Papers by Author: Manfred Geiger

Abstract: The adjustment of micro system components with laser forming using especially designed sheet metal actuator systems is a new and promising technology. However, due to the complexity of the design challenge and the contradicting targets that have to be considered computer assistance for the design of the actuator systems is needed. In order to build such a system several steps have to be taken. First, the actuators have to be modeled with all necessary data. Second, quality criteria have to be defined and fully automated assessment modules for every single objective have to be implemented. And third, an optimization system which utilizes the assessment modules must be developed to improve an initial design. This paper presents a solution for each of these steps. It closes with first results of a reduced version of the system as well as an outlook on the next development steps.

Abstract: Friction stir welding is a newer technology to join materials in the solid phase. Therefore plenty of problems which appear by melting phase welding technologies for aluminium alloys are avoided by this process. This is the main chance for friction stir welding to be accepted and integrated in forthcoming applications, especially for uses out of aluminium alloys. Starting from former results for friction stir welding of aluminium this article deals with the determination of the constitutive material properties of friction stir welded aluminium tailored blanks with regard to the finite-element (FE) simulation of sheet metal forming. The FE simulation of the formability of welded sheet metal demands the knowledge of the precise mechanical properties of the base material as well as the characteristic zones of the weld seam, which are affected by friction stir welding. While ordinary tensile tests can only determine the constitutive behaviour of a simple tensile specimen, an optical strain measurement can be used to determine flow stresses of the base and of the welded material with an adapted tensile specimen, respectively. By the usage of the so called rule of mixture the advantages of this new approach is demonstrated with a comparison of the tensile forces within FE simulations and experimental validations. For this purpose specimen with the weld seam oriented perpendicular and parallel to the uniaxial loading direction are utilized.

Abstract: The possibility to produce lightweight components with a complex geometry enhanced, in the last decades, the industrial application of the tube hydroforming and, more recently and restricted to specific industrial fields, of the sheet hydroforming technology. The integration in one tool of a tube and a double sheet hydroforming process represents an innovative technology which further emphasises the advantages offered by hydroforming in terms of costs reduction and complexity of the manufactured part. This paper describes the design and the construction of a complex hollow part resulting from the simultaneous hydroforming of two sheets and a tubular component in one tool. The focus is set in particular on the optimisation of the joining zone between tube and sheet pair, whose geometry allows a “metallic” sealing of the gap between the sheets and the tube, i.e. without using sealing components. The contact between tube and sheet pair allows the transmission of the axial force used to support the bulging of the tube to the sheet blanks, thus increasing their draw-in in the die and, consequently, avoiding the occurrence of tearing on the part. The paper describes the optimisation of different process parameters like the shape and the dimension of the blanks, their initial positioning in the tool, the value of the axial force applied to the tubular component and the blankholder force during the preforming and the calibrating stages.

Abstract: The increasing call for significant safety improvements of new cars requires, among other important relevant aspects, the application of high strength materials for the body in white. Quenchable manganese boron alloyed steels like 22MnB5 offer superior strength values, complex shapes and enhanced accuracy. For the evaluation of the crash appropriateness specimens in different annealed states are deformed in a so called drop test bench and hereupon judged in terms of absorbed energy, maximum deformation and other aspects like wrinkling behaviour. Microscopic analysis complements the evaluation of crash-appropriateness.

Abstract: Within the last years in sheet metal forming a trend towards forming at elevated temperatures as well as temperature assisted forming technologies can be observed. This development is caused by the increasing need on light and high strength materials in order to fulfill the demands of light weight structures. The decision which kind of temperature assistance is the most useful in order to improve the formability of the material depends on a hugh number of process influencing parameters, like e.g. the material itself, the geometry of the component, the number of forming operations etc.. In this paper the general possibility to separate different temperature assisted forming processes with regard to the used materials will be introduced. The different forming procedures will be explained and discussed. Examples with an industrial relevance are shown.

Abstract: Basic research concerning the material properties of the hot stamping steel 22MnB5 has been carried out. A survey is given about the as-delivered conditions with hardness tests, micrographs and flow curves. The process window of the austenitization time, before hot stamping can take place, is defined by austenitization tests. Also a new experimental set-up to detect the cooling rate in dependency on the contact pressure is presented. In addition to that the cooling experiments were simulated with ABAQUS and the heat transfer coefficient for each contact pressure is determined by inverse modeling.

Abstract: Thanks to the low weight, magnesium alloys feature high specific strength and stiffness properties. Thus they prove to be promising materials for todays ambitious automotive light weight construction efforts. Due to their comparative low formability at room temperature the process of magnesium sheet hydroforming can be improved at temperatures higher than 200 °C by the activation of additional sliding planes. This paper illustrates the determination of mechanical properties for the hydroforming of magnesium sheets at elevated temperature. In particular the mechanical behavior at elevated temperature was investigated by means of the tensile test and of the hydraulic bulge test. For the determination of the strains an optical measurement system was introduced into the experimental set-up. The exact knowledge of the strain condition in the area of diffuse necking enabled the determination of the flow curve in the tensile test also beyond the uniform elongation. The influence of temperature and strain rate was analyzed as well as the influence of uni- and biaxial stress state on the flow curve. Using circular and elliptic dies with different aspect ratio the hydraulic bulge test served to determinate the forming limit curves at three different elevated temperatures.

Abstract: Within the past decades tailored welded blanks have become a key product for the construction of lightweight automotive bodies. Following the principle of applying material only where it is really needed results in an improved stiffness and crash worthiness of the body-in-white, while at the same time weight and production costs may be reduced. An alternative to the concept of tailored blanks is the so-called patchwork blank technique. The principle of patchwork blanks is to add a flat piece of sheet metal onto the main blank in the areas where reinforcements are required. The assembly can be done by adhesive bonding, resistance spot welding or laser welding prior to forming. The major benefits of this technique in comparison to tailored blanks are the abolition of cutting operations and a high flexibility regarding the shape and position of the reinforcement. Even if there is a big application potential for patchwork blanks in automobile production, practical realisation of this innovative technique is hindered due to a lack of knowledge regarding the formability of patchwork blanks. In this study the formability of laser welded patchwork blanks is investigated by means of experimental trials and finite element analysis. In order to simulate accurately the forming processes of patchwork blanks using finite element analysis, knowledge about the characteristics of the weld metal, including the weld bead and the heat affected zone, is essential. Microhardness measurements have been applied to analyse the heat affected zone and to determine its lateral dimensions. The constitutive behaviour of the weld metal has been investigated by uniaxial tensile tests with a special sample geometry. The results obtained from these experimental investigations have been used to define different modelling techniques for the finite element analysis of patchwork blank forming processes. It can be shown that the consideration of the modified material properties along the weld seam leads to an improvement of the accuracy of the numerical calculation.

Abstract: In times of highest significance of process modelling and numerical simulation characterisation of material properties is of special importance for tools’ and components’ dimensioning. But in general material properties depend on many different influencing variables, e.g. temperature, humidity and many others. Especially in fields of sheet metal forming the mechanical behaviour of components highly differs according to real stress condition. In particular yield loci combine the information of beginning of yielding with a biaxial stress condition, but nevertheless for many materials they have not been determined yet. For all others the existing values are available only at room temperature. In this paper a novel concept of the experimental setup is shown, with which plastic yielding of sheet metal can be examined also at elevated temperatures. In usual biaxial tension tests cruciform specimen are drawn in plane. The new machine-concept, which is presented in this paper, is based on a punch-load moving perpendicular to the sheet. By clamping the specimen restoring forces are induced, which cause in dependence of special developed tool and work piece geometries defined stress conditions. Using an optical measurement system for determination of strains with CCDcameras of very high frame rate allows exact identification of starting plastification by offline analysis. Experiments at elevated temperatures are realised by local heating with a diode laser and a special optical system to reach a homogenous distribution of temperatures in the forming zone. On the one hand these investigations are necessary for many materials to achieve further information on characteristic properties in warm forming, because their data are only known at room temperature. On the other hand some materials, e.g. magnesium wrought alloys, are mostly formed at elevated temperatures (here in the range of 200°C to 250°C), because of its significant higher formability. Thus, material behaviour must be characterised at these temperatures.

Abstract: The multifarious applications of aluminum alloys in different industrial domains are based on the mechanical properties as well as the light weight characteristics, which allow energy saving for the products in use. Nevertheless aluminum alloys are considered to be difficult to weld by conventional welding processes. This paper deals with cost-effective friction stir welding of thin sheet aluminum alloys in thicknesses of about 1 mm to widen the possible range of applications. Based on former studies results will be shown how an optimized tool geometry increases the mechanical properties of friction stir welded parts. The characteristics of these friction stir welded thin sheets will be analysed in a statistical evaluation in order to detect the interactions and dependencies of the process parameters. By highlighting the main process parameters and their significances a process window for friction stir welded blanks of AA5182 and AA6016 is presented.