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Materials science and engineering / A 828, 142056 (2021). doi:10.1016/j.msea.2021.142056, Austenitic high manganese steels exhibit outstanding mechanical properties, such as high energy absorption, owing to various deformation-mechanisms such as dislocation slip, twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP). Here, we show a novel thermomechanical treatment to manufacture a high manganese steel Fe���18Mn-0.3C (wt.-%) with excellent mechanical performance by combining these three deformation-mechanisms. This process of mechanism-controlled rolling resulted in ultra-high tensile strength of the high manganese steel up to 1.6 GPa, simultaneously with uniform elongations up to 15%.A thermomechanical process was developed to establish this combination of properties. Warm rolling was conducted at 200 ��C, to suppress TRIP and activate TWIP as deformation mechanism. Thus, a high density of deformation twins and dislocations was introduced to the microstructure, avoiding martensite formation. During a subsequent recovery annealing at 520 ��C or 550 ��C, the dislocation density was reduced, yet the high density of deformation twins was preserved. The combination of warm rolling and recovery annealing resulted in an ultrafine microstructure with a high density of twins and moderate density of dislocations. The TRIP effect is predominant during plastic deformation at ambient conditions in the highly twinned microstructure. The resulting steel exhibits an ultra-high yield strength and sufficient ductility, favorable properties for lightweight construction in automotive or aerospace industry., Published by Elsevier, Amsterdam

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

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.

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.

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.

Materials 14(22), 7055 (2021). doi:10.3390/ma14227055 special issue: "Special Issue "Low-Loss Nonoriented Electrical Steel Sheet for Energy-Efficient Electrical Drives" / Special Issue Editor: Prof. Dr. Rudolf Kawalla, Guest Editor", Published by MDPI, Basel

Materials 14(21), 6588 (2021). doi:10.3390/ma14216588 special issue: "Special Issue "Low-Loss Nonoriented Electrical Steel Sheet for Energy-Efficient Electrical Drives" / Special Issue Editor: Prof. Dr. Rudolf Kawalla, Guest Editor", Published by MDPI, Basel

6th IFAC Workshop on Mining, Mineral and Metal Processing, MMM 2021, Nancy, France, 1 Sep 2021 - 3 Sep 2021; IFAC-PapersOnLine 54(11), 109-114 (2021). doi:10.1016/j.ifacol.2021.10.059 special issue: "6th IFAC Workshop on Mining, Mineral and Metal Processing MMM 2021 : Nancy, France, 1-3 September 2021 / Edited by Daniel Sbarbaro", Published by Elsevier, Frankfurt

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.

This manuscript aims at investigating the spreading dynamics of an additive-free Poly-(alpha)-olefin (PAO) oil on stainless steel surfaces (AISI 304) having multi-scale surface patterns with and without an applied temperature gradient. For this purpose, single-scale patterns were fabricated by micro-coining, direct laser interference patterning and ultrashort-pulse laser patterning. In a second step, multi-scale patterns were realized by superimposing micro-coined surfaces with the respective laser patterns. In order to precisely adjust the temperature gradient and to study the lubricant's migration on the patterned surfaces, a specially designed test rig was used. Experiments without temperature gradient demonstrated a preferential lubricant spreading parallel to the pattern for all samples. In this context, the multi-scale samples showed a faster lubricant spreading compared to the single-scale patterns, which can be attributed to additional capillary forces and an increased roughness. In case of an applied temperature gradient, purely micro-coined samples showed an increased rate of lubricant spreading compared to the other single-scale samples due to the larger volume to surface area ratio. The multi-scale surfaces demonstrated again the fastest lubricant spreading, which clearly underlines the ability to actively guide lubricant by multi-scale patterning. Keywords: Lubricant spreading, Micro-coining, Laser patterning, Multi-scale, Wetting

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.

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

In this work, a Fe23Mn0.3C1Al high manganese twinning-induced plasticity (TWIP) steel is subjected to varying warm rolling procedures in order to increase the yield strength and maintain a notable ductility. A comprehensive material characterization allows for the understanding of the activated deformation mechanisms and their impact on the resulting microstructure, texture, and mechanical properties. The results show a significant enhancement of the yield strength compared to a fully recrystallized Fe23Mn0.3C1Al steel. This behavior is mainly dominated by the change of the active deformation mechanisms during rolling. Deformation twinning is very pronounced at lower temperatures, whereas this mechanism is suppressed at 500 &deg, C and a thickness reduction of up to 50%. The mechanical properties can be tailored by adjusting rolling temperature and thickness reduction to desired applications.

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

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.

The design of industrial hot metal forming processes nowadays is mostly carried out using commercial Finite Element (FE) software codes. For precise FE simulations, reliable material properties are a crucial factor. In bulk metal forming, the most important material property is the materials flow stress, which determines the form filling and the necessary forming forces. At elevated temperatures, the flow stress of steels is determined by strain hardening, dynamic recovery and partly by dynamic recrystallization, which is dependent on strain rate and temperature. To simulate hot forming processes, which are often characterized by rapidly changing strain rates and temperatures, the flow stress is typically derived from flow curves, determined at arbitrary constant temperatures and strain rates only via linear interpolation. Hence, the materials instant reaction and relaxation behavior caused by rapid strain rate changes is not captured during simulation. To investigate the relevance of the relaxation behavior for FE simulations, trails with abrupt strain rate change are laid out and the effect on the material flow stress is analyzed in this paper. Additionally, the microstructure evolution due to the strain rate change is investigated. For this purpose, cylinder compression tests of an industrial case hardening steel are conducted at elevated temperatures and different strain rates. To analyze the influence of rapid strain rate changes, changes by one power of ten are performed at a strain of 0.3. As a reference, flow curves of the same material are determined at the initial and final constant strain rate. To investigate the microstructure evolution, compression samples are quenched at different stages, before and after the strain rate change. The results show that the flow curves after the strain rate change tend to approximate the flow curves measured for the final strain rate. However, directly after the strain rate change significant differences between the assumed instant flow stress and the real material behavior can be observed. Furthermore, it can be shown that the state of dynamic recrystallization at the time of the strain rate change influences the material response and relaxation behavior resulting in different slopes of the investigated flow curves after the strain rate change.

Nowadays, optimization of non-oriented (NO) electrical steels toward lower iron-loss, improved, and isotropic magnetizability is critical to the improvement of rotating electrical machines. The whole production process chain adjusts the microstructure evolution, e.g., grain size and crystallographic texture, determining the magnetic properties. In particular, the interdependence of raw material properties and the resulting mechanical stress distribution during final assembly, e.g., punching, leading to magnetic property deterioration is crucial for the optimization of NO steel properties of rotating machines. This paper studies the effect of different cold rolling strategies, annealing treatments, and sheet metal blanking (punching) regarding microstructure evolution, magnetic properties, and deterioration.

Hemispherical structures with different area densities and depths were fabricated by hot micro-coining on stainless steel samples. The tribological properties were studied using a ball-on-disc tribometer in rotational sliding mode with different sliding velocities in order to study the frictional behaviour under different lubricated conditions. For low sliding velocities (mixed EHL), all coined surfaces demonstrate a significant friction reduction, whereas only some of the structures lead to an improved frictional behaviour under full-film EHL (higher velocities). Furthermore, the influence of an additional pressure build-up and cavitation is discussed. The largest improvement (friction reduction by a factor of 3 for all velocities) is achieved using a pattern with a pocket depth of 25 μm and an area density of 5%.

Understanding the interdependencies of the processing steps of high silicon containing non-oriented (NO) electrical steel is a key factor for improving its final magnetic properties. Moreover, every process step in the production has a significant impact on the properties. Steels with a high silicon content are ferritic (α-Fe). Accordingly, the microstructure and texture are inherited from one processing step to the next. The presented research relates the hot strip microstructure and texture to the magnetic properties of the final annealed material. To investigate the impact of hot strip microstructure and texture for a ferritic 2.4 m.%Si silicon containing steel, the material was hot rolled from an initial thickness of 64 mm down to a thickness of 1 mm whereby different microstructures and textures were achieved by varied hot rolling conditions. Subsequently, the material was cold rolled to 0.25 mm and annealed at 800 °C or 1000 °C for 3 min. Finally, samples of each processing step were analyzed by means of light microscopy and XRD texture measurements. Samples after hot rolling and final annealing were tested in a single sheet tester for specific losses and magnetic polarization. Findings show an influence of the initial hot strip microstructure on the microstructure and magnetic properties of the final annealed samples.

Roll bonding is one of the most prevalent industrial processes to produce permanently joined metal products. Especially aluminum is often roll bonded for automobile or aerospace applications due to its low density and high corrosion resistance. During roll bonding, two or more layers of different materials are stacked and joined during several rolling passes. In the case of aluminum, the deformation in the roll gap leads to a fracture of oxide and film-layers enabling virgin metal-metal contact resulting in a firm permanent bond. However, the industrial process consists of multiple passes and corresponding inter-pass times. Since the bond forms in the first passes and persists throughout the process, it is subjected to multiple height reductions and waiting periods during inter-pass times. Therefore, an investigation of the influence of multi-pass conditions that enable microstructural changes, such as diffusion and recrystallization, on the bond strength is necessary. This paper hence investigates the observed bond strength as a function of waiting periods and multiple height reductions. To conduct the experiments, a plastometer is used, employing two opposing samples with the same geometry made of the two aluminum alloys to be bonded. This setup enables the bond formation and subsequent characterization of bond failure in one experiment while allowing precise control of the deformation conditions, such as temperature and height reduction, which can accurately resemble conditions during the industrial process. The results show that inter-pass times can lead to an increased bond strength especially when coupled with annealing. Multiple height reductions generally lead to a lower bond strength compared to a single height reduction of the same overall height reduction. Therefore, the whole pass schedule contributes to the final bond properties and their evolution. This insight can ultimately help to improve existing or open new production routes in the future.

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.

Due to increasing requirements regarding the flatness of sheet metals, the process of roller levelling is of particular importance. The process itself is influenced by a high number of parameters such as machine design, sheet dimension, and material properties. Therefore, it is desirable to provide an online process control to react on changes of those process parameters. One possible approach for the layout of a process control and the identification of reference values is the use of the Finite Element Method (FEM). Considering the alternate bending a sheet metal undergoes when passing through a roller leveller, kinematic hardening of the sheet material must be taken into account. Additionally, the initial stress and strain distribution of the sheet metal – e.g. induced by coiling – has an influence on the material behaviour and consequently on the process parameters. With respect to these effects, a coupled FE model, which accounts for the initial state of the sheet metal, is introduced. An inverse calculation of material parameters describing the behaviour under cyclic load conditions has been done for an aluminium alloy AA5005 and a mild steel DC01. Based on this numerical setup, the influence of the initial stress state in the pre-levelled sheet metal on the roller levelling process has been deduced. Accompanying experiments on a down-sized roller leveller were carried out for a validation of the numerical setup.

In this work, a high-manganese Fe–29Mn–0.3C TWIP steel was produced by twin-roll strip casting and subjected to additional thermo-mechanical treatment. The evolution of microstructure and texture in each processing condition was investigated and correlated with the corresponding mechanical properties. Due to pronounced microsegregations, chemical gradients and an inhomogeneous microstructure in the as-cast and hot-rolled material, regions of strongly varying stacking fault energy caused undesired austenite–martensite transformation and inhomogeneous mechanical properties. The specimens after additional cold rolling and annealing at temperatures as low as 900 °C revealed a microstructure with a homogeneous grain size distribution and significantly reduced microsegregations. The resulting mechanical properties were comparable to those of industrial advanced high strength steels and TWIP steel produced by conventional processing, where energy-consuming homogenization is necessary. Therefore, twin-roll strip casting offers the possibility for cost-effective processing of TWIP steel with competitive mechanical properties.

Multi-material components open up new possibilities for functional design. Such components combine beneficial physical properties of different materials in a single component as for instance chemical resistance, high strength or low density. The challenge is a reliable bond between both materials to enable a long term usage. This paper deals with a form closure connection to ensure a solid connection between steel strips and high pressure die cast aluminium. Two different sizes of channel structures with width ratios of 1.0 and 1.35 are produced on a steel sheet. An ensuing flat rolling pass is performed to create undercuts with a width of up to 50 µm, enabling an interlocking of the molten aluminium in the concluding casting process. For both rolling processes the resulting geometry is analysed depending on the thickness reduction. In a subsequent high pressure die casting process, aluminium is applied resulting in a complete form filling for the coarser structure. Comparing structures with and without undercuts, only structures suited with undercuts remain gap-free after solidification contraction. The finer structure could not be filled completely; nevertheless these structures result in shear strength of up to 45 MPa transversal to the channel-direction.

5th International Conference on New Forming Technology (ICNFT 2018) 5th International Conference on New Forming Technology, ICNFT 2018, ; Les Ulis : EDP Sciences, MATEC web of conferences 190, 14010 pp. (2018). doi:10.1051/matecconf/201819014010, Published by EDP Sciences, Les Ulis

Different experiments exist for the characterization of friction and wear in metal forming, which try to reproduce the tribological conditions of the forming process as precisely as possible. A particular challenge arises from processes such as cold extrusion, with contact stresses that are several times higher than the initial flow stress of the workpiece material. In this paper, a new experimental setup for wear testing is presented and compared with available tests concerning contact pressure, surface enlargement and sliding velocity. Using a special encapsulation of the specimen, the newly developed compression-torsion-wear-test is able to scale the contact pressure to a multiple of the initial flow stress of soft aluminium.

Ring rolling is an incremental bulk metal forming process used to produce seamless rings with a wide variety of sizes as well as processible materials. Combining the principle of roll bonding and ring rolling could enable production of seamless composite rings with different materials on the inside and outside, therefore giving the opportunity to manufacture rings tailored to specific applications. A fundamental condition to achieve this is a stronger growth tendency of the inner ring compared to the outer ring, which is satisfied at respective forming temperature by choosing a material combination of a stainless steel outer ring and a tempering steel inner ring. A closed-loop controlled Finite Element ring rolling model is used in conjunction with a contact interaction subroutine for modelling generation and rupture of local material bonds by implementation of an empiric model for bond generation. Efforts to prevent oxidation of the joining surface for ring rolling trials were successful but did not lead to a lasting material bond after rolling. A lasting material bond between both rings was only achieved after adjusting the process strategy to initially allow free lateral spread of the rings in the radial roll gap without reduction to the original ring height in the axial roll gap. FE simulations provided the crucial hints for this adjustment, as they were able to show repeated bond formation and rupture during the process as well as the impact of the adjustment on the combined stress state between both rings.

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.

The tribological properties of lubricated machine elements can be improved by micro lubrication pockets. Depending on the application's tribological system, the pocket dimension needs to be adjusted to achieve an optimum in friction and wear reduction. With respect to production efficiency, a forming operation would be suited to manufacture these pockets. In this work, finite element models were developed to study the steps of the hot micro-coining process: heating, cutting, and coining of micro lubrication pockets. Experiments have been conducted to validate the models and to investigate the emissivity and heat transfer coefficient between tool and specimen by inverse modeling. It was found that the results of the simulations agree well with the experiments as long as the elastic tool deformation is taken into account, which influences the process time. Thus, suitable process parameters and limits have been determined for the coining of hemispherical lubrication pockets in stainless steel AISI 304. An initial forming temperature above 540 °C is recommended to avoid plastic deformation of the die. Complete form filling was achieved at a normalized pressure of three due to the high specimen temperature of 785 °C.

Roll bonding is a joining-by-forming process, in which two or more metals are permanently joined through pressure and plastic deformation, which causes the creation of a metallic bond. The bond formation is a complex process based on various process conditions in the joining zone, such as strain, normal pressure, temperature, strain rate, shear strain and surface condition. Since an individual variation and analysis of the influencing parameters is usually not possible during the rolling process, a specific experimental setup for the investigation of the joining mechanisms is necessary. In this paper, a testing procedure has been developed to determine the bond strength in joining-by-forming processes. The material combination chosen was AA2024/AA1050 as used in aircraft applications. AA2024 sheets are cladded with pure aluminum to improve the corrosion resistance. The performed experimental parameter study confirms the expected influencing factors and is used to determine parameters of a bonding model, which can be integrated in a finite element simulation.

Fatigue properties in the very high cycle (VHCF) regime have been studied for Alclad 2024-T351 thin sheets. Furthermore, the influence of a riblet-like surface structure has been evaluated. For this we used a rolling technique to form a riblet structure into the Al clad layer. Fatigue testing was performed at ultrasonic frequencies of 20 kHz, in order to ensure a reasonable runtime of the experiments, with fully reversed axial loading (R = −1). The clad material shows a sharp transition from early failure at some 106 cycles to run-outs at ⩾ 2 × 109 cycles. The corresponding threshold is higher for the structured samples, due to a modified stress distribution near the interface between Al cladding and Al 2024 substrate as well as due to a shorter crack length of the fatigue cracks formed in the cladding. Fatigue cracks are initiated at the surface of the clad layer and grow easily to the interface (cladding/substrate). The fatigue threshold for this compound material can be attributed to the threshold for further crack growth into the substrate.

In aeronautics, economic and environmental aspects become increasingly important. As those are very much influenced by the frictional drag of the airplane, a reduction of skin friction which causes a major portion of total aerodynamic drag is desirable. One possible approach for passive drag reduction is the application of riblets small longitudinal grooves orientated in flow direction. Through an adapted rolling process, riblets can be brought into metal sheets on a large scale. For this process a thin high-strength steel wire is wound around a work roll to structure it with the negative riblet imprint. In a subsequent step the riblet profile is rolled into the sheet material. Different parameters can influence the process and the quality of the resulting riblet structure. Those parameters that depend on the sheets sheet thickness, material strength, and composition of the sheet are discussed in this paper. Form filling is used as an indicator for riblet quality. It is found that decreasing sheet thickness is beneficial for form filling, but a process dependent minimum sheet thickness exists for which this effect will reverse. Material strength is found to have a much smaller influence on form filling. Nevertheless, harder alloys seem to need a slightly smaller thickness reduction, but higher rolling forces and pressures to achieve desired form filling. Using clad instead of bare materials has a positive influence on form filling and riblet structuring. Furthermore, riblet rolling does not reduce the fatigue strength of the clad material.

A process to produce thin steel strips with an optimized cross-section has been recently developed at the Institute of Metal Forming (IBF) as possible alternative to tailor welding and cold profile rolling for the production of tailored flat products. This process was named profile strip casting and combines the advantages of twin-roll strip casting, such as energy efficiency and compact production route, with the attractiveness of tailored products for lightweight applications. A finite difference model for the thermal description of the complete process chain including solidification, air cooling, and rolling is presented in this work. The developed simulation tool is calibrated on the temperature measurements obtained with profile casting experiments and targets the investigation of thicknesses not achievable with the available equipment. The measured surface temperature difference between the thick and thin zone of the profile for a strip with a thickness of 1.5 mm in the thin and 2.2 mm in the thick section cast using selectively coated rolls was ΔT = Tthick - Tthin = 82 K after the casting gap and 109 K at the mill exit. The simulation indicates that increasing the thickness to 3.5 mm in the thin and to 5.1 mm in the thick profile zone the surface temperature difference results 26 K after the casting gap and 126 K at the mill exit.

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.

Friction and wear of lubricated machine elements can be reduced by the introduction of lubrication pockets produced by surface texturing. Different manufacturing methods can be taken into consideration whereas a forming process offers the possibility for mass production. Hot micro coining is a forming process, which allows manufacturing of surface textures with different shapes and dimensions into a flat and deformable material, e.g. seals made of steel. In this work hemispherical and ellipsoid pockets with a maximum depth of 100 µm have been embossed into stainless steel (AISI 304). To ensure that the used process parameters will not lead to tool damage, Finite Element simulations were performed and experimentally verified. First tribological experiments were conducted on a ball-on-disk tribometer in order to study the tribological properties of hemispherical structures with pockets depths of 50 and 100 µm. Different sliding velocities were applied to study the coefficient of friction and wear volume for different lubrication conditions. A comparison between coined and not‑coined specimens demonstrates that the micro coined surface textures lead to a significant reduction in the wear volume at boundary and mixed lubrication conditions.

Conventional strips can be converted into tailored strips by further processing such as rolling and welding. Tailored strips have a thickness or thickness distribution which is designed according to the expected loads. A new approach for the production of tailored strips is the twin roll casting of profiled strips. This technology combines the advantages of direct strip casting and the production of steel strip with an optimized cross section. In this paper the achievable process limits regarding the geometry of tailored strips with varying thickness in the cross section made by strip profile rolling, twin roll casting and welding are discussed and compared. Furthermore, experiments to demonstrate the suitability of twin roll casting to produce tailored strips made of AISI 304 stainless steel are treated. A selected tailored strip geometry of 150 x 1.5 mm2 (width x thickness) with a difference in strip thickness of 33% over the width was cast.

GLARE (GLAss-fiber REinforced aluminum) is a sandwich material that combines thin aluminum sheets with intermediate layers of glass fiber that are bonded using epoxy. Due to the resulting low specific weight and high strength as well as superior deterioration resistance the material has found its application in aircraft structures. GLARE parts are typically manufactured using the so-called self-forming technique, which is a very expensive and labor-intensive manufacturing process. If it was feasible to form GLARE from flat stock material using conventional forming processes, substantial savings could be achieved. Several attempts to form GLARE from flat stock reported in the literature are restricted by the limited formability of the glass fibers and/or delamination of the layers. This work analyses the possibilities to form GLARE using shot peen forming (SPF), which is an established forming process, e.g. for the production of fuselage parts. It is shown that GLARE shows a similar deformation behavior as monolithic sheets under quasi-static indentation with single steel balls. The process limits are analyzed using SPF tests and lock-in thermography, which is a non-destructive testing procedure for the detection of delamination. A process window for shot peen forming of GLARE is established, and it is shown that curvature radii of less than 2500 mm can be accomplished with no evidence of failure, which is a typical curvature radius of fuselage components for the Airbus A380.

Lightweight design is a major driving force of innovation, especially in the automotive industry. Using hybrid components made of two or more different materials is one approach to reduce the vehicles weight and decrease fuel consumption. As a possible way to increase the stiffness of multi-material components, this paper presents a process chain to produce such components made of steel sheets and high-pressure die cast aluminium. Prior to the casting sequence the steel sheets are structured in a modified rolling process which enables continuous interlocking with the aluminium. Two structures manufactured by this rolling process are tested. The first one is a channel like structure and the second one is a channel like structure with undercuts. These undercuts enable the formation of small anchors when the molten aluminium fills them. The correlation between thickness reduction during rolling and the shape of the resulting structure was evaluated for both structures. It can be stated that channels with a d...

IOP conference series / Materials science and engineering 181, 012028(2017). doi:10.1088/1757-899X/181/1/012028, Published by Institute of Physics, London [u.a.]

Cold extrusion processes are characterized by large relative contact stresses combined with a severe surface enlargement of the workpiece. Under these process conditions a high risk for galling of workpiece material to the tool steel occurs especially in processing of aluminum and aluminum alloys. In order to reduce adhesive wear lubricants for separation of workpiece and tool surfaces are used. As a consequence additional process steps (e.g. preparation and cleaning of workpieces) are necessary. Thus, the realization of a dry forming process is aspired from an environmental and economic perspective. In this paper a surface functionalization with self-assembled-monolayers (SAM) of the tool steels AISI D2 (DIN 1.2379) and AISI H11 (DIN 1.2343) is evaluated by a process-oriented tribological test. The tribological experiment is able to resemble and scale the process conditions of cold extrusion related to relative contact stress and surface enlargement for the forming of pure aluminum (Al99.5). The effect o...

Roll bonding is used to produce semi-finished products with optimized through-thickness material properties by combining different alloys. The bond between the materials is created during rolling due to surface enlargement occurring under high pressure. Depending on the process parameters of the rolling process and the materials flow, a bond is created and maintained or destroyed directly at the roll gap exit. This delamination of the bond can be caused by length differences of the individual layers, which introduce shear stresses into the bonding zone. Thus, the elongation of each layer can be used as an indicator to evaluate the process conditions. In order to investigate which process parameters are advantageous or detrimental for a successful bond formation, this study systematically investigates important process parameters and their effects on the elongation in rolling a two layer sandwich. To account for a wide parameter spectrum a FE sandwich rolling model is introduced and validated by rolling tr...

To evaluate if the deformation in the bulk of an oligocrystalline sample during a plane strain compression test can be accurately simulated numerically with the aid of a crystal plasticity model a detailed comparison between the experimentally found and numerically predicted deformation is made. In the experiment an incremental plane strain compression (PSC) test is conducted on an oligocrystalline specimen. The incremental PSC test is complemented by electron backscatter diffraction (EBSD) measurements. The experimental results enable qualitative observation of the specimen's microstructure and orientation distribution at intermediate stages of the plastic deformation. This incremental PSC test is numerically simulated with the aid of a crystal plasticity finite element model. The numerical model accounts for the specimen's microstructure and the orientations of the crystals. Mapping the microstructure and the grain orientations on the workpiece in the finite element model provides realistic boundary conditions. Both, the experiment and numerical simulation show that the deformation in the oligocrystalline specimen with only a few grains in the deformation zone considerably differs from that of well known continuum like materials. Qualitative comparisons of the incremental PSC test and its numerical simulation showed that most of the experimentally observed behaviour of the grains and their mutual interaction can be seen in the results of the CPFEM simulation as well. From quantitative comparisons it is concluded that the two-dimensional CPFEM simulation can predict the rotations of grains in the bulk of the three-dimensional microstructure of the oligocrystalline specimen only in a qualitative manner. The lack of quantitative agreement is most probably caused by the fact that this two-dimensional, plane strain CPFEM model does not account for the interaction of the grains in the microstructure beyond the analysed cross sectional surface.

In this paper, the capabilities of the thin strip (twin roller) casting method to produce high manganese steel strip are examined. For this purpose a FeMn22C0.6 strip has been cast both using a lab-scale twin roll casting line and a more conventional process chain. The experiments show that the production of high manganese steel strip is feasible with both methods. Differences become apparent in the microstructure and chemical composition. While the strip which was produced by ingot casting and hot forming shows a homogenous grain distribution, the thin-strip-cast one shows a typical casting microstructure, consisting of dendritic and globulitic structures. In addition, the thin-strip-cast steel contains the accurate carbon content, while the hot forming route led to a loss of carbon, which influences the mechanical properties. The good potential of thin-strip casting of high manganese steels, although it still has to be improved, is confirmed.

The process of hot forging with permanent moulds is a challenge in respect to the very high thermal, mechanical and tribological loads on tools. Ensuring sufficient lifetime application of protective films can be beneficial. Initial screening experiments using PVD coated compression plates show that one of the metastable phases of alumina, the γ-phase, exhibits high strength and toughness and fulfils the requirements for a protective coating. The next important step in the development towards an industrial application is the implementation on complex tool shapes and verification in real forming experiments. After coating deposition using an industrial coating unit, coated dies were tested in forming experiments under industrial conditions. The forming experiments show an improvement of the wear resistance after 1000 forming cycles for the coated dies compared to the uncoated dies.

The recovery behavior of a commercial aluminum alloy 3103 was investigated by the means of two alternative experimental methods: stress relaxation (SR) and double tension tests (DT). In case of SR, the stress-time evolution after deformation was recorded, and for DT the yield stress after several recovery times were measured. The DT tests were further sub-divided into tests with and without external load during recovery. The results revealed that the recovery kinetics is clearly accelerated by the external stress during the SR. However, the difference between the DT and SR stresses is much larger. It is caused by continued dislocation glide after the deformation, which causes continued plastic elongation of the specimens. This is demonstrated quantitatively by appropriate evaluation models for both experiments. In contrast to DT, the SR evaluation accounts for the elastic SR due to plastic elongation, but the recovery parameters are the same ones as for DT. This makes it possible to replace DT by SR experiments, which are materially less laborious.