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In this chapter, the focus lies on a predictive description of the material response to the thermomechanical loads within different process steps by means of physical and data-driven models. The modeling approaches are demonstrated in examples of innovative production technologies for components of a drive chain: Fine blanking of parts; powder metallurgical (PM) production of gears; open-die forging and machining of drive shafts. In fine blanking, material, process, and quality data are acquired to model interactions between process and material with data-driven methods. Interpretable machine learning is utilized to non-destructively characterize the initial material state, enabling an optimization of process parameters for a given material state in the long-term. The PM process chain of the gear includes sintering, pressing, surface densification, case hardening, and finishing by grinding. Several modeling and characterization approaches are applied to quantitatively describe the microstructure evolutions in terms of porosity during sintering, density profile after cold rolling, hardness and residual stresses after heat treating and grinding and the tooth root load bearing capacity. In the example of the open-die forging, a knowledge-based approach is developed to support the decision-making process regarding the choice of the proper material and optimized pass schedules. Considering the microstructure of the forged shaft, the elastoplastic material behavior is described by a dislocation-based, multiscale modeling approach. On this basis, process simulations could be carried out to predict the process forces, chip form, residual stresses, and the tool life among other output data.

Decision support systems can provide real-time process information and correlations, which in turn assists process experts in making decisions and thus further increase productivity. This also applies to well-established and already highly automated processes in continuous production employed in various industrial sectors. Continuous production refers to processes in which the produced material, either fluid or solid form, is continuously in motion and processed. As a result, the process can usually not be stopped. It is only possible to influence the running process. However, the highly nonlinear interactions between process parameters and product quality are not always known in their entirety which led to inferior product quality in terms of mechanical properties and surface quality. This requires accurate representations of the processes and the products in real-time, so-called digital shadows.Therefore, this contribution shows the necessary steps to provide a digital shadow based on numerical, physical models and process data and to couple the digital shadow with data analysis and machine learning to enable automatic decision support. This is exemplified at various stages throughout two different process chains with continuous processes: first, by using a thermoplastic production process called profile extrusion, and second, on the example of a metal processing process chain, from which three processes are described in more detail, namely, hot rolling, tempering, and fine blanking. Finally, the presented approaches and results are summarized.

In the original publication [...]

Archive of applied mechanics (2021). doi:10.1007/s00419-021-01916-2, Published by Springer, Berlin ; Heidelberg [u.a.]

Zunehmend steigende Anforderungen an die Effizienz und Leistung elektrischer Maschinen für den Einsatz in Fahrzeugen machen neue Herstellungsverfahren für Statorwicklungen notwendig. Daher wurde in Zusammenarbeit mit der RWTH Aachen University sowie den beiden Firmen Schaeffler und Breuckmann ein serientaugliches Herstellungsverfahren entwickelt, mit dem querschnittsvariable Einzelzahnspulen für den Einsatz in elektrischen Motoren mit hoher Drehmomentdichte hergestellt werden können. Increasingly high demands on the efficiency and performance of electrical machines for use in vehicles require new manufacturing processes for stator windings. Therefore, RWTH Aachen University collabaorated with the companies Schaeffler and Breuckmann to develop a manufacturing process suitable for series production, allowing for manufacturing variable-cross-section single-tooth coils for use in electric motors with high torque density.

Production engineering 14(1), 17-32 (2020). doi:10.1007/s11740-019-00949-5 special issue: "Damage controlled forming processes / Issue Editors: Gerhard Hirt, A. Erman Tekkaya …", Published by Springer, Heidelberg

Damage in the sense of voids influences material as well as product properties and thus is important for the performance of formed components. In this work, the influence of caliber rolling prior to a cold forward extrusion process in terms of damage evolution is investigated. To this end, an uncoupled Lemaitre-type damage model considering the effects of strain-rate and temperature dependent plasticity is employed. The damage-related parameters are identified in an inverse manner based on notched tensile tests. Numerical investigations of the forming processes show that damage increases throughout the process chain. The simulations are validated in terms of density measurements and microscopic investigations. The experimental measurements and numerical simulations are in good qualitative agreement. It is shown that the influence of rolling on subsequent cold forward extrusion is negligible for this experimental setup. For other process parameters, however, the influence could be significant.

Porosities, introduced into the material during continuous casting can impair the mechanical properties and performance of semi-finished material as well as final products. Healing of these porosities through closure and bonding of voids with subsequent dissolution of the interface in further hot working can reduce this detrimental effect. While large deformations under compressive stresses as well as high temperatures are beneficial for void healing, not all regions in the workpiece experience the same conditions. Gradients of these metrics in normal direction and over time create regions of varying probability for void healing. Knowledge on the process conditions and the void shapes in the respective regions is essential to predict void healing successfully and to produce a sound product, especially if the process window is limited. In the present work, a multiscale modelling approach is utilized to predict void closure and recrystallization along the normal direction in the roll gap. A practical void healing criterion is developed by combining these two mechanisms and then used to classify a typical workpiece in normal direction according to the likelihood of void healing. Optimal void healing conditions are found at a relative height of 80% while the center region (50%) of the workpiece exhibits the least favorable conditions for void healing.

Continuous casting leads to the formation of voids, which can be considered as an initial state of damage. During hot rolling these voids shall be closed, reducing this initial damage and avoiding its negative influence on the mechanical properties and performance of the produced material. However, hot rolling also influences the microstructure, which in turn affects the performance e.g. the fatigue strength. So far, little research has been published on the separation of those influencing factors. This paper is a first attempt to separate the influence of damage from the microstructure in hot rolling. Numerical simulation is utilized to study void closure throughout the multi-pass process while the evolution of microstructure and damage is investigated experimentally using numerous characterization methods. Two process routes with a large and a small pass reduction have been investigated and comparable microstructures have been achieved. A continuous damage reduction throughout the rolling process has been observed by means of void distribution and density measurements. The large pass reduction showed a slightly reduced damage and an increased fatigue strength in all considered thicknesses, however this could not be traced back to the damage reduction exclusively.

The amount of damage induced during hot forming depends not only on the stress state but also on material softening due to dynamic recrystallization (DRX), which can be used to delay or reduce damage initiation. An extension to the Gurson–Tvergaard–Needleman (GTN) model has been proposed recently that includes a new coupled DRX-damage nucleation criterion taking DRX and the stress state into account. In the present paper, a 4-pass caliber rolling process is considered where the bar stock is rotated by 90° after each rolling pass. The extended GTN model is applied to the caliber rolling process to predict the internal damage induced during the rolling process. The simulation results show that the highest void volume fraction (VVF) occurs during the first rolling pass. However, the variations of stress state during each pass assist in reducing the damage in the subsequent passes. The material softening due to increasing DRX in subsequent passes also helps to reduce the void nucleation. The microscopic analysis of the rolled bar confirms the damage distribution predicted by the simulations.

Ingots from casting processes usually contain defects such as voids from solidification. These voids can lead to failure during application, e.g., crack formation. Accordingly, it is important to close these voids in the forging process. Predicting the closure and healing of voids during forging has been the subject of numerous studies, most of which assume a constant loading direction. As multiple-pass open-die forging processes usually include workpiece rotations, in this study, an existing criterion found in the literature for the prediction of void closure is extended to consider changing loading directions. For model validation, an AISI 4140 ingot with artificial voids is forged, and the void volume is investigated by radiographic analysis. The results confirm the improvement in the model prediction provided by the proposed extension.

Purpose The paper aims to investigate the possibilities to control friction in lubricated systems by surface patterning, making use of a multi-scale approach. Surface patterns inside the tribological contact zone tend to directly reduce friction, whereas surface patterns located in the close proximity of the contact area can improve the tribological performance by avoiding lubricant starvation and migration. Finally, optimized surface patterns were identified by preliminary laboratory tests and transferred to a journal bearing, thus testing them under more realistic conditions. Design/methodology/approach Surface patterns on a large scale (depth > 10 µm) were fabricated by micro- and roller-coining, whereas surface patterns on a small scale (depth < 2 µm) were produced by direct laser interference patterning. The combination of both techniques resulted in multi-scale surface patterns. Tribologically beneficial surface patterns (verified in ball-on-disk laboratory tests) were transferred onto a journal bearing’s shaft and tested on a special test-rig. To characterize the lubricant spreading behavior, a new test-rig was designed, which allowed for the study of the lubricant’s motion on patterned surfaces under the influence of a precisely controlled temperature gradient. Findings All tested patterns accounted for a pronounced friction reduction and/or an increase in oil film lifetime. The results from the preliminary laboratory tests matched well, with results from the journal bearing test-rig, both tests showing a maximum friction reduction by a factor of 3-4. Numerical investigations, as well as experiments, have shown the possibility to actively guide lubricant over patterned surfaces. Smaller periodicities, as well as greater structural depths and widths, led to a more pronounced anisotropic spreading and/or greater spreading velocities. Multi-scale surfaces demonstrated the strongest effects regarding the lubricant’s spreading behavior. Originality/value Friction, as well as lubricant migration, can be successfully controlled by using micro-coined, laser-patterned and/or multi-scale surfaces. To the best of the authors’ knowledge, the study demonstrates for the first time the unique possibility to transfer results obtained in laboratory tests to a real machine component.

Open-die forging is an incremental bulk metal forming process providing excellent mechanical properties and lifetime. Therefore, open-die forged parts are used for highly loaded application purposes e.g., in energy technology. However, the range of producible geometries in open-die forging is limited mainly to long and straight workpieces with simple cross-sections. Here, open-die forging with superimposed manipulator displacements offers a large potential for the production of curved workpieces. The new process concept is based on the idea, that the manipulator, originally designed as a handling device, is now actively used to control the material flow towards an intended final geometry. The idea was successfully realized for the production of curved workpieces by open-die forging. The hereby-presented investigations now aim at an analysis of the process limits for bending in vertical direction. Based on a fundamental analysis, a tilting of the samples between the forging dies and an undesired contact of the curvature with the forging die were identified as main process limits leading to a reduction of the curvature. By numerical simulation and fundamental geometric analysis, a process window was identified, where the occurrence of process limits and thus a reduction of curvature can be avoided. Finally, experimental validation was carried out at the industrially scaled open-die forging setup at IBF proving that the identified process window can successfully be realized.

Wear is the main mechanism that reduces the lifetime of forging dies used in closed die forging. A newly proposed approach to decrease die wear is the concept of protective sheet metal forging die covers, where an inexpensive and easy-to-exchange sheet metal die cover is used to protect forging dies. After this concept has been fundamentally validated by previous studies, the presented work aims to further evaluate the protective effects of this concept regarding wear reduction. First, an application-oriented experiment of the die cover concept on forging dies for producing square flange was conducted. On both the forging dies with and without die covers 100 forging strokes were carried out. The wear depths of both forging dies were then measured and compared. The results indicated that the forging die with die covers has up to 98% less wear than the forging die without die covers. The expected tool life of the forging die with die covers is therefore 600% longer. In addition, the die cover applied in the experiment achieved the service life of 100 forging cycles without being distorted, which reached the maximum service life of the die cover concept so far.

The present article describes design, architecture, and implementation of the Aachen (“Aix”) Virtual Platform for Materials Processing—AixViPMaP®. This simulation platform focuses on enabling automatic simulation workflows in the area of microstructure evolution and microstructure property relationships by continuum models. Following a description of a variety AixViPMaP® functionalities like user management, the currently implemented software tools, simulation workflows, data storage, grid infrastructure, and many more, some example workflows which have been run on AixViPMaP® are presented in detail. These workflow examples—although each being specific—can readily be transferred to other materials or to similar processes as the major simulation tools used in these workflows are all generic and thus applicable to a wide range of metals and technical alloys. The article concludes with a discussion on the performance and benefits of the platform, an outlook on its future development and on its open, future availability for both academic and commercial use.

Steel research international 2200327 (2022). doi:10.1002/srin.202200327, Published by Wiley-VCH-Verl., Weinheim

Bistable fully closed shells can serve as long supporting structures that can be folded into a compact transport geometry and unfolded at the construction place. Bistability is achieved by introducing a specific distribution of residual stresses through the thickness of the shell, e.g. by incremental die-bending. In order to find a suitable bending radii combination a semi-analytical model was developed and experimentally validated for the steel 1.1274 in previous research. Nevertheless, minor deviations have occurred in the prediction of final curvatures of the different stable geometries and it is still unclear to what extent other influencing variables such as shell thickness or material properties influence the achievability of fully closed bistable shells. Therefore, in this paper, an enhancement and generalization of the existing semi-analytical model for different steels is described and the extended model is used for a comprehensive analysis of the influence of different variables on bistability and final shell geometries.

Aluminum-silicon-coated (AlSi-) boron-steels are regarded as standard material in direct hot stamping processes. Nevertheless, the material brings along some disadvantages such as the requirement of a long dwell time in the furnace for the generation of a resistant diffusion layer. In this paper, a multilayer steel sheet with a boron-steel core-layer and stainless steel outer-layers is introduced and the manufacturing process by hot roll-bonding is described. Due to the stainless steel surfaces, a coating for hot stamping is no longer necessary. The multilayer is characterized by hot tensile tests and compared with the monolithic multilayer-partners. Hot stamping experiments are conducted on a laboratory scale. Corresponding hardness measurements show that the core-layer is hardened while the outer-layers remain ductile. The new multilayer sheets offer the potential to deliver components with higher formability due to tailored properties along the sheet thickness and the use of rapid heating methods.

Besides achieving the intended final shape, one main aim of open-die forging is the adjustment of the mechanical properties by transforming the cast structure into a fine-grained microstructure. To achieve this, the process needs to be designed in a way that ensures achieving all the required part properties, such as grain size, which up to now often requires a lot of operator experience. This paper presents the concept of a forging assistance system, since during forging small deviations from the previously designed pass schedule might add up to unacceptable errors. Such an assistance system requires the evolution of part geometry and surface temperature as input, which are captured with a thermographic camera. The assistance system then uses fast models for equivalent strain, temperature, and microstructure which allow calculation of these properties for the core fibre within seconds on the basis of semi-empirical and physical formulae. However, in the context of an assistance system, which gives real-time advice in case of process deviations, these calculation times are still fairly long, if the hundreds of iteration necessary for process optimization are taken into account. Therefore, three scenarios of deviations, which have to be solved within different time frames, are examined to explore the limits of the chosen classical optimization algorithm.

Bistable fully closed shells can serve as long supporting structures that can be folded into a compact transport geometry and unfolded at the construction place. Bistability is achieved by introducing a specific distribution of residual stresses through the thickness of the shell, e.g., by incremental die-bending. In order to find a suitable bending radii combination a semi-analytical model was developed and experimentally validated for the steel 1.1274 in previous research. Nevertheless, minor deviations have occurred in the prediction of final curvatures of the different stable geometries and it is still unclear to what extent other influencing variables such as shell thickness or material properties influence the achievability of fully closed bistable shells. Therefore, in this paper, an enhancement and generalization of the existing semi-analytical model for different steels is described and the extended model is used for a comprehensive analysis of the influence of different variables on bistability and final shell geometries.

The industrial application of incremental sheet forming (ISF) still stays behind the expectations due to the low geometrical accuracy of the produced parts as well as the insufficient predictability of the forming result. This is due to the fact that some mechanisms which cause the geometrical deviations are not fully understood. Many investigations have already been carried out to determine which types of stresses induced by ISF influence the forming result. This paper tries to systematically categorize selected typical geometric deviations and to review their underlying mechanisms based on current literature as well as their own experience. The intention is to provide a structured basis for future scientific discussion and to stimulate an exchange of experience, with the goal that a better understanding may help to improve the geometrical accuracy of parts manufactured by incremental sheet forming.

In addition to the electromechanical properties, the design of the stator windings for electric drives depends strongly on the available installation space. In case of the wheel hub engine, the installation space is strongly limited by the rim diameter and the tyre width. Hence, stator windings for use in wheel hub engines should meet two requirements: On the one hand, the stator slot should be optimally filled with electrically conductive material so that a high torque can be guaranteed despite limited installation space. On the other hand, the end windings should be as compact as possible in order to ensure the greatest possible active length of the stator core. Conically shaped concentrated windings meet these requirements and are therefore suitable for use in wheel hub drives. In a cooperation between the Breuckmann company and the Institute of Metal Forming (IBF) of RWTH Aachen University, a multi-stage forming technology has been developed which enables the production of such a winding geometry. In this paper an automated multistage forming tool is described, which allows for an economical mass production of conically shaped concentrated windings. In addition, a 2D FE model is presented, which models the wire cross sectional development during forming. In this way, a suitable number and geometry of the forming stages could be designed prior to tool production and tool costs saved accordingly. Due to the high load on the primary insulation caused by the forming process, the effect of the forming process on the insulation layer thickness is investigated and alternative insulation processes after forming are described.

Action extraction from teleoperated robots can be a crucial step in the direction of full -or shared- autonomy of tasks where human experience is indispensable. This is especially important in tasks that seek dynamic goals, where a human operator needs more control on how the machine behaves to provide assistance or perform a task. Open-die forging is a basic metal-forming process that lacks non-destructive product quality measures. Human experience is therefore imperative. During the process, a robot-arm is operated to place the work-piece between the dies of the forge where it is striked several times to reach a specific geometry. In this paper, we apply a white-box computer vision technique to discretize open-die forging robot-arm teleoperation data into actions as a step in learning the operator’s behavior.

Model predictive control (MPC) has been used in a variety of industrial processes. Due to its large computation time application to fast and complex processes is limited. Therefore, reducing complexity has been focus of intense research. In this paper, a complexity reduction strategy for a linear MPC is developed. It is used for the control of an over-actuated roll gap with two different actuator types in a cold rolling mill. One of the actuators is able to accept new set points only at a much slower sample time than the fast actuator. In order to coordinate the actuators, a moving manipulation point scheme is introduced where a single time-varying optimization variable of the slow actuator moves through the control horizon of the fast actuator. The fast actuator ensures reference tracking regarding the roll gap when the slow one is idle. The proposed scheme reduces the number of optimization variables as well as constraints and thus enables control of faster processes. Simulation results show the proof of concept and an enhancement in computation time. Moreover, a real-time implementation is demonstrated on a cold rolling mill.

Open-die forging is an incremental forming process, which is mainly used for the production of large parts with high requirements regarding the mechanical properties and reliability of the forged parts. Finite element analysis (FEA) is able to simulate open die forging sequences. It is therefore very suitable to confirm, whether a selected schedule will be successful in terms of reaching the desired geometry and internal product quality. However, it is comparably slow and therefore not suitable for early process design, when out of an almost infinite number of potential sequences of strokes, an appropriate pass schedule needs to be designed. This is today usually achieved by pass schedule planning software, which takes into account volume constancy, empirical spread behavior and average temperature evolution. However, they do not account for product quality characteristics like microstructure and voids closure. In this paper recently developed fast models, which are able to calculate the temperature, equivalent strain and microstructure evolution along the core fibre of a forged workpiece are coupled with an optimization algorithm to allow automatic pass schedule layout and optimization. Different cost functions are evaluated regarding their impact on the resulting properties of the workpiece. The results indicate that for an overall optimization of open-die forging processes different phenomena and influencing parameters need to be considered, since all of these parameters have a significant influence on the resulting properties such as equivalent strain, temperature and grain size of the ingot.

Bistable fully closed metallic shells are of great interest as lightweight construction components, which could be transported without enclosure and unfolded at the construction place. The effect of bistability in metallic shell structures is achieved by a specific distribution of residual stresses over the shell thickness, which can be introduced by bending processes. So far no fully closed structures have been achieved in metallic shells. Hence, the aim of this paper is the identification of suitable bending radii combinations in order to produce a bistable shell with a fully closed tube shape in the deployed state. Therefore, a Finite Element (FE) model was established to determine whether bistability occurs in dependence on the bending radii. The numerical model indicates, that bistability can be achieved for 0.2 mm thick spring steel 1.1274 (AISI 1095) by applying two die-bending operations with small bending radii of 6–8 mm. Accompanying experiments with a closed-die incremental bending tool were performed. A good transferability of the numerical results regarding the occurrence of bistability and the calculated residual stresses is shown. For the second aspect, X-ray diffraction measurements of residual stresses were used for comparison. Finally, the present work presents a process route to generate a bistable shell with a fully closed tube shape structure in the deployed state, having a radius of 40.95 mm.

Improving tool life is one of the main challenges and research objectives in closed-die forging. One approach, first proposed in (Rosenstock et al. 2015) uses an inexpensive and easy-to-exchange sheet metal die cover, which is placed on the die surface during forging. By using this cover, die wear can be reduced since mechanical and thermal loads mostly affect the die cover instead of the die itself. Numerical and experimental investigations (Wolfgarten et al. 2015; Yu et al. 2016) have proven the general key features of the concept and the positive impact on the lifetime of the forging die. However, these studies have shown that the applicability of the concept and the corresponding lifetime of the die cover are strongly dependent on the investigated die and die cover geometries. For simple 2D die cover geometries, where the die cover can be produced by a simple bending operation, the maximum die cover lifetime of seven forging cycles was achieved. Since these geometries offer low structural stability, the die covers are prone to deformation or folding during forging. Hence, this work investigates the application of die covers in complex geometries. For this purpose, a 3D die cover geometry was proposed that offers higher structural stability and less challenging contact conditions regarding the material flow during forging. The die cover successfully experienced 40 forging cycles without visible distortions or folds. Regarding the thermal loads, the maximum temperature and the temperature amplitude measured in the forging die were reduced by 40 °C. Based on the experimental results, a numerical simulation model was built and validated regarding the temperature evolution and forging forces. The numerical model indicates a significant reduction in the mechanical loads on the dies through the application of the die cover.

The process of roller levelling is essential to ensure flatness of sheet metals and to reduce residual stresses within the material. Besides process parameters like machine geometry and roll positions, the process is sensitive to variations of material properties. In order to detect variations of the material properties, an inverse identification of material parameters from measured process data is desirable. Therefore, a semi-analytical levelling model based on bending theory that is capable to describe material behavior under low cyclic loading is introduced. This model calculates the levelling forces depending on given material data. Employing the model within an optimization loop, the model allows for an inverse determination of material data during levelling by comparing calculated and measured levelling forces. To validate the model, accompanying finite element (FE) simulations, levelling experiments as well as cyclic bending tests are performed. According to the results, the semi-analytical model is capable of calculating levelling forces in good agreement with FE results and much lower computation times. Furthermore, material data obtained directly from levelling experiments are in the same range as data determined from cyclic bending tests.

The present study experimentally examines the effect of selected single-scale and multi-scale surface patterns fabricated by roller-coining and/or direct laser interference patterning on the frictional performance of journal bearings. For this purpose, surface patterns showing beneficial effects in preliminary laboratory tests were selected and fabricated onto the shaft of journal bearings made of stainless steel (AISI 304). The frictional performance of these patterns was evaluated on a special test rig by recording Stribeck-like curves. The results show greatly reduced coefficients of friction and a shift in the transition from mixed to hydrodynamic lubrication to smaller rotational speeds for all patterned samples compared to the reference sample. The observed friction reduction matches well with results observed in the previous laboratory tests.

Guiding lubricant back to the tribological contact or preventing lubricant migration out of the contact zone can be considered as an efficient approach to significantly reduce friction and wear in machine components. This paper aims at studying the spreading behavior of an additive-free lubricant (PAO 4) on coined stainless steel surfaces (AISI 304) under the effect of a controlled temperature gradient of 2 °C/mm. Single channels and multi-channel samples were manufactured by roller coining and hot micro-coining, respectively. A systematic study of the influence of the geometrical parameters on the resulting spreading behavior has been performed. For polished reference samples, a preferential oil spreading parallel to the temperature gradient was observed which can be correlated with Marangoni forces. For single channels, the spreading velocity increases with an increase in structural depth. Multi-channels show a pronounced anisotropic spreading behavior. Lubricant migration towards the colder side of the sample can be prevented if the structures are oriented perpendicular to the temperature gradient. For multi-channel surfaces, the spreading behavior parallel to the temperature gradient is greatly influenced by the periodicity and area density. Samples with a smaller periodicity and greater area density show a higher spreading velocity of a droplet.

In cold forming of aluminum, various lubricants and coatings are typically used to reduce friction and wear, resulting in higher workpiece surface quality. The preparation of the workpiece surfaces and the cleaning of the products after the forming step generate a significant amount of environmentally hazardous residues. Therefore, current research focuses on the realization of dry metal forming processes. Instead of lubricants, modified tool surfaces can also optimize tribological conditions in the interaction zone of forming tool and workpiece. The applicability of these surfaces needs further examination before usage within an industrial manufacturing process. In this paper, different surface modifications are examined by using a conical tube-upsetting test setup that is based on the concept of the well-known ring-compression test. The conical tool surface homogenizes the relative displacement between tool and workpiece and suppresses the appearance of a neutral point. Conical tools from AISI H11 / DIN 1.2343 and AISI D2+ / DIN 1.2379+ are laser polished and functionalized with self-assembled monolayers. Friction conditions resulting from different surface modifications are analyzed and evaluated by the use of nomograms. Moreover, the applicability of different friction laws for dry metal forming of aluminum is investigated.

Roll bonding is a joining-by-forming process to permanently join two or more layers of different materials by hot or cold rolling. One of the typical industrial applications is aluminium sheets for heat exchangers in automobiles. During roll bonding the layers are fed into the rolling stand with parallel surfaces. Due to the plastic deformation in the roll gap metallic bonds between the layers are achieved. Several theoretical models have been published to describe the process, e.g. Zhang & Bay. These models have mostly been developed for cold rolling and describe the bond strength based on surface enlargement, contact pressure and flow stress. Since these models are developed for cold rolling, they are not temperature depending. Heat exchange is usually neglected and de-bonding after the roll gap is not accounted for. However, for hot roll bonding the above mentioned assumptions do not hold true. To understand the mechanisms of hot roll bonding industrial and laboratory scale investigations have previously been conducted. Based on the findings a FE framework for hot roll bonding was developed. This FE framework accounts for the possibility of de-bonding after the roll gap but is restricted to isothermal conditions. However, for a roll bonding simulation it is essential to take the temperature influence into consideration. Therefore, this paper presents an extended version of the FE framework which accounts for temperature dependent material flow, compatible definition of thermal & mechanical interactions and bonding status related heat exchange. To verify the new features of the extended FE framework a roll bonding test case is employed. Mechanical and thermal interactions as well as the current flow stress are calculated in subroutines in order to enable a fully coupled thermal stress simulation. The results show that with this extended FE framework the influence of non-isothermal conditions on material flow and bonding status as well as the feedback effects of bonding status to heat exchange have been successfully integrated in hot roll bonding simulations. This fully coupled thermal stress simulation is the first step towards multi-pass roll bonding simulations.

Particularly in fast rolling mills, conventional actuators reach their dynamic limits, when longitudinal thickness variations of the incoming strip shall be reduced with high accuracy by model-predictive roll gap control. Accordingly, the applicability of highly dynamic piezoelectric actuators in combination with electromechanical spindles and a high frequency precision measurement of the thickness in front of the roll gap was examined. Rolling tests in a cold rolling mill for narrow slit strips show that this novel concept is suitable to provide the required dynamic actuation especially at high rolling speed.

Advances in industrial and manufacturing engineering 3, 100045 (2021). doi:10.1016/j.aime.2021.100045, Published by Elsevier ScienceDirect, [Amsterdam]

Purpose The purpose of this study is to investigate the influence of hemispherical structures fabricated by hot micro-coining on the resulting wear performance. Hemispherical structures with different area densities (20 and 30 per cent), depths (50 and 100 µm) and diameters (100 and 200 µm) were fabricated by hot micro-coining on stainless steel samples. Design/methodology/approach The wear performance of these samples was studied using a ball-on-disk tribometer in rotational sliding mode using a normal load of 30 N and a fixed sliding velocity of 2 cm/s. Two different poly-(alpha)-olefin (PAO) oils without any additive having a kinematic viscosity of 4 and 40 cSt, were used to study the influence of the oil viscosity on the wear behavior. Findings Concerning the polished reference, an enlarged wear volume with an increase in the cycle number and the oil viscosity was observed. In the case of the micro-coined surfaces, all samples demonstrate a pronounced reduction in the wear volume (up to a factor of 100 for PAO 40) compared to the polished reference irrespective of the oil viscosity used. Originality/value This study details new research work studying the wear behavior of hot micro-coined surfaces.

Incremental Sheet Forming (ISF) has been developed as a flexible manufacturing technology for small batch production and prototyping. ISF can also be used to form additional features or stiffening elements such as hole flanges. Incremental Hole Flanging (IHF) operations seem to be a promising alternative to conventional hole flanging. If it was possible to exploit the extended formability of ISF while achieving accuracy and process times of conventional hole flanging, IHF could substitute conventional flanging operations in many cases. However, the long process times and limited geometrical accuracy hinder industrial take-up. In this work, two different tooling concepts which allow incremental hole flanging operations at high speeds are investigated. The first tool is designed as a single forming tool that offers high flexibility and a comparison to conventional Incremental Hole Flanging. The second tool consists of four forming tools to improve the geometrical accuracy of hole flanges. In order to achieve high speeds, the experimental setup is installed on a turning machine. Compared to hole flanging with a conventional CNC machine, the forming time to expand a hole from 50 mm to 100 mm could be reduced from 1680 s to 15.7 s. The geometrical accuracy of the parts formed with the second tool concept could be improved significantly (up to 3 times regarding to the mean surface deviation to at maximum speed). Furthermore, it is shown that forming at high speeds has no significant influence on the characteristics of sheet thickness, strain, forces or geometrical accuracy.

In case of wheel hub engines, the coil installation space is limited by the rim diameter and width. Due to their shape and a small end winding, conically shaped concentrated windings offer the possibility of increasing the torque density of wheel hub engines. After comparing different concepts, a multi-stage upsetting process seems to be suitable for an economical series production of these windings. In this paper a multi-stage tool, which allows for production of prototype windings from a pre-bent semi-finished product, is presented. The formability of enameled copper wires and the need for adjustment of the semi-finished product and the tool are investigated. Forming tests of first prototype coils show a necking of the deformed windings due to material flow in longitudinal direction of the wire. By adjusting the geometry of the semi-finished product, necking can be reduced. In addition, an adjustment of the prototype tool is presented, which allows for a narrowed cycle time of the upsetting process.

Piezoelectric actuators are used in a variety of technical processes. Due to their ceramic nature, they can be used in processes where high compressive loads occur. This enables them to be used as actuators for roll gap adjustment in a cold rolling mill for narrow strips. However, their local gain is significantly dependent on the current operating point with respect to applied voltage and loads. These fluctuations are investigated, quantified, and modeled in this paper within scope of the operation range of the actuators. The resulting model is used to develop a sliding mode controller with associated state and disturbance observer. The controller is used to impress a defined dynamic behavior on the actuators despite uncertain conditions and nonlinear disturbances. The experimental results show that the implemented sliding mode controller is advantageous in reference tracking compared to a formerly used PI-controller, regardless of the operating point.

Metals : open access journal 11(8), 1285 (2021). doi:10.3390/met11081285 special issue: "Special Issue "Modelling and Simulation of Microstructure Evolution in Manufacturing Processes" / Special Issue Editor: Prof. Dr. Hamid Assadi, Guest Editor", Published by MDPI, Basel

In forming simulations, flow curves are cardinal inputs to predict features, such as forming forces and material flow. The laboratory-scale experiments to determine them, like compression or tensile tests, are affected by deformation heating, restricting direct flow curve determination. In principle, the current analytical and inverse methods determine flow curves from these tests, but while the analytical methods assume a simplified temperature profile, the inverse methods require a closed-form flow curve equation, which mostly cannot capture complex material behavior like multiple recrystallization cycles. Therefore, the inverse piecewise flow curve determination method “FepiM” previously developed and published by the current authors is extended by introducing a two-step procedure to obtain isothermal flow curves at elevated temperatures and different strain rates. Thereby, the flow curve is represented as tabular data instead of an equation to reproduce complex flow curve shapes while also compensating the effect of inhomogeneous temperature profiles on the flow stress. First, a flow curve at the highest temperature is determined. In the second step, using this first flow curve as a reference, the flow curves at lower temperatures are obtained via interpolation. Flow curves from conventional compression tests for aluminum and copper in the temperature range of 20–500 °C are predicted, and it is shown that these flow curves can reproduce the experimental forces with a maximum deviation of less than 1%. Therefore, the proposed new piecewise method accurately predicts isothermal flow curves for compression tests, and the method could be further extended to highly inhomogeneous methods in the future.

Multi-material components made from aluminum and steel sheet metal are an innovative approach for weight reduction in automotive applications. However, lightweight components made from aluminum and steel require suitable joining technologies. A promising solid-state welding technology for producing dissimilar steel aluminum joints is Friction Stir Welding, which minimizes the formation of Fe-Al intermetallic phases due to process temperatures lower than the melting temperatures of the base material. The results obtained show a comparison of steel aluminum joints made by FSW using DC04 mild steel with the strain hardened aluminum alloy AA5754-H22 on the one hand and the precipitation hardened aluminum alloy AA6082-T6 on the other hand. The difference between achieved maximum tensile strengths of the joints in relation to those from both base materials is investigated. Due to the stirring and heat input of the welding process, the temper condition of the precipitation hardened aluminum alloy is changed. To improve the mechanical properties of the welded joints, post weld heat treatments are performed. The post weld heat treatments of the produced multi-material specimens from AA6082-T6 aluminum alloy and mild steel at various heat treatment conditions show substantial growth of intermetallic phase layer, which is characterized in detail within the present work. Tensile tests show a degradation of the mechanical properties resulting in a decreased tensile strength and insufficient connection of both materials. Investigations using a scanning electron microscope (SEM) show a distinct increase of the thickness of intermetallic phases in the transition between aluminum and steel.

International Conference on the Technology of Plasticity, ICTP 2017 : 17-22 September 2017, Cambridge, United Kingdom / Edited by Julian Allwood International Conference on the Technology of Plasticity, ICTP 2017, Cambridge, UK, 17 Sep 2017 - 22 Sep 2017; Amsterdam [u.a.] : Elsevier, Procedia engineering, 207, 1582-1587 (2017). doi:10.1016/j.proeng.2017.10.1052, Published by Elsevier, Amsterdam [u.a.]

International Conference on the Technology of Plasticity, ICTP 2017 : 17-22 September 2017, Cambridge, United Kingdom / Edited by Julian Allwood International Conference on the Technology of Plasticity, ICTP 2017, Cambridge, UK, 17 Sep 2017 - 22 Sep 2017; Amsterdam [u.a.] : Elsevier, Procedia engineering, 207, 437-441 (2017). doi:10.1016/j.proeng.2017.10.801, Published by Elsevier, Amsterdam [u.a.]

Transferring the principle of roll bonding to ring rolling could enable production of seamless radial composite rings tailored to specific applications. This process was further studied experimentally and numerically via FE simulations of the ring rolling process, which consider bonding and debonding behaviour of the rings by use of a contact subroutine. Model experiments used to estimate the material bond strength during the process show strong bonding behaviour. However, no bonding between both rings was observed after rolling trials. Using FE simulations, possible explanations for failure to achieve a lasting material bond between both rings are identified and necessary steps to successfully produce a composite rolled ring are introduced.

High-efficient, fully-automated strip processing lines rely on homogeneous conditions of the incoming metal strip. Roller levelling machines are therefore placed in the initial stage of sheet processing lines to ensure both the strip flatness and a desired residual stress distribution. Even though a sheet metal appears perfectly flat, the residual stress distribution may be disadvantageous and lead to flatness defects in the downstream production. Furthermore, the material properties of the metal strip may change along its length and make an adaption of the leveller’s setting necessary. To link both target values of sheet flatness and residual stress distribution, a numerical model of a seven-roll leveller is used to determine roll positions resulting in a flat sheet and a defined residual stress distribution. Thus, correlations between the initial sheet properties, the machine settings and the final sheet properties can be derived. Additionally, the influence of changing material parameters is shown. All results will finally be used to provide a reference for a feed-forward process control based on a force measurement in the first load triangle of a roller leveller. In order to validate the numerical results, experiments on a down-sized roller leveller are conducted which show that the changes of the forces in each load triangle correspond to the numerical results when changing both the initial sheet state and the intermesh. Thus, the general potential of changing the residual stress distribution via roller levelling can be investigated using numerical calculations.

For the electrical industry, clamps and springs made from strips and wires are produced by stamping and forming technologies. To realise complex geometries, multi-stage stamping and bending processes with cycle rates of up to 500 parts per minute are utilised. In the field of small-sized components, the demand for load optimised lightweight structures made from high strength materials increases. Due to the complexity of forming processes and materials used for clamps and springs, FE simulation of those processes is a challenging task. The first objective of the presented investigations is a holistic view of the influencing factors of multi-stage stamping and bending processes for springs and clamps. Therefore, clamps from high strength spring steel (X10CrNi18-8) with a thickness of 0.30 mm are formed on a hand-operated stamping and bending machine. In the experimental and corresponding numerical investigations, the gap between the tools, the tool radii as well as tool strokes are varied to evaluate the effects on the bending results. The main influencing factor on the product geometry is the tool stroke. The second objective of this work is to examine the transferability of the obtained results to production conditions. On a production machine, tool strokes were measured using different cycle rates. Regardless the cycle rate, the measured tool strokes were up to 2% shorter than defined in the process design. Simulations with measured tool strokes significantly enhance the predicted geometry after springback. The comprehensive goal is to describe the required geometrical accuracy in the FE model while minimising the required investigations to receive real data instead of nominal one. Thus, the industrial implementation of FE models for stamping and bending processes is accelerated.