Papers by Author: Markus Bambach

Abstract: In cold extrusion of aluminum alloys adhesive wear can be prevented by an excessive lubrication of the process. While this causes additional process steps also environmental risks have to be addressed. Hence, dry metal forming, i.e. avoiding lubrication by means of coatings and topography modifications is highly desirable. In this paper first results concerning the behavior of tailored surfaces under dry metal forming conditions for pure aluminum are presented. Different surface treatments (laser polishing and Mo₂BC coating) of the tool steel AISI H11 are tested in a compression-torsion-tribometer under conditions adapted from cold extrusion. Normal stresses six times higher than the initial yield stress of the tested workpiece material pure aluminum (AA1050-O) are applied. Furthermore, a strategy for the characterization of aluminum adhesions to the tool is introduced. The influences of different topographies and the presence of a coating on the loss of material due to adhesive wear are investigated.

Abstract: Surface curvature radii required for aircraft fuselage as well as structural components can be produced by peen forming processes. The innovative process idea of Rotary Peen Forming is a modification of the well-known Shot Peen Forming. Here, the impactors are flexibly connected to a rotating hub and thus moving on circular trajectories. As a consequence, there is no need to pressurize and recirculate the shots, as it is essential in Shot Peen Forming. Using a six axes robot, the rotating hub can be guided flexibly. The resulting machine design is more compact compared to traditional Shot Peen Forming.However, in Rotary Peen Forming not only principal stresses but also shear stresses are caused in the deformation zone which has a fundamental influence on the curvature. In order to generate defined curvatures on the workpiece, the capability to precisely adjust the intrusion depth of the impactors is essential.In this paper, a laser-assisted distance control for the Robot Controlled Rotary Peen Forming is introduced. By means of a point laser, the set-up allows for a distance control to adjust and keep a determined intrusion depth. This way, the machine design provides a mechanism to readjust the intrusion depth of the impactors while the desired curvature is formed during the process by the introduced plastic strains at the specimen’s surface. Using the distance control, the resulting curvature is two to four times bigger compared to experiments without a readjustment of the intrusion depth.

Abstract: For the full characterization of the hot working behaviour of a given material a large number of laboratory experiments have to be performed. The experiments themselves are time consuming and the required specimen material can be quite expensive. With the increasing versatility of the testing machines, like dilatometry with easily variable temperatures, overthinking the classical approaches for materials characterization becomes expedient.In this paper a new technique for the reduction of the experimental effort is presented at the example of a 25MoCrS4 case hardening steel. To analyse the potential for the reduction of the experimental effort the classical approach of a full experimental test matrix is chosen. Here 55 flow curves with temperatures between 700 and 1200°C and strain rates from 0.01 to 100/s are experimentally determined. Then a semi-empirical model for strain hardening and dynamic recrystallization is fitted using an automated routine for parameter determination, taking all available flow curves into account. Subsequently, the number of flow curves used to fit the model parameters is gradually reduced. The model accuracy obtained with the reduced experimental data is compared to the initial fit. The natural decrease in accuracy with the use of less data compared to the gain due to the reduction of experimental effort is analysed. In addition optimal distribution of the sampling points in the experimental matrix for a reduced number of experiments is discussed. It is shown that less than a quarter of the full matrix is sufficient to reach accuracies comparable to using the full matrix. Using the vertices and symmetrical distribution of the data within the full experimental matrix allows a drastic reduction of experimental effort while maintaining the initial accuracy. The results suggest that it might be possible to reduce the costs and effort for material characterization by 50-80%.

Abstract: Product development is complex due to the manifold requirements resulting from various perspectives, such as design, production, safety and sales. A concurrent engineering (CE) approach permits to respect all perspectives in the early development stage. However, in the architecture and construction sector for example, CE is particularly difficult to realize, because the central steering for this collaboration process is missing. Thus, the application of CE in the research sector can promote technical progress and cost reduction. In the specific field of freeform architecture, in most cases an individual shape of single components is unavoidable and the use of standard components impossible. Due to missing universal and mature construction concepts for freeform buildings, they are mostly realized with customized solutions often including material-consuming substructures, while the visible skin has only limited structural and functional properties.In this context the present paper proposes a novel universal panel system made of double-curved sheet metal layers enabling the assembly of self-supporting lightweight structures for the realization of freeform surfaces. The panel system has been developed in cooperation of architects, construction and production engineers, successfully applying an interdisciplinary CE approach. As a result, the concept allows for material and cost efficient solutions applicable for a wide range of freeform applications. The detailed development of the panel system is still in progress.Besides the general panel concept, the paper presents in particular the corresponding manufacturing chain and the tooling concept. Accounting for the varying part geometries in this application a flexible manufacturing chain based on the combination of stretch forming and incremental sheet forming has been developed. The entire production process is implemented in a single machine setup and successfully tested on a small-scale prototype.

Abstract: In recent years, hot stamping of sheet metal parts has emerged to satisfy the contrary demands of the automotive industry for components with increased strength at reduced weight. To analyse the material behaviour during these processes, a hot gas bulge test at high temperatures and high strain rates is promising, since the bulge test at room temperature has already proven itself as a useful test for the material characterization of sheet metals up to high strains. Therefore, a hot gas bulge test at elevated temperatures and high strain rates is being developed at the Institute of Metal Forming (IBF) in cooperation with the Institute for Fluid Power Drives and Controls (IFAS) at the RWTH Aachen. To verify if the concepts of the membrane theory, which are used for the evaluation of bulge tests at room temperature, are adaptable to such a hot gas bulge test, a simulation study using finite element calculations was conducted. The purpose of this simulation study is is to estimate the errors which occur if the equivalent stress at the bulge pole is calculated by using the membrane theory. In addition to this study several approaches were examined to obtain the sheet thickness at the bulge pole by measuring the bulge height. The study showed that a hot gas bulge test can be described very well by the membrane theory if the sheet thickness, the curvature at the bulge pole and the pressure inside the bulge are exactly known. However, substantial errors can occur if the sheet thickness at the bulge pole is determined by measuring the height of the bulge pole.

Abstract: Due to new material concepts (e.g. boron-manganese steels), hot stamping of sheet metal parts has emerged in order to produce high strength components. Thereby, the design of hot stamping processes by means of finite element simulations requires information about the thermo-mechanical material behaviour up to high strain levels at various temperatures as simulation input. It is known that hot tensile tests are only evaluable until low strain levels. Therefore, a hot gas bulge test for temperatures in the range of 600 °C to 900 °C and strain rates up to 1/s is being developed. In order to design such a hot gas bulge test, the requirements (e.g. forming pressure) are estimated by finite element simulations. The result is a test bench, which already enables a pneumatic forming of specimens at room temperature and pressures up to 200 bar without any unexpected side effects.

Abstract: Material models that couple the evolution of flow stress to the evolution of the microstructure are important for the simulation of hot working processes in which the microstructure undergoes large changes. Among the microstructural evolution mechanisms in hot working, dynamic recrystallization (DRX) plays a central role as it occurs during deformation. When the workpiece deforms, the element shape may deteriorate, which makes re-meshing necessary. At the same time, certain regions of the finite element mesh undergo DRX and a sharp interface between recrystallized and non-recrystallized portions of the workpiece develops. Elements of the old mesh that are cut by the interface contain nodes with a non-zero recrystallized volume fraction and nodes where the recrystallized volume fraction is zero. During re-meshing, when the microstructural data is transferred to the new mesh, nodes or integration points that are actually in region of the workpiece that is not yet recrystallized may be assigned a non-zero recrystallized volume fraction. As a consequence, the interface moves, which is unwanted and may produce large errors when re-meshing is frequently done. In this paper, the problem of the propagation of the DRX interface during re-meshing is treated. It is shown that the propagation occurs with standard data mapping algorithms and produces a large error at the interface. A re-meshing scheme is proposed that uses a smooth mesh-free interpolating function based on radial basis functions to interpolate the recrystallized volume fraction. The interface is the zero level set of this interpolant. Performing the mapping as a least squares fit of the interpolant allows for a substantial reduction of the mapping error and suppresses the propagation of the DRX front.

Abstract: Hybrid components made of steel and aluminum sheet metal are a promising approach for weight reduction for automotive applications. However, lightweight components made of steel and aluminum require suitable joining technologies, particularly if forming operations follow after the welding process. Friction Stir Welding (FSW) is a promising solid-state welding technology for producing dissimilar joints of steel and aluminum. Within this work dissimilar butt joints were produced using sheet metals of mild steel DC04 and the aluminum alloy AA6016 with a thickness of about 1 mm. The FSW joints show approximately 85 % of the tensile strength of the aluminum base material. In metallographic investigations of the produced FSW blanks it was found that the microstructure in the area of the weld seam changes in the aluminum alloy due to the process temperature and plastic deformation. Due to temperature dependent changes of precipitations of the aluminum alloy, temperature measurements have been carried out during the welding process. To find an explanation of the reduction in tensile strength of the FSW joints, short time heat treatment experiments in the temperature range between 250 °C and 450 °C were performed using the aluminum base material. Heat treatments in the temperature range of the measured process temperature result in a reduction of the tensile strength of about 20 % regardless the annealing time.

Abstract: Due to increasing demands for lightweight structures in automotive applications the use of sheet metal components made from aluminium alloys is a promising approach for weight reduction. The combination of steel and aluminium in car bodies may be an interesting alternative compared to a monolithic material design. The weight of structural parts of a car body shell can be reduced if dedicated parts consist of aluminium instead of steel. This approach allows for an optimal exploitation of the material properties of both materials, bringing high strength into highly loaded areas while areas subject to lower loads are equipped with lower strength and weight. However, a multi-material design combining steel and aluminium demands for suitable joining methods, especially if a forming operation is applied to the welded sheets. In conventional fusion welding processes the formation of intermetallic phases due to the metallurgical affinity of aluminium and iron is a serious problem. Recent developments in regulated cold metal transfer (CMT) welding technologies at the Institute of Welding Technology and Joining Technology (ISF) at the RWTH Aachen promise an appropriate solution to this problem. Due to a digitally regulated arc technology, the heat input in CMT is reduced to a minimum. However, the inevitable formation of a welding bead in arc processes with filler material is a criterion of exclusion in the case of production of welds for car body shells. To achieve an optimal appearance of the body shell, the welding beads need to be removed from both sides of the sheet in a second manufacturing step. Hence, to avoid further costs, it seems expedient to search for alternative welding technologies. Friction stir welded (FSW) joints show relatively even welding beads. Furthermore, this joining method is characterised by a low process temperature, which is considerably below the melting temperature of the base materials. Hence, FSW is a promising joining technique to produce tailored blanks out of aluminium and steel. The main objective of the present paper is the evaluation of suitable process parameters for the production of FSW butt joints with a thickness of 1 mm made from the aluminium alloy AA6016-T4 and the mild steel DC04. Welding experiments using a varying rotational speed, tool offset, tool velocity, tool plunge depth and tool tilt angle were carried out. To identify the best parameters in terms of the strength of the joint, tensile tests were performed. It is shown, that an amount of approximately 85% of the tensile strength of the base material AA6016 can be achieved. Using SEM the formation of the fracture surfaces was analysed. Different fracture types were identified and the possible reasons for their occurrence are discussed. It is shown that in the case of optimal joining procedure the failure occurs in the thermomechanically affected zone in the aluminium sheet, were the plastic deformation is low. Additionally, thermography has been employed to evaluate the temperature distribution during the process. In metallographic investigations it was found that during welding the microstructure of the aluminium base material changes due to plastic deformation and temperature increase in the area of the weld seam. Using hardness measurements the change of the mechanical properties in the contact zone of both base materials and in the heat affected zone was examined. Finally, an outlook is given with respect to the possibilities of producing FSW welded sheets that can be formed using conventional deep-drawing.

Abstract: Dynamic recrystallization (DRX) is widely used in industrial hot working processes to control the microstructure and properties of the workpiece and to keep the forming forces low. For the analysis and design of metal forming processes powerful simulation methods, must notably the Finite Element (FE) method, have been developed. Various models are available that consider the coupled evolution of microstructure and flow stress during hot deformation processes. Some of these models have been implemented into FE codes and are widely available now. However, for the implementation of flow stress models incorporating DRX into an FE formulation, special smoothness requirements exist that are not automatically fulfilled by the available flow stress models. This work reviews some conditions that a flow stress model incorporating DRX has to fulfill in order to be consistently embedded into an FE code for large plastic deformation. A specific Sellars-type model is analyzed for consistency with these conditions. It is shown that the use of a classical JMAK equation for the DRX kinetics within these models is problematic for Avrami exponents smaller than or equal to 3, for which the flow stress model is not sufficiently smooth. DRX kinetics based on the work of Cahn are proposed to remedy the differentiability issues.