Advanced Materials Research Vol. 769

Abstract: Orbital forming is an incremental bulk cold forming process with many advantages. It can produce net-shape or near-net-shape parts that have superior mechanical properties due to work hardening compared to manufacturing with cutting processes or hot forming. In this work the orbital forming process is employed in a closed-die configuration. A rising of the material thickness in the outer areas of a circular sheetmetal blank is enabled by preventing the lateral material flow. The main effects and subsequently the effects of the interaction of parameters were investigated by a two-step design of experiments. A screening plan was used to identify the statistically relevant parameters. The effects were then studied in a subsequent central-composite design of experiments. With the measured data a nonlinear response-surface model was parameterized to describe the dependency of the mould filling on the investigated process parameters. This model was validated experimentally and showed a good agreement to reality. Additionally a new concept for the tool system was developed and investigated. The form-defining cavity can be changed from the upper punch to the counterpunch.

Abstract: The demands on accuracy of forming simulation rised sharply in recent years due to increasing manufacturing costs and intensive application of lightweight materials in automotive industry. The accuracy of prediction of forming simulation depends mainly on the use of material models, pre-processing parameters and friction modelling. For isolated investigation of individual parameters a new test has been developed at the Institute for Metal Forming Technology (IFU), University of Stuttgart. This buckling-bending-test allows a frictionless investigation of bending processes. Within preliminary investigation buckling-bending-tests were carried out using an aluminium sheet material. The strain on the outer skin of the specimen was measured in-situ during the test by the use of an optical measuring system. According to the experiments, a forming simulation of the buckling-bending-test was generated using LS-Dyna. The parameters necessary for the pre-processing, such as element size, refinement levels and meshing parameters were examined in a separate simulation study to determine the optimum for the subsequent simulation study. The approaches of Ludwik, Hollomon, Gosh and Swift have been used to describe the flow curve. It was found out that the best match can be achieved by an extrapolation according to Ludwik. The improvement of prediction amounts to 5 per cent. The description of the yield locus depends on the material file used for the simulation. In this investigation the material models MAT_18, MAT_36, MAT_37, MAT122 and MAT_133 have been used. It was shown that the prediction of bending loads can be approved by the use of the yield locus description of Barlat.

Abstract: Parts, which dimensions do not exceed the micro range are gaining more importance in today´s goods due to miniaturization and function compaction of products. Thus, fabrication methods for these special, very small parts attract notice because whole processes or at least process parameters cannot be transferred into micro range. Nevertheless, this does not necessarily mean, that size-effects are always detrimental as there are some processes which can only be carried out in micro range such as the laser-based micro upsetting process. The basic characteristics of this process are presented in this paper such as its self-aligning capability. Due to the fact that the maximum achievable upset ratio in single stage upsetting decreases with decreasing sample size, the material accumulation represents a very good semi-finished product (preform) being formed in a succeeding upsetting process. A temperature- and time controlled forming unit has been developed which is directly adapted to the laser system which is required to generate the preform. Thus, either a pre-defined temperature of the preform or the down time gives the initial signal to start the forming process. In this paper, a mathematical model is very briefly presented to determine the process window for forming preforms at certain well defined temperatures regarding size-effects.

Abstract: In bending processes for tubes and profiles, the produced bending geometry depends on the characteristics of the semifinished product. Tubes are subjected to scatter and deviations from the standard’s dimensions caused by their fabrication process. This results in deviations of the forming behaviour. Depending on the bending process, the effect of this phenomenon is even larger, if the deviation of process forces leads to a significant deflection of the die elements. Particularly kinematic bending processes, e. g. the 3-roll-push-bending process have shown to be highly sensitive to low machine stiffness. The effect of this factor has not been identified among the influences of other disturbance factors so far. As an additional aspect, the tube dimensions affect the material characterization which leads to inaccurate material properties in numerical simulations. This results in wrong input data for the process design and has to be corrected in lengthy adjustment procedures. In this contribution the scatter of some geometrical tubes properties and its impact on the 3-roll-push-bending process is investigated. The tube geometry is measured by a tactile measurement system and is compared to the product standards. Observed deviations are categorized according to DIN 4760. The impact of the deviations on the bending process is examined by numerical variant calculations. Besides the tube dimensions, the stiffness of the machine is varied and the influence determined. By considering the errors resulting from wrong input data in the material characterization the overall error is quantified.

Abstract: Ground and satin stainless steel surfaces are often applied to lend various products a characteristic high-quality appearance. Examples can be found in interior and exterior architecture, passenger elevators, public ticket machines and cover panels of white goods. In everyday use the visual appearance of these products is impaired by soiling. Especially on the surfaces of household appliances fingerprints tend to accumulate. On satin stainless steel parts the cutaneous sebum sticks to the brush marks. Thus extensive cleaning effort is required. Since several years the manufacturers of stainless steel offer satin stainless-steel sheet metal equipped with thin clear coatings for this application. The clear coating fills the brush marks and also causes a better wetting behavior and optimal cleaning properties. In this regard, the term easy-to-clean surface is common.The manufacturers face the challenge of applying a very thin clear coating with an index of refraction adjusted to the satin stainless steel surface and the cutaneous sebum. Thus, ideally the coating should not be distinguishable, concerning haptic and optic properties. The thickness of these coatings amounts to a few micrometers. Therefore, the heat transfer coefficient of the surface only varies minimally and the surface does not feel painted. The advantageous wetting behavior and the slightly differing reflection properties of the clear coating contribute to reducing the adhesion and conspicuousness of fingerprints significantly. Because of these attributes the term anti-fingerprint coatings is used.A further prerequisite for the application of anti-fingerprint coatings in high-quality parts is a good scratch resistance. Therefore, nanoparticle-reinforced coating systems are applied. Typically hard particles like SiO₂ or Al₂O₃ are used. In addition to this, a curing procedure by ultraviolet radiation combined with a suitable solvent-free coating system can be used. This further improves the hardness and scratch resistance of the coating. Due to these requirements the clear coatings are applied in a coil-coating procedure. Consequently, the final painted sheets have to be processed to the respective sheet metal part. Common forming processes for white goods’ panels are bending and crash forming. After forming the final processor does not spend any effort on varnishing or curing on-site which leads to special cost savings in comparison to a piecework coating.The required performance characteristics of the anti-fingerprint coating allow no compromises concerning the coating system with regard to the formability. During forming operations damage is caused to the clear coating in the form of crack formation and delamination. Due to the optical properties of coating systems specifically adjusted to ground stainless steel surfaces, the defects in the clear coating are difficult to identify precisely. However, the properties of the final painted sheet metals’ surface are impaired. This also includes the corrosion resistance which must be taken into account especially for stainless steel applications, e.g. in the kitchen area or in outdoor applications.On the one hand this paper deals with experimental analyses for the identification of the defects caused to an anti-fingerprint clear coating by forming operations. The elaborated procedure is used to identify cracks and delamination due to bending operations. In the experiments mandrel bending specimens of the coated stainless steel sheet metal are formed. Due to different bending radii the intensity of cracks and delamination in the coating varies. But the orientation of the bending edge towards the grinding direction also influences the damage intensity and the damage characteristics. These specimens are also examined in a salt spray test to determine the influence of the different coating damages on the corrosion resistance of the stainless steel substrate. In comparison, the same stainless steel sheet metal material without clear coating is tested simultaneously. The comparison demonstrates the influence of the coating and the deformation-induced damages. On the other hand, to examine the damages as a result of a typical forming process a simplified oven panel is manufactured in a crash forming tool. The front side of the panel remains flat and shows no strains. But along the sharp bending radius, which is typical for the design of high-quality white goods’ applications, cracks in the clear coating can be observed. The experimental results show an influence on the coating damages resulting from the use of a deep draw foil. Furthermore, the forming tool features a heating device and panels are formed at higher temperatures with an effect on the coating damages.

Abstract: The main focus of the experimental observation deals with the investigation of the plasticyielding of DC06 and DP600 under biaxial tension in comparison to an identified material modelwith an isotropic hardening behaviour. The isotropic hardening law describes the hardeningbehaviour of the material by application and approximation of the flow curve. Prevalent hardeninglaws are proposed by Swift (1952) or Hockett and Sherby (1975) and lead to an expansion of theyield surface in the stress space. By reasons of good accordance in an earlier survey the givenmaterials are modeled by the yield criterion Yld2000-2d and the isotropic hardening law of Swift.In this case the yield loci at different experimental states of plastification are compared with theyield loci given from the isotropic expansion of the Yld2000-2d yield surface. Furthermore apossible approach for modelling the change of the yield criterion’s shape in the stress space duringplastification is shown. With respect to further investigations additional research work is neededincluding extended and complex hardening laws to describe the real material behaviour sufficiently.

Abstract: Since sheet-bulk metal forming processes inherit properties of both sheet and bulk metal forming processes, their analysis requires on one side following certain methods conventionally devised in these process classes analyses whereas on the other side leaving certain customs out. For instance, inherent anisotropy of the rolled sheet has to be taken into account whereas due to non-vanishing out of plane stress component, analysis with thin shells using the plane stress state assumption is no more applicable. Similarly, methods based on necking instabilities, i.e. forming limit diagrams, which are typically used in sheet metal formability assessment; fall short in sheet-bulk metal formability prediction. In the present study, we propose a local approach to fracture, more specifically a phenomenologically based Lemaitre variant CDM model, devised frequently in bulk metal forming analysis, as an alternative. For this purpose, a combined nonlinear isotropic-kinematic hardening plasticity with Hill48 type initial anisotropy is fully coupled with isotropic damage. Together with the concept of effective stress and equivalent strain principle, quasi-unilateral damage evolution is used, where the energetic contribution of the compressive stress state to the damage driving force is scaled with a so-called crack closure parameter, . For the quasi unilateral damage evolution is inactive whereas for it is fully active which completely suppresses the development of damage under compressive stress states. The framework devises state coupling between elasticity and damage and kinematic coupling between plasticity and damage which increases the relative effect of on the eventual damage development. To this end, a direct extension to the finite strains for metal forming analysis is realized using a corotational formulation and the developed framework is implemented as a VUMAT subroutine for ABAQUS Explicit. For evaluation of the predictive capability of the model, teeth forming process results for DC04 reported in Soyarslan et al. 2011, An Experimental and Numerical Assessment of Sheet-Bulk Formability of Mild Steel DC04, Journal of Manufacturing Science and Engineering, Vol. 133 6, (2011) S. (061008) 1-9, are used. Mechanical material characterization studies are realized using a hybrid experimental-numerical procedure. This methodology relies on minimizing the difference between the experimentally handled global clamp force demand diagrams and the diagrams from the simulations at the complete range of the experiments involving fracture. As known finite element solutions with softening material models are prone to pathological mesh dependence. For this fact, a crack band method is used where the minimum element size, as a controlling parameter of the localization size, is also fitted through the characterization studies and identically used in the process simulations. The simulations show that a correct prediction of the zone and time of fracture is possible for the selected process whereas since the teeth formation process is mainly a compressive process, once the quasi-unilateral damage development is not used, i.e. for , a premature crack prediction is recorded which is not compatible with the experimental findings.

Abstract: In sheet metal forming technology stamping machines are mainly used for an economical production of sheet metal workpieces. Apart from increasing the stroke rates of currently more than 3000 min^-1, which can be achieved with modern high-performance stamping machines, the demands on the periphery of the plant are rising as well. In particular, this concerns the material feeding systems used for a reliable feed of the sheet metal. The current technology is based primarily on the roll and gripper feed. Here the sheet metal is clamped between the grippers or rollers with a high contact pressure, which is required for a slip-free operation. To avoid an external damage of the surface or a roll out of the sheet, the clamping forces may not be increased indefinitely. In addition, contamination of the sheet metal or the elements of the feeding system should be excluded in order to avoid a permanent damage of the system and related maintenance costs. This means that the feed rates of previous feeding systems, currently up to 2000 min^-1, cannot be further increased, so that the performance potential of modern high-performance presses with large stroke rates cannot be exhausted. Thus the development of feeding systems in sheet metal processing with significantly higher forces is required.As part of a research project at the IFUM, facilitated by the German Machine Tools' Association (VDW), a novel method has been developed in which the sheet metal is fed completely without contact by means of electromagnetic forces. No mechanical elements are required for clamping the sheet metal, so that the inertia of the system can be reduced significantly. Thus higher dynamic properties of the feeder can be realized. The principle is based on the asynchronous linear motor with eddy current runner in a double cam arrangement. This feeder basically consists of two primary components, comprised of a laminated iron package and a three-phase winding. The primaries are symmetrically fixed positioned to compensate the forces of attraction in ferromagnetic materials as well as the repulsive forces in paramagnetic sheet metals such as aluminium or copper. The electrical conductive sheet metal acts as a secondary part and is located in the air gap between the two primary components. Thus the sheet is kept suspended in the air gap a damage to the sheet metal surface is prevented. Therefore surface-finished metal sheets can also be fed with high speed rates. The force initiation is performed entirely contactless to the sheet metal with the three-phase winding in the primaries which induce a sinusoidal magnetic traveling field in the air gap. During operation eddy currents are induced in the metal strip due to the speed of the traveling magnetic field relative to the sheet. By the interaction between the magnetic field and the eddy currents an advancing force is applied to the sheet metal according to the Lorentz law.For the design and optimization of the electromagnetic feeder extensive simulation-based studies have been performed using a parameterized finite element model. For this purpose the development of a three-dimensional model was necessary to represent the eddy currents in the sheet metal. The main subjects of the investigations were in particular the optimization of the iron core, the winding distribution and also to ensure an acceptable temperature in the primaries and the sheet metal during continuous operation. The studies show that, depending on the sheet material applied, very high feed forces can be achieved. For sheet metals with a width of about 100 mm more than 1000 N can be achieved by means of the electromagnetic feeding system. Compared to current mechanical feeders the forces can be more than doubled.To validate the simulation results and test the functional ability a demonstrator of the electromagnetic feeder was designed and manufactured. Due to the simulation-based optimization of the feeding system an external cooling is not required. The control of the feeder is realised via a conventional frequency converter, with which the voltage can be controlled in its amplitude and frequency, and thus indirectly the sheet metal position. The first experimental investigations were carried out on a specially designed force test bench. The results show a very good correlation obtained by simulation and the experimental measured feed forces. Future work objectives are to identify the feed characteristics and limitations as well as the implementation of a robust control algorithm for a reliable positioning of the sheet metal.

Abstract: Sheet metal forming processes, in particular deep drawing processes, are highly influenced by occurrence of latent and friction heat. Especially when forming metastable austenitic stainless steels, strain-induced martensite formation is suppressed by higher temperatures and therefore influences the material behavior and so called TRIP-effect. This study gives an overview about thermal influences on the deep drawing forming process of metastable austenitic CrNi-steel 1.4301 in comparison with ferritic stainless steels such as 1.4016. Measurements on serial and evaluation tools were carried out to determine occurring temperatures within forming tools. Attention is paid to effects on tribological aspects such as behavior of lubricants at higher temperatures, influence of temperature development on the martensite formation, mechanical properties, forming limit curves as well as heat flow within the forming tools. Lubricants with different temperature stability were compared to each other with determination of friction coefficient in strip drawing tests. Martensite and temperature development during forming of material was measured in non-isothermal tensile tests approving a high dependency of martensite formation on temperature. Forming limit curves for temperatures determined from RT to 140°C for EN 1.4301 are showing high dependency of necking behavior especially under plain strain conditions. Determination of thermal contact conductance coefficients for process and tool relevant material combinations allows interpreting heat flow mechanisms in forming tools and improving forming process to higher robustness. Results of this paper can be used to individually set boundary conditions for thermo-mechanical coupled forming simulation of austenitic stainless steel and process layout of tool temperature control systems.

Abstract: The unique process of sheet-bulk metal forming (SBMF) represents a combination of sheet and bulk metal forming by inducing a three-dimensional material flow in sheet metals in a single forming stage. Within this paper two different applications of sheet-bulk metal forming are demonstrated. Hereby two different combined drawing and upsetting processes to realize parts with symmetrically and asymmetrically arranged functional elements are analysed. Finally, this contribution introduces a new machine technique which provides an improvement of the working accuracy of a forming machine and thus has a positive influence on the parts quality. The idea is to use electromagnetic ram guidance to counteract the displacement of the ram due to horizontal forming forces while forming of asymmetric parts.