Prof. Dr. Marion Merklein

The hotstamping process has established itself as a manufacturing process for theproduction of ultra-high-strength materials. The process is based on amartensite transformation when quenching rates exceed a theoretical value of 27K/s. Investigations have already shown that the process-side parameters cansignificantly influence the critical cooling rate and thus the resultingmicrostructure formation. The goal of the research project is to develop afundamental understanding of the heat transfer mechanisms involved in hotstamping and the dominant influencing variables in order to derive a physicallybased model. This is the basic prerequisite for the precise representation ofall relevant sub-processes in hot stamping and will enable the transfer andapplication of simulation models and results to thermally assisted material processingmethods in the future.

Chargen-und Prozessschwankungen führen bei der Herstellung von Blechbauteilen zuAbweichungen der resultierenden Bauteileigenschaften, was nachfolgendeProzessschritte beeinflusst. Die Nutzung von mittels einer Prozessüberwachungakquirierten Daten bietet die Möglichkeit, Fertigungsprozesse in Abhängigkeitvariierender Randbedingungen anzupassen und dadurch eine konstanteBauteilqualität sicherzustellen. Voraussetzung hierfür ist die Kenntnis überprozesskettenübergreifende Zusammenhänge.

Indiesem Projekt wird die datenbasierte Prozessadaption anhand einer Prozesskettebestehend aus den Schritten Umformen, Spannen und Fügen untersucht. Ziel desForschungsvorhabens ist es, die gesamte Prozesskette mit Metamodellenabzubilden, welche mithilfe von automatisiertem maschinellem Lernen abgeleitetwerden. Dies soll die Prognose der Bauteilqualität sowie die inverse Anpassungder Teilprozessparameter zum Ausgleich von Chargen- und Prozessschwankungenermöglichen. Der Fokus am LFT liegt auf der umformtechnischenBauteilherstellung mittels Tiefziehen. Die nachfolgenden ProzessschritteSpannen und Fügen werden am Fraunhofer-Institut für Großstrukturen in derProduktionstechnik (Fh-IGP) in Rostock respektive am Laboratorium fürWerkstoff- und Fügetechnik (LWF) der Universität Paderborn untersucht.

Ausgehendvon der Definition der Modellanforderungen folgt die Datengenerierung für dieMetamodellierung durch die Korrelation der Prozessdaten undHalbzeugeigenschaften mit den resultierenden Bauteileigenschaften. Die Prognoseder Bauteileigenschaften nach dem Tiefziehen sowie die Modellierung derprozesskettenübergreifenden Zusammenhänge gestattet die Adaption derProzessparameter beim nachfolgenden Spannen und Fügen. Die Abbildung der Prozesskettedurch Metamodelle ermöglicht zudem die inverse Auslegung der Prozesskette durchdie Definition der Anforderungen an den Fügepunkt. Grundlage für die Vernetzungder Teilprozesse ist die Definition geeigneter Schnittstellen zur Realisierungdes prozessübergreifenden Datenaustauschs.

The challenge of this research project is the exact design of products made of sheet metal materials. In common practice, components manufactured in this way are nowadays designed by means of numerical simulations. This type of design is significantly influenced by the quality of the input parameters, such as the characteristic values from the material characterization. The assumption of wrong input parameters can predict a component failure too late or a component failure too early. For this very reason, large safety factors are often included in practical applications, leading to a conservative design with only moderate utilization of the material's limits. However, an improvement in the utilization of the material's potential is significantly influenced by an exact material characterization. In particular, the area of plane strain, which is often the cause of failure in deep-drawing or stretch-forming components, must be investigated and improved. Conventional characterization tests under plane strain can show nonlinearities in the strain path or are friction-induced or strongly affected by assumptions due to their test setup. Thus, the objective of this research project is to improve the characterization under plane strain for an improved failure prediction in order to shift the process limits to higher forming ratios and higher achievable drawing depths. The conventional characterization of forming limit change is to be improved to better predict failure under plane strain in order to increase the quality of input parameters for component design.

For the production of functionally integratedcomponents made of sheet metal materials, the processes of Sheet-Bulk MetalForming are suitable, which enable the production of planar components withlocal functional elements. Limitations in the diefilling of the functional elements and high die loads are a result of thedemanding forming and process conditions that occur within these processes. When transferring this process class to efficientproduction from coil, previous investigations also identified anisotropicmaterial flow and anisotropic die loads as relevant process influences whencarrying out extrusion processes from sheet coils.

Based on this, theinfluence of the component design on a multi-stage extrusion process from coilwill be analyzed numerically and experimentally within the scope of thisresearch project. In the first step,cause-effect relationships are to be investigated with regard to the local diefilling of the functional elements and the local die load when the blank andcoil geometry is varied. Furthermore, usingthese findings, the potential for controlling the material flow and thusimproving die filling by adapting the possible stage sequences is to bederived.

Tools and tool failure have major influence on the economic efficiencyof cold forging processes. In order to minimize losses due to machinedowntimes, it is necessary to accurately predict the service life. A challengein the prediction of tool life is that there is little data on the fatiguebehaviour of the used high strength steel materials. In addition, the qualityof the available data is limited, as it does not reflect the multi-axial stressconditions of cold forging. The objective of the research project is thereforeto investigate a new fatigue test, which can realistically model the stressstate occurring in cold forming tools considering both the multi-axiality andthe application of compressive prestresses. For this purpose, an elastomer madeof polyurethane is used as a pressure medium in a tool for fatigue testing. Acyclic hydrostatic pressure is applied by the repeated compression of theelastomer. The comparability of the tool stresses to the forming of steel isensured by analyzing the stresses both numerically and experimentally. Achallenge in the experimental concept is wear on the elastomer specimen, whichis to be minimized by adapting the test parameters. In order to illustraterealistic load cases, the transferability of the test results to systems underpreload is demonstrated. In a combined numerical-experimental approach, bothconventional reinforcements and new concepts for local prestressing will beinvestigated. This enables the identification of cause effect relationshipsbetween the stresses and strains caused by the reinforcement and the servicelife of the tools under realistic loads. Finally, the obtained results will beused to evaluate the potential of the test and the benefit compared toconventional fatigue tests.

A common method for increasing passenger safety invehicles is the use of high strength materials for structural safety-relevantparts. Due to the targeted adjustment of the mechanical properties, tailoredcomponents can be manufactured with locally adapted component strengths. Incase of a crash, they have the ability to absorb or transfer the occurringenergy to other regions of the vehicle. It is relevant that these componentscombine high-strength as well as ductile regions. Until now these componentsare only produced by modified press hardening operations of steel components.In association with the light-weight concept, a new opportunity to realizetailored components is the use of adapted, high-strength, hardenable aluminumcomponents. During the forming process, the properties of previously solution-annealedcomponents can be adjusted by simultaneous quenching and forming operations.The key influencing factor in quenching and forming operations is the coolingrate. In future, it will be possible to produce property-adapted structuralcomponents using high-strength aluminum alloys with the help of different localtool temperatures. Thermally coupled material models are necessary for thesimulation of this forming process.

The applicationof bulk forming processes to sheet metal, also known as sheet-bulk metalforming, extends the existing forming limits of established sheet and bulkforming processes. This enables the resource-efficient production offunctionally integrated lightweight components. In previous research, sheet-bulkmetal forming of pre-cut blanks was investigated. By using coil assemi-finished products, the advantages of manufacturing from coil in terms ofsimplified component handling and as a result higher output quantity arecombined with the advantages of sheet-bulk metal forming in terms of extendedforming limits.

The usage ofcoils, however, results in an anisotropic material flow and consequently an unequalshape of the formed components. This worsens the component application behaviorand is especially a challenge for cyclically symmetrical parts. In additionasymmetrical tool loads, critical for fatigue, occur. Considering this, theobjective is to elaborate a process understanding for extrusion of gears fromcoil. It should be investigated how the anisotropic material flow affects the formingof the individual functional elements of cyclically symmetrical components andthe tool loads.

In order to increasethe application potential of extrusion of gears from coil, another objective isto provide measures to reduce the anisotropic material flow and to improve thegeometrical accuracy of formed parts. For this purpose, different approaches formaterial flow control, such as adjusting the coil width, feed width, toolgeometry and local adaptation of friction should be analyzed for cyclicallysymmetrical parts. In addition to the effectiveness, the wear-behavior of the measureshas to be investigated in tool life tests.

Local carburization of semi-finished sheet material is a process variant to manufacture hot stamped parts with tailored properties. In this research project, the process combination of local carburization and hot stamping shall be qualified for carburization temperatures above 950 °C to reduce the required process time. The dependence of the mechanical properties and resulting microstructure on the process-relevant influencing factors will be determined by means of in-situ characterizations. Furthermore, the accuracy of the numerical simulation will be improved by extending existing material models. The material models will then be validated by additional experiments and the findings will be verified by application to a demonstrator geometry.

Part geometry is a major influence on the formation of local stress in cold forging tools. Non-constant asymmetrical cross-sections result in axial and tangential stress concentrations. This challenge can be seen in the processes of the industrial partners, where fatigue failure occurs in local elements. In order to counteract fatigue and improve process economics, the formation of local tool stresses is to be analysed using a model process. Using the process the influence of functional elements and changes in cross-section on the stress state of the die is researched. With an understanding of the mechanisms of the stress formation, adapted tool concepts will be developed, which enable a suitable improvement of the stress state depending on the part and tool geometry. To evaluate the effectiveness of the new tool designs, they will be implemented in the processes of the industrial partners. That way, the effect on tool life can be analysed for different die materials. These are usually cold working steels or cemented carbides, which react very sensibly to tensile stresses. Finally, the new tool concepts will be evaluated using the developed process understanding and the experimental results regarding the tool life.

Due totheir very high specific strength, the proportion of 7000 series aluminumalloys in vehicle bodies is steadily increasing. However, in the artificiallyaged condition T6 the materials generally exhibit a limited formability. Theaim within the project is to improve the joinability of high-strength aluminumalloys in mechanical joining processes by a process-adapted and locally limitedheat treatment and to control the material flow in order to improve the jointproperties. The use of laser radiation as a heat source enables the applicationof heat treatment layouts in the size of the joint and thus the adjustment oflocally different properties.

Consistent lightweight construction realized through high-performance and resource-efficient manufacturing processes represents a key technology for securing long-term competitiveness of the German industry while simultaneously reducing CO₂ emissions. By combining individual production technologies new opportunities are created, allowing the overcoming of limits that are currently set by conventional processes. HyConnect aims to combine forming technology with additive manufacturing in order to enhance the advantages of both process classes and to replace previous energy-intensive manufacturing methods (see Figure). In addition, digital solutions for a cross-company exchange and analysis of production data are being implemented, researched and evaluated. These are necessary to realize both the before described technical as well as environmentally added values and ensure that the safety requirements of the participating companies are met.

Within this project, the aim is to expand the existing fundamental knowledge of hybrid component manufacturing by combining laser metal deposition (LMD) and deep drawing. Interactions between the individual processes as well as influences resulting from the production sequence stand in the foreground of upcoming investigations. With the support of the below listed project partners, an exemplary bearing sleeve is planned to be manufactured with reduced material resources and CO₂ emissions.

The combination of lightweightmaterial and structural design should enable the manufacturing of componentswith high specific strengths. Aluminum alloys of the 7000 series are suitable,but they have only a low deformability at room temperature. In this context,heat-assisted process routes for hydroforming are being investigated within thescope of this project, in order to generate geometrical complex tube profiles. Forthis purpose, material models as well as process simulations of different heattreatment strategies will be created and validated. Based on this, the formingprocess will be designed and a demonstrator geometry will be produced. Finally,relevant process windows will be determined by varying the geometry and processparameters.

Inorder to increase the productivity of the production process of steel wires,the process chain of notch rolling and cyclic bending is fundamentallyanalyzed. During notch rolling, notches are formed on both sides of a sheetmetal strip, in whose areas the material fatigues and forms cracks during thesubsequent fulling process. The numerical and experimental implementation ofboth process steps enables the identification of relevant influencingparameters and their interactions. Parameters taken into account are, amongothers, the notch radius, notch angle and web thickness in notch rolling, andthe bending angle and number of cycles to breakage or to the desired residualweb thickness in cyclic bending. Numerical and experimental studies of ductiledamage are required to evaluate material separation.

The aim of this project is to conduct fundamental scientific research into joining without auxiliary element using metallic pin structures produced by forming technology, which are pressed into the joining partner or caulked after insertion into a perforated joining partner, and the joint properties that can be achieved with this. This includes the development of a fundamental understanding of the acting mechanisms with a focus on feasibility in phase 1, the optimisation of the pin structure with regard to geometry and arrangement as well as the joining process for the targeted adjustment of joining properties in phase 2 and the transferability of the technology to an extended range of applications in phase 3. The aim in phase 1 is therefore to develop a fundamental understanding of the extrusion of defined metallic pin geometries from the sheet plane using local material accumulation in order to be able to determine local changes in the material properties, such as strength. Simultaneously, different process control strategies for joining metal and FRP as well as different metals will be fundamentally researched and process windows will be derived.In the case of FRP, various process routes will be investigated with a focus on fibre-friendly injection of the pin structures or hole forming for caulking of the pin structures without delamination of the FRP. Ultrasound, vibration, infrared radiation or combinations of these methods are used to melt the matrix with the goal of identifying suitable process routes and generating an understanding of the mechanisms at work.  Based on the findings of the pin manufacturing and the results regarding the joining processes, a fundamental understanding of the process will be developed, which will allow the further development of the pin geometry and the definition of suitable simple, regular pin arrangements and dimensions in the next step. In order to meet the different requirements of the pin manufacturing process and the joining method, the adaptability of the tool and joining technology is essential. Accordingly, the adaptation on the tool side and the specific process control during pin production will be investigated in order to demonstrate the possible variations. In addition, the adaptability of the joining operation will be achieved by adapting the process control, especially in the case of metal-FRP joints, in order to react to different conditions, such as the fibre layer and layer structure of the FRP. Finally, the direction-dependent joint properties and the application behaviour of the multi-material joints joined with the developed pin geometries will be characterised and evaluated depending on the pin dimensioning and arrangement in order to identify the decisive influencing factors on the joint properties.

The operational behavior of steel components is significantly influenced by their residual stress state. On the basis of forward rod extrusion of stainless steel, methods for the controlled generation of residual stresses are being investigated, their stability under typical operating conditions is being analyzed and their effects on the operating behavior are being identified in this research project. In the first phase, basic mechanisms of the generation of residual stresses were identified. In the current second phase, parameters for a robust adjustment of the residual stress state during forming were developed, whereby lubrication in particular was identified as relevant. Furthermore, the influence of thermal and mechanical loads on the stability of the residual stresses in the components is being investigated.

Joining is an important method of productionengineering, for which reason the efficiency of such manufacturing processes ishighly relevant. Self-piercing riveting is a mechanical joining process, usinga rivet as fastener to join two or more sheets. This makes it possible to joindissimilar materials and to realize multi-material design. However, the rivetproduction is a time-consuming process, including the steps hardening andcoating in order to achieve an adequate strength and a high ductility as wellas corrosion resistance. The use of high strain hardening materials as rivetmaterials, such as high nitrogen steels, shows a huge potential concerning thereduction of production steps and thus a shortening of the rivet manufactureprocess chain since the conventional hardening, tempering and coating steps afterforming are not necessary anymore. However, the challenging high tool loadsduring cold bulk forming of high nitrogen steels represent a major challengefor the manufacturing process.

The objective is to investigate fundamentalinfluencing factors on the forming process for manufacturing rivets using highstrain hardening materials, the resulting rivet properties, the joining processand the achievable joint properties. The LFT is working on this project incollaboration with the  Laboratory for material and joining technology (LWF) at Paderborn University. At the LFT, the projectfocus is on the development of the forming tools and appropriate formingstrategies in order to realise the forming process despite the high tool loads.In this context, fundamental correlations between the forming temperature, theachievable die filling during forming and the mechanical properties of theformed rivets are investigated. By choosing a suitable forming strategy and rivetgeometry, the mechanical properties of the rivets are to be adapted accordingto the requirements of the joining process.

The comprehension of geometric part deviationsand their manufacturing and assembly related sources as well as the investigationof their effects on the function and quality of technical products builds theframework for the planned research group “process-oriented tolerance managementbased on virtual computer-aided engineering tools”. The aim of this researchgroup is the provision of holistic methods and efficient tools for thecomprehensive management of geometric deviations along the product originationprocess, which are to be validated in a model factory. In doing so, aparticular focus is set on the development of a procedure for the fruitfulcooperation of all departments involved in geometric variations management,from product development, to manufacturing, to assembly and to metrology, whichwill enable companies to quickly specify functional tolerances, which aremanufacturable and measurable, and consequently to save costs and to reduce thetime to market.

In this regard, the vision of the researchgroup is to enable the close collaboration of product development,manufacturing, assembly and metrology in computer-aided tolerancing, i. e.the joint formulation of functional tolerances, which are manufacturable andmeasurable. By enabling this close collaboration, all manufacturing andassembly related sources of later problems regarding the product function andquality can be considered already during early phases of virtual product andprocess development. As a consequence, tolerances can be specified efficientlyand optimized inspection plans as well as robust manufacturing and operatingwindows can be identified, which allows the development of robust products tobe manufactured and measured at low costs.  

Since geometric part deviations are inevitableand affect the function and quality of technical products, their managementalong the product origination process is essential for the development offunctioning products, which conform to the quality and usage requirements ofcustomers and are successful on international markets. As a consequence,tolerance management is a fundamental task in product development and reachesvarious fields of industry, from consumer to industrial goods. Due to steadilyincreasing requirements on quality and efficiency, it strongly gains importancenot only with large, but also small and medium-sized enterprises. In thiscontext, the industrial application of the scientific findings of the researchgroup will contribute to the success of the German economy.  

The subproject TP4 is part of the DFG research group 2271 - process-oriented tolerance management with virtual validation methods.

In order to achieve an integrated tolerance management, it is necessary to consider not just the process-related causes of geometric component deviations but also the deviations resulting from operation. The aim of the subproject is to identify variables influencing the operational behavior of extruded gears in the material pairing metal-plastic and to derive functional relationships between the process-related component properties and the resulting wear behavior of the pairing. Based on these findings, an operation-optimized design of the extrusion process for the production of ready-to-use gears is to be carried out and compliance with the function-relevant tolerances over the service life is to be ensured. Within the framework of the research group's overall objectives, an important contribution is made to accomplish a holistic approach within process-orientated tolerance management.

The project is based on the findings of the first funding period. During this period, the focus was on the identification of influencing variables and process-related fluctuations and its effects on the geometric accuracy and mechanical properties of steel gears produced by full forward extrusion. For this purpose, the impact extrusion process was mapped using the finite element method and a reference process was designed. The FEM model has been validated by forming experiments. The validated model was then used to identify influencing variables on the component and process side. The effects of process-related fluctuations on the resulting geometric and mechanical component properties were investigated by further forming tests. From the results, a process window was derived to ensure compliance with the functionally relevant tolerances.

In the second funding period, the effects of the process-related geometric, mechanical and tribological properties of extruded gears on geometric deviations as a result of the characteristic wear behavior of the metal-plastic material pairing during operation will be investigated. The resulting decrease in tooth width limits the adherence to the tolerances as well as the lifetime of the pairing due to tooth wear. The investigations regarding the wear behavior of the material pairing metal-plastic of the first funding period show that the properties of the metallic partner have a significant influence on the wear of both gears. Against this background, the choice of the gear material and the specific adaptation of the application-relevant component properties of the gearing within the extrusion process offer potential. Within the framework of the sub-project, the essential wear mechanisms within the material pairing are identified and the application-relevant component properties of the metallic gearing are determined on a transmission test bench. Finally, recommendations regarding the operationally optimized design of extrusion processes are derived and verified.

Aim of this sub-projectis to investigate the combination of additive manufacturing and formingtechnology for the production of tailored, functionalized and individualizedtitanium hybrid parts, fundamentally. Besides the sheet metal body also theadditively manufactured elements of the hybrid parts will be formed. This leadsto a defined work-hardening of the material and locally adjusted properties. Bythe spatially resolved application of additives in combination with in situalloying during Laser Beam Melting (LBM) a local material modification isintended. This modification is supposed to be adjustable to the demands of theforming operation as well as to the demands of the later use case.

The Center for Nanoanalysis and Electron Microscopy (CENEM) is a facility featuring cutting-edge instrumentation, techniques and expertise required for microscopic and analytical characterization of materials and devices down to the atomic scale. CENEM focuses on several complementary analysis techniques, which closely work together: Electron Microscopy, X-ray Microscopy, Cryo-TEM, Scattering Methods, Scanning Probes and Atom Probe Microscopy. With the combination of these methods new materials, particles, structures and devices are characterized not only microscopically and analytically on all length scales even down to the atomic scale but also by various in situ investigations and 3D methods. The knowledge gained through the versatile characterization methods is then used to further develop and improve materials and devices.

CENEM was established in 2010 to provide a forefront research center for the versatile characterization of materials and devices with state-of-the-art instrumentation and expertise and to intensify the interdisciplinary research. The big CENEM network represents the strong collaborations within the University of Erlangen-Nürnberg as well as the collaboration with other universities, dedicated research institutes and industry.

The support of the core facility CENEM by the German Science Foundation (DFG) and the Cluster of Excellence EXC 315 “Engineering of Advanced Materials” is gratefully acknowledged.