Papers by Author: Gerhard Hirt

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Authors: Michael Pietryga, Johannes Lohmar, Gerhard Hirt
Abstract: Roll bonding is a process to join two or more different materials permanently in a rolling process. A typical industrial application is the manufacturing of aluminum sheets for heat exchangers in cars where the solder is joined onto a base layer by roll bonding. From a modelling point of view the challenge is to describe the bond formation and failure of the different material layers within a FE-process model. Most methods established today either tie the different layers together or treat them as completely separate. The problem for both assumptions is that they are not applicable to describe the influence of tangential stresses that can cause layer shifting and occur in addition to the normal stresses within the roll gap. To overcome these restrictions in this paper a 2D FE-model is presented that integrates an adapted contact formulation being able to join two bodies that are completely separated at the start of the simulation. The contact formulation is contained in a user subroutine that models bond formation by adhesion in dependence of material flow and load. Additionally if the deformation conditions are detrimental already established bonds can fail. This FE-model is then used to investigate the process boundaries of the first passes of a typical rolling schedule in terms of achievable height reductions. The results show that passes with unfavorable height reduction introduce tensile and shear stresses that can lead to incomplete bonding or can even destroy the bond entirely. It is expected that, with adequate calibration, the developed FE-model can be used to identify conditions that are profitable for bond formation in roll bonding prior to production and hence can lead to shorter rolling schedules with higher robustness.
Authors: E. Maidagan, Joachim Zettler, Markus Bambach, P.P. Rodríguez, Gerhard Hirt
Abstract: Nowadays many industrial sectors use forming processes in order to produce sheet metal components. The most widely used processes are stamping and deep drawing, which are based on big, costly dies and presses. These processes require large initial investment and specific dies for each part, which makes them inflexible and only profitable for large batches. A possible approach to small series production is based on the incremental sheet forming technique (ISF), which consists of a gradual plastic deformation of flat sheet metal by the action of a CNC controlled tool. Equipment such as a 3-axis milling machine can be used for ISF, such that the initial investment costs in ISF are around 5-10% of those required to set up a production line for conventional stamping. In its current stage of development, dedicated dies are often used as support tools in ISF. However, due to the fact that the forming forces are low in ISF, the dies can be made out of cheap materials like resin or wood. Although this is an additional advantage over stamping, the need to use additional tools still reduces the flexibility of the process. The present paper details the concept of a truly “dieless” incremental forming process. In the framework of the SCULPTOR EU project, the authors are working on an innovative concept of incremental sheet metal forming which is based on the replacement of the commonly used dies by a second forming tool which moves in a coordinated way with the first forming tool, thus creating a flexible die system, which does not depend on the specific geometry of the part to be formed. The present work summarizes the results obtained up to now in two fields: (i) the development of a prototype for the flexible die system to be included both in milling machines or combined with robots and (ii) process modelling to improve the understanding of the process.
Authors: Markus Grüber, Marius Oligschläger, Gerhard Hirt
Abstract: Within today’s sheet processing lines, roller levellers are included in the production chain to eliminate initial curvature and reduce internal stresses of the sheet material. Despite the desire to achieve fully automated industrial processes, roller levellers still have to be set manually by an operator based on his experience and empirical data. Therefore, this paper evaluates an enhanced numerical approach to predict the vertical roll position, the so called roll intermesh, in the last load triangle. To gain the respective machine setting, a closed-loop control based on an actual curvature measurement is implemented in the finite element (FE) programme Abaqus utilising a user-subroutine. Thus, the presented FE model allows the adjustment of the roller leveller leading to a flat strip in a single simulation run within the accuracy of the FE prediction. Additionally, the FE model provides the chance to develop and test closed-loop controls for roller levelling. Complementing the results gained from the FE model, experiments have been conducted on a down-sized roller leveller with aluminium sheets (AA5005). First results obtained with the presented numerical model proved that the roll intermesh of the last load triangle was determined successfully and the use of an actual curvature measurement within the FE model provides enhanced accuracy.
Authors: Michele Vidoni, Markus Daamen, Gerhard Hirt
Abstract: Direct thin strip casting is an economically end energetically smart process for the production of steel strip. In a single process step, liquid steel can be cast and directly rolled to hot strip in thicknesses ranging from one to four millimeters. With the use of specifically profiled casting rolls it is possible to produce strip with optimized cross-sections, allowing this process to compete with tailor welded and tailor rolled blanks for the production of a class of products already widely applied in industry. Numerical and experimental studies proved the feasibility of this concept and additional simulations were used to optimize the profile to be used for the experiments. A thickness variation of one millimeter from the edge to the center could be successfully achieved. However, the dimensional precision and the roughness distribution along the cross section of the produced strip were not satisfactory. Additional profiles were applied for the experimental analysis leading to better roughness distribution and geometrical accuracy. In order to further improve the uniformity of properties along the profiled section it is necessary to increase the homogeneity of the microstructure. The coating and surface preparation of the casting rolls play a very important role in the strip casting process as they strongly affect the solidification behavior. This observation lead to the idea of selectively coating the casting rolls, applying a less conductive layer on the areas where the casted profile is thinner. Thus, a more homogeneous solidification front can be obtained. The effect of a locally modified casting roll coating on the solidification is numerically investigated and the results applied for the selection of the coating parameters to be used for the experiments.
Authors: H. Justinger, Gerhard Hirt
Abstract: With the increasing trend towards miniaturization and the enhanced demand for small components, reliable processes for mass production are needed. Today the deep drawing process is already used to produce large numbers of small parts (diameter < 1 mm) at low costs per part. But a better understanding of the process in relation to miniaturization is required to improve process stability, because several aspects of the process change when scaled down. For example, product accuracy and process parameters can be influenced by changing the ratio of surface to volume or the ratio of grain size to foil thickness. For the analysis of these effects experiments with geometrically scaled deep drawing tool sets from 8 mm to 1 mm punch diameter have been carried out, using CuZn37 foils in different annealed conditions and a foil thickness ranging from 0.3 mm to 0.04 mm. Additionally, the deep drawing process is simulated via FE-methods to consider influences that cannot be measured using the available experimental setup, such as temperature conditions resulting from the heat generated due to plastic dissipation and friction.
Authors: René Baadjou, H. Shimahara, Gerhard Hirt
Abstract: The thixoforming processes join the advantages of conventional forming technologies as forging and casting in respect of the mechanical properties and the practicable geometries. Within the framework of the Collaborative Research Centre 289 at the RWTH Aachen University intensive investigations on semi-solid processing with some steel grades have been running. For this purpose an automated thixoforging plant (thixo-cell) has been developed in a closed cooperation with several industrial partners. With this equipment multi material demonstrator components have been successfully produced by thixojoining using semi-solid X210CrW12 tool steel.
Authors: Babak Taleb Araghi, Markus Bambach, Gerhard Hirt
Abstract: Asymmetric incremental sheet forming (AISF) is a new sheet metal forming process in which sheet metal parts are produced by CNC-controlled movements of a simple ball-headed forming tool. Despite its flexibility and successful application in many cases, AISF has not yet been established in an industrial context due to some still existing process limits such as severe thinning, which strongly depends on the inclination of the part surface, as well as a limited geometric accuracy due to springback. Furthermore, there is little knowledge available about the properties of parts produced by AISF, especially in comparison to deep-drawn parts. The aim of the present paper is to compare cylindrical cups manufactured by deep-drawing and AISF regarding the resulting strain and thickness distribution. For AISF, different forming strategies were applied. Comparisons of the wall thickness and surface strain distributions show similar results for the cup produced by deep-drawing and the best cup produced by AISF, but the surface strains and the sheet thinning in the parts formed by AISF were larger than in the deep-drawn part.
Authors: H. Voswinckel, Heinrich Schleifenbaum, Markus Bambach, Gerhard Hirt
Abstract: This paper details an integrated product process design model that represents process capabilities by a set of key indicators and allows for the design of products taking into account constraints set out by the process. The model is applied to Incremental sheet forming (ISF) processes and their variants. ISF processes have been developed over the past 20 years and have reached a state of development now allowing for a transition from scientific research to broader industrial application. ISF with its low part specific tooling represents a technology suitable for individualized production down to one-piece-flow. Hence, it might satisfy the growing demand for individualized products in the field of sheet metal production. However, an industrial use of ISF requires that general design rules are provided to designers to enable designs that are compatible with the capabilities of ISF. Today’s product design typically is more suitable for stamping operations than for ISF which makes the fabrication of parts by ISF difficult and increases lead time and costs. Also, different variations of ISF processes exist that are based on different machines (industrial robots, CNC machines,…) and are characterized by different capabilities, e.g. in terms of accuracy. The objective of this work is the development of an integrated product process design model and its application to ISF. The capabilities of currently available ISF processes are determined and compared to the requirements of selected products from the automotive and aero-space industry.
Authors: Alina Melzner, Gerhard Hirt
Abstract: Innovative product characteristics can be realized by hot roll bonding of two or more layers of different materials. To optimize the roll bonding process, an approach to align the strength differences in both materials by a temperature difference between the layers has been proposed. Therefore, the temperature distribution has to be investigated by finite element (FE) simulations. In these simulations the heat transfer coefficient (HTC) between the two aluminum layers is of great importance. With this coefficient the temperature transfer between the two layers can be determined in order to estimate the temperature field and the material strength difference in the layers.In hot roll bonding there are two ranges for the HTC depending on whether bond formation takes place or not. This effect can be used to determine at which pressures bond formation starts. To evaluate the HTC for this application and to determine its value ranges, a simple setup has been developed. This setup allows conducting experiments under defined temperature and pressure conditions. The resulting force-time measurements were used as input values for inverse FE-simulations, with the goal to gather the HTC by inverse modelling the temperature distributions of the specimens. First results show that the range of the pressure dependent HTCs leads up to 21 kW/(m²K) in the unbonded range. In the range where bonding occurred between the specimens, values over 150 kW/(m²K) were estimated. The data for the HTC was implemented in roll bonding simulations as an interaction property. A comparison between the simulated temperature curve and a measured temperature curve during roll bonding showed a good agreement between the temperature values.
Authors: Sven Stockert, Matthias Wehr, Johannes Lohmar, Gerhard Hirt, Dirk Abel
Abstract: Almost all metal strips with thicknesses of < 2 mm are produced by cold rolling. Thickness variations of cold rolled strips are caused by various factors like fluctuation in strength of the material, the eccentricity of the rolls or thickness variation of the incoming strip. As the demands concerning the thickness variation are ever increasing the Institute of Automatic Control and the Institute of Metal Forming aim at reducing the thickness tolerance of thin, cold-rolled steel and copper strips to 1 μm. As high frequency disturbances are expected, it is assumed that this goal can only be achieved by using a predictive controller in combination with a high precision strip thickness gauge and, for roll adjustment, a piezoelectric actuator in addition to the existing electromechanical actuator. The objective of this work is the constructive implementation and the testing of a thickness gauge based on laser triangulation. The gauge includes guide rollers to prevent strip vibration, a C-frame to allow an inline calibration and mechanical adjustment of the measuring range so that even flexible strip thicknesses can be measured. The designed gauge showed a high repeat accuracy of 0.4 μm for two different metal strips. Furthermore the gauge was used to investigate the dynamics of the thickness change of a steel strip at maximum rolling speed of 5 m/s using a Fourier transformation. This frequency analysis supports the need for a piezoelectric actuator that can also subsequently be dimensioned based on the obtained frequency data.
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