomplished by compressing or expanding cylindrical profiles using rigid tools for rolling-in processes, fluid active medium for hydro-forming as well as active energy for electromagnetic forming.

Abstract: The air bending process is one of the most widely used process for the manufacturing of sheet metal bending parts made of thin as well as of thick sheet metal. Although the air bending process offers a very high production potential due to its great flexibility, it is associated with certain problems which can negatively influence the shape and dimensional accuracy of the bending parts. Examples for such negative influences are the springback of the material, the batch variations, or the deflections of the bending machine and tools. These differences have to be considered either in the determination of the process parameters or they have to be compensated later on in the manufacturing process itself. A well established approach to calculate process data for forming processes is the use of a process simulation. At the Institute of Forming Technology and Lightweight Construction (IUL) a simulation software called Sheet Metal Bending Simulation (SMBS) has been developed and successfully been tested for the field of sheet metal bending, based on semi-analytical approaches. Although it already provides very satisfactory results in general, disturbances such as material and batch variations as well as the deflections of C-frame, machine table, and press brake ram can still negatively affect the prediction of the punch displacement necessary to achieve a certain bend angle. While material and batch variations cannot properly be considered in a process simulation at present, the afore mentioned influences offer a promising potential for improvements. Therefore, in order to further improve the accuracy of predicted quantities such as punch displacement and bending angle, a new module describing the elastic machine behaviour of press brakes has been developed and successfully been integrated in the process simulation SMBS. Experimental investigations have been carried out on a conventional CNC press brake to verify the efficiency of the newly implemented approach.

Abstract: The phenomenon of springback of thin-walled sheet metal parts after forming is a well known problem of forming technology in general, but particularly since the finite element simulation offers the opportunity to predict geometrical and material properties after forming. Irrespective of the intensive efforts in the previous years, a reliable and accurate prediction of springback deviations by use of the finite element simulation is still not possible. This paper deals with the numerical and experimental analysis of the springback effect itself, which dependents on the final stress states of a part after the forming process. Experimental investigations have been carried out to analyze geometrical accuracy in loaded and unloaded conditions to isolate the springback effect. Additional finite element simulations have been conducted in order to compare the experimental and numerical results and to determine the geometrical differences and their reasons. Two experimental set-ups are being discussed: Air bending on the one hand, which offers good access to the specimen in the testing equipment, and draw bending on the other hand, which is characterized by a simple strain state, but also by strain reversal within the tests. Both experiments were carried out using DP600 and X5CrNi18.10 with three different sheet thicknesses and bend radii and were compared with according FE-models. An additional shear test experiment has been developed to characterize the material behavior of the tested sheet metals for strain reversal. Furthermore, the importance of the Bauschinger effect and usable hardening models were analyzed. This study intended to investigate reasons for insufficient form and dimensional accuracy between simulations and experiments after springback and to propose modeling methods to improve the accuracy.